1. Crystallization behaviour of a polymeric membrane
based on the polymer PVdF–HFP and the ionic
liquid 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 and
polymer electrolytes is an attractive area for researchers
because it has a direct relationship to the structures and
properties of the polymeric materials.1,2
Polymer electrolytes
are 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-
vinylideneuoride (PVdF), but these polymer electrolytes
are relatively poorly conducting at room temperature and are
not 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 polymer
electrolytes are volatile in nature. Therefore, these polymer
electrolytes are not electrochemically and thermally stable,
which limits their application in some devices. The previously
mentioned problem can be addressed by incorporating ionic
liquids (ILs) into the polymer matrix. Recently, the incorpo-
ration of room temperature ionic liquids (RTILs) into
polymers, and polymer electrolytes have been found to be a
very promising approach for enhancing the ionic conductivity
as well as for maintaining mechanical and thermal stability of
polymer electrolyte membranes.12–14
Ionic liquids are gener-
ally considered to be salts that have melting temperatures
below 100
C, and are mainly composed of dissociated
cations and anions, and RTILs are those ionic liquids that are
in the liquid state at room temperature. However, it has
been recently found that IL ions exist in layers.15
ILs play
important roles in electrochemical devices, especially in
rechargeable batteries, because of some exotic properties
such as non-volatility, non-ammability, high thermal
stability, wide electrochemical window and excellent ionic
conductivity up to their decomposition temperatures.16–18
Ionic liquids act as suppliers of large numbers of free charge
carriers and also as plasticizers.19
Polymeric membranes
based on poly(vinylideneuoride-co-hexauoropropylene)
(PVdF–HFP) (which consist of both crystalline and amor-
phous phases) have drawn much research attention because
of their high dielectric constant (3 ¼ 8.4) which facilitates
high charge dissociation. The crystalline phase of the polymer
acts as a mechanical support for the polymer electrolyte,
whereas the amorphous phase of the polymer helps in ion
conduction.20,21
Generally, in polymer electrolytes, the amor-
phous phase is found to be highly conducting when compared
to the crystalline phase.22
Therefore, it is very important to
study the crystallization kinetics of the polymer. Several
studies have reported changes that occur to the crystallization
behavior of various polymers such as PEO, PMMA, PVdF, PVA,
and PAN upon changing the polymer’s molecular weight,
Ionic Liquid Solid State Ionics Laboratory, Department of Physics, Banaras Hindu
University, Varanasi-221005, India. E-mail: rksingh_17@rediffmail.com;
rajendrasingh.bhu@gmail.com; Fax: +91 542 2368390; Tel: +91 542 6701541
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07085b
Cite this: RSC Adv., 2014, 4, 50914
Received 14th July 2014
Accepted 15th September 2014
DOI: 10.1039/c4ra07085b
www.rsc.org/advances
50914 | RSC Adv., 2014, 4, 50914–50924 This journal is © The Royal Society of Chemistry 2014
RSC Advances
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2. adding complexing salts, using inorganic llers such as SiO2
and TiO2, adding ferrite nanoparticles such as CoFe2O4 and
NiFe2O4, using carbon nanotubes and also when the polymer
is conned.23–28
However, very few studies are available that
have shown the effect of an IL on the crystallization behavior
of the polymers and polymer electrolytes.29
To the best of our
knowledge, results are not available for the crystallization
kinetics of PVdF–HFP and for the role of the IL in modifying
its crystallization behavior. The present study reports on the
crystallization kinetic behavior of PVdF–HFP, and of PVdF–
HFP combined with different percentage weights of the ionic
liquid: 1-butyl-3-methylimidazolium tetrauoroborate
(BMIMBF4). This behavior was investigated by using
isothermal and non-isothermal crystallization methods and
differential scanning calorimetry (DSC).
