Solid State Ionics 252 (2013) 102–108

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Solid State Ionics

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Effect of ethylene carbonate on proton conducting polymer electrolytebased on poly(ε-caprolactone) (PCL)

H.J. Woo, S.R. Majid, A.K. Arof ⁎Centre for Ionics University of Malaya, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia

⁎ Corresponding author. Tel.: +60 379674085; fax: +6E-mail address: akarof@um.edu.my (A.K. Arof).

0167-2738/$ – see front matter. Crown Copyright © 2013http://dx.doi.org/10.1016/j.ssi.2013.07.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 January 2013Received in revised form 14 June 2013Accepted 7 July 2013Available online 7 August 2013

Keywords:Polymer electrolytePolycaprolactonePermittivityVTFCrystallinity

Polymer electrolytes (PE) comprised of poly(ε-caprolactone) (PCL), ethylene carbonate (EC) and ammoniumthiocyanate (NH4SCN)were preparedby solution casting. Thermal properties, conductivity and structural studieswere characterized by differential scanning calorimetry (DSC), impedance spectroscopy and X-ray diffraction(XRD), respectively. EC is intended to be used as plasticizing solvent to enhance the conductivity. It is foundlater that EC can also promote ion dissociation in polymer electrolyte system. From DSC studies, the introductionof EC as additive into PCL-NH4SCN complexes reduces the Tm from 56.4 °C to 44.1 °C, drops Tg from −59.7 °Cto −76.7 °C and suppresses the crystallinity. The conductivity of PE improves from 4.6 × 10−8 S cm−1 to3.8 × 10−5 S cm−1 at 50 wt.% EC. Relative permittivity studies show that EC contributes to higher dissociationrate of charge ions. In the temperature dependence conductivity analysis, VTF equation fits verywellwith regres-sion value close to unity for all the samples demonstrating a close coupling between the conductivity and thepolymer segment mobility. The pre exponential factor A increases as more EC is added, reflecting the growthin the number of charge carriers. Structural analysis using XRD displays an apparent reorganization of polymermatrix due to the EC effect. This is in agreement with DSC and EIS observations. Degree of crystallinity calculatedusing XRD two phase model shows a decrease from 32.7% of zero EC to 16.0% of 50 wt.% EC. These results arecomparable to the DSC analysis.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction

In 1973, Fenton, Parker and Wright [1] reported their first polymerelectrolyte (PE) study and a few years later Armand and co-workers[2] have highlighted the potential application in solid-state-battery.Since then, many efforts have been spent over the past few decades toimprove the room temperature (RT) conductivity through plasticization[3], polymer blending [4] and addition of fillers [5]. The uniqueness ofPE compared to liquid or solid electrolyte is that polymer matrix is re-sponsible for the solvation of ions and the ion site positions are flexibleto move along with polymer segments. This solid-state polymer mem-brane is light weight, mechanically stable and can be rolled or foldedaccording to packaging design.

Extensive research on PE has been focused on PEO [6], PMMA[7], PVdF-HFP [8] and PMAN [9] achieving RT conductivity around10−4 Scm−1. There are several projects aiming at exploring eco-friendly materials as polymer host to reduce environmental loads.Such materials include the natural (e.g. cellulose [10], starch [11],chitosan [12]) and synthetic (e.g. poly(vinyl alcohol) [13], poly(ε-caprolactone) (PCL) [14]) polymers. PCL contains a Lewis base (ester

0 379674146.

Published by Elsevier B.V. All rights

oxygen) that can coordinate cations and possesses low glass transitiontemperature, Tg (−60 °C) similar to PEO that enables the polymerchain to exhibit segmental motion at RT. A number of lithium saltssuch as lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate(LiCF3SO3), and lithium tetrafluoroborate (LiBF4) have been added toPCL in smart window and battery applications [15].

