Crystallization behaviour of a polymeric membrane

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PAPER

Crystallization be

Ionic Liquid & Solid State Ionics Laboratory

University, Varanasi-221005, India.

[email protected]; Fax: +91 54

† Electronic supplementary informa10.1039/c4ra07085b

Cite this: RSC Adv., 2014, 4, 50914

Received 14th July 2014Accepted 15th September 2014

DOI: 10.1039/c4ra07085b

www.rsc.org/advances

50914 | RSC Adv., 2014, 4, 50914–509

haviour of a polymeric membranebased on the polymer PVdF–HFP and the ionicliquid BMIMBF4†

Shalu, S. K. Chaurasia and R. K. Singh*

The crystallization behaviour of the polymer poly(vinylidenefluoride)–hexafluoropropylene (PVdF–HFP) in

the presence and absence of the ionic liquid (IL) (1-butyl-3-methylimidazolium tetrafluoroborate;

[BMIMBF4]) were studied by isothermal and non-isothermal crystallization processes using differential

scanning calorimetry. The well-known Avrami equation is used to describe the isothermal crystallization

process of pristine PVdF–HFP or PVdF–HFP + x wt% of IL BMIMBF4, where x ¼ 10 and 30, respectively. It

was found that the presence of the IL BMIMBF4 in the PVdF–HFP matrix suppresses the crystallization of

the polymer PVdF–HFP, resulting in low crystal growth rates. Three kinetic methods (i.e., those of

Jeziorny, Ozawa and Mo) were used to analyze the non-isothermal crystallization process. The Avrami

equation modified by Jeziorny could only describe the initial stage of crystallization and the Ozawa

method failed to describe the non-isothermal crystallization behavior, but Mo’s method explains the

results better.

Introduction

The study of the crystallization kinetics of polymers andpolymer electrolytes is an attractive area for researchersbecause it has a direct relationship to the structures andproperties of the polymeric materials.1,2 Polymer electrolytesare very important for the development of solid-state elec-trochemical devices with electro-active properties. Generally,these polymer electrolytes are formed by using ionic salts(e.g., LiClO4, LiBF4, NaClO4, NH4ClO4, Mg(ClO4)2, and so on)with polymer matrices such as polyethylene oxide (PEO),polypropylene oxide (PPO), polyvinyl acetate (PVA), and poly-vinylideneuoride (PVdF), but these polymer electrolytesare relatively poorly conducting at room temperature and arenot thermally very stable.3–6 To obtain higher conductivity,many approaches7–11 such as addition of ceramic llers,plasticizers, copolymerization and blending have been adop-ted but the solvents used for the preparation of these polymerelectrolytes are volatile in nature. Therefore, these polymerelectrolytes are not electrochemically and thermally stable,which limits their application in some devices. The previouslymentioned problem can be addressed by incorporating ionicliquids (ILs) into the polymer matrix. Recently, the incorpo-ration of room temperature ionic liquids (RTILs) into

, Department of Physics, Banaras Hindu

E-mail: [email protected];

2 2368390; Tel: +91 542 6701541

tion (ESI) available. See DOI:

24

polymers, and polymer electrolytes have been found to be avery promising approach for enhancing the ionic conductivityas well as for maintaining mechanical and thermal stability ofpolymer electrolyte membranes.12–14 Ionic liquids are gener-ally considered to be salts that have melting temperaturesbelow 100 �C, and are mainly composed of dissociatedcations and anions, and RTILs are those ionic liquids that arein the liquid state at room temperature. However, it hasbeen recently found that IL ions exist in layers.15 ILs playimportant roles in electrochemical devices, especially inrechargeable batteries, because of some exotic propertiessuch as non-volatility, non-ammability, high thermalstability, wide electrochemical window and excellent ionicconductivity up to their decomposition temperatures.16–18

