allows the measurement on product wafers, but for the creation of a kinetic model, a special oven with integrated measurement equipment is necessary. The discrete IR wavelength gets into resonance with the rotation or vibration of some molecule groups. The rate of reactants and products of the polymerization could be estimated by the peak height [15]. The samples were measured before and after the curing by the "FT-IR 2000 System" from Perkin-Elmer.

The broadly used method for the investigation of thermal processes like polymerization is difference scanning calorimetric. The DSC is preferred because of an accurate temperature regulation and good recording of the energy dissipation [12] [16]. The DSC-analysis was done with 7 mg CYCLOTENE 4024-40 in an inert atmosphere using a "Q2000" machine from the TA Instruments Company. The samples were dried and exposed before they were analyzed. The DSC analyzer could perform high heat-up and cool-down rates, up to 300°C/min. Higher heat-up rates leads to more inaccuracy based on a stronger overshoot of

The DSC records the heat flow to a sample during a tempering in comparison to a reference cell. The integration of the heat flow over time gives the heat dissipation of the sample. The polymerization reaction is an exothermic reaction and occurs in DSC plot by a dip/peak of the heat flow curve [14]. The integration of the heat flow peak over time allows the

The DSC experiments for the estimation of the BCB polymerization has been done in isothermal and non-isothermal mode with a constant heat rate. In figure 6 a non-isothermal signal of a BCB DSC sample is shown. The heat of the reaction of 294.4 J/g is calculated by a linear baseline. A more accurate calculation can be done by using a blind curve, which means a second measurement of the fully polymerized sample. The difference between the two curves gives the energy dissipation of the polymerization process. The blind curve also compensates the parasitic endothermic signal which came from heating up the aluminum

The heat of the reaction could be set in relation to the extent of reaction. The reaction rate dα/dt at any state of the polymerization can be estimated at any stage of the reaction by the

> *d dH dt dt H*

The total heat of reaction ΔHtotal describes the integral of heat flow from the initiation of reaction up until the full polymerized material form. The constant heat rate β allows for applying the extent of reaction over the heat rate, instead of the extent of reaction over time,

> *d dH dt dT H*

/ *total*

> / *total*

(1)

(2)

estimation of the heat transformation (enthalpy change) of the reaction.

**Figure 6.** DSC plot of the polymerization of a photo BCB sample during a non-isothermal experiment with a heat rate of 2 K/min

This is the reason why non-isothermal experiments are often used to analyze the reaction. The degree of cure at any state could be estimated by the rate of the heat of reaction ΔH to the total reaction heat ΔHtotal of a fully cured layer:

$$
\alpha = \frac{\Delta H}{\Delta H\_{total}} + \alpha\_i = \frac{1}{\Delta H\_{total}} \int \frac{dH}{dt} dt \tag{3}
$$

where αi is the initial degree of cure of the b-stage resin. The estimation has included an inaccuracy by the calculation of the total heat reaction of the complete polymerization. Linear heating experiments at different heating rates are performed. The ΔHtotal of the nonisothermal experiments with different heat rates are printed in table 2. The blind curve corrected DSC-curves are shown in figure 7. By increasing the heat rate, the reaction is shifted more and more into higher temperatures. The average total heat of reaction of the fully cured BCB film is about 281.8 ± 5.3 J/g. The results are in good correlation to the measurement of CHAN, who measured a heat of reaction for CYCLOTENE 4024-40 of 291.3 ± 8.8 J/g [17]. The measurement of the heat reaction is strongly dependent on base line settings. The estimation of the total heat reaction has in general an inaccuracy of 3 to 5%. It should be noted that the photosensitive CYCLOTENE will be shipped out with an initial degree of cure of about 47 %. Based on this, the true total heat of the reaction is around 531.7 J/g. The value is confirmed by BAIR et. al. which shows a value of 515 ± 10 J/g for the complete polymerization of a monomer BCB resin [18]. T An increased heat rate leads to more intensive reaction peak but with less time resolution, which increases the inaccuracy for a kinetic model development.

**Table 2.** Heat reaction and the peak temperature of photo BCB

**Figure 7.** Non-isothermal DSC measurement of CYCLOTENE 4026-46 versus temperature and time

The state change at the glass transition temperature caused more or less a peak in the DSC curve. This peak increases significantly with higher heat rates [19]. The uncured coated BCB has a Tg of around 60°C. In figure 7 the parasitic peak which is caused by the overwhelming of the glass transition temperature (gelation) could be seen for the two heat rates at 20K/min and 40K/min in the temperature range from 60°C up to 100°C.

The vitrification of the polymer is the important factor for the reaction kinetic caused by the gradual decreasing of the rate. When the material returns to a glassy state, the same peak is generated, but the high slope of the reaction signal absorbs the additional peak. The vitrification could only be observed by the gradual change of the reaction rate.

The gradual change is in the case of non-isothermal experiments softened by the permanent increase in the temperature. The heat signal of the state change is calculated into the total heat of reaction.

122 New Polymers for Special Applications

Heat rate [K/min] Reaction heat ΔH [J/g] TPeak [°C]

**Figure 7.** Non-isothermal DSC measurement of CYCLOTENE 4026-46 versus temperature and time

and 40K/min in the temperature range from 60°C up to 100°C.

The state change at the glass transition temperature caused more or less a peak in the DSC curve. This peak increases significantly with higher heat rates [19]. The uncured coated BCB has a Tg of around 60°C. In figure 7 the parasitic peak which is caused by the overwhelming of the glass transition temperature (gelation) could be seen for the two heat rates at 20K/min

The vitrification of the polymer is the important factor for the reaction kinetic caused by the gradual decreasing of the rate. When the material returns to a glassy state, the same peak is generated, but the high slope of the reaction signal absorbs the additional peak. The

vitrification could only be observed by the gradual change of the reaction rate.

Average ΔHtotal 281,8 ± 5,3

**Table 2.** Heat reaction and the peak temperature of photo BCB

2 289,6 233,35 5 283,6 245,24 10 276,4 255,52 20 281,8 266,00 40 277,4 277,12

Based on the knowledge of the Tg (figure 5), the influence of the vitrification could be estimated for the different heat rate experiments. The theoretical development of the Tg for the non-isothermal experiments is plotted in figure 8. The influence of the diffusion controlled part decreases with higher heat rates and has shifted the vitrification to higher temperatures.

**Figure 8.** Prediction of the glass transition temperature for the non-isothermal experiments of photo BCB

Low heat rates or isothermal experiments allow to analyze better the influence of the virtification on the reaction rate. An isothermal measurement is based on a heat up phase, a soak temperature and a cool down phase. An isothermal experiment also gives a better prediction for the real cure process in a convection oven. But an isothermal measurement method has some disadvantages in contrast to an experiment with a continuous heat rate. The extent of cure is calculated by the integration of the reaction peak signal to get the reaction heat ΔHds over the soak temperature. The higher time resolution of the reaction signal is in relation to a much smaller reaction peak. The small peak and the possibility of not a fully polymerized layer at the soak temperature, caused by vitrification or short process time, are critical impacts in estimating the total heat reaction ΔHtotal. A simple estimation of the total heat reaction can lead to an inaccurate interpretation of cure degree (see Eq.(3)). The reaction already starts during the heat up and cool down phases, which takes into account the integration of heat flow over time. The discontinuity of the temperature function produce parasitic signals. The DSC signal of a BCB soft cure at 210°C at 30 minutes is plotted in figure 9. The heat flow signal shows that there is a parasitic signal peak between isothermal and dynamic heat phases. The reaction start during the heat up phase, which can also be seen. The short process time leads only to a soft cured layer, which

means that the measured reaction heat does not equates to the total reaction heat. After the soft cure, the sample was again heated up to 350°C with a constant heat rate. A leftover heat of reaction ΔHres can be measured. The sum of ΔHds and ΔHres leads to the total heat of reaction ΔHtotal [12].

**Figure 9.** DSC measurement of a BCB soft cure process with additional non-isothermal heat up step to estimate the cure degree.

### **5. Development of a kinetic model of the polymerization reaction**

The extent of cure for a dedicated cure program can be measured by DSC experiments. But the prediction of the extent of cure and the polymerization behavior based on any temperature – time program is necessary for any process optimization with respect to energy consumption, process time or material properties. On account of the DSC experiments, a kinetic model can be set up to characterize the polymerization reaction and can be used to design and optimize the cure process. The reaction is described by a mathematical relation between time, temperature and the extent of cure. In general, special software is required to develop the kinetic equation. The kinetic model in combination with a chemical-rheological model allows a simulation of the complete polymerization reaction [14] [8].

The kinetic models are generally sorted into two types:

A phenomenological model and A chemical model.

The phenomenological model describes the reaction by a relative simple equation. This model type intentionally ignores details of the reaction with the benefit of simple application. Detailed relations and progression of the different chemical reaction steps and the relationship of the reaction groups will be ignored. The chemical reaction model takes into account the chemical steady state of the partial reactions. These types of models have a much better prediction and interpretation potential, but it is often not useable, especially for complex reactions. In contrast to a phenomenological model, the efforts involved for such a model are much higher, because many more reactions parameters are necessary, and the users need to do a chemical analysis of every reaction step. The phenomenological model can be used without chemical understanding, and can be created only by a comparison of reactants and products. The extent of cure behavior, which has been estimated by DSC experiments, was used to create a phenomenological model for BCB. The simple application makes the phenomenological model the most preferred for polymerization reactions [20] [21].

124 New Polymers for Special Applications

The kinetic models are generally sorted into two types:

A phenomenological model and

A chemical model.

estimate the cure degree.

[14] [8].

reaction ΔHtotal [12].

means that the measured reaction heat does not equates to the total reaction heat. After the soft cure, the sample was again heated up to 350°C with a constant heat rate. A leftover heat of reaction ΔHres can be measured. The sum of ΔHds and ΔHres leads to the total heat of

**Figure 9.** DSC measurement of a BCB soft cure process with additional non-isothermal heat up step to

The extent of cure for a dedicated cure program can be measured by DSC experiments. But the prediction of the extent of cure and the polymerization behavior based on any temperature – time program is necessary for any process optimization with respect to energy consumption, process time or material properties. On account of the DSC experiments, a kinetic model can be set up to characterize the polymerization reaction and can be used to design and optimize the cure process. The reaction is described by a mathematical relation between time, temperature and the extent of cure. In general, special software is required to develop the kinetic equation. The kinetic model in combination with a chemical-rheological model allows a simulation of the complete polymerization reaction

The phenomenological model describes the reaction by a relative simple equation. This model type intentionally ignores details of the reaction with the benefit of simple application. Detailed relations and progression of the different chemical reaction steps and the relationship of the reaction groups will be ignored. The chemical reaction model takes into account the chemical steady state of the partial reactions. These types of models have a much better prediction and interpretation potential, but it is often not useable, especially for complex reactions. In contrast to a phenomenological model, the efforts involved for such a

**5. Development of a kinetic model of the polymerization reaction**

The two-step polymerization reaction of BCB can be described by a pseudo first-order equation. Based on that simplification, the BCB reaction could described as follows:

$$\frac{d\alpha}{dt} = k\_e(T)f(\alpha) \tag{4}$$

Where α is the degree of cure, ke(T) is the reaction rate constant which depends on the temperature, and f(α) is the reaction model which describes the reaction order [22]. The kinetic equation such as Eq.(4) is a simplified assumption which describes the reaction rate as a product of a temperature-depending function and a fraction-transformed-depending function. There is a range of empirical mathematical forms for the reaction model [23]. Some particularly idealized reaction models are proposed in table 3.

**Table 3.** A range of reaction models which are often used in kinetic analytics

The reaction rate constant could be realized for the simplest case by the Arrhenius relation:

$$k\_e = A \exp\left(\frac{-E\_A}{RT}\right) \tag{5}$$

Where R is the universal gas constant, EA is the activation energy, T is the temperature and A is the reaction rate constant at an infinite temperature [24]. The Arrhenius equation describes a reaction in gas phase. In approximation, the relation can be used for liquid and solid materials. But influence by molecule mobility and vitrification are ignored and could be added by the adaptation of the reaction function.

The determination of the three important kinetic parameters, the two Arrhenius parameter EA and A and the reaction model f(α) is an interlinking problem. The measurement of one of the parameters, and especially the accurate estimation, influence the other two parameters.

Based on this problem they are often called a kinetic triplet [25]. There are many different methods for the determination based on DSC measurements. The method of KISSINGER has been used for the investigation of the polymerization process of BCB. It allows for a simple application with a small inaccuracy, which is also described in detail by STARINK.