The PE system with ammonium salt has been reported as a protonconductor. With an ideal tetrahedral structure, the NH4

+ cation hasfour N\H bonds but one of the four hydrogen atoms is loosely bound[16].When an electric field is applied, thisweakly bound proton can dis-sociate easily and jump from one PCL-NH4SCN complex site to another,leaving a vacant site to be filled by another proton from nearby site. 1HNMR studies by Binesh and Bhat [17] on PEG-NH4ClO4 systemhave con-cluded that the conductivity is protonic in nature. A coulombmetricstudy done by Srivastava and co-workers [18] on PEO-NH4SCN systemhas also identified that themobile cation is H+ ion. In our previous stud-ies [19,20], PE based on PCL and ammonium thiocyanate (NH4SCN) hasexhibited RT conductivity around 10−4 Scm−1 but it was investigatedunder high relative humidity level (70%). The low relative humiditylevel (b50%) suffers low RT conductivity issue. In this study, we aim toenhance the conductivity by incorporating additive to reduce the de-gree of crystallinity. The effect of adding Ethylene carbonate (EC) onthe proton conducting PCL-NH4SCN system at 50% relative humiditylevel is investigated.

reserved.

103H.J. Woo et al. / Solid State Ionics 252 (2013) 102–108

2. Experimental

2.1. Materials

The poly(ε-caprolactone) (PCL) with Mn = 80 kDa and ethylenecarbonate (EC)were procured from Sigma Aldrich and used as received.Ammonium thiocyanate (NH4SCN) with Mw = 76.12 g/mol wasobtained from R&M Marketing and dried in an oven at 45 °C for 24 hprior to use as the incorporating salt. Proton conducting polymer elec-trolytes consisting of PCL complexed with NH4SCN in various composi-tions were prepared by solution casting technique. Fig. 1 shows thechemical structure of poly(ε-caprolactone) (PCL), ethylene carbonate(EC) and ammonium thiocyanate (NH4SCN).

2.2. Sample preparation

EC is added as additive to the highest conducting sample of PCL-NH4SCN system. Mixture of PCL and NH4SCN was kept at PCL:NH4SCNweight ratio 74:26 and varying the EC content at (PCL-NH4SCN):ECweight ratio from 0 wt.% to 50 wt.%. All composition was dissolved in30 ml aprotic solvent tetrahydrofuran (THF) with purity N99% fromJ.T. Baker. The mixtures were stirred continuously with a magnetic stir-rer for several hours at room temperature until complete dissolution.The solutions were then cast into different glass Petri dish and allowslow evaporation at 50 °C for 10 h. The films were further dried slowlyat room temperature inside a desiccator for two weeks to remove anytraces of solvent. The thickness of the free standing film ranges from100 μm to 150 μm.

2.3. Differential scanning calorimetry (DSC)

Thermal analyses were performed with TA Instruments DSC-Q200.The samples were heated to 80 °C at a heating rate of 10 °C/min (iso-thermal hold for 10 min), cool down to −90 °C (with fast coolingrate, 150 °C/min) and then heated to 90 °C at 10 °C/min under drynitrogen atmosphere. The glass transition temperature Tg (midpoint),melting temperature, Tm andmelting enthalpy, ΔHm of PCL were deter-mined from the second heating scan.

2.4. Impedance spectroscopy

Impedancesweremeasured by using theHIOKI 3532 Z LCRHi-testerwithin the 50 Hz to 5 MHz frequency range. The polymer electrolytefilms were cut into small disks and sandwiched between two polishedstainless steel disks under spring pressure. The diameter of the stainlesssteel disks is 2 cm. The electrical conductivity was then calculated usingthe equation:

σ ¼ dRB � A

: ð1Þ

Here, σ is the conductivity in units of S cm−1, d is thickness of theelectrolyte film in unit of cm, A is the surface area of contact in unit ofcm2 and RB is the bulk electrolyte resistance in unit ofΩ. RBwas obtained

ECPCL NH4SCN

Fig. 1. Chemical structure of poly(ε-caprolactone) (PCL), ethylene carbonate (EC) andammonium thiocyanate (NH4SCN).