Ionic liquids act as suppliers of large numbers of free chargecarriers and also as plasticizers.19 Polymeric membranesbased on poly(vinylideneuoride-co-hexauoropropylene)(PVdF–HFP) (which consist of both crystalline and amor-phous phases) have drawn much research attention becauseof their high dielectric constant (3 ¼ 8.4) which facilitateshigh charge dissociation. The crystalline phase of the polymeracts as a mechanical support for the polymer electrolyte,whereas the amorphous phase of the polymer helps in ionconduction.20,21 Generally, in polymer electrolytes, the amor-phous phase is found to be highly conducting when comparedto the crystalline phase.22 Therefore, it is very important tostudy the crystallization kinetics of the polymer. Severalstudies have reported changes that occur to the crystallizationbehavior of various polymers such as PEO, PMMA, PVdF, PVA,and PAN upon changing the polymer’s molecular weight,

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Fig. 1 DSC exothermic curves for the isothermal crystallization (a)pristine PVdF–HFP, PVdF–HFP + xwt% of IL BMIMBF4 where (b) x¼ 10and (c) 30 at different crystallization temperatures (Tc).

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adding complexing salts, using inorganic llers such as SiO2

and TiO2, adding ferrite nanoparticles such as CoFe2O4 andNiFe2O4, using carbon nanotubes and also when the polymeris conned.23–28 However, very few studies are available thathave shown the effect of an IL on the crystallization behaviorof the polymers and polymer electrolytes.29 To the best of ourknowledge, results are not available for the crystallizationkinetics of PVdF–HFP and for the role of the IL in modifyingits crystallization behavior. The present study reports on thecrystallization kinetic behavior of PVdF–HFP, and of PVdF–HFP combined with different percentage weights of the ionicliquid: 1-butyl-3-methylimidazolium tetrauoroborate(BMIMBF4). This behavior was investigated by usingisothermal and non-isothermal crystallization methods anddifferential scanning calorimetry (DSC).

Experimental detailsMaterials

Starting materials PVdF–HFP, molecular weight ¼ 400 000 gmol�1 and the ionic liquid BMIMBF4 (purity >99.99%) werepurchased from Sigma-Aldrich. The IL was dried under avacuum of �10�6 torr for two days before use. The PVdF–HFPplus ionic liquid gel membranes were prepared using aconventional solution cast method. In order to synthesizepolymeric gel membranes, the desired amount of host poly-mer (PVdF–HFP) was dissolved in acetone by stirring at 50 �Cfor 2 hours until a clear homogeneous solution was obtained.Different amounts of ionic liquid BMIMBF4 were added to theresulting solution under continuous stirring for about �5–6hours at 50 �C until a clear, viscous, homogeneous mixture wasobtained. The resulting viscous solution was cast over poly-propylene petri dishes and, aer complete evaporation ofthe solvent, freestanding rubbery lms of polymeric gelmembranes containing different amounts of the IL wereobtained.

Isothermal and non-isothermal crystallization kineticmeasurements were carried out using DSC with a MettlerToledo DSC 1 system that was calibrated with indiumand zinc metals. All the DSC measurements were performedin a nitrogen atmosphere (at a ow rate of 25 ml min�1)and the weight of the samples was kept constant (at �10 mg)to here.

Results and discussionIsothermal crystallization kinetics

DSC analysis is one of the most suitable methods for studyingthe crystallization and phase transition occurring in semi-crystalline polymers. The original plug-in soware developedby Lorenzo et al. was used to perform the isothermal crystal-lization kinetics calculations and for the comparison of theexperimental and theoretical results.30 DSC curves for pristinePVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼10 and 30, respectively) are shown in Fig. 1. For isothermalcrystallization, the samples were heated to 165 �C (which isabove the �147 �C melting temperature of pure PVdF–HFP),

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held there for 10 min to remove any thermal history, and thenquickly cooled (at a rate �50 �C min�1) to various crystalliza-tion temperatures (Tc). In the present case, Tc was kept

