poly(vinylidene fluoride ) 
 ( pvdf ) is a widely studied and used polymer , 
 especially for industrial application due to its excellent chemical , 
 mechanical , and uv stability properties .
pvdf membranes have been 
 extensively used in ultrafiltration ( uf ) and microfiltration ( mf ) 
 applications for separation processes and wastewater treatment and 
 currently are explored as contactors and in bioseparation applications .
an interesting new application 
 is involving the use of pvdf blends to make artificial muscles .
in addition , there is increasing interest in 
 the modification of pvdf membranes with environmentally sensitive 
 functionalities for many diverse applications such as drug delivery , 
 cell encapsulation , electronic devices , sensors , and water softening . to meet the needs of the desired application ,
various methods are currently 
 used such as coating , adsorption , the grafting of functional groups or graft polymerization 
 to the membranes , and chemical 
 modification of membrane materials . in 
 our previous work , we described methods for obtaining pvdf membranes 
 with ph and temperature responsive properties via in situ hydrogel 
 cross - linking or in situ polymerization in solvent phase .
these functionalized membranes 
 were subsequently used as a support to immobilize fe or fe / pd nanoparticles 
 to remove toxic chloro - organics from groundwater . in this work ,
we extend previous studies and present a new method 
 to form covalently attached polymers onto the pvdf membranes involving 
 chemical treatment of the pristine membrane with alkaline solutions 
 to obtain dehydrofluorinated pvdf membranes .
the reaction mechanism 
 of alkaline degradation of pvdf is shown as follows : where x = na , k , or li . the dehydrofluorinated 
 pvdf polymer forms conjugated double bonds 
 or a polyene structure which can be attacked by specific reactants 
 to introduce functional groups to the membrane , such as acrylic acid 
 ( aa ) .
the introduction of covalent bonding can eliminate the concerns 
 on the stability of poly(acrylic acid ) chains on the pvdf membrane 
 prepared by the in situ hydrogel cross - linking method .
as mentioned 
 above , the pvdf membrane support is further used to 
 immobilize metal nanoparticles for a dechlorination reaction .
as expected , 
 a higher metal loading leads to a higher reaction rate , and a higher 
 surface area of the support pvdf membrane is required to achieve a 
 greater degree of metal loading .
therefore , in this work , another 
 improvement was achieved through the modification of pristine pvdf 
 structure .
pvdf membranes are usually made by a phase inversion method 
 induced by immersion of a cast solution in a polymer nonsolvent bath .
the membrane morphology , porosity , flux and retention properties 
 can be altered by changing the casting and immersion parameters . in this work ,
a sponge - like pvdf 
 membrane is designed to have higher ion adsorption capacity which 
 results in higher nanoparticle loading . the dehydrofluorination 
 of pvdf membranes has been reported .
however , 
 the application of dehydrofluorinated pvdf for covalent attachment 
 of different polymers for reaction and separation has been reported 
 very little .
the main goals of the present 
 study are to ( 1 ) create dehydrofluorinated membranes for the introduction 
 of proper functional groups through covalent bonding ; ( 2 ) investigate 
 the effect of alkaline treatment time on the membrane structure ; ( 3 ) 
 study the ph responsive behavior of functionalized pvdf membranes ; 
 and ( 4 ) prepare sponge - like pvdf membranes with high surface area 
 and metal loading .
sodium hydroxide 
 ( naoh ) , acrylic acid ( aa ) , ammonium persulfate ( aps ) , ferrous chloride 
 tetrahydrate ( fecl24h2o ) , sodium borohydride 
 ( nabh4 ) , trichloroethylene ( tce ) , n , n - dimethylfomamide ( dmf ) , and lithium chloride ( licl ) were 
 purchased from sigma - aldrich . deionized ultrafiltered water ( diuf ) 
 was purchased from fisher scientific .
the pvdf powder was a commercial 
 product ( kynar 761 ) ( mw = 350  000 
 g / mol ) kindly offered by ultura inc .
ultrahigh purity ( uhp ) nitrogen gas used in 
 flux experiments was purchased from scott specialty gases .
a piece of pvdf membrane was 
 soaked in 15 wt % naoh solution in 
 40 ml diuf for 10 min .
then , the membrane was sandwiched between two 
 glass plates and placed in an oven at 70 c to react for 1 , 3 , 
 and 22 h. the final membrane was washed with diuf until the ph became 
 neutral .
the dehydrofluorinated pvdf membrane was 
 functionalized with paa by in situ polymerization with acrylic acid . 