Experimental details
Materials
Starting materials PVdF–HFP, molecular weight ¼ 400 000 g
molÀ1
and the ionic liquid BMIMBF4 (purity 99.99%) were
purchased from Sigma-Aldrich. The IL was dried under a
vacuum of $10À6
torr for two days before use. The PVdF–HFP
plus ionic liquid gel membranes were prepared using a
conventional solution cast method. In order to synthesize
polymeric gel membranes, the desired amount of host poly-
mer (PVdF–HFP) was dissolved in acetone by stirring at 50
C
for 2 hours until a clear homogeneous solution was obtained.
Different amounts of ionic liquid BMIMBF4 were added to the
resulting solution under continuous stirring for about $5–6
hours at 50
C until a clear, viscous, homogeneous mixture was
obtained. The resulting viscous solution was cast over poly-
propylene petri dishes and, aer complete evaporation of
the solvent, freestanding rubbery lms of polymeric gel
membranes containing different amounts of the IL were
obtained.
Isothermal and non-isothermal crystallization kinetic
measurements were carried out using DSC with a Mettler
Toledo DSC 1 system that was calibrated with indium
and zinc metals. All the DSC measurements were performed
in a nitrogen atmosphere (at a ow rate of 25 ml minÀ1
)
and the weight of the samples was kept constant (at $10 mg)
to here.
Results and discussion
Isothermal crystallization kinetics
DSC analysis is one of the most suitable methods for studying
the crystallization and phase transition occurring in semi-
crystalline polymers. The original plug-in soware developed
by Lorenzo et al. was used to perform the isothermal crystal-
lization kinetics calculations and for the comparison of the
experimental and theoretical results.30
DSC curves for pristine
PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼
10 and 30, respectively) are shown in Fig. 1. For isothermal
crystallization, the samples were heated to 165
C (which is
above the $147
C melting temperature of pure PVdF–HFP),
held there for 10 min to remove any thermal history, and then
quickly cooled (at a rate À50
C minÀ1
) to various crystalliza-
tion temperatures (Tc). In the present case, Tc was kept
Fig. 1 DSC exothermic curves for the isothermal crystallization (a)
pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10
and (c) 30 at different crystallization temperatures (Tc).
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3. different for different concentrations of IL because incorpo-
ration of IL into the polymer matrix reduces the melting
temperature due to the decrease in crystallite size and
increase in its interfacial area.31
Fig. 1 shows the exothermic
curves of the prepared samples during isothermal crystalli-
zation. It can be seen from Fig. 1 that, at higher Tc, the
exothermic peak becomes atter and the polymeric
membranes are taking more time to crystallize. Fig. 1(a) shows
the exothermic curves for pristine PVdF–HFP at Tc ¼ 138 and
136
C while Fig. 1(b) and (c) show the exothermic curves for
PVdF–HFP + x wt% of IL BMIMBF4 for x ¼ 10%, Tc ¼ 116 and
114
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 the
crystallization temperature (Tc) and (ii) IL-containing samples
take a much longer time to crystallize as compared to pristine
PVdF–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 DSC
exothermic curves (Fig. 2). The relative crystallinity (Xt) is
dened as the ratio of crystallinity at any time t, to the crys-
tallinity as time approaches innity, and can be calculated by
the equation32
Xt ¼
DHt
DHN
¼
ðt
0
dH
dt
dt
ðN
0
dH
dt
dt
(1)
where dH/dt is the rate of heat evolution, DHt is the total heat
evolved at any time t, and DHN is the heat evolved when time
approaches innity (N).
The plots of relative crystallinity (Xt) (expressed as relative
DH values) vs. time (t) for the isothermal crystallization of
pristine PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (for
x ¼ 10 and 30) at different crystallization temperatures are
shown in Fig. 2. For pristine PVdF–HFP, the time required for
complete crystallization is around 14 min at a crystallization
temperature of $136
C, and this crystallization time increases
with increasing crystallization temperature. It can be seen
from Fig. 2 that as the crystallization temperature increases, a
typical sigmoidal-shaped curve is obtained for all the samples,
and these curves shi towards higher time scales (i.e., take
longer time to crystallize). In the present work, the Avrami
equation33,34
Xt ¼ 1 À exp(Àktn
) (2a)
Fig. 2 Plots of relative crystallinity (Xt) (expressed as relative DH values) vs. time t for the isothermal crystallization of (a) pristine PVdF–HFP,
PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 at different crystallization temperature (Tc).