from the electrochemistry Nyquist plot by fitting to an equivalent circuitmodel as shown in Fig. 2 inset. From the electrochemistryNyquist plot, adepressed semicircle arc is observed at high frequency region, making

an anglep1π2

with the vertical axis. The tilted spike is depicted at low

frequency region, making an anglep2π2

with the horizontal axis. Ac-

cording to Linford [21], this depressed semicircle arc could be represent-ed by a parallel combination of a resistor and constant phase element(CPE). The titled spike could also be represented by a CPE connectedin series. The impedance of the constant phase element (CPE) can beexpressed by

ZCPE ¼ k jωð Þ−p ð2Þ

where k is CPE coefficient and p is a dimensionless experimental param-eter between zero and unity [22]. When p = 1, the CPE would becomeidentical to capacitorwith inverse of k corresponding to the capacitance,C value and ZC = (jωC)−1. A fitted electrochemistry Nyquist plot isshown in Fig. 2.

2.5. X-ray diffraction (XRD)

Structural studies were performed in a Bragg–Brentano reflectiongeometry using an Alpha Instruments Supplies XRD D5000 diffractom-eter with monochromatic Cu-Kα radiation (λ = 1.5418 Å) at 40 kV,40 mA and a scattering angle range from 2θ = 5° to –80° with a stepsize of 0.1° at a temperature of 25 °C.

3. Results and discussion

Fig. 3 shows theDSC thermograms of the pure PCL and EC plasticizedcomplexes in wt.%. The DSC results of pure PCL and EC plasticized com-plexes are presented in Table 1.

The addition of EC into the PCL-NH4SCN system has changed the de-gree of crystallinity of the sample. Inset of Fig. 3 shows an endothermicpeak indicating melting of pure EC at 36.7 °C. The endothermic peak asshown in Fig. 3(a) indicates the crystallinemelting of pure PCL at 57 °C.This is attributed to the presence of uncomplexed PCL crystalline phase.Fonseca and coworkers [14] have observed a similar melting tempera-ture, Tm at 55.7 °C. The highest conducting polymer–salt film (beforeadding EC) has recorded a lower Tm at 56.4 °C. When 10 wt.% EC isintroduced to the polymer–salt complexes, the melting peaks of thePCL-based film are observed to shift from 56.4 °C to 56.1 °C. Tm keepsdecreasing with increasing of EC content to 54.3 °C and 49.0 °C at20 wt.% and 30 wt.%, respectively. Moreover, the endothermic peak at30 wt.% becomes broader and a secondmeltingpeak appears as a shoul-der. EC content beyond 30 wt.% displays multiple endothermic peaks asshown in Fig. 3(f) and (g). The appearance of multiple melting peakscould be attributed to electrolyte aggregates of polymer segments ofdifferent chain length [23] or occurrence of recrystallization duringmelting [24].

The enthalpy of melting, ΔHm can be calculated from the area underthemelting peak as presented in Table 1. It is then used to calculate thedegree of crystallinity by comparing ΔHm with that of completely crys-talline PCL according to the following equation.

χC ¼ ΔHm

ΔH0m� 100% ð3Þ

where ΔHm is the melting enthalpy of completely crystalline PCL and istaken as 139.5 J/g [25,26]. ΔHm is found to decrease with increasingEC concentration. Thus, the relative degree of crystallinity, χC is alsofound to reduce from 35.6% of 0 wt.% EC to 28.3% after incorporationof 10 wt.% EC. χC drops continuously until 20.1% and 16.8% of 40 wt.%and 50 wt.% EC. ECmolecules are trapped into the PCLmatrix, interruptpolymer–polymer interaction either by occupying or increasing inter

ω

-Im

(Z)(

Ω)

Re(Z) (Ω)

Rb

CPE1 (k1, p1)

CPE2 (k2, p2)

Rb = 1320 Ωk1 = 4 x 10-6 F-1

p1 = 0.61k2 = 6.2 x 10-4 F-1

p2 = 0.69

21p

22p

π

π

Fig. 2. The electrochemistry Nyquist plots of 30 wt.% EC plasticized complex. Inset shows the equivalent circuit model of the fitting.

