polyacrylonitrile ( pan ) are commercially important and used for the precursor for high - performance carbon fiber . in the fiber applications , pan is dissolved in a suitable solvent and spin to fibers by a wet or dry spinning process .
the extensively used solvents are aqueous sodium sulfocyanate ( nascn ) , n , ndimethylformamide ( dmf ) , n , n - dimethylacetamide ( dmac ) , and n , n - dimethyl sulfone ( dmso ) .
unfortunately , all the solvents mentioned above are unfriendly to the environment . to meet the requirements of  green chemistry , scientists have been searching for a green solvent to be an alternative to these solvents .
in recent years , room temperature ionic liquids ( rtils ) have received a lot of attention as the potential  green  and  designable  solvents . although the ionic liquids have been studied since the 1950s , but they can not to be handled under an inert atmosphere .
these are not part of our studies until some kinds of water - stable and air - stable ionic liquids are found such as [ bmim]cl . because of its extremely low volatility , some of the rtils are promising environment - friendly solvents instead of the volatile organic solvents in a range of science and technology applications such as media for organic and inorganic reactions , materials processing   electrochemistry   and chemical separation . during the copolymerization of an and mma in rtils our research group find that rtils are good solvents for pan copolymer , and pan copolymer could be precipitated from the rtils solutions by the addition of water . therefore rtils are promising to be green solvents for pan .
understanding the rheological properties of a solution is a convenient and effective way to gain a fundamental knowledge of the spinnability and structure - property relationships for the spinning solution .
thus , this paper investigates the rheological properties of pan copolymer / bmimcl concentrated solutions in some detail .
the pan copolymer , chlorobutane , 1-methylimidazole , ethyl acetate and acetone were used as supplied .
firstly , pan powder was swollen by [ bmim]cl at room temperature to give a white , little viscous pulp .
secondly the pulp was heated to 90c until a homogeneous , transparent solution was formed .
the pan/[bmim]cl solution of 5 wt% to 22 wt% concentration was obtained within 5 hours at 90c .
figure 1 shows the process of the dissolution of pan with ionic liquids [ bmim]cl at 90c at the concentration of 10 wt% .
steady state rheological measurements were performed on a haake rs150l rotational rheometer using a 35 mm , 1 cone and plate .
the apparent viscosity a is shown as a function of shear rate for the pan in two different solvents , [ bmim]cl and dmf ( see figure 2 ) . over the experimental range ,
the pan / dmf solution is newtonian fluid , while the pan/[bmim]cl solution is pseudoplastic , that is , the apparent viscosity decreases with increasing shear rate .
according to the so - called power law equation , where k and n are constants , for pseudoplastic liquids the viscosity should decrease nearly linearly with shear rate on the log - log plot and the value of n is less than one .
however , it is surprising to find that the data of pan/[bmim]cl solution should be considered having two linear stages ( see figure 2 ) .
the value of n of the first stage is less than 1 , according with the shear thinning rheological behavior .
while at the second stage the apparent viscosity decreases so dramatically that even the value of n is negative .
it is known that n is used to indicate the non - newtonian property of fluids .
the more the value of n deviates from 1 , the more non - newtonian the fluids would be . usually , the value of n of pan / traditional organic solvents solutions is between 0 and 1 .
therefore , it could be postulated that there would be some much stronger interaction between pan and the new solvent [ bmim]cl which will be discussed in the following text .
the data in figure 3 shows that the viscosity - shear rate curves for the pan/[bmim]cl solutions of different concentrations .
these curves show that solutions of lower concentration remain newtonian in behavior at high shear rates than concentrated solutions . at high concentration ,
the rheological behavior of pan/[bmim]cl solutions acts like that of liquid crystalline polymers ( lcp ) .
the solvent [ bmim]cl reduces the number of entanglements . reducing the number of entanglements at a given shear rate
since the orientation of macromolecular is the major cause of non - newtonian behavior , increasing the shear rate would make the non - newtonian behavior more noticeable .
besides , it could be found in figure 3 that when at high shear rate , the viscosity of high concentration solutions is lower than that of low concentration , for example , the viscosity of 22wt% solution is the lowest among all the solutions when the shear rate is close to 1000 1/s .
as we known , liquid crystalline polymers usually have rigid chain segment such as aromatic polyamide and aromatic polyester . the flexibility of the c - c bond of the pan main chain is smaller than the c - o bond , c
- n bond and decrease because of the strong polarity of the cn , but comparing to the liquid crystalline polymers , the pan chain segment can not be oriented because of lack of strong rigid chain segment . in this case
, high concentration means more entanglements , which indicates that only low shear rates would be needed to orient the macromolecules . and
the amount of entanglements is so large that there is no enough time and much more difficulties for most of them to slip and disengage . with the increasing of the shear rate the number of the oriented segments increases , so that the viscosity of the high concentration pan/[bmim]cl solution decreases greatly .
commonly , the viscosity follows the andrade or arrhenius equation to a good approximation : in this equation , a is a constant , e is the activation energy .
the slope of the straight line plotted by ln versus 1/t is e / r .