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different for different concentrations of IL because incorpo-ration of IL into the polymer matrix reduces the meltingtemperature due to the decrease in crystallite size andincrease in its interfacial area.31 Fig. 1 shows the exothermiccurves of the prepared samples during isothermal crystalli-zation. It can be seen from Fig. 1 that, at higher Tc, theexothermic peak becomes atter and the polymericmembranes are taking more time to crystallize. Fig. 1(a) showsthe exothermic curves for pristine PVdF–HFP at Tc ¼ 138 and136 �C while Fig. 1(b) and (c) show the exothermic curves forPVdF–HFP + x wt% of IL BMIMBF4 for x ¼ 10%, Tc ¼ 116 and114 �C, and for x ¼ 30%, Tc ¼ 108 and 106 �C, respectively.From Fig. 1a–c it is found that (i) IL BMIMBF4 reduces thecrystallization temperature (Tc) and (ii) IL-containing samplestake a much longer time to crystallize as compared to pristinePVdF–HFP due to the plasticization effect of the IL. The rela-tive degree of crystallinity (Xt) (expressed as relative DH values,i.e., total heat evolved) with time t can be calculated using DSCexothermic curves (Fig. 2). The relative crystallinity (Xt) isdened as the ratio of crystallinity at any time t, to the crys-tallinity as time approaches innity, and can be calculated bythe equation32

Fig. 2 Plots of relative crystallinity (Xt) (expressed as relative DH valuesPVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 at differe

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Xt ¼ DHt

DHN

¼

ðt0

�dH

dt

�dt

ðN0

�dH

dt

�dt

(1)

where dH/dt is the rate of heat evolution, DHt is the total heatevolved at any time t, and DHN is the heat evolved when timeapproaches innity (N).

The plots of relative crystallinity (Xt) (expressed as relativeDH values) vs. time (t) for the isothermal crystallization ofpristine PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (forx ¼ 10 and 30) at different crystallization temperatures areshown in Fig. 2. For pristine PVdF–HFP, the time required forcomplete crystallization is around 14 min at a crystallizationtemperature of�136 �C, and this crystallization time increaseswith increasing crystallization temperature. It can be seenfrom Fig. 2 that as the crystallization temperature increases, atypical sigmoidal-shaped curve is obtained for all the samples,and these curves shi towards higher time scales (i.e., takelonger time to crystallize). In the present work, the Avramiequation33,34

Xt ¼ 1 � exp(�ktn) (2a)

) vs. time t for the isothermal crystallization of (a) pristine PVdF–HFP,nt crystallization temperature (Tc).

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Fig. 3 Plots of log[�ln(1 � Xt)] versus log t of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 at differentcrystallization temperatures (Tc).

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is used to study the isothermal crystallization kinetics,where Xt is the relative crystallinity at any time t (and isplotted in Fig. 2 for different Tc values), n is the Avramiexponent and k is the crystallization rate constant, whichdepends on the nature of nucleation and growth geometryparameters. The above equation can be converted to thefollowing linear equation:

log[�ln(1 � Xt)] ¼ log k + n log t (2b)

Fig. 3 shows a graphic representation of log[�ln (1�Xt)]versus log t for the pristine PVdF–HFP and PVdF–HFP + x wt%of IL BMIMBF4 (where x ¼ 10 and 30 respectively). The value ofthe Avrami exponent n (slope of the straight line in Fig. 3) andcrystallization rate constant k (intersection with the ordinateaxis in Fig. 3) are determined by tting the data to a doublelogarithm plot using Avrami t soware. The Avrami equationrarely explains the complete crystallization conversion processwhich is a measure of the extent or degree of crystallisationand is usually applicable only for primary crystallization.35