104 H.J. Woo et al. / Solid State Ionics 252 (2013) 102–108

and intra-chain separations [27]. This disorders the original polymercrystalline structure and causes reorganization of polymer chain. Poly-mer backbone becomes more flexible showing an apparently lower Tgfrom 20 wt.% of EC onwards. Tg of PCL has dropped from −59.7 °C of0 wt.% EC to −56.9 °C, −66.0 °C and −76.7 °C of 20 wt.%, 30 wt.%and 40 wt.% EC, respectively. Lower Tg means increase of polymer seg-mental mobility.

The RT conductivity versus EC concentration of plasticized PCL-NH4SCN polymer electrolytes is plotted in Fig. 4. RT conductivity aslow as 4.6 × 10−8 S cm−1 is obtained for unplasticized PCL-NH4SCNfilm. Upon addition of 10 wt.% EC, conductivity increases one order of

Temperature (ºC)

(g) 50 wt.%

(f) 40 wt.%

(e) 30 wt.%

(d) 20 wt.%

(c) 10 wt.%

(b) 0 wt.%

(a) Pure PCL

Hea

t F

low

(E

xo)

-80 -40 0 40 80

-80 0 80

Fig. 3.DSC thermograms of the pure PCL and ECplasticized complexes inwt.%. Inset showsthe pure EC thermogram.

magnitude to 8.2 × 10−7 S cm−1. Conductivity keeps on increasingmonotonically with EC content up to 3.8 × 10−5 S cm−1 at 50 wt.%EC. This indicates that the polymer is capable to contain a large amountof ECmolecules. The increase of conductivitywith increasing EC contentis attributed to the enhancement of ion density and ion mobility whichwill be explained in the following section.

Real part of complex relative permittivity or dielectric constant, ε'study of PE is a measure of the capacity of the polymer film to retentmobile charges and dipoles in a given volume. The dimensionless ε'can be calculated using the impedance data according to the followingrelation:

ε0 ¼ Z00

ω � C0 Z02 þ Z0 02� � ð4Þ

where C0 is vacuum capacitance (C0 = ε0A/d) in units of F or sΩ−1, d isthickness of the electrolyte film, A is the surface area of contact andω isangular frequency (ω = 2πf). Fig. 5 shows the plot of dielectric constantof plasticized complexes versus frequency with various EC concentra-tions at RT.

Fig. 5 is plottedwith an x-axis of log-scale. This is to show the disper-sion at low frequency. The value of ε' builds up exponentially with de-creasing frequencies and is attributed to electrode polarization. In thelow frequency region, ions will move along the direction of the electric

Table 1DSC results of pure PCL and EC plasticized complexes.

EC content(wt.%)

Tg(°C)

Tm (°C) ΔHm

(Jg−1)χ

C

(%)Peak 1 Peak 2 Peak 3

Pure PCL −59.8 57.0 – – 60.4 43.3EC plasticizedcomplexes

0 −59.7 56.4 – – 49.6 35.610 −59.6 56.1 – – 39.5 28.320 −56.9 54.3 – – 38.9 27.930 −66.0 49.0 46.2 – 32.4 23.240 −76.7 46.3 43.0 17.0 28.0 20.150 – 44.1 40.8 23.8 23.4 16.8

EC content (wt.%)

log

(S

cm

-1)

σ

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

200 40 60

Fig. 4. EC concentration dependence of ionic conductivity at RT.

105H.J. Woo et al. / Solid State Ionics 252 (2013) 102–108

field but are unable to be transported to the external circuit becauseof the blocking electrodes. As a result, the ions are trapped near theelectrode–electrolyte interface leading to the formation of a hetero-charge layer [28] called electrode polarization. As frequency increases,ε' value drops drastically and eventually saturate in the high frequencyregion. This is because the ionswith finitemassfind it hard to follow thefast periodic orientation of the applied field.