the value of the energy of activation depends strongly on whether the viscosities at various temperatures are evaluated at constant shear stress or at constant shear rate . here
since low molecular weight polymers has fewer entanglements than high molecular weight ones , it is not surprising that deviations from newtonian behavior start at higher shear rates for the low molecular weight solutions . however , in figure 5 , when the shear rate up to 210 1/s the viscosity of the high molecular weight pan/[bmim]cl solution is lower than the other one .
the viscoelasticity of polymer is usually characterized by dynamic rheological experiment in which the sample is subjected to a sinusoidal strain of infinitesimal amplitude and fixed angular frequency .
the term g , called the storage modulus , is the in - phase component of the modulus and represents the energy stored and recovered per cycle .
correspondingly , the term g  , called the loss modulus , is the out - of - phase component of the modulus and represents the energy dissipated as heat per cycle of deformation .
figure 6 shows the modulus - angular frequency curves for the pan/[bmim]cl solutions of different concentrations(90c)(a ) and temperature(14wt%)(b ) .
the data shows that the loss modulus g  is much higher than the storage modulus g. either module increases when the concentration increases or the temperature decreases .
the storage modulus g increases faster than the loss modulus g  because the elasticity of the solution increases . the data in fig .
7 shows that the complex viscosity - angular frequency curves for the pan/[bmim]cl solutions of different concentrations(90c)(a ) and temperature(14wt%)(b ) .
. the complex viscosity of the solution increases when the concentration increases or the temperature decreases .
this non - newtonian behavior is of tremendous practical importance in the processing and fabrication of polymer material .
first , the decreased viscosity makes the polymer solution easier to process or squirt through small channels , such as spinning .
second , the decrease in viscosity is associated with the development of elasticity in the polymer solution .
this elasticity produces such phenomena as die swell , or the  puff - up  of extruded strands .
the possible dissolution mechanism of pan in ionic liquid [ bmim]cl is shown in figure 8 .
it has been proved that the miscibility of ionic liquid with polymer is the function of their structural characteristic .
we here focus on the polar group of pan which has strong interaction with the ionic liquid . because the electronegativity of the nitrogen atom on the nitrile grouping is very strong
when the electron density is very large , the nitrogen atom on the nitrile grouping donate electrons and the carbon atom on the nitrile grouping attract electrons . as a result , there is strong interaction between the cation and the nitrogen atom on the nitrile grouping .
similarly , there is strong interaction between the anion and the carbon atom on the nitrile grouping .
the pan copolymer , chlorobutane , 1-methylimidazole , ethyl acetate and acetone were used as supplied .
firstly , pan powder was swollen by [ bmim]cl at room temperature to give a white , little viscous pulp .
secondly the pulp was heated to 90c until a homogeneous , transparent solution was formed .
the pan/[bmim]cl solution of 5 wt% to 22 wt% concentration was obtained within 5 hours at 90c .
figure 1 shows the process of the dissolution of pan with ionic liquids [ bmim]cl at 90c at the concentration of 10 wt% .
steady state rheological measurements were performed on a haake rs150l rotational rheometer using a 35 mm , 1 cone and plate .
the apparent viscosity a is shown as a function of shear rate for the pan in two different solvents , [ bmim]cl and dmf ( see figure 2 ) . over the experimental range ,
the pan / dmf solution is newtonian fluid , while the pan/[bmim]cl solution is pseudoplastic , that is , the apparent viscosity decreases with increasing shear rate .
according to the so - called power law equation , where k and n are constants , for pseudoplastic liquids the viscosity should decrease nearly linearly with shear rate on the log - log plot and the value of n is less than one .
however , it is surprising to find that the data of pan/[bmim]cl solution should be considered having two linear stages ( see figure 2 ) .
the value of n of the first stage is less than 1 , according with the shear thinning rheological behavior . while at the second stage the apparent viscosity decreases so dramatically that even the value of n is negative .
it is known that n is used to indicate the non - newtonian property of fluids .
the more the value of n deviates from 1 , the more non - newtonian the fluids would be . usually , the value of n of pan / traditional organic solvents solutions is between 0 and 1 .
therefore , it could be postulated that there would be some much stronger interaction between pan and the new solvent [ bmim]cl which will be discussed in the following text .
the data in figure 3 shows that the viscosity - shear rate curves for the pan/[bmim]cl solutions of different concentrations .
these curves show that solutions of lower concentration remain newtonian in behavior at high shear rates than concentrated solutions . at high concentration ,
the rheological behavior of pan/[bmim]cl solutions acts like that of liquid crystalline polymers ( lcp ) .
the solvent [ bmim]cl reduces the number of entanglements . reducing the number of entanglements at a given shear rate reduces the amount of orientation of macromolecular . since the orientation of macromolecular is the major cause of non - newtonian behavior , increasing the shear rate would make the non - newtonian behavior more noticeable . besides , it could be found in figure 3 that when at high shear rate , the viscosity of high concentration solutions is lower than that of low concentration , for example , the viscosity of 22wt% solution is the lowest among all the solutions when the shear rate is close to 1000 1/s .