Therefore, in order to determine the value of the conversion

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degree of crystallinity that yields the best t, we have used theaforementioned soware.30 For a good t, the value of thecorrelation coefficient should be very high (i.e. in our case r2 $0.999 in all cases). The tted line (shown by an arrow in eachplot of Fig. 3) is plotted separately in Fig. 4. Fig. 4 shows thetting with a relative conversion of 5–30%. In the presentstudy, the Avrami plots for PVdF–HFP and PVdF–HFP + x wt%of IL BMIMBF4 membranes give rise to a series of straightlines, as shown in Fig. 4. By knowing the slope and intercept ofthese straight lines, the values of the Avrami exponent (n) andthe crystallization rate constant (k) can be obtained, and cor-responding values of n and k at different crystallizationtemperatures are listed in Table 1. It can be seen from Table 1that the value of the Avrami exponent n for all the membranesis between 1 and 2 at various crystallization temperatures (Tc),indicating a 2-dimensional crystal growth.36 The crystalliza-tion half-life (t1/2, which is dened as the time required toachieve 50% crystallization) (listed in Table 1) is also animportant parameter for the discussion of crystallizationkinetics. The values of t1/2 (theoretical as well as experimental

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Fig. 4 Linear fitting plots of log[�ln(1 � Xt)] vs. log t with therelative conversion of 5–30% (i.e., at the initial stage of nucleationgrowth) of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of ILBMIMBF4 where (b) x ¼ 10 and (c) 30 at different crystallizationtemperatures (Tc).

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values) for the prepared membranes at different crystallizationtemperatures are given in Table 1. By determining the value oft1/2, the crystallization rate (which is the inverse of t1/2) can beestimated. It can be seen from Table 1 that the crystallizationhalf-life (t1/2) increases (or 1/t1/2 decreases) when increasingthe crystallization temperature (Tc) as well as IL content in the

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membranes, indicating that the overall crystallization ratedecreases.37,38

A small change of the Avrami exponent with crystallizationtemperature and IL content indicates that the crystallizationmechanism does not change within the investigated crystalli-zation temperature range despite the variation of IL content.Incorporation of ionic liquid in the semi-crystalline/semi-amorphous polymer PVdF–HFP matrix leads to the disruptionof the crystalline phase of PVdF–HFP by reducing the interac-tions between the polymer chain segments, which results inincreased polymer chain exibility. Therefore, the incorpora-tion of IL into the polymeric matrices can subsequently inu-ence the crystallization kinetics of the crystalline segment of thepolymer.

Non-isothermal crystallization kinetics

The non-isothermal crystallization kinetics of pristine PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼ 10 and30) were also studied, and the corresponding exothermic curvesat different cooling rates (i.e., 5, 10, 15 and 20 �C min�1) areshown in Fig. 5. It can be seen from Fig. 5 that, as we increasethe cooling rate, the exothermic crystallization peaks of pristinePVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 membranes(where x ¼ 10 and 30) shi to lower temperatures and becomebroader. On the basis of the crystallization exotherms of theprepared membranes, the relative crystallinities (Xt) at differentcooling rates (f) were calculated (using eqn (1)). The procedureemployed to calculate the relative crystallinity (Xt) here issimilar to that used in the isothermal crystallization study.Fig. 6 shows the relative crystallinity (Xt) with respect totemperature for the non-isothermal crystallization kineticsprocess. In this process, crystallization temperature (T) could betransformed into the time scale to correlate the relative crys-tallinity (Xt) and crystallization time (t) using the followingequation (eqn (3)).

T ¼ (T � T0)/f (3)

where T0 is the initial temperature when crystallization starts(i.e., at t ¼ 0).

Using eqn (3), curves for Xt versus t can be obtained, as shownin Fig. 7. The exothermic curves of the prepared membranesbroadened, the crystallization peak temperature (Tp) shied to alower temperature, and the amount of heat released uponcrystallization decreased as cooling rates increased. Because themobility and exibility of the polymer chain decreased at highcooling rates, the segments of the polymer took a longer time tocrystallize, which further lowered Tp (crystallization peaktemperature, see Fig. 5). Hence, the exothermic trace of the twosamples became wider and shied to lower temperatures whenincreasing the cooling rate.

For the present study, various approaches were employed toanalyze the non-isothermal crystallization kinetics of pristinePVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4.