The KISSINGER method is focusing on the determination of activation energy as first parameter with the most accuracy possible. The reaction rate in Eq.(4) is calculated by the Eq.(1) from a DSC experiment. The Eq.(5) is inserted into the Eq.(4). The logarithm of Eq.(4) leads to the following equation:

$$
\ln \frac{d\alpha}{dt} = -\frac{E}{RT} - \ln f(\alpha) \tag{6}
$$

For non-isothermal experiments with constant heat rate (β=dT/dt) the Eq.(6) could be changed to:

$$
\ln \frac{d\alpha}{dT} \beta = -\frac{E}{RT} - \ln f(\alpha) \,. \tag{7}
$$

The activation energy could be obtained from the slope of plots of ln(dα/dT) versus 1/T. It is estimated without any knowledge of the reaction model f(α). In literature, the principle is also termed model free kinetic (MFK). The slope in the plot should be constant. If there is a change in the slope, it indicates a phase change in the reaction. The calculation of the reaction rate by Eq.(1) leads to an inaccuracy which could have a huge impact on the kinetic equation caused by the triplet relation. The assumption of the reaction rate can be replaced by correlation of a set of non-isothermal DSC experiments. Based on Kissinger, the Eq.(7) is integrated by separation of variables and the following approach was postulated:

$$\int\_{0}^{a\_f} \frac{d\alpha}{f(a\_f)} = \frac{A}{\beta} \Big|\_{0}^{T\_{\text{inv}}} \exp(-\frac{E\_A}{RT}) dT \approx \frac{AE\_A \exp(\frac{-E\_A}{RT})}{\beta \text{R} \cdot \exp(\frac{-E\_A}{RT})^\kappa} \tag{8}$$

The Eq.(5) have been logarithm and lead to following equation:

$$\ln \frac{\beta}{T\_f^{\kappa}} = -\frac{E}{RT} + C \tag{9}$$

C is a constant for different heating rates at a constant degree of polymerization and belongs to the reaction model. Tf means the temperature of experiments with different heat rates where the same extent of cure occurs. The variable κ is a correction factor in the range of 1,92 up to 2. With the Eq.(9) the EA can determined as the slope of the plot ሺߚȀܶ ச) versus 1/ܶ independent of constant C. Three or more experiments with different heating rate β are necessary to determine the activation energy with MFK. The correct estimation of Tf is the main challenge at this method. For the normal reaction, analyzing the maximum rate method or peak method is used, which is described also in the ASTM E698 norm [26]. The peak method avoids the defective calculation of cure rate by Eq.(1). It was found that the extent of cure is approximately fixed at the maximum reaction rate, but a small error will be introduced. The peak temperatures of the non-isothermal experiment are shown in table 2. The temperature of the peak is shifted with the heat rate. The results of Eq.(9) for the five non-isothermal experiments are plotted in figure 10.

126 New Polymers for Special Applications

leads to the following equation:

changed to:

Based on this problem they are often called a kinetic triplet [25]. There are many different methods for the determination based on DSC measurements. The method of KISSINGER has been used for the investigation of the polymerization process of BCB. It allows for a simple application with a small inaccuracy, which is also described in detail by STARINK.

The KISSINGER method is focusing on the determination of activation energy as first parameter with the most accuracy possible. The reaction rate in Eq.(4) is calculated by the Eq.(1) from a DSC experiment. The Eq.(5) is inserted into the Eq.(4). The logarithm of Eq.(4)

ln ln ( ) *d E f dt RT*

For non-isothermal experiments with constant heat rate (β=dT/dt) the Eq.(6) could be

ln ln ( ) *d E f dT RT*

The activation energy could be obtained from the slope of plots of ln(dα/dT) versus 1/T. It is estimated without any knowledge of the reaction model f(α). In literature, the principle is also termed model free kinetic (MFK). The slope in the plot should be constant. If there is a change in the slope, it indicates a phase change in the reaction. The calculation of the reaction rate by Eq.(1) leads to an inaccuracy which could have a huge impact on the kinetic equation caused by the triplet relation. The assumption of the reaction rate can be replaced by correlation of a set of non-isothermal DSC experiments. Based on Kissinger, the Eq.(7) is

exp( ) ( ) exp( )

*T RT*

C is a constant for different heating rates at a constant degree of polymerization and belongs to the reaction model. Tf means the temperature of experiments with different heat rates where the same extent of cure occurs. The variable κ is a correction factor in the range of

1/ܶ independent of constant C. Three or more experiments with different heating rate β are necessary to determine the activation energy with MFK. The correct estimation of Tf is the main challenge at this method. For the normal reaction, analyzing the maximum rate method or peak method is used, which is described also in the ASTM E698 norm [26]. The

*E C*

*T A A f A*

*E AE d A E RT dT f RT E R*

(8)

(6)

. (7)

exp( )

*RT*

(9)

*A*

ச) versus

integrated by separation of variables and the following approach was postulated:

100%

ln *f*

1,92 up to 2. With the Eq.(9) the EA can determined as the slope of the plot ሺߚȀܶ

0 0

The Eq.(5) have been logarithm and lead to following equation:

*f*

**Figure 10.** Non-isothermal measurement of BCB. The slope approximate the activation energy of the polymerization reaction cure degree of around 73%

A value of 149.5kJ/mol ±1.75 kJ/mol was measured, which acknowledges the declarations of DIBBS et. al.. Different literature sources describe an activation energy in a range from 146 kJ/mol up to 197,6 kJ/mol [27] [17] [28]. A disadvantage of this method is that the values are only estimated at one degree of cure (around 73 % conversion). Therefore, in relation to the used heat rates, the value describes only the chemically controlled kinetic. With a first-order reaction model, the A has a value of 1,35 h-1 for a reference temperature T0 of 210°C:

$$\frac{da}{dt} = 1,35h^{-1}\exp\left[-\frac{149,5kJ/mol}{T}\left(\frac{1}{T} - \frac{1}{T\_0}\right)\right](1-a) \tag{10}$$

The plot for the non-isothermal experiments (Figure 11) shows that the model fits the chemically controlled regime very well. The estimated model is in good agreement with the already proposed kinetic model for BCB [2] [6]. There is a poor estimation for the diffusion controlled part, which is also described in previous publications [6] [28]. With the focus on lower cure temperatures, the influence of the diffusion controlled part increases.

The polymerization analysis aims for a better understanding of the influence of the vitrification. That demands a modulation of the normal kinetic model Eq.(10). There are many different methods for correcting the conventional reaction model by a conversion-dependent diffusion contribution. One of the most popular methods for thermosetting systems is to modify the kinetic equation by a "diffusion control function" which is described by SCHAWE [29].

**Figure 11.** Extent of polymerization based on the non-isothermal experiments and calculation based on Eq.(10)

$$\frac{d\alpha}{dt} = k\_e(T)f(\alpha)f\_d(\alpha; T) \tag{11}$$

The diffusion control function works in an electro-technical analogy like a low-pass filter. During the polymerization up to the change in the solid state, the function should not influence the reaction which means fd(T<Tg)=1. After the vitrification, the function decreases down to zero. A function which achieves this condition is proposed by SCHAWE:

$$f\_d(\alpha; T) = f\_d(T\_g(\alpha)) = 1 - \left( 1 + \left( \frac{T\_0 - T\_g(\alpha)}{D} \right)^3 \right)^{-1} \tag{12}$$

where T0 is the temperature at which the diffusion control function is zero and A is an empirical constant to smooth the function. The function is focused for a reaction which is nearly stopped in a glassy state. The difficulty of this method is to estimate the empirical diffusion control function which leads to the best fit. Another problem is the influence of the diffusion control function for the chemically controlled part.

MENCZEL describe an alternative model from RABINOWITCH for small-molecule reactions, where the rate constant k is based on a sum of a chemically controlled and diffusion controlled rate constant [8]:

$$\frac{1}{k\_{\epsilon}(T,\alpha)} = \frac{1}{k\_{r}(T)} + \frac{1}{k\_{d}(T,\alpha)}\tag{13}$$

where ke is the overall rate constant, kr is the Arrhenius ratio constant for the chemical reaction and kd is the diffusion rate constant. The overall reaction rate constant ke is governed prior the vitrification by the Arrhenius rate constant, and after vitrification is dominated by the diffusion rate constant. The disadvantage is an extended interlink problem of the parameters.

A combination of both models from SCHAWE and RABINOWITCH has been used to create an advanced model for the polymerization reaction of CYCLOTENE. Two different rate constants (kr;kd) were calculated. The overall reaction rate constant is the sum of both, but the terms are combined with a step-function, which works like a switch and allows the change between gel and glassy state kinetic models.

128 New Polymers for Special Applications

Eq.(10)

**Figure 11.** Extent of polymerization based on the non-isothermal experiments and calculation based on

*d dg*

diffusion control function for the chemically controlled part.

diffusion controlled rate constant [8]:

problem of the parameters.

()() (; ) *e d d kTf f T dt*

(11)

(12)

1 3

(13)

*T T*

The diffusion control function works in an electro-technical analogy like a low-pass filter. During the polymerization up to the change in the solid state, the function should not influence the reaction which means fd(T<Tg)=1. After the vitrification, the function decreases

down to zero. A function which achieves this condition is proposed by SCHAWE:

*f T fT D*

0 ( ) ( ; ) ( ( )) 1 1 *g*

where T0 is the temperature at which the diffusion control function is zero and A is an empirical constant to smooth the function. The function is focused for a reaction which is nearly stopped in a glassy state. The difficulty of this method is to estimate the empirical diffusion control function which leads to the best fit. Another problem is the influence of the

MENCZEL describe an alternative model from RABINOWITCH for small-molecule reactions, where the rate constant k is based on a sum of a chemically controlled and

> 111 (,) () (,) *e rd kT kT kT*

where ke is the overall rate constant, kr is the Arrhenius ratio constant for the chemical reaction and kd is the diffusion rate constant. The overall reaction rate constant ke is governed prior the vitrification by the Arrhenius rate constant, and after vitrification is dominated by the diffusion rate constant. The disadvantage is an extended interlink

$$\frac{da}{dt} = k\_r(T)f(a)f\_{step}(a;T) + k\_d(T)f(a)(1 - f\_{step}(a;T))\tag{14}$$

The step-function belongs to the process temperature and the glass transition temperature and gets a value between one and zero. For both reaction regimes, a first-order reaction model was used based on a good fit, but the model allows for the declaration of two different reaction models for chemically controlled and diffusion controlled states. The following expression was used for the step-function:

$$f\_{step}(\alpha; T) = \frac{1}{1 + \exp\{-(T - T\_{\mathcal{g}}(\alpha))B\}}\tag{15}$$

where B is a parameter to smooth the changeover between the two reaction functions. The value was set at 0.1 to get a change in temperature range of ±5°C around the glass transition temperature. The glass transition temperature depends on the extent of cure (see figure 5), and the following relation was estimated based on the measurements of TOEPPER:

$$T\_g = \frac{35}{0.054}a^2 - \frac{10865}{27}a + \frac{3070}{27} \tag{16}$$

For the kinetic model, two Arrhenius terms need to be calculated, one for the chemically controlled part and one for the diffusion controlled part. The Eq.(9) allows the calculation of the activation energy for different extents of reaction. In contrast to the peak method, the extent of reaction need to be calculated by Eq.(3). The consideration of the vitrification-heat rate relation for the estimation of the activation energy of the two regimes is necessary to see any difference. For example, the highest heat rate of 40 K/min leads to a vitrification above 90% in contrast to the slowest heat rate of 2 K/min, which vitrificates around 80%. Therefore the heat rate of 2 K/min is used to calculate the activation energy for the chemically controlled state up to 80%, above this value it is used for the calculation of the activation energy of the diffusion controlled state. The calculated activation energies for different extents of reaction are shown in figure 12.

The change in the reaction kinetic is marked by the change in the activation energy. The activation energy for the chemically controlled region has an approximate value of 157.1 ± 3.6 kJ/mol. The activation energy is slightly higher than the value which was estimated by the peak method /ASTM E698 norm (pointed line). For the diffusion controlled kinetic, a higher activation energy of around 166.1 ± 8.1 kJ/mol was measured. The increase in the activation energy in a diffusion controlled regime shows a higher temperature dependency for the reaction, which could be explained by the more temperature-dependent molecule mobility.

**Figure 12.** Activation energy estimated by the ASTM E698 norm and dependent of the degree of cure

The results for the reaction rate at the reference temperature are 1.41 hr-1 for the chemically controlled regime, and 0.27 h-1 for the diffusion controlled regime. In figure 13, the calculated degree of polymerization for the conventional (Eq.(10)) and modified kinetic model (Eq.14) is compared to a measured curve for a cure at 190°C. The modified model shows a much better fit in comparision with the real behavior above a degree of cure of approximately 80 %.

**Figure 13.** Degree of cure of a photo BCB sample at 190°C

### **6. Benefits of the kinetic model for the BCB polymerization and the relation for the polymer properties**

130 New Polymers for Special Applications

approximately 80 %.