the polymerization solution contained 11.1 wt % acrylic acid ( monomer ) 
 and 0.4 wt % of aps ( initiator ) .
the def - pvdf membrane was dipped 
 in the polymerization solution for 5 min , sandwiched between two glass 
 plates , and placed in an oven at 90 c for 2 h. the undehydrofluorinated 
 pvdf membrane was also functionalized with paa as a control by using 
 the same pore - filling method .
the casting solution was made of 20 wt % pvdf , 2 wt % pvp , 2.25 wt 
 % licl , and 75.75 wt % dmf and heated to 50 c .
a film of pvdf 
 solution was first cast on a glass plate ( 2325% humidity ) 
 and then immersed in diuf water at 50 c for coagulation .
after 
 10 s , the formed membrane was put in pure diuf water at 23 c 
 to wash and let it dried in an oven at 70 c .
the full - scale 
 spvdf was also fabricated under the same conditions by ultura inc . 

attenuated total reflectance 
 fourier transform infrared ( atr - ftir ) spectroscopy ( varian 7000e ) 
 was used to determine the presence of functional groups in dehydrofluorinated 
 pvdf and functional membranes .
the samples were placed on the diamond 
 crystal , and the spectrum was obtained between 500 and 4000 cm for 32 scans at a resolution of 8 cm . the surface and cross - section morphology of the blank pvdf , def - pvdf , 
 functionalized membranes , and spongy pvdf were studied using a hitachi 
 s-4300 scanning electron microscope ( sem ) .
the samples were mounted 
 on sample studs , and a thin layer of gold was sputtered on the sample 
 surface for imaging purpose .
the sem measurements were performed at 
 an accelerating voltage of either 10 or 3 kv .
the surface composition of membrane was 
 characterized using an x - ray photoelectron spectroscopy ( thermo scienctific 
 k - alpha ) with al / k ( h = 1486.6 ev ) anode mono x - ray source . 

the sample was directly mounted on a sample holder then transferred 
 into the analyzer chamber .
the entire spectra of all the elements 
 were recorded with very high resolution using avantage software .
the water 
 permeability was measured at different ph levels to study the ph responsive 
 flux behavior of paa - def - pvdf .
the tested membrane was mounted in 
 a stirred cell ( millipore ) which contains feedwater of varying ph . 

once the membrane flux reached steady - state , volume flux was measured 
 in triplicates by recording the volume passed through the membrane 
 in a given time interval .
na2so4 rejection experiments 
 were performed 
 using a stirred membrane cell provided by millipore with a membrane 
 cross - sectional area of 13.2 cm which included a stirring 
 device to minimize the effects of concentration polarization .
the 
 permeate sodium concentration was measured with a varian aa220 series 
 atomic absorption spectrophotometer .
the method for the synthesis of nanoparticles 
 was developed by our group and described previously .
the fe / pd nanoparticles were made by an ion - exchange with fe , the reduction with sodium borohydride , and a post - palladium 
 coating .
the dechlorination was conducted in a 40 ml vial loaded with 
 nanoparticles and fed a tce solution at 30 mg / l .
the tce concentration 
 was determined after a certain time interval by extracting the sample 
 with pentane and analyzing by gas chromatography ( hp series ii model 
 58590 ) with a mass spectrometer ( gc - ms ) .
the pvdf membranes were treated with 15 
 wt % of sodium hydroxide for different treatment times .
after treatment , 
 a color change was observed for all the membranes from white to a 
 light yellow or deep yellow or even black as shown in figure 1 .
the atr - ftir spectra of blank pvdf , def - pvdf , 
 and paa - def - pvdf membranes are shown in figure 2 .
the characteristic peaks at 1403 and 1600 cm are corresponding to ch2 and c = c , respectively 
 ( figure 2b  d ) .
this is due to the formation 
 of polyene in the membrane as the fluorocarbon groups of pvdf are 
 changed by the treatment ( 10001250 cm ) 
 ( figure 2a ) . as the treatment time is increased , 
 the intensity of the carbon double bond response increases .
it should 
 be noted that the low absorbance intensity at 1600 cm is due to the inactive c = c bond in infrared .
the hydrocarbon at peaks 13501450 and 800900 
 cm do not change significantly with treatment . change 
 in color of the pvdf membranes after treatment with 15
wt 
 % of naoh for different hours at 70 c : ( a ) 0 , ( b ) 1 , ( c ) 3 , 
 ( d ) 22 h. atr - ftir spectrum of blank pvdf , def - pvdf , and 
 paa - def - pvdf membranes . 