Similar polarization effect in low frequency regionwas also observedbefore addition of EC in our previous report [20]. However, the additionof EC into polymer–salt complexes seems to amplify the value of ε'many folds. ε' value of the unplasticized film is only around 102 at lowfrequency as shown in [20]. Addition of 10 wt.% and 20 wt.% EC hasmoved ε' value up 10 times and 100 times, respectively. The ε' value at50 wt.% EC plasticization is as high as 9.0 × 104. This skyrocket jumpcould be understood by high ε' value (89.8) of pure EC. A high ε' valueadditive is always taken to have high dissociation ability to prevention-pair formation or high effectiveness on shielding the interionic cou-lombic attraction between cations and anions. According to Bjerrumtheory quoted in [29], the critical distance, r for ion pair formation is re-ciprocal to ε' value as the following equation:

r ¼ jziz jje28πε0ε

0kT: ð5Þ

Here, z is the valency of the cations and anions, e is the elementary elec-tron charge in unit of C, ε0 is the dielectric constant in vacuum, k isthe Boltzmann's constant ( = 8.6173 × 10−5 eV K−1) and T is tem-perature in unit Kelvin. Ion pair formation is likely to occur when the

log f

ε

1.2E+05

8.0E+04

4.0E+04

0.0E+001 3 5 7

0 wt. %

10 wt. %

20 wt. %

30 wt. %

40 wt. %

50 wt. %

Fig. 5.Real part of complex permittivity of plasticized complexes as a function of frequencyfor various EC concentrations at RT.

distance between cation and anion is smaller than r. Higher ε' valuemeans smaller q.

In a polymer system incorporated with high ε' material like EC, ionassociation is unlikely to happen where most ions would stay free.Thus, this boosting of PE system's ε' proves that EC is also acting as anadditive to promote ion dissociation. Although EC is intended to beused as plasticizing solvent to enhance the conductivity by reducing Tgand degree of crystallinity, it is found that EC can also increase numberdensity of free ions in polymer electrolyte system and help to increaseconductivity. Chintapalli and Frech have shown higher dissociationrate of free ions using FTIR result by adding EC into their PEO-LiCF3SO3

system [30]. On the other hand, Forsyth and co-workers used NMR re-sult in their PEG-NaCF3SO3-PC system [31] to draw the same conclusion.

How does the increase of number density free ions be reflected inthe real part of complex permittivity of polymer complexes as displayedin Fig. 5? The real part of complex permittivity is a measure of thematerial's polarization, associated with the amount of dipoles (in thiscase, salt ions that dissociate into cation–anion pairs) in a given volume.Therefore, an increase of ε' in Fig. 5 would reflect an increase of numberdensity of free ions and would be of useful data in conductivity studieslater.

Following the above discussion, a plot of both EC concentration andconductivity versus dielectric constant at selected frequenciesmay shedsome light on the conductivity study. The conductivity in the units ofS cm−1 can also be viewed based on the following equation:

σ ¼X

i

ni � zie � μ i ð6Þ

where ni is the number density of charge carriers of type i in unit ofcm−3; zi is the valency of the charge carriers on each type, e is theelementary electron charge and μi is the ionic mobility in units ofcm2 V−1 s−1. The increasing conductivity could be attributed tothe increase in number density of free ions in the polymer electro-lyte. In order to observe the EC concentration dependence of ε' forthe electrolyte, a fix frequency value must be chosen randomly.300 Hz and 3 kHz are in the region of electrode polarization whereit is easier to see that ε' varies with EC content. 30 kHz is in the regionwhere the relative permittivity can be considered as the dielectricconstant. Fig. 6 shows the EC concentration dependence of log ε' at300 Hz, 3 kHz and 30 kHz.

It is observed that ε' for increasing EC content behaves in the sametrend as RT conductivity. In other words, the increase of electrolyte rel-ative permittivity does play an important role in conductivity enhance-ment. Although a direct relationship between conductivity and relativepermittivity or dielectric constant of an electrolyte in solid phase is notknown as to-date, Petrowsky and Frech [32,33] have shown that there isa strong dependence of conductivity on dielectric constant for liquid

EC concentration (wt.%)

log

εε

5.0

4.0

3.0

2.0

1.0

0.00 20 40 60

300 Hz3 kHz

30 kHz

Fig. 6. EC concentration dependence of real part of complex permittivity of PE system at300 Hz, 3 kHz and 30 kHz.

1000/|T-T0| (K-1)

ln(S

cm

-1K

0.5 )

σσT

0.5

-6.0

-9.0

-12.05.5 6.5 7.5 8.5

1020304050

Fig. 7. Temperature dependence of conductivity at various EC content in wt.%.