50916 | RSC Adv., 2014, 4, 50914–50924 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

4. is used to study the isothermal crystallization kinetics,
where Xt is the relative crystallinity at any time t (and is
plotted in Fig. 2 for different Tc values), n is the Avrami
exponent and k is the crystallization rate constant, which
depends on the nature of nucleation and growth geometry
parameters. The above equation can be converted to the
following linear equation:
log[Àln(1 À Xt)] ¼ log k + n log t (2b)
Fig. 3 shows a graphic representation of log[Àln (1ÀXt)]
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 of
the Avrami exponent n (slope of the straight line in Fig. 3) and
crystallization rate constant k (intersection with the ordinate
axis in Fig. 3) are determined by tting the data to a double
logarithm plot using Avrami t soware. The Avrami equation
rarely explains the complete crystallization conversion process
which is a measure of the extent or degree of crystallisation
and is usually applicable only for primary crystallization.35
Therefore, in order to determine the value of the conversion
degree of crystallinity that yields the best t, we have used the
aforementioned soware.30
For a good t, the value of the
correlation coefficient should be very high (i.e. in our case r2
$
0.999 in all cases). The tted line (shown by an arrow in each
plot of Fig. 3) is plotted separately in Fig. 4. Fig. 4 shows the
tting with a relative conversion of 5–30%. In the present
study, the Avrami plots for PVdF–HFP and PVdF–HFP + x wt%
of IL BMIMBF4 membranes give rise to a series of straight
lines, as shown in Fig. 4. By knowing the slope and intercept of
these straight lines, the values of the Avrami exponent (n) and
the crystallization rate constant (k) can be obtained, and cor-
responding values of n and k at different crystallization
temperatures are listed in Table 1. It can be seen from Table 1
that the value of the Avrami exponent n for all the membranes
is 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 dened as the time required to
achieve 50% crystallization) (listed in Table 1) is also an
important parameter for the discussion of crystallization
kinetics. The values of t1/2 (theoretical as well as experimental
Fig. 3 Plots of log[Àln(1 À 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 different
crystallization temperatures (Tc).
This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 50914–50924 | 50917
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5. values) for the prepared membranes at different crystallization
temperatures are given in Table 1. By determining the value of
t1/2, the crystallization rate (which is the inverse of t1/2) can be
estimated. It can be seen from Table 1 that the crystallization
half-life (t1/2) increases (or 1/t1/2 decreases) when increasing
the crystallization temperature (Tc) as well as IL content in the
membranes, indicating that the overall crystallization rate
decreases.37,38
A small change of the Avrami exponent with crystallization
temperature and IL content indicates that the crystallization
mechanism 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 disruption
of the crystalline phase of PVdF–HFP by reducing the interac-
tions between the polymer chain segments, which results in
increased polymer chain exibility. Therefore, the incorpora-
tion of IL into the polymeric matrices can subsequently inu-
ence the crystallization kinetics of the crystalline segment of the
polymer.
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 and
30) were also studied, and the corresponding exothermic curves
at different cooling rates (i.e., 5, 10, 15 and 20
C minÀ1
) are
shown in Fig. 5. It can be seen from Fig. 5 that, as we increase
the cooling rate, the exothermic crystallization peaks of pristine
PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 membranes
(where x ¼ 10 and 30) shi to lower temperatures and become
broader. On the basis of the crystallization exotherms of the
prepared membranes, the relative crystallinities (Xt) at different
cooling rates (f) were calculated (using eqn (1)). The procedure
employed to calculate the relative crystallinity (Xt) here is
similar to that used in the isothermal crystallization study.