( a ) blank pvdf ; ( b ) def - pvdf with 1 h naoh treatment ; ( c ) def - pvdf 
 with 3 h naoh treatment ; ( d ) def - pvdf with 22 h naoh treatment ; ( e ) 
 paa - def - pvdf ( paa functionalization of sample c ) .
the percentage of 
 transmission scales is varied in the range 10040% . as for the paa
functionalized def - pvdf membranes , 
 the appearance 
 of an obvious peak at 1720 cm belongs to the cooh 
 stretching vibration . to prove the covalent attachment of paa with 
 def - pvdf
this result together with the atr - ftir spectra obviously demonstrates 
 that the paa was successfully covalently grafted onto the surface 
 of the pvdf membrane .
the 
 morphologies of 
 the blank and functionalized pvdf membranes were characterized by 
 sem as shown in figure 3 .
a pristine pvdf membrane 
 ( figure 3a ) shows fairly porous structure with 
 mostly circular shape and uniform pore size .
the effect of treatment 
 time with naoh on the structure of membrane was also investigated 
 by sem , and the results can be seen in figure 3b and c. when treated for a short amount of time ( such as 1 or 3 
 h ) , no change in the membrane surface was observed as shown in figure 3b . on the other hand ,
large pores are formed on 
 the membrane surface ( figure 3c ) after being 
 treated for 22 h. it was also found that the membrane was easy to 
 crack with less mechanical force due to the loss of fluorine by the 
 dehydrofluorination treatment . additionally , from figure 3d , it can be clearly seen that paa is grafted on 
 the membrane surface and also has little effect on the structure of 
 the pvdf membrane .
sem image of blank pvdf ( a : sample a in figure 2 ) , dehydrofluorinated pvdf-3h ( b : sample c in figure 2 ) and -22h ( c : sample d in figure 2 ) , and paa - def - pvdf ( d : sample e in figure 2 ) .
the surface compositions of the blank 
 membrane , def - pvdf , and paa - def - pvdf 
 membranes were investigated by xps .
compared 
 with a blank pvdf membrane ( figure 4a ) , the 
 dehydrofluorinated pvdf membrane ( figure 4b ) 
 shows a higher level of noise due to the change in surface roughness 
 after treatment with naoh solution . the dehydrofluorination is proved 
 by a reduction of fluorine intensity , and relative increase of carbon 
 intensity . the decrease in fluorine concentration and the increase 
 in carbon concentration with time
are also shown in figure 5 , which indicate that most of the dehydrofluorination 
 happens in the first hour of the treatment , and it is also seen that 
 the rate of reaction slows down as treatment time increases . as shown 
 in figure 4c , the increase of carbon and oxygen 
 intensity indicates that the paa was grafted on the membranes .
figure 6 show the respective c 1s core - level spectra of 
 the blank pvdf ( figure 6a ) and paa - def - pvdf 
 ( figure 6b ) membranes .
the blank pvdf membrane 
 can be fitted with two main peaks at 286 ev for ch2 groups 
 and 290.6 ev for cf2 groups .
on the other 
 hand , the c 1s spectra of the paa - def - pvdf membrane can be fitted 
 with five chemical species .
the peak with binding energy of 288.5 
 ev belongs to the c = o species of the grafted acrylic acid ( aa ) 
 groups .
the peak with binding energy 
 at 286.4 ev can be assigned to c  o of aa groups and the peak 
 with binding energy at 284.6 ev is assigned to ch2 groups 
 from the grafted paa polymer .
xps survey 
 spectra of pvdf ( a : sample a in figure 2 ) , 
 def - pvdf ( b : sample c in figure 2 ) , and paa - def - pvdf 
 ( c : sample e in figure 2 ) .
changes in xps elemental percentages of fluorine and carbon with 
 different treatment time using15 wt % naoh .
xps c 1s core - level spectra of pristine pvdf ( a : sample a in figure 2 ) and paa - def - pvdf ( b : sample e in figure 2 ) membranes .
figure 7 shows the dsc curves of the pristine 
 pvdf , def - pvdf , and paa - def - pvdf membranes .
it is well - known that 
 pristine pvdf membrane is a partially crystalline polymer because 
 of its symmetrical structure and a melting point of about 173 c .
after dehydrofluorination , the melting point 
 for the def - pvdf membrane does not change as seen in figure 7b .
however , after the covalent binding of paa on 
 the dehydrofluorinated pvdf membranes , the symmetrical structure of 
 the pristine pvdf is partially destroyed resulting in the decrease 
 of the melting point to 168 c ( figure 7d ) . on the other hand ,
the pvdf membranes functionalized with paa 
 by pore - filling methods show no obvious change in the melting point 
 ( figure 7c ) due to the formation of polymer 
 blends .
thus , the dsc results suggest that the grafting of paa chains 
 on the pristine pvdf membrane structure should impart the membranes 
 with enhanced and stable functionalities .
dsc thermogram of ( a ) 
 pristine pvdf ( sample a in figure 2 ) , ( b ) def - pvdf 
 ( sample c in figure 2 ) , ( c ) paa - pvdf by the 
 pore - filling method , and ( d ) paa - def - pvdf 
 ( sample e in figure 2 ) .
since paa was present in the functionalized membranes , a response 
 of water permeation through the membrane with ph would be expected . 
 the ph effect on the water permeability of paa - def - pvdf functionalized 
 membranes is shown in figure 8 . for comparison , 