The Avrami equation was used to analyze the non-isothermalcrystallization process at the initial crystallization state and isgiven as

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Table 1 Different crystallization parameters of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 obtained byAvrami plots using an isothermal crystallization method

Sample Tc (�C) n K min�n t1/2/mine t1/2/mint t0 (min) DH (J g�1) R2

Pure PVdF–HFP 136 1.63 0.035 7.016 6.18 0.63 12.58 0.999138 1.64 0.023 9.33 7.89 1.15 11.75 0.999

PVdF–HFP + 10% BMIMBF4 114 2.01 0.062 3.61 3.33 0.55 15.09 0.999116 1.95 0.036 4.92 4.564 1.08 14.96 0.999

PVdF–HFP + 30% BMIMBF4 106 2.03 0.033 5.46 4.46 0.16 6.94 0.998108 1.72 0.034 6.63 5.72 0.91 7.00 0.999

PVdF–HFP + 10% BMIMBF4+5% SiO2 114 2.32 0.11 2.32 2.18 0.33 8.24 0.999116 2.61 0.035 3.4 3.10 0.016 9.31 0.999

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1 � Xt ¼ exp(�Zttn0) (4)

where Xt is the relative degree of crystallinity, which is a func-tion of crystallization temperature T; the exponent n0 is amechanism constant depending on the types of nucleationparameters and growth process parameters, and Zt is a crys-tallization rate constant involving both nucleation and growthrate parameters. Fig. 8 shows plot of log[�ln(1� Xt)] versus log t.It can be seen from these curves that the non-isothermal crys-tallization kinetics can be tted by the Avrami equation only atthe initial stage of crystallization. Plotting log[�ln(1 � Xt)]against log t for the initial stage of crystallization gives astraight line for each cooling rate (see Fig. 9, see also below).Thus two parameters, n0 and Zt, are obtained from the slope andintercept respectively of the straight-line portion of the plot. Itshould be noted here that the values of n0 and Zt for the non-isothermal crystallization rate do not have the same physicalsignicance as for isothermal crystallization, because in non-isothermal crystallization, the temperature changes at aconstant rate.29 The values of these parameters affect the ratesof both nuclei formation and spherulite growth, which dependon temperature.

Since the crystallization rate depends upon the cooling rate(f), Jeziorny39 suggested that the non-isothermal crystallizationrate (Zc) should be corrected by the cooling rate (f) to obtain thecorresponding corrected rate constant (Zc).

log Zc ¼ log Zt/f (5)

However, the nonlinear dependence of log[�ln(1 � Xt)]against log t (see Fig. 8) suggests that the Avrami equationmodied by Jeziorny is not suitable for the entire non-isothermal crystallization process because this modied equa-tion is valid only at the initial stage of the non-isothermalcrystallization process.

Linear ttings of the log[�ln(1 � Xt)] against log t plots atthe initial stage of crystallization for pristine PVdF–HFP andPVdF–HFP + x wt% of IL BMIMBF4 are shown in Fig. 9, asmentioned above. The values of the Avrami constant n0 and Zcobtained by the modied Avrami equation in the non-isothermal crystallization method are given in Table 2. FromTable 2, it can be seen that the value of n0 varies slightlybetween 1 and 2, suggesting that the non-isothermalcrystallization mechanism for the pristine PVdF–HFP and

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PVdF–HFP + x wt% of IL BMIMBF4 membranes did not changemuch as the heating rate changed. It can also be concludedfrom Table 2 that the value of Zc, i.e., the corrected rateconstant, increases with increasing the heating rate, since thetime needed for the complete crystallization decreased as theheating rates increased.