Table 2The VTF fitted parameters from temperature dependence of conductivity at various ECcontent.

EC content(wt.%)

Tg(K) R2 VTF fit

T0(K) A (Scm−1 K0.5) B (K)

10 213.45 0.99 173.25 4.42 × 10−5 101.520 216.25 0.98 166.75 4.40 × 10−4 312.330 207.15 0.99 164.85 1.27 × 10−3 357.440 196.45 0.99 149.35 1.84 × 10−3 332.550 – 0.99 146.00 1.07 × 10−2 373.4

106 H.J. Woo et al. / Solid State Ionics 252 (2013) 102–108

electrolytes. The present work is a preliminary result of similar observa-tion in solid phase to breed for more work in future.

Temperature dependence of conductivity at various EC content isplotted in Fig. 7. The Vogel–Tamman–Fulcher (VTF) equation [34] isused for the linear fit as:

ln σ T0:5 ¼ ln A− BT−T0ð Þ : ð7Þ

Here A is the pre-exponential factor in units of Scm−1 K0.5 which is pro-portional to the number density of charge carriers, B is the pseudo-

(1

2θθ (de

Rel

ativ

e In

tens

ity

(a.u

)

0 20 4

17

Fig. 8. XRD pattern of (a) pure NH4SCN, pure PCL and EC plasticized complexes

activation energy in unit of K for the redistribution of free volume, T isabsolute temperature and T0 is the quasi-equilibrium glass transitiontemperature at which free volume disappears or ionic conduction be-comes frozen. T0 is usually 50 K lower than Tg [35]. Table 2 displaysthe VTF parameters determined by fitting the experimental data. T0 isobtained by trial and error beginning from (Tg − 50) and change subse-quently to get the maximum regression value approaching unity.

Fig. 7 presents a good fit of conductivity to the VTF equation withregression value of ~0.98–0.99 for all the samples. The parameter Aincreases with increasing EC concentration. This reflects the growth inthe number of charge carriers, supporting the idea that EC helps to dis-sociatemore free ions in the plasticized polymer electrolyte system. Theparameter B is also found to increase with EC content unexpectedly. Asurvey on literature shows that a similar observation has been reported[36]. Pseudo-activation energy does not necessarily reflect the energybarrier for ion conduction but rather the energy to encourage segmentalmotion [37]. When the temperature is elevated, this pseudo energy istransmitted to the polymer chain, promoting segmental motion. Thefree volume enlarges allowing polymer matrix to expand.

It is good to note that the Tg (b−60 °C) is well below the room tem-perature at the rubbery-state of the current plasticized system. This so-called viscoelastic behavior corresponds to the anisotropic liquid at themicroscopic scale. Assuming the critical free volume is larger than thatof segmental motion, the ions should be mobile at temperatures aboveTg. Hence, the conductivity is the cause of ionic motion coupled withthe segmental movement of the polymer chain [38] whereby VTF givesthe perfect fit. The value of parameter B is determined in the range of102 K (or 10−2 eV or 1 kJ mol−1). These values are of the same magni-tude as those report for the PEO [36,39], P(DMS-co-EO) [37] and PEG[40] systems.

Fig. 8 displays the XRD pattern of (a) pure NH4SCN, (b) pure PCL andEC plasticized complexes in various weight percent. Inset shows theexpanded XRD pattern of pure PCL showing its semi-crystalline nature,three sharp crystalline peaks at 2θ = 21.4°, 22.0° and 23.7° correspond-ing to the (110), (111) and (200) orientations, respectively, of theorthorhombic crystal structure [41] and a small continuous spectrumcentered on 2θ = 21.0° showing amorphous contribution.

Plasticizing the PCL-NH4SCN with EC generates significant disorderinto the polymer structure with higher amorphousness. IndividualXRD pattern is plotted and analyzed. With addition of EC from 10 wt.%to 50 wt.%, the amorphous spectrum has become stronger and broaderindicating structural re-organization of PCL. The widely used two phase

10)

(111)

(200)

2θ (degree) (h) 50 wt.%.