**Figure 13.** Degree of cure of a photo BCB sample at 190°C

**Figure 12.** Activation energy estimated by the ASTM E698 norm and dependent of the degree of cure

The results for the reaction rate at the reference temperature are 1.41 hr-1 for the chemically controlled regime, and 0.27 h-1 for the diffusion controlled regime. In figure 13, the calculated degree of polymerization for the conventional (Eq.(10)) and modified kinetic model (Eq.14) is compared to a measured curve for a cure at 190°C. The modified model shows a much better fit in comparision with the real behavior above a degree of cure of The investigation of the cure behavior of BCB shows that an ongoing polymerization reaction in solid state is also possible with the requirement of a minimal process temperature above 150°C caused by the thermal activation. The enhanced model which is described in Eq.(14) allows a much better prediction for the polymerization of BCB in glassy state. The polymerization in vitrificated state is relative slow and should only use if the device is temperature critical. But the cure in glassy state could be interesting with the focus on the properties of the polymer. The stress development of BCB during the cure at 250°C is plotted in figure 14. The thermal stress of a polymer film on a substrate could be calculated as followed:

$$
\sigma\_{th} = \frac{E\_f}{1 - \nu\_f} (\alpha\_f - \alpha\_s) \Delta T \tag{17}
$$

Where Ef and υf are the Young's modulus and Poisson ratio of polymer. For silicon as substrate CTE αs are approximately 2.6 µm/mK, and BCB has a CTE of 42 µm/mK. Delta T is the temperature difference between ambient temperature and the vitrificated temperature. The polymer is in gel phase during the heat up phase and the stress is nearly zero. When the polymer vitrificates, the stress state at that point is set to zero. During the cool down, the thermal tensile stress in the layer increases based on the coefficient of thermal expansion mismatch to the substrate [8] [30] [31]. A nearly fully cured BCB has a Tg near the decomposition temperature, which means the zero stress point could not reset by processing at a higher temperature. A reheating of the BCB after the cure at 250°C above the temperature where the polymer vitrificate leads to a compressive stress, which could be seen in figure 15. The stress measurements were done by Toho Technology with a Flexus-2320-S system. In comparing the stress values at room temperature between the measurement of figure 14 and figure 15 a slightly difference occurs. This could be explained by a visco elastic behavior of the BCB which is discussed elsewhere [32].

There are trends in microelectronics towards enlarging the wafer size in combination with thinner wafers to decrease costs. In contrast to this, the signal frequency gets higher and higher, which means packaging thick dielectric polymer layers to assure good signal integrity. It results in two major challenges. The first is in handling, because the stress leads to a bow of the wafer, which is a problem for automatic handling tools and also for the processes because of the topography. The second point is that the stress in the layer stack is a continual problem for reliability. It force cracks through the layers and delamination. The decrease of the stress in the layer stack becomes more and more a focus in the future. This could also be seen by the polymer supplier who presented a couple of new polymers with low temperature cure properties in the last few years.

The stress in the polymer layer can be minimized by the optimization of the cure process with the help of a kinetic model. There are already some works which try to decrease the stress by cure process optimization [33] [34], but they unaccounted the meaning of the vitrification for the stress formation. A set of BCB experiments was done to examine the

**Figure 14.** The stress in the material during a cure at 250°C

**Figure 15.** Stress curve of a cured BCB layer on a silicon substrate measured with Flexus-2320-S

potential of stress decreasing by process optimization. In relation to the lowest temperature where vitrification occurs, and a nearly fully cured layer is obtainable, a theoretical minimal stress of around 18 MPa should be achievable. There has been shown a possible decreasing of the stress in a BCB layer from 28 MPa based on a normal cure at 250°C down to a stress value of 19 MPa [32]. Nevertheless, the process time will increase exponentially with a decrease of the temperature, and for higher volumes a decrease of the temperature down to 150°C is not economically feasible. The kinetic model allows for the creation of curing processes with more than one soak temperature, with the benefit that the processing temperature before and after the vitrification is higher to decrease the processing time, and the temperature is set lower in the phase change to a solid state. The cure of the polymer is generally performed in convection ovens. The slow temperature change in a convection oven limits the difference between the high and low temperature levels. A programmable control of the oven makes a dynamic heat process possible, which reaches the shortest possible time in combination with low stress in the polymer layer. Such a theoretical temperature profile is plotted in figure 16.

132 New Polymers for Special Applications

**Figure 14.** The stress in the material during a cure at 250°C

**Stress (MPa)**

**Stress (MPa)**

0 100 200

**Temperature (°C)** heat up cool down

**Figure 15.** Stress curve of a cured BCB layer on a silicon substrate measured with Flexus-2320-S

**Temperature (°C)**

heat up cool down

**Figure 16.** Calculated process profile with a focus on a vitrification at low temperature

The oven will heat up to a high peak temperature to achieve fast polymerization. After that the temperature is decreased to reach a vitrification at low temperature. The temperature will be ramped up after the vitrification, but the process temperature is controlled to be below the glass transition temperature, to decrease the process time and avoid devitrification. The described program aims for a vitrification at low temperature in relation to the stress in the polymer layer. DIBBS described a seven days process, which also aims to get a vitrification at a low temperature and slowly increase the temperature after that [2]. The process involves a long time processing at 150°C, and after that a slow ramp up to 250°C. The expected cure profile of DIBBS process is plotted in figure 17. The long heat up phase of 6 days results due the lack of a detailed kinetic model for the diffusion controlled part.

**Figure 17.** Calculated process profile of the low stress cure of DIBBS

He realized only a small decrease of the stress down to 24 MPa. The results of DIBBS and own experiments lead to the assumption that the ongoing of the polymerization in glassy state also influences the stress. A BCB layer on substrate was soft cured at 210°C up to 70%, which has a Tg of around 190°C. After soft cure, the layer was stored at 150°C for 100 hours and reached 93% polymerization. The stress in the layer decreased during the storage from 24 MPa down to 19 MPa [32]. A layer which was fully cured at 250°C shows no strong relaxation after the storage. This could be explained by two things. First, the partly cured film has a higher viscoplastic behavior and the material relaxed easier. Another possible reason is stress relaxation caused by the ongoing polymerization at low temperature. Further ongoing experiments are necessary to clarify in more detail the stress behavior in solid state BCB layers.

The polymer dielectric layer is an essential component in packaging technology. The temperature-activated polymerization processing of polymers has increasingly become a key parameter for the process workflow. The investigation of polymerization processes allows the optimization of the cure process down to lower temperatures. Based on the example of BCB, a simple model was developed, which also described the polymerization reaction of BCB in a solid state. The model allows for significantly decreasing the stress in the layer and also decreasing down to very low cure temperatures. The mechanical properties like Young's modulus or tensile strength of BCB will not be affected by the processing parameter changes [32]. In combination with the low-temperature curing possibilities of BCB, a decrease in the stress of about approximately 30% is possible.

### **Author details**

134 New Polymers for Special Applications

solid state BCB layers.

0°C 50°C 100°C 150°C 200°C 250°C 300°C 350°C 400°C

approximately 30% is possible.

part.

**Temperature**

phase of 6 days results due the lack of a detailed kinetic model for the diffusion controlled

He realized only a small decrease of the stress down to 24 MPa. The results of DIBBS and own experiments lead to the assumption that the ongoing of the polymerization in glassy state also influences the stress. A BCB layer on substrate was soft cured at 210°C up to 70%, which has a Tg of around 190°C. After soft cure, the layer was stored at 150°C for 100 hours and reached 93% polymerization. The stress in the layer decreased during the storage from 24 MPa down to 19 MPa [32]. A layer which was fully cured at 250°C shows no strong relaxation after the storage. This could be explained by two things. First, the partly cured film has a higher viscoplastic behavior and the material relaxed easier. Another possible reason is stress relaxation caused by the ongoing polymerization at low temperature. Further ongoing experiments are necessary to clarify in more detail the stress behavior in

0 h 50 h 100 h 150 h

**Time**

Temperature

Degree of cure

Tg

45.00%

55.00%

65.00%

75.00%

**Degree of cure**

85.00%

95.00%

The polymer dielectric layer is an essential component in packaging technology. The temperature-activated polymerization processing of polymers has increasingly become a key parameter for the process workflow. The investigation of polymerization processes allows the optimization of the cure process down to lower temperatures. Based on the example of BCB, a simple model was developed, which also described the polymerization reaction of BCB in a solid state. The model allows for significantly decreasing the stress in the layer and also decreasing down to very low cure temperatures. The mechanical properties like Young's modulus or tensile strength of BCB will not be affected by the processing parameter changes [32]. In combination with the low-temperature curing possibilities of BCB, a decrease in the stress of about

**Figure 17.** Calculated process profile of the low stress cure of DIBBS

Markus Woehrmann *Technische Universität Berlin (TUB), Berlin, Germany*

Michael Toepper *Fraunhofer Institute for Reliability and Microintegration (IZM), Berlin, Germany*

### **Acknowledgement**

The authors want to emphasize the contributions from the colleagues at Fraunhofer IZM who are not mentioned here by name. Financial support from the Deutsche Forschungsgemeinschaft (International Research Training Group GRK 1215, "Materials and Concepts for Advanced Interconnects") is gratefully acknowledged.

### **7. References**

[27] J. Kawahara, A. Nakano, K. Kinoshita, Y. Harada, M. Tagami, M. Tada und Y. Hayashi, "Highly thermal-stable, plasma-polymerized BCB polymer film," *Plasma Sources Sci. Technol.,* Bd. 12, pp. 80-88, 2003.

136 New Polymers for Special Applications

Chemical Co., 2005.

*Proceedings, 227*, 1991.

128, 1976.

301, 1989.

Bd. 26, pp. 2402-2422, 1992.

*Acta,* Bd. 404, pp. 163-176, 2003.

pp. 41-51, 2006.

[10] W. R. Roth, T. Ebbrecht und A. Beitat, Chinodimethan-Benzocyclobuten-Gleichgewicht;

[11] J. Yang, *Processing Procedures for CYCLOTENE 3000 Series Dry Etch Resins,* DOW

[12] H. Schindlbauer, G. Henkel, J. Weiss und W. Eichberger, "Quantitative Untersuchungen zum Aushärtungsverhalten von Phenoplasten mittels differentialthermoanalytischer Messungen (DSC)," *Die Angewandte. Makromolekulare Chemie,* Bd. 49, Nr. 725, pp. 115-

[13] H. E. Kissinger, "Reaction Kinetics in Differential Thermal Analysis," *Analytical*

[15] T. Stokich, W. M. Lee und R. A. Peters, "Real-Time FT-IR studies of the reaction kinetics for the polymerization of Divinyl Siloxane bis-Benzocyclobutene monomers," in *MRS*

[16] J. Dupuy, E. Leroy und A. Maazouz, "Determination of Activation Energy and Preexponential Factor of Thermoset Reaction Kinetics Using Differential Scanning Calorimetry in Scanning Mode: Influence of Baseline Shape on Different Calculation

[17] K. Chan, M. Teo und Z. Zhong, "Characterization of low-k benzocyclobutene dielectric

[18] H. E. Bair und C. A. Pryde, "Curing and glass transition behavior of a benzocyclobutene polymer," in *Society Plastics Engineers Proceedings*, Montreal, Quebec, Canada, 1991.

[20] V. M. Gonzales-Romero und N. Casillas, "Isothermal and Temperature Programmed Kinetic Studies of Thermosets," *Polymer Engineering and Science,* Bd. 29, Nr. 5, pp. 295-

[21] A. Yousefi, P. G. Lafleur und R. Gauvin, "Kinetic studies of thermoset cure reactions,"

[22] S. White und H. Hahn, "Process Modeling of Composite Materials: Residual Stress Development during Cure. Part I Model Formulation," *Journal of Composite Materials,*

[23] S. Vyazovkin, "Modell-free kinetics," *Journal of Thermal Analysis and Calorimetry,* Bd. 83,

[25] M. J. Starink, "The determination of activation energy from linear heating rate experiments: a comparison of the accuracy of isoconversion methods," *Thermochimica*

[26] B. Battenstein und H. Ritter, "Composite Membranen. Einfluß der Glastemperatur und der Zusammensetzung von Acrylsaure-Methylmethacrylat Copolymeren auf das

Permeationsverhalten von Decan," *Makromol. Chem.,* Bd. 7, pp. 767-770, 1986.

Methods," *Journal of Applied Polymer Science,* Bd. 78, pp. 2262-2271, 2000.

thin film," *Microelectronics International,* Bd. 20, pp. 11-22, 2003.

[19] R. B. Prime, "An Introduction to thermosets," 2005.

*Polymer Composites,* Bd. 18, Nr. 2, pp. 157-168, 1997.

[24] K. J. Laidler, Chemical Kinetics, Harper & Row, 1987.

Sauerstoffabfangtechnik, Chem. Berichte 121, pp. 1357-1358, 1988.

*Chemistry,* Bd. 29, Nr. 11, pp. 1702-1706, Nov. 1957. [14] G. Widmann und R. Riesen, Thermoanalyse, Hüthig, 1990.

**Chapter 5**

## **New Polymer Networks for PDLC Films Application**

Ana Isabel Mouquinho, Krasimira Petrova, Maria Teresa Barros and João Sotomayor

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48203

### **1. Introduction**

138 New Polymers for Special Applications

105-117, 1975.

1999.

pp. 793-805, May 2001.

coatings," *Wear,* Bd. 246, pp. 20-33, 2000.

