Solid State Ionics 278 (2015) 260–267
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Solid State Ionics
journal homepage: www.elsevier.com/locate/ssi
Effects of potassium iodide (KI) on crystallinity, thermal stability, and
electrical properties of polymer blend electrolytes (PVC/PEO:KI)
Reddeppa Nadimicherla a,b, Ramamohan Kalla b, Ravi Muchakayala b, Xin Guo a,⁎
a
b
Laboratory of Solid State Ionics, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
Laboratory of Thin films, Department of Physics, Sri Venkateswara University, Tirupati 517502, India
a r t i c l e
i n f o
Article history:
Received 11 May 2015
Received in revised form 2 July 2015
Accepted 2 July 2015
Available online xxxx
Keywords:
Structural property
Crystallinity
Melting enthalpy
Impedance analysis
Optical band gap
Transference number
a b s t r a c t
Potassium ion conducting solid polymer blend electrolytes based on PVC/PEO:KI of various compositions were
prepared by solution-casting technique. The structural changes along with K+ ion–polymer blend electrolyte interactions and the thermal stability of the samples were studied by X-ray diffraction, Fourier transform infrared
spectroscopy, differential scanning calorimetry, scanning electron microscopy, and thermo-gravimetric analysis.
Electrical properties were measured as a function of composition and temperature using complex impedance
spectroscopy. The electrical conductivity exhibited Arrhenius type of behavior, with the activation energy decreasing with increasing KI concentration. The conductivity of the polymer blend (PVC/PEO) sample was obtained at ambient temperature (303 K) as 3.09 × 10−6 S cm−1, and for the PVC/PEO:KI (42.5:42.5:15) electrolyte as
3.66 × 10−4 S cm−1, the conductivities were greatly enhanced at 363 K to 8.23 × 10−4 and 1.30 × 10−2 S cm−1,
respectively. The ionic transference number of the PVC/PEO:KI (42.5:42.5:15) electrolyte was determined to be
0.98. The increase in amorphous phase and free K+ ion concentration was responsible for the conductivity improvement in the electrolyte system.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Polymer electrolytes have the prospective to play an important role
in large-scale electrochemical energy storage devices such as batteries.
In the past two decades, they have been tailored as electron or ion conductors. When combined with appropriate salts, their ionic conductivity
extends to the level of an electrolyte. Polymer electrolytes possess several advantages over traditional liquid electrolytes, e.g. safety and multifunctionality. Single-ion conductors are a unique class of polymer electrolytes that have no classical counterpart.
Among various solid-state electrolytes, gel polymer electrolytes
(GPEs), which are generally composed of polymer matrix and liquid
electrolyte, are widely used in lithium-ion batteries owing to their excellent ionic conductivity, low rates of safety failure, and mechanical
flexibility [1–3]. In general, conventional GPEs are prepared using
a predesigned frame via solution casting of liquid state mixtures
(i.e., liquid electrolytes and polymers dissolved in organic solvents or
liquid electrolytes/polymerizable monomers), followed by solvent
evaporation or chemical cross-linking for solidification. Conducting
polymers contain π-electron backbone responsible for their unusual
electronic properties such as high electrical conductivity, low energy
optical transitions, low ionization potential, and high electron affinity.
This extended (π-conjugated) system of the conducting polymers has
⁎ Corresponding author. Tel./fax: +86 27 87559804.
E-mail address: [email protected] (X. Guo).
http://dx.doi.org/10.1016/j.ssi.2015.07.002
0167-2738/© 2015 Elsevier B.V. All rights reserved.
single and double bonds alternating along the polymer chain. The
higher values of the electrical conductivity obtained in such organic
polymers have led to the name ‘synthetic metals’. Since the discovery
of high electrical conductivity from blending polyethylene oxide (PEO)
with potassium salts by Fenton et al. [4], polymer electrolytes have
attracted a lot of interest, because of their potential use in thin film batteries [4]. Polymer electrolytes consist of polar polymers and ionizable
salts. The progress of polymeric systems with high ionic conductivity
is one of the main objectives in polymer research. This is due to their
potential applications in solid state batteries [5–8]. Polymer batteries
possess advantages such as high ionic conductivity, high energy density,
solvent-free condition, leak proof, wide electrochemical stability windows, easy processability, and light weight.