(g) 40 wt.%

(f) 30 wt.%

(e) 20 wt.%

(d) 10 wt.%

(c) 0 wt.%

(b) Pure PCL

(a) Pure NH4SCN

gree) 0 60 80

22 27

in wt.%. Inset is the expanded XRD showing crystalline orientation of PCL.

2θθ (degree)

Rel

ativ

e In

tens

ity

(a.u

)

10 20 30 40

experimental data

(110) peak fitting

(111) peak fitting(200) peak fitting

halo fittingcombine fitting

Fig. 9. The deconvoluted XRD of 40 wt.% EC plasticized complex.

107H.J. Woo et al. / Solid State Ionics 252 (2013) 102–108

model [42,43] was applied to estimate the relative degree of crystal-linity, χC according to the following equation:

χC ¼ ICIC þ Iað Þ � 100% : ð8Þ

Here, IC refers to the total crystalline intensity of the three sharp peaksand Ia is amorphous intensity of the halo. Literature review shows thatI could be taken as integrated intensity [44] (area under the curve) orintensity [45] (magnitude of ordinate) of the respective peak. This re-port uses the formal definition. In all spectra, the baseline of XRD spec-tra was corrected and then the sharp crystalline peaks were separatedfrom amorphous spectrum using peak separation software. The good-ness of fit, R2 is around 0.99.

Fig. 9 illustrates a sample of deconvoluted XRD of 40 wt.% EC plasti-cized complex. The relative degree of crystallinity as a function of ECconcentration is then calculated from the respective integrated areaand is depicted in Fig. 10. The degree of crystallinity, χC is found todecrease slightly from 32.7% to 31.2% upon addition of 10 wt.% EC. χC

of the ternary system is observed to drop continuously to 24.8%, 23.1%,20.5% and 16.0% with addition of 20 wt.%, 30 wt.%, 40 wt.% and50 wt.% EC, respectively. EC has interrupted the crystalline phase ofthe polymer matrix therefore reducing the degree of crystallinity. Thisobservation is consistent with thermal studies and comes alongsidewith the increase of ionic conductivity. It is also noted that theχC value calculated from XRD is comparable to that of DSC.

4. Conclusions

EC is intended to be used as plasticizing solvent to enhance the con-ductivity. It is found later that EC can also promote ion dissociation inpolymer electrolyte system. From DSC studies, the introduction of ECas additive into PCL-NH4SCN complexes is found to reduce the Tmfrom 56.4 °C to 44.1 °C. EC content beyond 30 wt.% displays multiplemelting peaks possibly due to electrolyte aggregates of polymer seg-ments of different chain length. Compare to our previous studies [19],

EC content (wt.%)

χχ C (

%)

40

30

20

100 20 40 60

Fig. 10. Degree of crystallinity as a function of EC content, estimated from XRD.

the addition of EC suppresses the crystallinity and decreases the Tgfrom −59.7 °C to −76.7 °C. The conductivity of PE improves from4.6 × 10−8 S cm−1 to 3.8 × 10−5 S cm−1 at 50 wt.% EC. In permittivitystudies, ε' value increases 100 times showing that EC contributes tohigher dissociation rate of charge ions. Thus conductivity is enhanced.In the temperature dependence conductivity analysis, VTF equationfits very well with regression value close to unity for all the samples.The value of (T0 − Tg) lies in the range of 40–50 K, demonstrating aclose coupling between the conductivity and the polymer segmentmobility. The parameter A increases with increasing EC concentration.This reflects the growth in the number of charge carriers. Structuralanalysis using XRD displays an apparent reorganization of polymermatrix due to the plasticizing effect. This is in agreement with DSCand EIS observations. Degree of crystallinity calculated using XRD twophase model shows a decrease from 32.7% of unplasticized system to16.0% of 50 wt.% EC. These results are comparable to the DSC analysis.

Acknowledgment

The authors would like to thank the University of Malaya (PV056/2011B) and the Ministry of Higher Education (FP035-2012A) forsupporting this research work. H.J. Woo thanks the University for thescholarship award.

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