the permeability of a pristine pvdf membrane and def - pvdf membrane 
 are 964 and 1474 l / mhbar , respectively .
as 
 expected , when the membrane was functionalized with paa , the flux 
 decreases due to the filling of pores with paa polymer .
it is also 
 seen that as the ph of the feed solution increases , the water flux 
 decreases .
this is consistent with previous findings where similar 
 tests were done with paa functionalized pvdf membrane by the pore - filling 
 method .
the change in flux as a response 
 to the change in ph can be caused by the conformational change of 
 acrylic acid polymer chains in the membrane .
the linear relationship 
 between water flux and applied pressure resulting in constant flux 
 at ph 4 for different experimental runs indicate that the paa functionalization 
 on the membrane is very stable .
the water permeability data can be 
 fitted with the following equations to 
 obtain the pka:12where , lp , max and lp , min are the maximum permeability ( 760 l / mhbar ) 
 and minimum permeability ( 29.1 l / mhbar ) when 
 the ph is increased from 3 to 8.5 .
the experimental 
 data were fitted with both equations and the pka was found to be 5.2 , which is consistent with the literature 
 reported pka value for paa .
ph 
 effect on the pure water permeability of the paa - def - pvdf functionalized 
 membrane .
the amount 
 of cooh groups in the functionalized membrane was characterized 
 by the amount of ca entrapped by the membrane due to 
 the high affinity of the free cooh groups to ca . to do this , 100 ml of a 1000 mg / l ca solution
the feed and permeate samples were collected , 
 and the ca concentrations were analyzed using a varian 
 aa220 series atomic absorption spectrophotometer .
based 
 on ion exchange principles , the assumption is that 1 mol ca can be bound to 2 mol carboxyl groups . therefore , the calculated 
 cooh quantity is 0.03 mmol / cm .
our previous publication showed that for paa - pvdf membranes functionalized 
 by pore - filling methods , ca pick - up can range from 0.008 
 to 0.014 mmol / cm for different degrees of cross - linking . 
 here
, the proper amount of cross - linker is required to keep the paa 
 functionalization stable without washing out .
on the other hand , it 
 has been found that too much cross - linking will block membrane pores 
 and increase the mass transfer resistance .
the covalent binding of 
 paa onto the membrane surface offers a solution to avoid this problem .
the effect of ph on the ion exclusion of the paa functionalized def - pvdf 
 membrane is evaluated , and the results are shown in figure 9 . with a ph of 8 ,
meanwhile at lower ph ( around ph 4 ) , the observed rejection 
 is decreased to 10% .
this dramatic decrease of solute rejection is 
 because of the helix  coil transitions of the paa chain in response 
 to the ph change . at lower ph ,
the paa chain is 
 protonated , which can reduce the overall charge and thus decrease 
 the electrostatic interaction between the membrane and permeate ions . when the ph increases to ph 8 , the paa is ionized 
 with enhanced electrostatic interaction and thus a higher rejection 
 is seen .
effect of feed solution ph on the ion exclusion ( p = 0.7 bar ) of a 100 mg / l na2so4 solution through paa - def - pvdf 
 membrane .
the relationship between na2so4 rejection 
 and permeate flux for a paa functionalized pvdf membrane is shown 
 in figure 10 .
the inset figure shows the linear 
 relationship between the pressure and flux , which indicates the absence 
 of membrane fouling .
the observed decline in rejection at a higher 
 flux is in direct contrast to experimental trends generally observed 
 for ion separations in dense media such as reverse osmosis ( ro ) and 
 nanofiltration ( nf ) . for nf type membranes
, it is seen that the solution 
 rejection increases with increasing flux due to the reduced effect 
 of diffusion .
the microporous membranes used in this study show a 
 nonuniform distribution within the pore geometry .
this is primarily 
 due to the incomplete pore coverage resulting from nonuniform functionalization 
 or a low chain length to pore ratio . at higher applied pressures , 