One of the most studied polymers is polyethylene oxide (PEO), also
known as polyethylene glycol (PEG). Polyethylene oxide (PEO) is considered to be one of the most capable conducting polymers due to its
high pseudo-capacitance, good environmental stability, safety, low
cost, facile synthesis, and good conductive ability at doping state,
which is beneficial to the increase of electron and ion exchange rate
[9–11]. Polyethylene oxide (PEO) can act as a host for sodium and potassium salts, thus producing a solid electrical conductor polymer/salt
complex. The resistance of PEO to protein adsorption is generally attributed to a steric repulsion effect, by which the polymer prevents the protein from reaching the substrate surface [12,13]. Indeed, in aqueous
environments, PEO molecules are highly mobile and strongly hydrated,
attaining extremely large exclusion volumes [14,15]. PEO is the most
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
appropriate base material due to its ability to play a host to various
metal–salt systems for a wide range of concentrations. It also acts as
a good binder for other phases and has excellent chemical stability.
The chemical structure of PEO consists of sequential oxy-ethylene
group –CH2–CH2–O– that has a series of polar group –O–, which can associate with metal cations. Hence, PEO can solvate a wide variety of salts
even at very high salt concentrations, e.g. LiX (X = F, Cl, Br, I, ClO4, SCN,
CF3SO3, BF4, AsF6, etc. [16,17]). However, PEO has C–C, C–O, C–H bonds
only, and the reactivity is exceedingly low owing to the good chemical
and electrochemical stability [18].
PVC is the only general purpose plastic that allows free, wide and
seamless adjustment of the required physical properties of products
such as flexibility, elasticity, and impact resistance, by adding plasticizers, additives, and modifiers. Polyvinyl chloride (PVC) has an amorphous nature with polar chlorine atoms in the molecular structure;
therefore, it mixes well with various other substances. The required
physical properties of end products (e.g., anti-fouling, prevention of microbial growth, anti-mist, fire retarding) can be freely designed through
formulation with plasticizers and various additives, modifiers and coloring agents. Polyvinyl chloride (PVC) has a high mechanical strength and
wear-resistance [19]. Compared with polyimides and celluloses, PVC is
relatively cheap and ideal for the formation of PVC/PEO blends. It has
been demonstrated that the addition of PVC into PEO improves the mechanical property of PEO membranes [20], and suppresses the crystallization of PEO [21,22]. These approaches include synthesizing new
polymers [23], cross linking two polymers [24], blending of two polymers [25], adding plasticizers to polymer electrolytes [26], adding inorganic inert fillers [27] to make composite polymer electrolytes (CPEs).
Potassium salt was selected as a dopant to prepare the present polymer blend complex electrolyte system. Potassium iodide is a component
in the electrolyte of dyes sensitized solar cells (DSSC) along with iodine.
The activity of a salt in an electrolyte has a direct impact on the performance of electrochemical cells, because it can affect both the potential
between phases and transport through the electrolyte. The usage of potassium complexed electrolyte films has been found to exhibit several
advantages over lithium counterparts. The mobility of smaller ions
(Li+ and/or Mg+2) is lower than that of cations with larger sizes (K+
and/or Zn+2) in the polymer electrolytes [28], because smaller cations
are embedded or captured by the polymeric network. Furthermore,
the interaction between Li+ ions and the polar groups of polymer is
stronger than K+ ions, thus Li+ ion transfer requires higher activation
energy in the polymer electrolytes [29,30]. For the present work, we
have adopted one of the above techniques, namely blending, because
of the ease of preparation and easy control of physical properties within
the compositional regime. The inherent merits of using blend-based
polymer electrolytes have been exemplified by several research groups
[31,32], detailed studies of the blend-based polymer electrolytes can
furnish valuable information on the relative importance of various factors, which affect the electrical, thermal and mechanical properties of
the polymer blend electrolytes.
Although solid polymer electrolytes have great potential in solid
state batteries, there are only limited studies on potassium ion complexed PVC/PEO electrolyte systems. In the present work, K+ ion
conducting solid polymer blend electrolytes based on PVC/PEO:KI of different compositions were prepared by solution-casting technique, and
the structure, thermal stability, and ionic conductivity of the solid polymer blend electrolytes were systematically studied.
261
Because of its higher polar nature, the electrical insulating property is
inferior to non-polar polymers such as polyethylene and polypropylene.
Electric dipoles are formed in the mers due to polarity of the C = Cl
bond. It creates a physical, or secondary, bond between adjacent chains
owing to permanent dipoles. The structure of PEO is (–CH2–CH2–O–).
The helix conformation of PEO that is the basis of structural unit in the
crystalline phase has two turns in a fiber identity period of 19.3 Å. This
polymer, in its pure form, is chemically and electrochemically stable,
since it contains only strong unstrained C–O, C–C, and C–H bonds [33].