as we known , liquid crystalline polymers usually have rigid chain segment such as aromatic polyamide and aromatic polyester . the flexibility of the c - c bond of the pan main chain is smaller than the c - o bond , c
- n bond and decrease because of the strong polarity of the cn , but comparing to the liquid crystalline polymers , the pan chain segment can not be oriented because of lack of strong rigid chain segment . in this case
, high concentration means more entanglements , which indicates that only low shear rates would be needed to orient the macromolecules . and
the amount of entanglements is so large that there is no enough time and much more difficulties for most of them to slip and disengage . with the increasing of the shear rate the number of the oriented segments increases , so that the viscosity of the high concentration pan/[bmim]cl solution decreases greatly .
commonly , the viscosity follows the andrade or arrhenius equation to a good approximation : in this equation , a is a constant , e is the activation energy . the slope of the straight line plotted by ln versus 1/t is e / r .
the value of the energy of activation depends strongly on whether the viscosities at various temperatures are evaluated at constant shear stress or at constant shear rate . here
the two molecular weights are high enough to become entangled . since low molecular weight polymers has fewer entanglements than high molecular weight ones , it is not surprising that deviations from newtonian behavior start at higher shear rates for the low molecular weight solutions . however , in figure 5 , when the shear rate up to 210 1/s the viscosity of the high molecular weight pan/[bmim]cl solution is lower than the other one .
the viscoelasticity of polymer is usually characterized by dynamic rheological experiment in which the sample is subjected to a sinusoidal strain of infinitesimal amplitude and fixed angular frequency .
the term g , called the storage modulus , is the in - phase component of the modulus and represents the energy stored and recovered per cycle .
correspondingly , the term g  , called the loss modulus , is the out - of - phase component of the modulus and represents the energy dissipated as heat per cycle of deformation .
figure 6 shows the modulus - angular frequency curves for the pan/[bmim]cl solutions of different concentrations(90c)(a ) and temperature(14wt%)(b ) .
the data shows that the loss modulus g  is much higher than the storage modulus g. either module increases when the concentration increases or the temperature decreases .
the storage modulus g increases faster than the loss modulus g  because the elasticity of the solution increases . the data in fig .
7 shows that the complex viscosity - angular frequency curves for the pan/[bmim]cl solutions of different concentrations(90c)(a ) and temperature(14wt%)(b ) .
the complex viscosity of the solution increases when the concentration increases or the temperature decreases .
this non - newtonian behavior is of tremendous practical importance in the processing and fabrication of polymer material .
first , the decreased viscosity makes the polymer solution easier to process or squirt through small channels , such as spinning .
second , the decrease in viscosity is associated with the development of elasticity in the polymer solution .
this elasticity produces such phenomena as die swell , or the  puff - up  of extruded strands .
the possible dissolution mechanism of pan in ionic liquid [ bmim]cl is shown in figure 8 .
it has been proved that the miscibility of ionic liquid with polymer is the function of their structural characteristic .
we here focus on the polar group of pan which has strong interaction with the ionic liquid . because the electronegativity of the nitrogen atom on the nitrile grouping is very strong
when the electron density is very large , the nitrogen atom on the nitrile grouping donate electrons and the carbon atom on the nitrile grouping attract electrons . as a result , there is strong interaction between the cation and the nitrogen atom on the nitrile grouping .
similarly , there is strong interaction between the anion and the carbon atom on the nitrile grouping .
within the limits of the experimental techniques accessible to us , the results obtained for pan/[bmim]cl solutions show that they are pseudoplastic , similar to the pan / traditional organic solvents solutions .
the dependence on concentration and molecular weight shows that entanglements play an important role in the rheological behaviors of pan/[bmim]cl concentrated solutions .
the dynamic rheological measurement indicates the elasticity of the solution increases when the concentration is increasing or the temperature decreasing .
the rheological behaviors of the concentrated solution at high share rate are directive to the spinning of pan fiber .
 one of the room temperature ionic liquids ( rtils ) , 1-butyl-3-methylimidazolium chloride ( [ bmim]cl ) was chosen to prepare the concentrated solutions of polyacrylonitrile ( pan ) . 
 the rheological behaviors of the solutions were measured with rotational rheometry under different conditions , including temperatures , concentration , and molecular weight of pan . 
 the solutions exhibited shear - thinning behaviors , similar to that of pan / dmf solutions . 
 the viscosities decreased with the increasing of shear rates . 
 however , the viscosity decreased sharply at high shear rates when the concentration was up to 16wt% . 
 the dependence of the viscosity on temperature was analyzed through the determination of the apparent activation energy . 
 unusually , the viscosity of solutions of higher concentration is lower than that of lower concentration . 
 similarly , the viscosity of low molecular weight pan was higher than high molecular weight pan at high shear rates . 
 the dynamic rheological measurement indicates the loss modulus is much higher than storage modulus . 
 the trend of complex viscosity is similar with the result of static rheological measurement . 
 the interaction between pan and ionic liquid [ bmim]cl was discussed . 
1. Introduction
2. Experimental
2.1 Materials
2.2 Preparation of concentrated solutions
2.3 Rheometry
2.4 Results and discussion
Conclusions