[41] S. Hogmark, S. Jacobson und M. Larsson, "Design and evaluation of tribological

[42] D. Dollimore, "The application of thermal analysis in studying the thermal

[43] M. Wagner, G. CzerlinskiI und M. Pring, "The application of fitting methods to chemical kinetics," *Compur. Biol Med. Pergamon Press 1975. Vol 5. pp 105-117.,* Bd. 5, pp.

[44] A. A. Skordos und I. K. Pazitridge, "Cure Kinetics Modeling of Epoxy Resins Using a Non-Pa rametric Nu me rica I Procedure," *Polymer Engineering and Science,* Bd. 41, Nr. 5,

[45] S. Vyazovkin und C. A. Wight, "Model-free and model-fitting approaches to kinetic analysis of isothermal and nonisothermal data," *Thermochimica Acta,* Bd. 340, pp. 53-68,

decomposition of solids," *Therrnochimica Acta,* Bd. 203, pp. 7-23, 1992.

### **1.1. Polymer Dispersed Liquid Crystals**

Polymer dispersed liquid crystal (PDLC) films are a mixed phase of nematic liquid crystals (LC) commonly dispersed as inclusions in a solid polymer1. They have remarkable electrooptical behaviour since they can be switched from an opaque to a transparent state simply by application of an electric field 2.

PDLCs have been prepared by two general methods: one in which the system remains heterogeneous during the process, and another in which the system becomes heterogeneous. The first case includes the solvent induced phase separation (SIPS): the LC is mechanically dispersed in solution of a polymer. After evaporation of the solvent, the composite structure obtained is stabilised because of the polymer morphology, but is poorly controlled due to a coalescence of droplets during the preparation process. The second case is called thermally induced phase separation (TIPS) or polymerisation induced phase separation (PIPS)1.

The main advantage of preparation by the PIPS method is the possibility to obtain a composite directly between glass plates coated with conductive indium tin oxide (ITO) film without additional laminating procedures. Thus, the PDLC film is produced in one technological step, in which the phase separation of the initially homogeneous mixture and the polymerisation occur simultaneously. The PIPS process was particularly suitable for our purpose because it is quite simple and allows for high degree of control over the nal properties of the PDLC films.3

In this work the PIPS method was performed by thermal or photochemical polymerisation of a homogeneous mixture of polymerisable monomers, initiator and liquid crystal. These

© 2012 Sotomayor et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Sotomayor et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

types polymerisations occur by the mechanism of free radical polymerisation, as explained bellow.

#### **1.2. Free radical polymerisation**

Free radical polymerisation has been widely practiced method for chain polymerisation and has been used in this work. This type of polymerisation can be described in three steps: initiation, propagation and termination. It is initiated by radicals - chemical species characterised by unpaired electrons which subsequently initiate polymerisations. Radicals can be formed by monomers themselves without added initiators (self-polymerisation) but normally by the action of initiators which were added deliberately. The decomposition of initiators in radicals can be achieved electrochemically, thermally or photochemically. Depending on the polymerisation conditions, the same monomer can originate polymers that differ in configuration, molecular mass and therefore in properties. In this work more attention was given to the thermal and photochemical polymerisation.

In thermal polymerisation the N,N-azobisisobutyronitrile (AIBN) is one of the most common initiator which fragments mainly into isobutyronitrile radicals (figure 1).

**Figure 1.** Reaction scheme for the thermal decomposition of N, N-azobisisobutyronitrile (AIBN) in two isobutyronitrile radicals.

Thermal initiator decomposition normally depends on temperature, but also on the solvent used and its solubility. AIBN decomposition normally occurs to an appreciable extent at temperatures higher than 64ºC.

In photochemical polymerisation, light sensitive compounds are required as initiators. These compounds absorb light in the ultraviolet / visible wavelength range and undergo fragmentation leading to the formation of radicals 4. An example is the photochemical decomposition of 2,2-dimethoxy-2-phenylacetophenone (DMPA) which is illustrated in figure 2. The main advantage of polymerisation with UV radiation is the possibility to achieve high polymerisation rates in a fraction of second 5.

**Figure 2.** Reaction scheme for UV induced decomposition of 2,2-dimethoxy-2-phenylacetophenone (DMPA) into a benzoyl and an acetal radical fragments.

However, for thermal and photochemical initiators, it is necessary to take into account that some of the radicals formed may recombine in secondary reactions to form compounds that cannot decompose further into radicals and therefore only some radicals are useful to initiate a chain reaction.

The initiation step includes radical production (described above for the AIBN and DMPA) and the attack of these radicals to the monomer molecules. The overall mechanism for free radical chain polymerisation can be described as:

Initiation:

140 New Polymers for Special Applications

isobutyronitrile radicals.

temperatures higher than 64ºC.

CH3

CN

H3C C

achieve high polymerisation rates in a fraction of second 5.

OCH3 OCH3

(DMPA) into a benzoyl and an acetal radical fragments.

O

**1.2. Free radical polymerisation**

bellow.

types polymerisations occur by the mechanism of free radical polymerisation, as explained

Free radical polymerisation has been widely practiced method for chain polymerisation and has been used in this work. This type of polymerisation can be described in three steps: initiation, propagation and termination. It is initiated by radicals - chemical species characterised by unpaired electrons which subsequently initiate polymerisations. Radicals can be formed by monomers themselves without added initiators (self-polymerisation) but normally by the action of initiators which were added deliberately. The decomposition of initiators in radicals can be achieved electrochemically, thermally or photochemically. Depending on the polymerisation conditions, the same monomer can originate polymers that differ in configuration, molecular mass and therefore in properties. In this work more

In thermal polymerisation the N,N-azobisisobutyronitrile (AIBN) is one of the most

CH3

CN

H3C

2 N2 64ºC

O OCH3

OCH3

**Figure 1.** Reaction scheme for the thermal decomposition of N, N-azobisisobutyronitrile (AIBN) in two

Thermal initiator decomposition normally depends on temperature, but also on the solvent used and its solubility. AIBN decomposition normally occurs to an appreciable extent at

In photochemical polymerisation, light sensitive compounds are required as initiators. These compounds absorb light in the ultraviolet / visible wavelength range and undergo fragmentation leading to the formation of radicals 4. An example is the photochemical decomposition of 2,2-dimethoxy-2-phenylacetophenone (DMPA) which is illustrated in figure 2. The main advantage of polymerisation with UV radiation is the possibility to

**Figure 2.** Reaction scheme for UV induced decomposition of 2,2-dimethoxy-2-phenylacetophenone

hv

common initiator which fragments mainly into isobutyronitrile radicals (figure 1).

CH3

CH3

CN

attention was given to the thermal and photochemical polymerisation.

N N C

i. Generation of free radicals (R·) by homolytic dissociation of the initiator (I).

$$\mathsf{I} \stackrel{\*}{\to} \mathsf{2R} \mathsf{\bullet}$$

(*kd*- rate constant for the dissociation of the initiator)

ii. Reaction between radicals (R**·**) previously formed and monomer´s double bond (M1) producing new active species (M1·).

$$\mathsf{R}\bullet\mathsf{+M}\_{\mathsf{i}}\stackrel{\mathsf{k}\_{\mathsf{i}}}{\to}\mathsf{M}\_{\mathsf{i}}\bullet$$

(*k1*- rate constant for the initiation step)

Propagation:

i. Chain extension by successive addition of monomer molecules (M) to the monomer radical unities (Mn·) formed in the initiation step.

$$\mathbf{M\_n} \bullet \mathbf{M\_m} \xrightarrow{k\_n} \mathbf{M\_{n+m}}$$

(*kp*- rate constant for the propagation step)

Termination:

In last step, radicals combine or disproportionate to terminate the chain growth and form polymer molecules.

i. Combination (a simple interaction between two reactive species, Mn· and Mm**·**).

$$\mathbf{M\_{n}} \bullet \mathbf{M\_{m}} \xrightarrow{k\_{n}} \mathbf{M\_{n+m}}$$

(*ktc*- rate constant for the termination step by combination)

ii. Disproportionation (when hydrogen atom is transferred from one chain to another).

$$\mathbf{M\_{n}} \bullet \mathbf{M\_{m}} \xrightarrow{k\_{\mathrm{id}}} \mathbf{M\_{n}} + \mathbf{M\_{m}}$$

(*ktd*- rate constant for the termination step by disproportionation)

#### **1.3. The nematic liquid crystal mixture E7**

The liquid crystal used in this work was thermotropic calamitic nematic, know as E7. It is a mixture composed by three different cyanobiphenyls and one cyanotriphenyl in different proportions. The molecular structures of the different components of the nematic liquid crystal E7 are shown in figure 3. The mass percentage and physical properties of E7 components are shown in table 1 6, 7.

**Figure 3.** Molecular structures of the components of the nematic liquid crystal mixture E7.

E7 is widely used in polymer dispersed liquid crystals, and it was selected to be studied in this work, because it offers a wide range of operating temperatures in which it maintains anisotropic characteristics. The refractive indices of E7 at T=20ºC are given as: no=1.5183 (ordinary refractive index) and ne=1.7378 (extraordinary refractive index)2. It exhibits a nematic to isotropic transition at nearly TNI=58ºC. At room temperature it still exhibits a nematic phase and no other transitions between 58 and -62ºC, where it shows a glass transition. Therefore, liquid crystalline properties are extended down to the glass transition7. These features are possibly due to the multicomponent nature of E76.

**Table 1.** Components and mass composition of the Merck E7 liquid crystal.

#### **1.4. Microstructure of the polymer matrix**

During polymerisation of what is initially a homogeneous solution of monomers and LC molecules, the polymeric components grow in molecular weight, and when the two components become sufficiently incompatible, there is a decrease of LC solubility which induces formation of separate phases. The polymer matrix acquires a particular morphology with liquid crystal dispersed in its clusters1. The polymerisation conditions8, the chemical nature of the liquid crystal9 and the polymerisable monomers10 determine the morphology of polymer matrix. Conventional PDLCs have two main morphologies: Swiss cheese or polymer ball types each one with different characteristics. The observation of the microstructure of the polymer matrix was carried out by scanning electron microscopy (SEM). The dark areas in the SEM microphotographs reveal the absence of the material, which would have corresponded to the original liquid crystal domains.

142 New Polymers for Special Applications

(*ktd*- rate constant for the termination step by disproportionation)

The liquid crystal used in this work was thermotropic calamitic nematic, know as E7. It is a mixture composed by three different cyanobiphenyls and one cyanotriphenyl in different proportions. The molecular structures of the different components of the nematic liquid crystal E7 are shown in figure 3. The mass percentage and physical properties of E7

5CB C N

C N

C N

O C N

**Figure 3.** Molecular structures of the components of the nematic liquid crystal mixture E7.

**Designation Molecular formula IUPAC name Composition**

**5CB** C18H19N 4-cyano-4´-pentyl-1,1´-biphenyl 51% 35.3 **7CB** C20H23N 4-n-heptyl-4´cyanobiphenyl 25% 42.8 **8OCB** C21H25NO 4, 4´-n-octyloxycyanobiphenyl 16% 80 **5CT** C24H23N 4´n-pentyl-4-cyanotriphenyl 8% 240

During polymerisation of what is initially a homogeneous solution of monomers and LC molecules, the polymeric components grow in molecular weight, and when the two components become sufficiently incompatible, there is a decrease of LC solubility which induces formation of separate phases. The polymer matrix acquires a particular morphology with liquid crystal dispersed in its clusters1. The polymerisation conditions8, the chemical

**(w/w) TNI (ºC)**

These features are possibly due to the multicomponent nature of E76.

**Table 1.** Components and mass composition of the Merck E7 liquid crystal.

**1.4. Microstructure of the polymer matrix**

E7 is widely used in polymer dispersed liquid crystals, and it was selected to be studied in this work, because it offers a wide range of operating temperatures in which it maintains anisotropic characteristics. The refractive indices of E7 at T=20ºC are given as: no=1.5183 (ordinary refractive index) and ne=1.7378 (extraordinary refractive index)2. It exhibits a nematic to isotropic transition at nearly TNI=58ºC. At room temperature it still exhibits a nematic phase and no other transitions between 58 and -62ºC, where it shows a glass transition. Therefore, liquid crystalline properties are extended down to the glass transition7.

**1.3. The nematic liquid crystal mixture E7**

7CB

80CB

5CT

components are shown in table 1 6, 7.

The Swiss cheese morphology type (figure 4A) is characterised by liquid crystal randomly dispersed in a polymer matrix, as microdroplets. The size and shape of the LC microdroplets are in strong dependence on the parameters of preparation and the type of polymeric matrix 11, 12.

**Figure 4.** SEM micrograph for the microstructure of the polymer matrix with a A) Swiss cheese morphology type and B) polymer ball morphology type.

In PDLC morphologies with isolated LC microdroplets, two types of distribution of liquid crystal molecules are typically observed, as shown in figure 5.

The bipolar and radial configurations (figure 5) are related to droplet size and shape and depend on whether the liquid crystal prefers to align parallel or perpendicular to the polymer surface, respectively. Due to these different configurations, liquid crystal molecules would be randomly oriented within each droplet or from droplet to droplet, causing dispersion of the incident light.