Fig. 6 shows the relative crystallinity (Xt) with respect to
temperature for the non-isothermal crystallization kinetics
process. In this process, crystallization temperature (T) could be
transformed into the time scale to correlate the relative crys-
tallinity (Xt) and crystallization time (t) using the following
equation (eqn (3)).
T ¼ (T À 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 shown
in Fig. 7. The exothermic curves of the prepared membranes
broadened, the crystallization peak temperature (Tp) shied to a
lower temperature, and the amount of heat released upon
crystallization decreased as cooling rates increased. Because the
mobility and exibility of the polymer chain decreased at high
cooling rates, the segments of the polymer took a longer time to
crystallize, which further lowered Tp (crystallization peak
temperature, see Fig. 5). Hence, the exothermic trace of the two
samples became wider and shied to lower temperatures when
increasing the cooling rate.
For the present study, various approaches were employed to
analyze the non-isothermal crystallization kinetics of pristine
PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4.
The Avrami equation was used to analyze the non-isothermal
crystallization process at the initial crystallization state and is
given as
Fig. 4 Linear fitting plots of log[Àln(1 À Xt)] vs. log t with the
relative conversion of 5–30% (i.e., at the initial stage of nucleation
growth) of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL
BMIMBF4 where (b) x ¼ 10 and (c) 30 at different crystallization
temperatures (Tc).
50918 | RSC Adv., 2014, 4, 50914–50924 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

6. 1 À Xt ¼ exp(ÀZttn0
) (4)
where Xt is the relative degree of crystallinity, which is a func-
tion of crystallization temperature T; the exponent n0
is a
mechanism constant depending on the types of nucleation
parameters and growth process parameters, and Zt is a crys-
tallization rate constant involving both nucleation and growth
rate parameters. Fig. 8 shows plot of log[Àln(1 À 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 at
the initial stage of crystallization. Plotting log[Àln(1 À Xt)]
against log t for the initial stage of crystallization gives a
straight line for each cooling rate (see Fig. 9, see also below).
Thus two parameters, n0
and Zt, are obtained from the slope and
intercept respectively of the straight-line portion of the plot. It
should be noted here that the values of n0
and Zt for the non-
isothermal crystallization rate do not have the same physical
signicance as for isothermal crystallization, because in non-
isothermal crystallization, the temperature changes at a
constant rate.29
The values of these parameters affect the rates
of both nuclei formation and spherulite growth, which depend
on temperature.
Since the crystallization rate depends upon the cooling rate
(f), Jeziorny39
suggested that the non-isothermal crystallization
rate (Zc) should be corrected by the cooling rate (f) to obtain the
corresponding corrected rate constant (Zc).
log Zc ¼ log Zt/f (5)
However, the nonlinear dependence of log[Àln(1 À Xt)]
against log t (see Fig. 8) suggests that the Avrami equation
modied by Jeziorny is not suitable for the entire non-
isothermal crystallization process because this modied equa-
tion is valid only at the initial stage of the non-isothermal
crystallization process.
Linear ttings of the log[Àln(1 À Xt)] against log t plots at
the initial stage of crystallization for pristine PVdF–HFP and
PVdF–HFP + x wt% of IL BMIMBF4 are shown in Fig. 9, as
mentioned above. The values of the Avrami constant n0
and Zc
obtained by the modied Avrami equation in the non-
isothermal crystallization method are given in Table 2. From
Table 2, it can be seen that the value of n0
varies slightly
between 1 and 2, suggesting that the non-isothermal
crystallization mechanism for the pristine PVdF–HFP and
PVdF–HFP + x wt% of IL BMIMBF4 membranes did not change
much as the heating rate changed. It can also be concluded
from Table 2 that the value of Zc, i.e., the corrected rate
constant, increases with increasing the heating rate, since the
time needed for the complete crystallization decreased as the
heating rates increased.