there is enhanced flow through the region of the membrane that is 
 not covered by paa chains due to the reduction in hydrodynamic thickness 
 of the paa layer .
commercial nanofiltration 
 membranes are reported to have a 95 to 99% na2so4 rejection . however , these membranes have a pore size of about 1 
 nm and are operated at pressures ranging from 10 to 40 bar .
therefore , the development of much more effective membranes will require 
 much higher paa loading and the limitation of the core region .
effect of 
 operating pressures on the ion exclusion of a 100 mg / l 
 na2so4 solution at ph = 6.5 .
the inset is the water flux 
 ( ph = 6.5 ) dependence on pressure .
nanoparticles 
 were used for the dechlorination of trichloroethylene ( tce ) , a ubiquitous 
 pollutant in groundwater .
figure 11 shows the 
 successful degradation of tce with time by fe / pd nanoparticles .
this 
 reaction can be modeled as a pseudo - first - order heterogeneous reaction:3where c is the tce 
 concentration 
 in water ( mg / l ) ; ksa is the surface area 
 normalized reactivity ( l / mh ) ; s is the specific surface area of nanoparticles ( m / g ) ; 
 m is the mass concentration of nanoparticles ( g / l ) , 
 and t is the time ( h ) . under the reaction conditions 
 leading to the results in figure 11
, ksa is calculated to be 0.11 l / mh 
 which is consistent with the literature reported dechlorination reactivity 
 by fe / pd nanoparticles .
tce dechlorination by fe nanoparticles immobilized in paa - def - pvdf 
 membrane : vol = 43 ml , initial ph = 6.8 , initial tce concentration 
 = 30 mg / l , iron loading amount = 0.2
g / l ( pd = 1.4 wt % of fe ) , temperature 
 = 25 c .
as 
 discussed in previous session , the effective dechlorination or ion 
 separation require high loading of paa into the membrane pores .
therefore , 
 spongy pvdf membranes were first prepared by the phase - inversion method 
 in lab - scale .
the full - scale spvdf was later developed to assess the 
 feasibility of continuously manufacturing these membranes .
it is clearly seen that the overall structure of the membrane cross - section 
 is asymmetrical consisting of a skin surface ( figure 12a ) on the top layer supported by a macroporous spongelike 
 sublayer ( figure 12b  d ) .
these macrovoids 
 became more accentuated and extended throughout the entire cross - section . 

the void volume of the spongelike pvdf membrane can be estimated as 
 follows:4where vm and vp are the membrane volume and volume occupied 
 by pvdf polymer respectively , l is the pvdf membrane 
 thickness ( cm ) , a is the membrane area ( cm ) , wm is the membrane mass ( g ) , and p is the density of pvdf ( about 1.78 g / cm ) .
the 
 properties of the spvdf membranes were compared with our previous 
 pvdf membranes from millipore as shown in table 1 .
the spvdf membrane shows high porosity at about 78.6% and almost 
 4 times higher surface area compared with that of pvdf ( millipore ) . 