**Figure 5.** Schematic illustration of bipolar (A) and radial (B) director configurations of the liquid crystals inside spherical droplets.

In the polymer ball morphology type (figure 4B) the polymerisable monomers are phase separated from the continuous liquid crystals and form micro-sized polymer balls. These micro-sized polymer balls merge and form a large polymer network structure with diversely

shaped voids in which LC exists. In this case, the LC is in a continuous phase and fills the irregular shaped voids of the polymer network, which are more or less interconnected12.

### **1.5. PDLC films transmittance**

As mentioned before, PDLC devices can be switched electrically from an opaque scattering state to a highly transparent state when a film of liquid crystal-polymer mixture is sandwiched between two conductive glass slides and the electric field is applied. The polymer matrix material is optically isotropic so it has a single refractive index (np). The liquid crystal within the micro-domains has an ordinary refractive index (no) and an extraordinary refractive index (ne) when light ray travel through the liquid crystal.

When no electric field is applied although the liquid crystal molecules would be oriented within each droplet, this orientation changes from droplet to droplet and light propagation normal to the film surface will experiment a range of refractive indices between (no) and (ne). Since the optical anisotropy of LC molecules used in PDLC is sufficiently large, the effective refractive index is not generally matched with the refractive index of the polymer (np), light will be scattered and the PDLC is opaque (OFF state). To maximize off-state scattering, the birefringence (∆n= ne-no) should be as large as possible9. On the other hand, when an electric field with sufficient strength to overcome the interactions between polymer matrix and liquid crystal at interfaces of LC domains–polymer matrix, is applied across the film liquid crystal directors within each droplet become uniformly oriented parallel to the direction of the field. If refractive index of the liquid crystal matches the refractive index of the polymeric matrix (np) the film become transparent (ON state)13.

Normally, when the applied electric field is removed, the nematic directors return to their random distribution. The film begins to appear opaque again. A schematic representation of a functional PDLC film is shown in figure 6.

**Figure 6.** Schematic representation of the averaged molecular orientation of the liquid crystal within the microdroplets without (a) and with (b) an applied electric field.

#### **1.6. Electro-optical properties of PDLCs**

Three main types of electro-optical response can be observed (figure 7). The factors contributing to the different response are many and still poorly understood. Electro-optical response of PDLC is usually measured by ramping a PDLC up and down in voltage and comparing the optical response at each voltage1. The different types of change of transmittance of PDLC films as a function of the electric field are shown in figure 7.

144 New Polymers for Special Applications

**1.5. PDLC films transmittance**

shaped voids in which LC exists. In this case, the LC is in a continuous phase and fills the irregular shaped voids of the polymer network, which are more or less interconnected12.

As mentioned before, PDLC devices can be switched electrically from an opaque scattering state to a highly transparent state when a film of liquid crystal-polymer mixture is sandwiched between two conductive glass slides and the electric field is applied. The polymer matrix material is optically isotropic so it has a single refractive index (np). The liquid crystal within the micro-domains has an ordinary refractive index (no) and an

When no electric field is applied although the liquid crystal molecules would be oriented within each droplet, this orientation changes from droplet to droplet and light propagation normal to the film surface will experiment a range of refractive indices between (no) and (ne). Since the optical anisotropy of LC molecules used in PDLC is sufficiently large, the effective refractive index is not generally matched with the refractive index of the polymer (np), light will be scattered and the PDLC is opaque (OFF state). To maximize off-state scattering, the birefringence (∆n= ne-no) should be as large as possible9. On the other hand, when an electric field with sufficient strength to overcome the interactions between polymer matrix and liquid crystal at interfaces of LC domains–polymer matrix, is applied across the film liquid crystal directors within each droplet become uniformly oriented parallel to the direction of the field. If refractive index of the liquid crystal matches the refractive index of the

Normally, when the applied electric field is removed, the nematic directors return to their random distribution. The film begins to appear opaque again. A schematic representation of

**Visible light Visible light**

**LC domain**

**Figure 6.** Schematic representation of the averaged molecular orientation of the liquid crystal within the

**ON State Transparent material** **ITO**

(a) (b)

**Polymer matrix**

Three main types of electro-optical response can be observed (figure 7). The factors contributing to the different response are many and still poorly understood. Electro-optical

extraordinary refractive index (ne) when light ray travel through the liquid crystal.

polymeric matrix (np) the film become transparent (ON state)13.

microdroplets without (a) and with (b) an applied electric field.

**1.6. Electro-optical properties of PDLCs**

**OFF State Opaque material**

a functional PDLC film is shown in figure 6.

One of the parameters used to evaluate the efficiency of electro optical response of PDLC is the electric field required to achieve 90% of the maximum transmittance and is designated as E90. Thus, the lower the value of E90, the more easily PDLC devices switch from the OFF state to ON state.

The most common electro-optical response reported in the literature for PDLC is when the increasing voltage curve is coincident to the decreasing voltage curve as shown in figure 7A. It was observed that when the electric field is removed liquid crystal molecules relax back, so that the long shaped molecules which were oriented in the same direction in each droplet return to their original random orientation.

**Figure 7.** Example of electro-optical response of PDLC A) with no hysteresis, B) with hysteresis and C) with permanent memory effect.

When these curves are not coincident the PDLC shows electrical hysteresis. The transmission with increasing voltage is lower than the transmission when the electric field is decreased. This effect is illustrated on figure 7B and can be defined as the difference between the increasing voltage curve and the decreasing voltage curve.

This difference is commonly attributed in literature to the fact that liquid crystal molecules have a tendency to remain with a certain degree of alignment even after removal of the electric field. The PDLC film is more transparent under decreasing field voltage than with increasing field, because liquid crystals remain with certain degree of alignment caused by the increasing field voltage. However, at the end of measured electro-optical response, the value of the transmittance for the initial opaque state is coincident with that of the final opaque state14.

In particular cases, not only the transmission with increasing voltage is lower than the transmission when the voltage is decreased but also a high transparency state is obtained for a long period of time at room temperature even after the applied voltage has been switched off, starting from an opaque state and later reaching a transparent state (figures 7C and 8)1.

In figures 7C and 8, the initial OFF state corresponds to the transmittance of the initial opaque state (zero electric field), the ON state to the transmittance upon applying electric field and the final OFF' state is the transmittance after removal of the applied field. The PDLC film with this electro-optical response has a permanent memory effect and this still remains a poorly understood aspect of PDLC electro-optical behaviour.

**Figure 8.** Images of PDLC devices with permanent memory effect: (a) initial OFF state, (b) upon applying electric field and (c) OFF state after removed electric field.

Various factors can influence the performance of a PDLC and therefore the permanent memory effect. However, the most frequently explanation mentioned in literature is related to the anchoring effect6, 11, 15, 16. This effect consists of the interaction between the liquid crystal molecules and polymer matrix at the interface of LC domains and polymer matrix. When an electric field is applied inducing the orientation of LC molecules, it opposes the anchoring effect. If this orientation remains even after the applied voltage has been switched off, the liquid crystal does not relax back completely but remains aligned with the electric field and a high transparency state is displayed without any more energy consumption. Therefore for this to happen there must be a weak force between LC molecules and the polymer surface.

The permanent memory effect is very revolutionary in the study of PDLCs because it allows a switch in transparency and the new state is kept without the need for energy to be spent. The only energy required is that needed to switch the PDLC from the OFF state to the ON state, therefore these devices become low power consumers and environmentally friendly.

#### **1.7. Parameters that influence the performance of PDLC films**

Despite the fact that PDLC films can be switched between a highly scattering opaque state and a clearly transparent state, these devices sometimes have disadvantages such as the high driving voltage (E90) and the insufficient maximum transmittance (TMAX)17. The electrooptical properties of PDLC films prepared by the polymerisation induced phase separation (PIPS) method depend on a numbers of factors such as the type of liquid crystal, molecular structure of polymerisable monomers and polymeric conditions. These factors could seriously affect the microstructure of the polymer matrix, the size and the shapes of the LC domains and molecular interactions between the LC molecules and the polymer matrix (anchoring effect)8, 18-20. In general and in a simple way, in a nematic liquid crystal there is a relationship between the size and the shape of LC domains and the anchoring effect. When sizes decreased the anchoring effect increase. This effect is directly related with the voltage needed to align the liquid crystal molecules in the domain. Strong anchoring forces hinder the alignment of liquid crystal molecules. Therefore a high voltage is needed to align liquid crystal molecule along the electric field, and the reverse is valid for lower anchoring effects18. Therefore, an understanding of the relationships between preparation conditions and molecular structure of the monomers on electro-optical performance of PDLCs is crucial to control device properties.

#### **1.8. Polymerisation conditions**

146 New Polymers for Special Applications

polymer surface.

field and the final OFF' state is the transmittance after removal of the applied field. The PDLC film with this electro-optical response has a permanent memory effect and this still

**Figure 8.** Images of PDLC devices with permanent memory effect: (a) initial OFF state, (b) upon

Various factors can influence the performance of a PDLC and therefore the permanent memory effect. However, the most frequently explanation mentioned in literature is related to the anchoring effect6, 11, 15, 16. This effect consists of the interaction between the liquid crystal molecules and polymer matrix at the interface of LC domains and polymer matrix. When an electric field is applied inducing the orientation of LC molecules, it opposes the anchoring effect. If this orientation remains even after the applied voltage has been switched off, the liquid crystal does not relax back completely but remains aligned with the electric field and a high transparency state is displayed without any more energy consumption. Therefore for this to happen there must be a weak force between LC molecules and the

The permanent memory effect is very revolutionary in the study of PDLCs because it allows a switch in transparency and the new state is kept without the need for energy to be spent. The only energy required is that needed to switch the PDLC from the OFF state to the ON state, therefore these devices become low power consumers and environmentally friendly.

Despite the fact that PDLC films can be switched between a highly scattering opaque state and a clearly transparent state, these devices sometimes have disadvantages such as the high driving voltage (E90) and the insufficient maximum transmittance (TMAX)17. The electrooptical properties of PDLC films prepared by the polymerisation induced phase separation (PIPS) method depend on a numbers of factors such as the type of liquid crystal, molecular structure of polymerisable monomers and polymeric conditions. These factors could seriously affect the microstructure of the polymer matrix, the size and the shapes of the LC domains and molecular interactions between the LC molecules and the polymer matrix (anchoring effect)8, 18-20. In general and in a simple way, in a nematic liquid crystal there is a relationship between the size and the shape of LC domains and the anchoring effect. When sizes decreased the anchoring effect increase. This effect is directly related with the voltage needed to align the liquid crystal molecules in the domain. Strong anchoring forces hinder the alignment of liquid crystal molecules. Therefore a high voltage is needed to align liquid

**1.7. Parameters that influence the performance of PDLC films**

remains a poorly understood aspect of PDLC electro-optical behaviour.

applying electric field and (c) OFF state after removed electric field.

The phase separation phenomenon between LC and polymeric matrix in PDLCs is a kinetic process where the transport parameters can play an important role in determining the domain size and amount of LC separated from the polymer matrix. The rate of polymerisation and also some physical parameters, such as the viscosity of the systems change the LC domain size. In general, the thermal polymerisation is slower (can take several hours), which combined with the effect of temperature that significantly decreases the viscosity of the medium may cause a promotion of the growth of LC domains by diffusion and coalescence. On the other hand, photochemical polymerisation produces a higher polymerisation rate. Smaller domain size can be achieved by higher viscosity of the systems and poor diffusion of free radicals of the polymerisable monomers during polymerisation and hence the reduced coalescence of LC domains. Therefore, small LC domains can be obtained8,20.

#### **1.9. Molecular structure of polymerisable monomers**

The electro-optical response of PDLC films is greatly influenced by molecular structure of the monomers to be incorporated as polymeric matrix10, 19, 20. In literature reports there is not a great variety of monomers used in research for applications for PDLC films, besides the most common commercial ones. To the best of our knowledge, published results of the synthesis of monomers to be incorporated as polymer matrix, were mainly dedicated to the preparation of monomers bearing a cyanobiphenyl group21, 22. These monomers with a molecular structure similar to the E7 liquid crystal molecules, could lead to better miscibility and compatibility between the polymeric matrix and LCs during the preparation of PDLCs. This could lead to a uniform phase separation and control of the LC domains, which could enhance the performance of PDLCs. On the other hand, the low miscibility with the LC molecules can lead to a premature separation from the matrix. Therefore, one of the main objectives of this work was the design and synthesis of new photochemically and thermally polymerisable monomers, mimicking some structural elements of the E7 liquid crystal molecules. Besides the specific cyanobiphenyl groups, also aromatic systems in general with and without a linear chain spacer with five methylene units were synthesised. Thus, a number of aromatic mono- and dimethacrylates with and without spacer, as well as vinylic monomers, were synthesised under microwave irradiation. A linear chain spacer with five methylene units was introduced into some of the structures between the aromatic systems and the methacrylate group to mimic the structure of 5CB, which is the liquid crystal component present with the highest percentage in E7.