Ozawa’s method. According to the Ozawa theory,40
non-
isothermal crystallization is the result of an innite number
of small isothermal crystallization steps. The corresponding
equation for relative degree of crystallinity is given by
1 À Xt ¼ exp(ÀK(T)/fm
) (6)
where K(T) is the cooling crystallization function, which is
related to the overall crystallization rate and indicates how fast
crystallization occurs, and m is the Ozawa exponent, which
depends on the dimensions of crystal growth. The double-
logarithm form of eqn (6) is
log[Àln (1 À Xt)] ¼ log(ÀK(T) À m log f (7)
By studying the non-isothermal crystallization process at
different cooling rates, from log[Àln(1 À Xt)] vs. log f plots at a
given temperature, a straight line should be obtained and
values of m and K(T) can be found out by the slope and the
intercept, respectively. But in our case, this theory was not
valid. The non-linear dependence of log[Àln(1 À Xt)] vs. log f
(see Fig. S1 in ESI†) shows that the Ozawa equation is not
appropriate to illustrate the non-isothermal crystallization
process.
Mo’s method. In order to understand the crystallization
behaviour, a method proposed to describe the non-isothermal
crystallization process by Mo’s group41
has been used. By
combining the Ozawa and Avrami equations, Mo derived the
following equation for non-isothermal crystallization kinetics
behaviour:
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 unit
crystallization time when the system has a dened degree of
crystallinity. For the non-isothermal crystallization kinetics of
pristine PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4, a
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 by
Avrami plots using an isothermal crystallization method
Sample Tc (
C) n K minÀn
t1/2/mine
t1/2/mint
t0 (min) DH (J gÀ1
) R2
Pure PVdF–HFP 136 1.63 0.035 7.016 6.18 0.63 12.58 0.999
138 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 0.999
116 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 0.998
108 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 0.999
116 2.61 0.035 3.4 3.10 0.016 9.31 0.999
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7. Fig. 5 Exothermic curves for the non-isothermal crystallization
kinetics of (a) pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4
where (b) x ¼ 10 and (c) 30 at different cooling rates (f).
Fig. 6 Relative crystallinity (Xt) with respect to temperature 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 at
different cooling rates (f).
50920 | RSC Adv., 2014, 4, 50914–50924 This journal is © The Royal Society of Chemistry 2014
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8. good linear relationship between log f vs. log t could be seen for
all the prepared membranes (see Fig. 10) and the values of
log F(T) and b as the intercept and the slope respectively are
given in Table 3. It is shown that the F(T) systematically
decreases and the value of b increases with a rise in the relative
degree of crystallinity.
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 at
different cooling rates (f).
Fig. 8 Plot of log[Àln(1 À 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 at different cooling
rates (f).
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9. In addition, an approach oen 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/Tp
2
]/d(1/Tp)] ¼ ÀDE/R (9)
where R is the gas constant and DE is the activation energy for
the crystallization. The slopes of the plots of log(f/Tp
2
) vs.
log(1/Tp) were used to determine the activation energies (DE)
for the non-isothermal crystallizations of pristine PVdF–HFP
and 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
Fig. 9 Linear fitting plots of log[Àln(1 À 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 at
different cooling rates (f).