Potassium iodide (KI) salt (Sd. Fine-Chem. Limited, China) was used as
dopant. Aqueous solutions of potassium iodide are usually neutral of
slightly alkaline; a pH of 7 to 9 is typical. Tetrahydrofuran (THF)
(Merck) was used as common solvent. Circular disks of aluminum electrodes (area = 1.132 cm2) were used as blocking electrodes in the electrochemical measurements.
2.2. Preparation of polymer electrolyte films
Appropriate amounts of PVC, PEO, and KI were dissolved in THF and
the solutions, and were magnetically stirred for 12–15 h at room temperature to obtain a homogeneous solution. Pristine blend films of
PVC/PEO (50:50) and various compositions of KI doped PVC/PEO were
obtained in various weight percentage ratios (47.5:47.5:5, 45:45:10,
42.5:42.5:15). The stirred solutions were cast onto polypropylene
dishes and allowed to evaporate slowly at room temperature, followed
by vacuum drying. The final composite product was vacuum dried at
45 °C thoroughly under a vacuum of 10 mbar to remove any residual
solvent. The reaped films were stored in highly evacuated desiccators
to avoid any moisture absorption. Film thickness was measured by the
capacitance method, and later verified by the gravimetric method. The
values were found to approximately 100 μm with an accuracy of about
±5 μm.
2.3. Characterization techniques
The X-ray diffraction (XRD) patterns of prepared samples were obtained on an X-ray diffractometer (type PANalytical, Netherlands)
using monochromatized Cu Kα (0.15418 nm) at a scan rate (2θ) of
0.05°s−1. The accelerating voltage and the applied current were 40 kV
and 30 mA, respectively. The Fourier transform infrared spectroscopy
(FTIR) of pristine PVC/PEO and KI salt complexed PVC/PEO films was recorded using EO–SXB IR spectrometer at a resolution of 4 cm−1. The
spectra were obtained in the wave number range of 400–3800 cm−1.
Differential scanning calorimetry (DSC) measurements were carried
out using NETZSCH DSC 204 in the range of 47–77 °C, and all the measurements were taken at a heating rate of 5 °C min−1 under nitrogen atmosphere. The morphology of the samples was characterized by JOEL
JSM 840A scanning electron microscope (SEM). The conductivity of
these solid polymer blend films was investigated with the cell
consisting of two blocking stainless steel electrodes. Impedance measurements were carried out in the temperature range of 303–363 K
using 3532-50 LCR Hi–Tester over a frequency range of 100–1 MHz.
The instrument was interfaced to a computer for data collection. The optical absorption profiles of these samples were recorded at room temperature in the wavelength range of 200–600 nm using UV–VIS–NIR
(Model UV–3100) spectrophotometer.
3. Results and discussion
2. Experimental
3.1. Structural properties
2.1. Materials
PVC and PEO were obtained from Aldrich (USA) and used without
further purification to prepare solid polymer blend electrolytes. PVC,
PEO, and KI were dried by heating at 50 and 60 °C under vacuum for 5
and 10 h, respectively. PVC is a polymer with good insulation properties.
X-ray diffraction studies were carried out to investigate the semi‐
crystalline nature of PVC/PEO blend and possible changes in semi‐crystalline behavior due to the addition of KI. XRD patterns of pristine, complexed PVC/PEO blend membranes (Fig. 1) exhibit significant peaks at
around 19.63°, 23.31°, and 24.06° and low intense peaks at around
262
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
Fig. 1. XRD patterns of (a) PVC/PEO, (b) PVC/PEO:KI (47.5:47.5:5), (c) PVC/PEO:KI
(45:45:10), (d) PVC/PEO:KI (42.5:42.5:15), and (e) KI salt.
15.5°, 22.47°, and 24.48°, which can be attributed to the crystalline nature of PEO. These peaks overlap on a broad hump between 15° and
25°. This broad hump indicates the amorphous nature of PEO, whereas
a weak peak centered circa 14.5–14.9° is considered to be due to amorphous PVC [34]. These observations confirm that the present polymer
blend system possesses semi‐crystalline nature. When the salt was
added to polymer blend samples, the intensity of all crystalline peaks
was found to decrease gradually and their width increased with increasing KI salt. The interaction between the blend polymer and salt leads to
the decrease in the intermolecular interaction between polymer blend
chains, and form the co-ordination interaction between the salt and C–
O–C group of PEO and/or C = Cl group of PVC, which implies marked decrease in the degree of crystallization, thereby increase in amorphous region [35]. The sharp peak at around 36.19° (220) and low intensity peak
at 25.34° (200) (JCPDF card no. 01-073-0382) pertaining to potassium iodide salt are absent in Fig. 1 in the case of complexes, demonstrating the
complete dissolution of the salt in the polymer matrix [34]. These observations demonstrate the complexation of the polymer blend with KI and
the suppression of the crystallinity of the polymer blend samples.