### **2. Synthesis and characterisation**

As mentioned before, a number of photochemical and thermal polymerisable monomers, as well in some cases, their starting materials were prepared under microwave irradiation. The synthetic methods and the full characterisation of the compounds obtained have been described elsewhere23. All the compounds were synthesised using a microwave synthetic reactor under mild or solventless procedures. For some reactions, it was necessary to add a small amount of DMF (dimethylformamide) to obtain better homogenisation of the mixture. DMF was chosen because of its dipolar nature and the high ability to absorb microwave energy and convert it into heat, coupled with a comparatively high boiling point of 153ºC24. The protocol employed consisted in placing equivalent amounts of the corresponding reagents in an open quartz tube and then subjecting the mixture to microwave irradiation at 200W. The reaction time was optimised by following the reactions by thin layer chromatography (TLC) every minute and stopped when no starting material remained. The authors have described23 the data of compounds characterisation by 1H and 13C NMR, FTIR spectroscopy, elemental analysis and melting points. Also, a comparison between the results described in the literature for the syntheses of the compounds under classical synthetic methods with the results obtained under microwave irradiation have been made. Part of the Gel Permeation Chromatography (GPC), Differential Scanning Calorimetry (DSC) and electro-optical studies presented here have been previously published26.

### **2.1. List of synthesised monomers**

The molecular structures of all synthesised monomers are shown in figure 9. Some of the synthesised monomers have been already mentioned in the literature, but were lacking full characterisation, which was provided in ref. 23. This was the case for compounds: 125, 227, 325,4 28, 529, 630, 729, 931, 10, 1132, 1333, 14 34,15 35, 1636, 25 37, 26 38. To the best of our knowledge, the compounds 8, 12, 17, 18, 19, 20, 21, 22, 23 and 24 had not been synthesised before.

#### **2.2. Selected monomers**

Among all the synthesised monomers, the monomers containing cyano and/or spacer chain group (figure 10) were studied further due to higher chemical structural compatibility with E7 liquid crystal molecules. The monomers (with cyano and/or with spacer chains groups) properties will be compared to the analogous monomers without cyano group and/or spacer chain and the results will be presented later on.

#### **2.3. Calorimetric characterisation**

In order to determine the transition temperatures and their associated enthalpies differential scanning calorimetry (DSC) was employed. Figures 11-14 present the DSC thermograms collected on heating and on cooling run of cycle I. This cycle corresponds to the first heating and cooling runs and the cycle II to the second heating and cooling runs.

#### New Polymer Networks for PDLC Films Application 149

**Figure 9.** Molecular structures of synthesised monomers.

148 New Polymers for Special Applications

**2. Synthesis and characterisation**

**2.1. List of synthesised monomers**

chain and the results will be presented later on.

**2.3. Calorimetric characterisation**

**2.2. Selected monomers**

As mentioned before, a number of photochemical and thermal polymerisable monomers, as well in some cases, their starting materials were prepared under microwave irradiation. The synthetic methods and the full characterisation of the compounds obtained have been described elsewhere23. All the compounds were synthesised using a microwave synthetic reactor under mild or solventless procedures. For some reactions, it was necessary to add a small amount of DMF (dimethylformamide) to obtain better homogenisation of the mixture. DMF was chosen because of its dipolar nature and the high ability to absorb microwave energy and convert it into heat, coupled with a comparatively high boiling point of 153ºC24. The protocol employed consisted in placing equivalent amounts of the corresponding reagents in an open quartz tube and then subjecting the mixture to microwave irradiation at 200W. The reaction time was optimised by following the reactions by thin layer chromatography (TLC) every minute and stopped when no starting material remained. The authors have described23 the data of compounds characterisation by 1H and 13C NMR, FTIR spectroscopy, elemental analysis and melting points. Also, a comparison between the results described in the literature for the syntheses of the compounds under classical synthetic methods with the results obtained under microwave irradiation have been made. Part of the Gel Permeation Chromatography (GPC), Differential Scanning Calorimetry (DSC) and

electro-optical studies presented here have been previously published26.

The molecular structures of all synthesised monomers are shown in figure 9. Some of the synthesised monomers have been already mentioned in the literature, but were lacking full characterisation, which was provided in ref. 23. This was the case for compounds: 125, 227, 325,4 28, 529, 630, 729, 931, 10, 1132, 1333, 14 34,15 35, 1636, 25 37, 26 38. To the best of our knowledge,

Among all the synthesised monomers, the monomers containing cyano and/or spacer chain group (figure 10) were studied further due to higher chemical structural compatibility with E7 liquid crystal molecules. The monomers (with cyano and/or with spacer chains groups) properties will be compared to the analogous monomers without cyano group and/or spacer

In order to determine the transition temperatures and their associated enthalpies differential scanning calorimetry (DSC) was employed. Figures 11-14 present the DSC thermograms collected on heating and on cooling run of cycle I. This cycle corresponds to the first heating

and cooling runs and the cycle II to the second heating and cooling runs.

the compounds 8, 12, 17, 18, 19, 20, 21, 22, 23 and 24 had not been synthesised before.

As mentioned before, the melting temperature (Tm) was obtained at the peak of the melting endothermic transition, and in analogues way the crystallisation temperature either for cold- (Tcc) or for melt- (Tc) crystallisation was obtained at the peak of the respective exothermic transition peak. The glass transition temperature (Tg) was taken at the inflection point of the specific heat capacity variation in the transition. The melting and crystallisation enthalpies (∆Hm) and (∆Hcr), respectively, were determined from the areas under the curve that represents the respective transitions. The information obtained on the temperatures and enthalpies is summarised in table 2.

Figure 11A presents the DSC thermogram collected on cycle I for liquid monomer 19. The heat flux presents a discontinuity characteristic of the glass transition at Tg=-86.01ºC. At higher temperatures an exothermic peaks characteristic of cold crystallisation emerges at - 40.19ºC with an enthalpy of -97.45 Jg-1 followed by endothermic peak due to melting at Tm=- 3.62ºC with an enthalpy of 115.52 Jg-1. During the first cooling rate no transitions were detected, so it is possible that the liquid remained in a supercooled state. During cycle II the same calorimetric behaviour as that obtained for cycle I was observed.

**Figure 10.** Molecular structure of synthesised monomers with and without cyano and spacer chain group.

During cycle I for liquid monomer 1 (figure 11B) the thermogram showed endothermic peak at 18.86ºC with an enthalpy of 108.97 Jg-1 due to the melting of the crystalline form; on cooling rate no transitions were detected, thus the liquid could have remained in a supercooled state. However, during the cycle II the thermograms showed endothermic transitions assigned to a glass transitions at Tg=-89.02ºC and melting temperature at Tm=18.86ºC with an enthalpy of 107.67 Jg-1. Cold crystallisation was also observed at Tcc=- 55.13ºC with an enthalpy of -71.0 Jg-1. These thermal transitions were very similar to those observed in the thermogram obtained for liquid monomer 19 (figure 11A).

group.

enthalpies is summarised in table 2.

NC O (CH2)5O

NC O

4'-isocyanobiphenyl-4-yl methacrylate

5-(4'-cyanobiphenyl-4-yloxy)pentyl methacrylate

O (CH2)5 O

5-phenoxypentyl methacrylate

(∆Hm) and (∆Hcr), respectively, were determined from the areas under the curve that represents the respective transitions. The information obtained on the temperatures and

Figure 11A presents the DSC thermogram collected on cycle I for liquid monomer 19. The heat flux presents a discontinuity characteristic of the glass transition at Tg=-86.01ºC. At higher temperatures an exothermic peaks characteristic of cold crystallisation emerges at - 40.19ºC with an enthalpy of -97.45 Jg-1 followed by endothermic peak due to melting at Tm=- 3.62ºC with an enthalpy of 115.52 Jg-1. During the first cooling rate no transitions were detected, so it is possible that the liquid remained in a supercooled state. During cycle II the

**Figure 10.** Molecular structure of synthesised monomers with and without cyano and spacer chain

observed in the thermogram obtained for liquid monomer 19 (figure 11A).

O

Biphenyl-4-yl methacrylate

O

O

During cycle I for liquid monomer 1 (figure 11B) the thermogram showed endothermic peak at 18.86ºC with an enthalpy of 108.97 Jg-1 due to the melting of the crystalline form; on cooling rate no transitions were detected, thus the liquid could have remained in a supercooled state. However, during the cycle II the thermograms showed endothermic transitions assigned to a glass transitions at Tg=-89.02ºC and melting temperature at Tm=18.86ºC with an enthalpy of 107.67 Jg-1. Cold crystallisation was also observed at Tcc=- 55.13ºC with an enthalpy of -71.0 Jg-1. These thermal transitions were very similar to those

O

Phenyl methacrylate

O

5-(biphenyl-4-yloxy)pentyl methacrylate

O (CH2)5O

5-(4-cyanophenoxy)pentyl methacrylate

O

(1)

O

(21)

(20)

(7)

NC O

NC O (CH2)5 O

4-isocyanophenyl methacrylate

O

same calorimetric behaviour as that obtained for cycle I was observed.

O

O

(22)

(19)

(5)

(8)

**Figure 11.** DSC measurement obtained for A) monomer 19 and B) monomer 1 during cycle I at 10ºC min-1.

The solid monomers**,** 7, 5, 22, 21 and 20 had different thermal behaviour compared with the liquid monomers (19 and 1) described above. The thermograms for monomers 7, 5, 22 and 21 in figures 12 and 13, showed reversible systems that on heating gave endothermic peaks characteristic of melting transition and on cooling rate - exothermic peaks of crystallisation. The second heating and cooling rates were characterised by the same thermal behaviour.

**Figure 12.** DSC measurement obtained for A) monomer 7 and B) monomer 5 during cycle I at 10ºC min-1.

**Figure 13.** DSC measurement obtained for A) monomer 22 and B) monomer 21 during cycle I at 10ºC min-1.

Monomer 20 (figure 14A), despite having the same thermal behaviour as described above, it was not reversible. The thermogram of monomer 20 showed that the crystalline form melted and partially crystallised on cooling. During the first heating rate it exhibited endothermic peak due to melting transition with ΔHm=57.84 Jg-1 and during the cooling - exothermic peak

due to melt-crystallisation but with ∆Hcr=-18.65 Jg-1. During the second heating this crystalline solid formed melted with ΔHm=14.93 Jg-1 and partially crystallised again on cooling, but also with lower value of ΔHcr=-2.86 Jg-1. The difference between the values of enthalpies associated with endothermic and exothermic transitions suggested that part of the material previously melted could have remained in supercooled state, degraded or polymerised, and only a small fraction of the monomer crystallised during cooling run. This possibility was supported by previous studies of DSC between 25 and 200ºC, which indicated that the temperature of 100ºC seemed to be enough to initiate polymerisation of the monomer.

All these monomers revealed high tendency to organise in crystalline form that melted during the heating run followed by melt-crystallisation during the cooling run. The only thermal event during the heating run was the melting transition that showed that until melting the material was in a crystalline phase.

The insertion of a spacer chain, in monomers 20, 21 and 22 decreased the melting temperatures, compared to the analogue monomers without the spacer chain, which have made the later structures more rigid.

**Figure 14.** DSC measurement obtained for A) monomer 20 and B) monomer 8 during cycle I at 10ºC min-1.

It can be seen in figure 14B that for monomer 8 on the heating run there were two endothermic peaks at 114.95 and 125.90ºC with enthalpies of 29.09 and 60.40 Jg-1, respectively. However on the cooling run only one exothermic peak emerged at 107.72ºC with enthalpy of -14.74 Jg-1. During the second heating run part of the amorphous material underwent cold crystallisation at Tcc=63.61ºC with an enthalpy of -10.43 Jg-1 followed by two endothermic transitions at 109.60 and 123.95 ºC with 7.48 and 12.13 Jg-1, respectively. On the second cooling run no transitions were detected, so it was possible that the liquid have remained in supercooled state or that it had polymerised previously. This last possibility was supported by DSC studies between 25-200ºC. POM studies allowed better understanding of the thermal transitions, especially those involved in the thermal behaviour of monomer 8. Thus, the data from DSC and POM indicated that crystalline material transited to a mesophase state which could be assigned to the first endothermic peak, followed by transition to an isotropic state, assigned to the second endothermic peak. The peak at 114.95ºC could be assigned to solid to mesophase transition temperature (TSM) and at 125.90ºC to mesophase to isotropic transition temperature (TMI).

### **2.4. Polarised optical microscopy characterisation**

152 New Polymers for Special Applications

melting the material was in a crystalline phase.

made the later structures more rigid.

min-1.

due to melt-crystallisation but with ∆Hcr=-18.65 Jg-1. During the second heating this crystalline solid formed melted with ΔHm=14.93 Jg-1 and partially crystallised again on cooling, but also with lower value of ΔHcr=-2.86 Jg-1. The difference between the values of enthalpies associated with endothermic and exothermic transitions suggested that part of the material previously melted could have remained in supercooled state, degraded or polymerised, and only a small fraction of the monomer crystallised during cooling run. This possibility was supported by previous studies of DSC between 25 and 200ºC, which indicated that the

temperature of 100ºC seemed to be enough to initiate polymerisation of the monomer.