Table 2 Different crystallization parameters of (a) pristine PVdF–HFP,
PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10 and (c) 30 obtained
by Avrami plots using the non-isothermal crystallization method
PVdF–HFP +
x wt% of
BMIMBF4
Heating rate
(4) (
C minÀ1
) n0
Zt (minÀn0
) Zc t1/2 (min)
X ¼ 0 5 1.05 0.037 0.517 11.13
10 1.16 0.076 0.773 5.23
15 1.45 0.141 0.877 3.23
20 1.57 0.2 0.922 2.35
X ¼ 10 5 1.03 0.051 0.552 7.46
10 1.11 0.115 0.805 3.50
15 1.34 0.230 0.906 2.20
20 1.51 0.374 0.952 1.55
X ¼ 30 5 1.04 0.053 0.556 7.17
10 1.09 0.120 0.814 3.02
15 1.13 0.178 0.897 2.17
20 1.62 0.382 0.951 1.39
Table 3 Non-isothermal crystallization kinetics parameters of (a)
pristine PVdF–HFP, PVdF–HFP + x wt% of IL BMIMBF4 where (b) x ¼ 10
and (c) 30 at different degrees of crystallinity
PVdF–HFP +
x wt% of BMIMBF4 X0
T (%) F(T) b
X ¼ 0% 10 0.0828 0.9423
20 0.0453 1.0325
30 0.0269 1.1094
40 0.0172 1.1663
50 0.0146 1.1252
60 0.0129 1.0875
70 0.0115 1.0638
80 0.0098 1.0703
90 0.0093 1.0320
100 0.0085 1.0267
X ¼ 10% 10 0.0880 1.0959
20 0.0723 0.9929
30 0.0546 0.9873
40 0.0309 1.1046
50 0.0215 1.1293
60 0.0177 1.1053
70 0.0146 1.1014
80 0.0129 1.0698
90 0.0107 1.0959
100 0.0094 1.0943
X ¼ 30% 10 0.0841 1.1416
20 0.0383 1.2058
30 0.0348 1.1160
40 0.0277 1.1334
50 0.0221 1.1498
60 0.017 1.1756
70 0.0146 1.1563
80 0.0123 1.1573
90 0.0106 1.1565
100 0.0095 1.1488
50922 | RSC Adv., 2014, 4, 50914–50924 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

10. determined to be $162, 124 and 108 KJ per mole respectively.
The decreasing activation energy with increasing amount of
ionic liquid in PVdF–HFP indicates easier ionic transport in
PVdF–HFP + IL gel membranes having higher amount of ionic
liquid, as a result of increasing plasticization/amorphization
of the polymeric membranes, which suppresses the crystalli-
zation rate.
Fig. 10 Plots of log f vs. 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 at different relative degrees of
crystallinity (Xt).
Fig. 11 ln(f/t2p) versus a (1/tp) plot for evaluating the non-isothermal
crystallization activation energy for (a) PVdF–HFP, PVdF–HFP + x wt%
of IL BMIMBF4 where (b) x ¼ 10 and (c) x ¼ 30.
This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 50914–50924 | 50923
Paper RSC Advances

11. Conclusions
In the present study, the crystallization behaviours of pristine
PVdF–HFP and PVdF–HFP + x wt% of IL BMIMBF4 (where x ¼ 10
and 30) were studied by isothermal and non-isothermal crystal-
lization processes using DSC. The isothermal crystallization
process of pristine PVdF–HFP and prepared membranes was well
described by the Avrami equation. The values of the Avrami
exponent n lie between 1 and 2 for all the prepared membranes
indicating two-dimensional growth of spherulites. It has been
found that the presence of the ionic liquid BMIMBF4 in the
PVdF–HFP matrix suppresses the crystallization of polymer
PVdF–HFP, resulting in low crystal growth rates. This effect
occurs because the presence of IL (which amorphizes/
plasticizes the polymers) hinders the chain folding, and
thereby increases the time it takes for crystallization to occur
when the crystallization depends on a folded polymer. The
exible nature of a polymer allows it to sample the conforma-
tions necessary for joining a crystal, but such exibility is
decreased in the presence of IL. Various kinetics methods such
as those of Jeziorny, Ozawa and Mo have been employed to
study the non-isothermal crystallization process. The Avrami
equation modied 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
(i.e., the combination of the Avrami and Ozawa equations)
claries the results better. All parameters such as the Avrami
exponent, crystallization rate constant and crystallization half
time are found to be strongly dependent on the cooling rate and
concentration of IL. The activation energy (DE) of the prepared
membranes varies with IL loading.
Acknowledgements
R.K. Singh is grateful to the BRNS-DAE, India, for nancial
assistance. Shalu and S.K.C wish to thank the U.G.C. and CSIR
New Delhi, India, respectively, for their Research Fellowships.
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RSC Advances Paper