Ozawa’s method. According to the Ozawa theory,40 non-isothermal crystallization is the result of an innite numberof small isothermal crystallization steps. The correspondingequation for relative degree of crystallinity is given by

1 � Xt ¼ exp(�K(T)/fm) (6)

where K(T) is the cooling crystallization function, which isrelated to the overall crystallization rate and indicates how fastcrystallization occurs, and m is the Ozawa exponent, whichdepends on the dimensions of crystal growth. The double-logarithm form of eqn (6) is

log[�ln (1 � Xt)] ¼ log(�K(T) � m log f (7)

By studying the non-isothermal crystallization process atdifferent cooling rates, from log[�ln(1 � Xt)] vs. log f plots at agiven temperature, a straight line should be obtained andvalues of m and K(T) can be found out by the slope and theintercept, respectively. But in our case, this theory was notvalid. The non-linear dependence of log[�ln(1 � Xt)] vs. log f

(see Fig. S1 in ESI†) shows that the Ozawa equation is notappropriate to illustrate the non-isothermal crystallizationprocess.

Mo’s method. In order to understand the crystallizationbehaviour, a method proposed to describe the non-isothermalcrystallization process by Mo’s group41 has been used. Bycombining the Ozawa and Avrami equations, Mo derived thefollowing equation for non-isothermal crystallization kineticsbehaviour:

log f ¼ log F(T) – b log t (8)

where F(T) ¼ [K(T)/Zt]1/m, m is the Ozawa exponent, and b is the

ratio between the Avrami exponent and Ozawa exponent (b ¼ n/m). F(T) refers to the value of the cooling rate chosen at unitcrystallization time when the system has a dened degree ofcrystallinity. For the non-isothermal crystallization kinetics ofpristine PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4, a

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Fig. 5 Exothermic curves for the non-isothermal crystallizationkinetics of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4where (b) x ¼ 10 and (c) 30 at different cooling rates (f).

Fig. 6 Relative crystallinity (Xt) with respect to temperature for thenon-isothermal crystallization kinetics process of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 atdifferent cooling rates (f).

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Fig. 7 Relative crystallinity (Xt) with respect to time for the non-isothermal crystallization kinetics process of (a) pristine PVdF–HFP,PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 atdifferent cooling rates (f).

Fig. 8 Plot of log[�ln(1 � Xt)] versus log t for the non-isothermalcrystallization kinetics process of (a) pristine PVdF–HFP, PVdF–HFP +x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 at different coolingrates (f).

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good linear relationship between log f vs. log t could be seen forall the prepared membranes (see Fig. 10) and the values oflog F(T) and b as the intercept and the slope respectively aregiven in Table 3. It is shown that the F(T) systematicallydecreases and the value of b increases with a rise in the relativedegree of crystallinity.

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Fig. 9 Linear fitting plots of log[�ln(1 � Xt)] versus log t for the non-isothermal crystallization kinetics process of (a) pristine PVdF–HFP,PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 atdifferent cooling rates (f).

Table 2 Different crystallization parameters of (a) pristine PVdF–HFP,PVdF–HFP + xwt% of IL BMIMBF4 where (b) x¼ 10 and (c) 30 obtainedby Avrami plots using the non-isothermal crystallization method

PVdF–HFP +x wt% ofBMIMBF4

Heating rate(4) (�C min�1) n0 Zt (min�n0) Zc t1/2 (min)

X ¼ 0 5 1.05 0.037 0.517 11.1310 1.16 0.076 0.773 5.2315 1.45 0.141 0.877 3.2320 1.57 0.2 0.922 2.35

X ¼ 10 5 1.03 0.051 0.552 7.4610 1.11 0.115 0.805 3.5015 1.34 0.230 0.906 2.2020 1.51 0.374 0.952 1.55

X ¼ 30 5 1.04 0.053 0.556 7.1710 1.09 0.120 0.814 3.0215 1.13 0.178 0.897 2.1720 1.62 0.382 0.951 1.39

Table 3 Non-isothermal crystallization kinetics parameters of (a)pristine PVdF–HFP, PVdF–HFP + xwt% of IL BMIMBF4 where (b) x¼ 10and (c) 30 at different degrees of crystallinity

PVdF–HFP +x wt% of BMIMBF4 X 0

T (%) F(T) b

X ¼ 0% 10 0.0828 0.942320 0.0453 1.032530 0.0269 1.109440 0.0172 1.166350 0.0146 1.125260 0.0129 1.087570 0.0115 1.063880 0.0098 1.070390 0.0093 1.0320