All these monomers revealed high tendency to organise in crystalline form that melted during the heating run followed by melt-crystallisation during the cooling run. The only thermal event during the heating run was the melting transition that showed that until

The insertion of a spacer chain, in monomers 20, 21 and 22 decreased the melting temperatures, compared to the analogue monomers without the spacer chain, which have

**Figure 14.** DSC measurement obtained for A) monomer 20 and B) monomer 8 during cycle I at 10ºC

125.90ºC to mesophase to isotropic transition temperature (TMI).

It can be seen in figure 14B that for monomer 8 on the heating run there were two endothermic peaks at 114.95 and 125.90ºC with enthalpies of 29.09 and 60.40 Jg-1, respectively. However on the cooling run only one exothermic peak emerged at 107.72ºC with enthalpy of -14.74 Jg-1. During the second heating run part of the amorphous material underwent cold crystallisation at Tcc=63.61ºC with an enthalpy of -10.43 Jg-1 followed by two endothermic transitions at 109.60 and 123.95 ºC with 7.48 and 12.13 Jg-1, respectively. On the second cooling run no transitions were detected, so it was possible that the liquid have remained in supercooled state or that it had polymerised previously. This last possibility was supported by DSC studies between 25-200ºC. POM studies allowed better understanding of the thermal transitions, especially those involved in the thermal behaviour of monomer 8. Thus, the data from DSC and POM indicated that crystalline material transited to a mesophase state which could be assigned to the first endothermic peak, followed by transition to an isotropic state, assigned to the second endothermic peak. The peak at 114.95ºC could be assigned to solid to mesophase transition temperature (TSM) and at The thermal properties of monomers were complemented with polarised optical microscopy (POM) studies. The polarised optical microscope equipped with a hot stage only allows the observation of transitions from 20 to 300ºC and therefore for the liquid monomers an analysis by POM with the same conditions used in DSC (-130 to 25ºC) was not possible. Optical observations allowed the determination of not only temperatures for each transition, but also the texture acquired by the sample. Furthermore, polarised optical microscopy can evaluate the possibility of the existence of liquid crystalline phases. An isotropic phase placed between crossed polarisers does not allow polarised light to pass through and what is observed is simply a black uniform image, unlike what is observed for a birefringent phase (optical anisotropy). The values of the thermal proprieties determined by POM are summarised in table 3.

**Table 2.** Thermal properties of monomers obtained by DSC during different cooling/heating cycles

The texture for liquid monomers 19 and 1 which may be analysed in the optical microphotographs presented in figure 15, revealed an isotropic phase on heating and the cooling runs between 30 and 100ºC and no birefringent phase was observed. The spheres that appear in the images are spacers of ITO cell.

Optical microscopy pictures of monomers 7, 5, 22, 21 and 20 are present in figures 16-20. The analysis of these figures revealed a birefringent texture in the solid state. However, this property was lost with melting transition and the liquid state showed an optic isotropic structure that remained until the crystallisation temperature where again the monomers acquire a birefringent texture.

As mentioned before, in DSC analysis monomer 8 showed a particular thermal behaviour with two endothermic transitions one after the other in the heating run. The results of combined POM and DSC studies allow a better understanding of the type of transitions involved in all the monomers and in particular in monomer 8. The optical microscopy picture presented in figure 21 shows in the heating run that the birefringent solid material first transition to a mesophase at 114ºC (TSM). This mesophase was assigned based on the observation of birefringence and fluidity. However this phase with the increase of temperature transited to an isotropic phase at 124ºC (TMI). During the cooling run, at 102ºC the isotropic phase crystallised out, so that in the second heating run transit to an isotropic phase at 124ºC. This isotropic phase crystallised at 100ºC into a birefringent material. From POM studies it was possible to observe the onset of polymerisation at 140 ºC which is in agreement with the DSC analysis.

**Figure 15.** Optical micrographs of A) monomer 19 and B) monomer 1 at specific temperatures during the heating and cooling runs at 10ºC min-1.

The melting temperature determined by DSC for all the monomers analysed seemed to be corroborated by the melting temperature determined by capillarity. However, the transition temperatures determined by POM showed small differences. Although the programme for the heating and the cooling runs was the same (10ºCmin-1) it was difficult to keep exactly the same ramp throughout the POM analysis. During POM measurements the runs of heating and cooling were stopped momentarily to take optical micrographs.

acquire a birefringent texture.

agreement with the DSC analysis.

the heating and cooling runs at 10ºC min-1.

Optical microscopy pictures of monomers 7, 5, 22, 21 and 20 are present in figures 16-20. The analysis of these figures revealed a birefringent texture in the solid state. However, this property was lost with melting transition and the liquid state showed an optic isotropic structure that remained until the crystallisation temperature where again the monomers

As mentioned before, in DSC analysis monomer 8 showed a particular thermal behaviour with two endothermic transitions one after the other in the heating run. The results of combined POM and DSC studies allow a better understanding of the type of transitions involved in all the monomers and in particular in monomer 8. The optical microscopy picture presented in figure 21 shows in the heating run that the birefringent solid material first transition to a mesophase at 114ºC (TSM). This mesophase was assigned based on the observation of birefringence and fluidity. However this phase with the increase of temperature transited to an isotropic phase at 124ºC (TMI). During the cooling run, at 102ºC the isotropic phase crystallised out, so that in the second heating run transit to an isotropic phase at 124ºC. This isotropic phase crystallised at 100ºC into a birefringent material. From POM studies it was possible to observe the onset of polymerisation at 140 ºC which is in

**Figure 15.** Optical micrographs of A) monomer 19 and B) monomer 1 at specific temperatures during

The melting temperature determined by DSC for all the monomers analysed seemed to be corroborated by the melting temperature determined by capillarity. However, the transition temperatures determined by POM showed small differences. Although the programme for the heating and the cooling runs was the same (10ºCmin-1) it was difficult to keep exactly the same ramp throughout the POM analysis. During POM measurements the runs of heating

and cooling were stopped momentarily to take optical micrographs.

**Figure 16.** Optical micrographs of monomer 7 at specific temperature during the heating and cooling run at 10ºC min-1.

**Figure 17.** Optical micrographs of monomer 5 at specific temperatures during the heating and cooling runs at 10ºC min-1.

**Figure 18.** Optical micrographs of monomer 22 at specific temperatures during the heating and cooling runs at 10ºC min-1.

**Figure 19.** Optical micrographs of monomer 21 at specific temperatures during the heating and cooling run at 10ºC min-1.

runs at 10ºC min-1.

run at 10ºC min-1.

**Figure 18.** Optical micrographs of monomer 22 at specific temperatures during the heating and cooling

**Figure 19.** Optical micrographs of monomer 21 at specific temperatures during the heating and cooling

**Figure 20.** Optical micrographs of monomer 20 at specific temperatures during the heating and cooling runs at 10ºC min-1.

**Figure 21.** Optical micrographs of monomer 8 at specific temperatures during the first and second heating at 10ºC min-1.

**Table 3.** Thermal properties of monomers obtained by POM during heating/cooling runs.

### **3. Structural and electro-optical characterisation**

#### **3.1. Molecular weights and polymer structures**

The molecular weight of the polymer matrix plays an important role in the morphologies of the matrix and therefore in the liquid crystal domain size and shape. If the polymer chain increases with an increase in molecular weight leading to a higher network density, the size of liquid crystal domains should decrease. This promotes the interaction between polymer and liquid crystal molecules; the opposite is valid for smaller chains. As previously mentioned, the size and the shapes of liquid crystal domains are correlated with the anchoring strength on the interface between the liquid crystal molecules and polymer molecules. The anchoring strength determines the electric field needed to achieve the transparent state in the PDLC. Generally, the strength of the field needed is inversely proportional to the liquid crystal domain size. A number of factors influence the electrooptical response of PDLCs and the investigation of these parameters allows control of the performance of these devices. So, it is important to understand and establish the effect of the molecular structure of the polymerisable monomers on the molecular weight of the polymer and the effect of molecular weight on the morphology and electro-optical properties.

### **3.2. Characterisation of molecular weights**

Gel permeation chromatography (GPC) is a liquid chromatography technique that separates molecules according to sizes and not according to chemical affinities toward the porous substrate39.Large polymer molecules with higher molecular weights can move into pores but since there are only few large pores available to them, they reside, on average, less time in pores than smaller molecules. Therefore, larger molecules are eluted first. On the other hand, smaller polymer molecules with lower molecular weights can fit into the small pores and penetrate into a larger number of pores. So, the elution time increases with decreasing molecular size40. This correlation between elution time and molecular weight was observed for the polymers analysed. The values of average molecular weights (Mn and Mw) and polydispersity index (PDI) for mixture of monomer/glycidyl methacrylate (50/50 w/w) after thermal and photochemical polymerisations are summarised in Tables 4 and 5, respectively. It was possible to observe differences between polymer matrixes prepared by thermal polymerisation and by photochemical polymerisation. For thermal polymerisation, the polymers had a number average molecular weight, Mn, around e8 or e12, and for photochemical polymerisation polydispersions were higher and Mn was around e12. Generally, the polymers with lower molecular weight possessed lower polydispersion, indicating that there were chains with more uniform lengths (sizes), but no correlation was observed with the molecular structure of the respective monomers.

#### **3.3. Microstructure of the polymer matrix**

158 New Polymers for Special Applications

**Heating run at 10ºC min-1 Cooling run at 10ºC min-1 Melting Melt-crystallisation**

**Monomer Initial optical property T/ºC Optical property Tc/ºC Optical property**  Isotropic - - - - Isotropic - - - - Birefringent 84(Tm) Isotropic 35 Birefringent Birefringent 108(Tm) Isotropic 87 Birefringent Birefringent 78(Tm) Isotropic 63 Birefringent Birefringent 49(Tm) Isotropic 30 Birefringent Birefringent 35(Tm) Isotropic - -

**8**Birefringent 114 (TSM) Birefringent 102 124 (TMI) Isotropic Birefringent

The molecular weight of the polymer matrix plays an important role in the morphologies of the matrix and therefore in the liquid crystal domain size and shape. If the polymer chain increases with an increase in molecular weight leading to a higher network density, the size of liquid crystal domains should decrease. This promotes the interaction between polymer and liquid crystal molecules; the opposite is valid for smaller chains. As previously mentioned, the size and the shapes of liquid crystal domains are correlated with the anchoring strength on the interface between the liquid crystal molecules and polymer molecules. The anchoring strength determines the electric field needed to achieve the transparent state in the PDLC. Generally, the strength of the field needed is inversely proportional to the liquid crystal domain size. A number of factors influence the electrooptical response of PDLCs and the investigation of these parameters allows control of the performance of these devices. So, it is important to understand and establish the effect of the molecular structure of the polymerisable monomers on the molecular weight of the polymer

and the effect of molecular weight on the morphology and electro-optical properties.

Gel permeation chromatography (GPC) is a liquid chromatography technique that separates molecules according to sizes and not according to chemical affinities toward the porous substrate39.Large polymer molecules with higher molecular weights can move into pores but since there are only few large pores available to them, they reside, on average, less time in pores than smaller molecules. Therefore, larger molecules are eluted first. On the other hand, smaller polymer molecules with lower molecular weights can fit into the small pores and penetrate into a larger number of pores. So, the elution time increases with decreasing molecular size40. This correlation between elution time and molecular weight was observed for the polymers analysed. The values of average molecular weights (Mn and Mw) and

**Table 3.** Thermal properties of monomers obtained by POM during heating/cooling runs.

**3. Structural and electro-optical characterisation**

**3.1. Molecular weights and polymer structures**

**3.2. Characterisation of molecular weights**

The microscopic images of the microstructure of the polymeric matrix were evaluated by scanning electronic microscopy (SEM). Some microscopic images can be assigned to the Swiss cheese (figure 22A) or polymer ball (figure 22B) types of morphology, but for some samples these morphologies were not uniform across the sample. For other samples it was not possible to identify any morphology. These results can suggest that the chemical affinity of polymerisable monomers with liquid crystal molecules due to their similarities in chemical structure produced smaller phase separation and therefore the E7 liquid crystal molecules

**Table 4.** Values of average molecular weights (Mn and Mw) and polydispersity index (PDI) for polymers prepared with a mixture of monomer/glycidyl methacrylate (50/50 w/w) by thermal polymerisation.

**Table 5.** Values of average molecular weights (Mn and Mw) and polydispersity index (PD) for polymers prepared with a mixture of monomer/glycidyl methacrylate (50/50 w/w) by photochemical polymerisation.

**Figure 22.** Examples of SEM micrographs for the microstructure of the polymer matrix prepared by thermal polymerisation of A) monomer 19 and B) monomer 7.

would be highly embedded in the matrix. It was also possible to observe that there could not be established a correlation between the molecular structure of the polymerisable monomers and polymerisation conditions. Although the samples for analysis by GPC and SEM were not prepared under the same conditions, it was not possible to establish a correlation between the molecular weight of polymers and the microstructure of the polymer matrix.