although the spvdf showed less permeability , the skin layer can enhance 
 antifouling properties due to the prevention of large particles deposition 
 into membrane pores . while the highly porous structure below the skin 
 layer without tortuosity can reduce the residence time of solute and 
 eliminate the mass transfer resistance .
sem images of cross - section spongy pvdf morphology under different 
 magnification : ( a ) whole picture , 300 ; ( b ) skin surface and 
 macroporous matrix , 4000 ; ( c ) interconnected fibrous structure 
 in the microporous matrix , 10  000 ; ( d ) microporous matrix 
 under skin surface , 10  000 . supplied by manufacturer . with more surface area , a higher 
 grafting yield will be expected . 
 therefore , these two membranes were functionalized with acrylic acid 
 under the same conditions , and the results are shown in table 2 .
as expected , the spvdf shows high grating yield 
 at about 81.1 wt % compared to 13% for pvdf ( millipore ) .
the ion exchange 
 capacity was evaluated by ca pickup on these two functionalized 
 membranes .
the adsorption capacity of the paa - spvdf increases in the 
 first hour and then remains relatively constant at 3.2 mg / cm ( 265.1 mg / g ) .
the paa - spvdf membrane shows high ion - exchange performance 
 with an adsorption capacity that is 10 times higher than paa - pvdf 
 ( millipore ) as shown in table 2 .
the sem - eds 
 spectra shows the elemental composition and distribution of species 
 and gives the atom ratio of 3.3 ( oxygen per calcium ) ( figure 13 ) .
the pvdf membrane treated with alkaline solution leads to a modified 
 membrane ( def - pvdf ) with conjugated double bonds which were confirmed 
 by atr - ftir and xps spectra .
the introduced double bonds make the 
 membrane suitable for the covalent attachment of paa polymer .
the 
 paa - def - pvdf membrane shows ph responsive behavior for both the hydraulic 
 permeability and solute retention .
this porous medium offers less 
 resistance to solvent transport , allowing for higher permeation rates 
 at much lower operating pressures with less energy consumption comparing 
 to traditional nf membrane .
the paa - def - pvdf membrane was also utilized 
 as a support to immobilize metal nanoparticles to remove toxic chlorinated 
 compounds ( such as , tce ) from water .
asymmetric membranes with a macroporous 
 sponge - like structure were successfully prepared by the phase inversion 
 method in both lab - scale and large - scale .
the modified pvdf membrane 
 shows high surface area , high porosity and higher yield of paa functionalization 
 compared to traditional microfiltration pvdf membrane .
the ion - exchange 
 capacity of ca is 3.2 mg / cm ( 265.1 mg / g ) , 
 which is 10 times higher than that reported in our previous publications . 
 the paa - spvdf membrane can be used to remove heavy metal ions from 
 water due to its good ion - exchange properties .
these two modifications 
 of pvdf can extend the development of functionalized membranes with 
 advanced applications .
 poly(vinylidene fluoride ) ( pvdf ) 
 was modified by chemical treatments 
 in order to create active double bonds to obtain covalent grafting 
 of poly(acrylic acid ) ( paa ) on membrane . 
 the attenuated total reflectance 
 fourier transform infrared ( atr - ftir ) spectrum confirms the formation 
 of conjugated c = c double bonds with surface dehydrofluorination . 
 
 the membrane morphology was studied by scanning electron microscopy 
 ( sem ) . 
 the surface composition was characterized by x - ray photoelectron 
 spectroscopy ( xps ) . 
 the thermal stability of the dehydrofluorinated 
 membrane ( def - pvdf ) and functionalized membranes were investigated 
 by differential scanning calorimetry ( dsc ) analysis . 
 the influence 
 of covalently attached paa on def - pvdf membrane has been investigated 
 to determine its effect on the transport of water and charged solute . 
 
 variations in the solution ph show an effect on both permeability 
 and solute retention in a reversible fashion . 
 metal nanoparticles 
 were also immobilized in the membrane for the degradation of toxic 
 chlorinated organics from water . 
 in addition , pvdf membranes with 
 an asymmetric and sponge - like morphology were developed by immersion - precipitation 
 phase - inversion methods in both lab - scale and large - scale . 
 the new 
 type of spongy pvdf membrane shows high surface area with higher yield 
 of paa functionalization . the ion - capacity with ca2 + ions 
 was also investigated . 
Introduction
Experimental Section
Results
and Discussion
Conclusion