100 0.0085 1.0267X ¼ 10% 10 0.0880 1.0959

20 0.0723 0.992930 0.0546 0.987340 0.0309 1.104650 0.0215 1.129360 0.0177 1.105370 0.0146 1.101480 0.0129 1.069890 0.0107 1.0959

100 0.0094 1.0943X ¼ 30% 10 0.0841 1.1416

20 0.0383 1.205830 0.0348 1.116040 0.0277 1.133450 0.0221 1.149860 0.017 1.175670 0.0146 1.156380 0.0123 1.157390 0.0106 1.1565

100 0.0095 1.1488

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In addition, an approach oen used to evaluate the activa-tion energy at different cooling rates was proposed by Kis-singer,42 based on the following equation:

d[ln f/Tp2]/d(1/Tp)] ¼ �DE/R (9)

where R is the gas constant and DE is the activation energy forthe crystallization. The slopes of the plots of log(f/Tp

2) vs.log(1/Tp) were used to determine the activation energies (DE)

50922 | RSC Adv., 2014, 4, 50914–50924

for the non-isothermal crystallizations of pristine PVdF–HFPand PVdF–HFP + x wt% of IL BMIMBF4 (Fig. 11). These acti-vation energy values for pristine PVdF–HFP and the PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼ 10 and 30) were

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Fig. 10 Plots of log f vs. log t for the non-isothermal crystallizationkinetics process of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of ILBMIMBF4 where (b) x ¼ 10 and (c) 30 at different relative degrees ofcrystallinity (Xt).

Fig. 11 ln(f/t2p) versus a (1/tp) plot for evaluating the non-isothermalcrystallization activation energy for (a) PVdF–HFP, PVdF–HFP + x wt%of IL BMIMBF4 where (b) x ¼ 10 and (c) x ¼ 30.

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determined to be �162, 124 and 108 KJ per mole respectively.The decreasing activation energy with increasing amount ofionic liquid in PVdF–HFP indicates easier ionic transport inPVdF–HFP + IL gel membranes having higher amount of ionicliquid, as a result of increasing plasticization/amorphizationof the polymeric membranes, which suppresses the crystalli-zation rate.

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Conclusions

In the present study, the crystallization behaviours of pristinePVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼ 10and 30) were studied by isothermal and non-isothermal crystal-lization processes using DSC. The isothermal crystallizationprocess of pristine PVdF–HFP and prepared membranes was welldescribed by the Avrami equation. The values of the Avramiexponent n lie between 1 and 2 for all the prepared membranesindicating two-dimensional growth of spherulites. It has beenfound that the presence of the ionic liquid BMIMBF4 in thePVdF–HFP matrix suppresses the crystallization of polymerPVdF–HFP, resulting in low crystal growth rates. This effectoccurs because the presence of IL (which amorphizes/plasticizes the polymers) hinders the chain folding, andthereby increases the time it takes for crystallization to occurwhen the crystallization depends on a folded polymer. Theexible nature of a polymer allows it to sample the conforma-tions necessary for joining a crystal, but such exibility isdecreased in the presence of IL. Various kinetics methods suchas those of Jeziorny, Ozawa and Mo have been employed tostudy the non-isothermal crystallization process. The Avramiequation modied by Jeziorny could only describe the initialstage of crystallization and the Ozawa method failed to describethe non-isothermal crystallization behavior, but Mo’s method(i.e., the combination of the Avrami and Ozawa equations)claries the results better. All parameters such as the Avramiexponent, crystallization rate constant and crystallization halftime are found to be strongly dependent on the cooling rate andconcentration of IL. The activation energy (DE) of the preparedmembranes varies with IL loading.

Acknowledgements

R.K. Singh is grateful to the BRNS-DAE, India, for nancialassistance. Shalu and S.K.C wish to thank the U.G.C. and CSIRNew Delhi, India, respectively, for their Research Fellowships.

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