#### **3.4. Electro-optical characterisation**

The PDLCs prepared with the monomers synthesised showed poor electro-optical response (figure 23). In most of the studies the PDLC cells showed good opacity after polymerisation with transmittance of the initial opaque state TOFF�0%. However, the electro-optical study did not reveal significant difference on the transmission of the sample with the application of electric field up to 400V (20V μm-1), the maximum transmittance, TMAX, being less than 4%. The reason for this non-ideal behaviour is unclear, however, it was possible to suggest explications.

**Figure 23.** General electro-optical response for PDLC films prepared by the PIPS method with the monomers synthesised + E7 in a ratio of 30/70 (w/w).

As mentioned before, the electro-optical properties of PDLC films depend on numerous parameters, namely, the structure and the molecular weight of the polymeric matrix (size and shape of the liquid crystal domains). These factors control the dynamics between alignment and random distribution when the electric field is applied and after removal of the electric field, respectively. These dynamics are very much dependent on anchoring force. This force is related to the interactions between the liquid crystal molecules and the polymeric matrix at the interface of LC domains and polymer matrix. The electro-optical results for PDLC films studied in this work suggested that higher molecular affinity between the polymeric matrix and the liquid crystal could increase the anchoring effect.

The similar molecular structure between the molecules of monomer and the LC could have led to higher affinity between molecules of polymer and molecules of liquid crystal in the interfaces matrix–LC domains. In this way, the electric field applied could not overcome the interactions at the interface and the PDLC remained in an opaque state. This was due to the fact that if the liquid crystal molecules do not align themselves along the electric field, the PDLC films appear opaque. The higher affinity between molecular structure of polymer matrix and LC molecules caused an increase in anchoring strength, i.e. it hindered the alignment of the liquid crystal molecules and could be the cause of the low electro-optical response of PDLC films.

In conclusion, the chemical affinity between the polymerisable monomers and the liquid crystal molecules was able to provide better affinity between the two which created more homogeneous mixture with consequent phase separation, in which the liquid crystal was uniformly dispersed in the polymer matrix. However, this affinity could be high enough to increase the anchoring effect not allowing a good electro-optical response.

### **4. Conclusions**

160 New Polymers for Special Applications

**Figure 22.** Examples of SEM micrographs for the microstructure of the polymer matrix prepared by

molecular weight of polymers and the microstructure of the polymer matrix.

would be highly embedded in the matrix. It was also possible to observe that there could not be established a correlation between the molecular structure of the polymerisable monomers and polymerisation conditions. Although the samples for analysis by GPC and SEM were not prepared under the same conditions, it was not possible to establish a correlation between the

(19) NC O

O

(7)

The PDLCs prepared with the monomers synthesised showed poor electro-optical response (figure 23). In most of the studies the PDLC cells showed good opacity after polymerisation with transmittance of the initial opaque state TOFF�0%. However, the electro-optical study did not reveal significant difference on the transmission of the sample with the application of electric field up to 400V (20V μm-1), the maximum transmittance, TMAX, being less than 4%. The reason

for this non-ideal behaviour is unclear, however, it was possible to suggest explications.

**Figure 23.** General electro-optical response for PDLC films prepared by the PIPS method with the

0 5 10 15 20 **Electric field (V/μm)**

thermal polymerisation of A) monomer 19 and B) monomer 7.

O

**3.4. Electro-optical characterisation**

O (CH2)5 O

monomers synthesised + E7 in a ratio of 30/70 (w/w).

0 20

40 60

**Transmittance(%)**

80 100 A series of new monomers with structurally diverse functionalisation was successfully synthesised under microwave irradiation. These results were compared with those obtained following classical methods. The comparison of these methodologies clearly indicated the considerable reduction in time and amount of solvent used in the reactions performed under microwave irradiation. Therefore, two important advantages, reduction in time and solvents, obtained by the use of microwave irradiation were demonstrated. Beyond that, the majority of monomers were synthesised in one step and in some cases in two steps, which provided simple and effective methods of synthesis. The GPC analysis of co-polymers (monomers synthesised and glicidyl methacrylate) revealed that it was not possible to establish a correlation between the molecular structure of the polymerisable monomers and the molecular weight of polymers. The microstructure of the polymer matrix was evaluated by SEM. From these studies, it was also not possible to establish a correlation between the molecular structure and molecular weight of the respective monomers. Nevertheless, the absence of a typical morphology on most microscopic images seemed to indicate that the liquid crystal remained highly impregnated in the polymer matrix and thus, their microdomains could not be visualised. The phase transformations of the monomers were investigated by DSC and complemented by POM. In general, the thermal behaviour seems to

be that of a reversible system and the monomers show a high tendency to melting followed by a melt-crystallisation. The spacer chain in the molecular structure of the monomers decreases the melting temperature in comparison with the respective monomers without this chain. The POM analyses confirmed the transition determined by DSC and allowed an observation of the texture acquired by the sample. The analysis of solid monomers showed a birefringent texture in the solid state that where lost in the melting transition where an isotropic phase is acquired. However, monomer 8 revealed a different thermal behaviour with a mesophase between 114-124ºC. On the other hand, the liquid monomers showed isotropic properties in all temperature ranges.

The electro-optical responses of all PDLCs prepared by the PIPS method with the composites of monomers synthesised–E7 exhibit a lower transmittance upon the application of an electric field. This behaviour can be interpreted as the result of a higher anchoring effect. Thus, even the application of a higher electric field could not orient the E7 molecules along the direction of the electric field and they scattered strongly the incident light. The similarity in molecular structure between the molecules of polymerisable monomers and LC should allow a good chemical affinity between them and a better homogenisation of the mixture and a uniform phase separation can be achieved but it should not be too high to avoid a higher anchoring effect which resulted in a low electro-optical response of the PDLCs.

### **Author details**

Ana Isabel Mouquinho, Krasimira Petrova, Maria Teresa Barros and João Sotomayor *REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal*

### **Acknowledgement**

This work was supported by Fundação para a Ciência e Tecnologia through Project PTDC/CTM/69145/2006.

### **5. References**

[6] Bedjaoui, L.; Gogibus, N.; Ewen, B.; Pakula, T.; Coqueret, X.; Benmouna, M.; Maschke, U., Preferential Solvation of the eutectic mixture of liquid crystal E7 in a polysiloxane. *Polymer* 2004, 45, 6555-6560.

162 New Polymers for Special Applications

**Author details**

**Acknowledgement**

PTDC/CTM/69145/2006.

**5. References**

isotropic properties in all temperature ranges.

*Universidade Nova de Lisboa, Caparica, Portugal*

effect which resulted in a low electro-optical response of the PDLCs.

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Ana Isabel Mouquinho, Krasimira Petrova, Maria Teresa Barros and João Sotomayor *REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia,*

This work was supported by Fundação para a Ciência e Tecnologia through Project

[1] Drazaic, P. S., Liquid Crystal Dispersions World Scientific Publishing: Singapore, 1995. [2] Benkhaled, L.; Coqueret, X.; Traisnel, A.; Maschke, U.; Mechernene, L.; Benmouna, M., A Comparative Study of UV and EB-Cured PDLC Films via Electro-Optical Properties.

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be that of a reversible system and the monomers show a high tendency to melting followed by a melt-crystallisation. The spacer chain in the molecular structure of the monomers decreases the melting temperature in comparison with the respective monomers without this chain. The POM analyses confirmed the transition determined by DSC and allowed an observation of the texture acquired by the sample. The analysis of solid monomers showed a birefringent texture in the solid state that where lost in the melting transition where an isotropic phase is acquired. However, monomer 8 revealed a different thermal behaviour with a mesophase between 114-124ºC. On the other hand, the liquid monomers showed

The electro-optical responses of all PDLCs prepared by the PIPS method with the composites of monomers synthesised–E7 exhibit a lower transmittance upon the application of an electric field. This behaviour can be interpreted as the result of a higher anchoring effect. Thus, even the application of a higher electric field could not orient the E7 molecules along the direction of the electric field and they scattered strongly the incident light. The similarity in molecular structure between the molecules of polymerisable monomers and LC should allow a good chemical affinity between them and a better homogenisation of the mixture and a uniform phase separation can be achieved but it should not be too high to avoid a higher anchoring

## **Photopolymers for Use as Holographic Media**

Michael R. Gleeson, Jinxin Guo and John T. Sheridan

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/46242

### **1. Introduction**

164 New Polymers for Special Applications

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[25] http://www.sigmaaldrich.com/ (December,2011).

Photopolymers were first introduced as a holographic recording material by Close *et al*. in 1969, [1]. Since then numerous systems have been examined, but only a small number have become commercially available, [2]. Polymer materials have several advantages. Because thick layers can be fabricated they act as true volume materials giving high diffraction efficiency and good angular selectivity. Most of the materials are self–developing or require only some simple post–processing, such as an exposure to light or heat treatment. This eliminates the need for wet chemical development, which makes photopolymers suitable for applications such as holographic embedded photopolymer waveguides, [3-6] and holographic data storage [7-18].

Photopolymers generally consist of a monomer, a photosensitive dye and an initiator. They can either be liquid or dry layer systems. The dry photopolymers usually contain a polymeric binder in addition to the other components. As mentioned above the first photopolymer material used for holographic recording, was reported by Close *et al*. [1]. This liquid state material consisted of a mixture of acrylamide and metal acrylate monomers and a photocatalyst methylene blue. Sadlej and Smolinska [19] improved the original system proposed by Close by including a poly-vinylalcohol (PVA) binder which allowed the production of dry photopolymer layers.

In the eighties, Calixto [20] continued the work on acrylamide-based systems. The material contained acrylamide monomer, TEA as an electron donor, methylene blue photosensitizer and PVA as a binder. Blaya *et al*., [21], improved the sensitivity of the acrylamide material for recording at 633 nm by changing the crosslinker, *N*,*N*-dihydroethylenebisacrylamide. A hybrid material containing acrylamide and acrylic acid as monomers was proposed by Zhao *et al*., [22]. The material uses methylene blue as the photosensitizer, TEA and ptoluenesulfonic acid as sensitizers and gelatin as a binder.

While acrylamide based free-radical systems have received much attention in the literature other materials offer advantages when it comes to the development of practical storage

media. Trentler *et al*., [23] developed an epoxy resin photopolymer material with a solid matrix, which is formed in situ as the epoxy cures at room temperature. The unreacted vinyl monomers within the material are subsequently photo-polymerized during hologram recording. One of the key features of this type of material is the separation of the epoxy and vinyl polymerizations. This separation allows for a large index contrast to be developed in holograms when components are optimized. This material is functional in thick formats (several millimetres), which enables narrow angular bandwidth and high diffraction efficiency. A dynamic range (M/#) up to 13 has been measured in these materials.

More recently, works have been carried out on the development of a commercial grade acrylate photopolymer with the same two step polymerisation [24], offering very fast response of the refractive index modulation with respect to the recording dosage, particularly at lower power densities, very high storage resolution, and very large refractive index modulation [25,26].

Extensive work has been carried out, in both industry and academia, on the development of these photopolymer materials and more recently on the understanding of the photochemical kinetics associated with them. In order to maximise the potential of these materials for various applications, the necessity for a physically comprehensive theoretical model of the effects which occur during photo-polymerization is becoming ever more important. Providing such a model will enable potential trends in a material's performance to be recognized and optimised, [27]. Such models allow simulations of ratios of various key material components to be made, yielding indications of the most suitable material compositions in order to improve material performance.

In this chapter we examine some recently published results on the Non-local Photopolymerization Driven Diffusion (NPDD) model, [26-32]. This model provides a comprehensive theoretical representation of the processes, which occur during free radical photo-polymerization. The physically realistic model enables predictions to be made about a number of very different photopolymer materials [26-32]. We present several extensions to the previous model in particular allowing for spatially and temporally varying primary radical generation, oxygen inhibition, dark reactions and chain transfer effects. We then apply this model to analyse a number of effects observed to take place during holographic grating formation in an acrylamide/polyvinylalcohol (AA/PVA) based photopolymer and compare experimental results and the predictions of the model with the aim of characterising these effects.

The chapter is structured as follows: In Section 2 we will briefly examine some of the methods used to measure and monitor the optical performance of holographic gratings in photopolymers. In Section 3 we briefly describe the fundamental photo-kinetic processes, which occur in photopolymers during holographic exposure. Then based on these photokinetic processes we construct a set of first order coupled differential equations which represent the temporal and spatial variation of the concentrations of constituents of the photopolymer, which form the basis of the Non-local Photo-polymerization Driven Diffusion (NPDD) model. We then analyse some of the simulated behaviour of this model under given physical conditions to examine its physicality and present some of the predictions made which offer potential methods to improve a photopolymers holographic performance. Following these predictions, in Section 4 we then attempt to improve an acrylamide/polyvinylalcohol (AA/PVA) based photopolymer's spatial frequency response which will increase its high density storage resolution. In Section 5 a brief conclusion is presented offering potential direction for future advancements in the area of photopolymer development.