vibrational modes and various functional groups of PVC and PEO are
given in Table 1. The co-existence of well resolved bands of oxygen
groups (C–O–C) of PEO and vinyl groups (C = Cl) of PVC indicates
that PVC and PEO are miscible. The spectral region 1004–831 cm−1 consists of a mixture of CH2 rocking and C–O stretching vibrational modes.
Spectral changes in this region reflect changes occurring in the local
structure of the polymer backbone. The vibrational mode responsible
for the band at 847 cm−1 is primarily due to CH2 rocking motion with
a little C–O stretching motion mixed in, while the band at 998 cm− 1
originates primarily from C–O stretching motion with some contributions from CH2 rocking motion. The presence of these bands, assigned
to CH2 rocking vibrations, indicates gauche conformations of –CH2–
CH2– groups in the crystalline portion of PEO, which has all –O–C–C–
O– torsion angles in gauche conformations. The absence of the characteristic band near 1320 cm−1, assigned to the CH2 stretching vibration
of ethylene groups, indicates the trans conformations. These conformations are responsible for helical conformation of PEO. Particularly, KI
complexes of PEO based polymer electrolytes are believed to be in the
crystalline state and have a helical configuration for the polymer [37].
The dopant effect on the modes of vibrations was observed in terms
of decrease in the intensity, broadening of the bands with increasing salt
content and shifting of the bands to lower wave numbers, which resulted from the formation of cross-links between the cations and ether oxygen atoms in PEO, and coordination between the K+ cation and the
chlorine atom of vinyl groups of PVC. In these interactions, some of
PEO chains possibly wind around the K+ cations and C–O single bond
strength decreases in PEO. In PVC, the spectral perturbation of vinyl
band becomes extensive due to the increase in the basicity of C = Cl
group with increased salt concentration, implying that the vinyl group
is able to act as a strong electron donor to interact with K+ cation
[44]. These interactions lead to the interruption of crystallization, and
the fraction of amorphous content increases. The intensity of two
bands at 955 and 847 cm−1, related to helical structure of PEO, decreases and their position is shifted to lower wave numbers with increasing salt concentration. It leads to the conclusion that the PEO
helical conformation is distorted, or at least, stretched with increasing
salt concentration [37].
Additionally, a small band at 1320 cm− 1, which is attributed to
amorphous PEO, showed higher intensity when 10 wt.% KI was added.
This indicates the transformation of crystalline regions of PEO into
3.2. Fourier transform infrared spectroscopy
The FTIR spectra of pristine PVC/PEO, complexed PVC/PEO with different concentrations of KI and KI salt are depicted in Fig. 2. The
Fig. 2. FTIR spectra of (a) PVC/PEO, (b) PVC/PEO:KI (47.5:47.5:5), (c) PVC/PEO:KI
(45:45:10), (d) PVC/PEO:KI (42.5:42.5:15), and (e) KI salt.
Table 1
FTIR transmittance band positions and their assignments.
Band assignment
C–Cl valence band
CH2 rocking
C–O stretching
C–O–C (sym. & asym.) stretching
C–C stretching
Asymmetric CH2 twisting
Swinging vibration of C–H in CH2 group
CH2 symmetric twisting
C–H second overtone
CH2 bending
CH2 wagging
C–H bending of CH2
C–H stretching or C–O stretch second
over tone combination
C–O stretching
Asymmetric C–H stretch/C–H deformation
combination
C–H stretching/C = O combination
CH2 bending second over tone
C–H stretching/C–C and C–O–C stretching
Asymmetric C–H stretching of CH2
Symmetric C–H stretching
PVC
PEO
Wavenumber
(cm−1)
Wavenumber
(cm−1)
663 [36]
847
955
–
1159
1250 [36]
1320 [36]
1211 [36]
1258
1333
1279 [42]
1465
–
–
847 [37]
955 [36]
1103 [38]
1159 [39]
1264 [40]
1394 [40]
1212 [36]
1258 [41]
1324 [42]
1369 [43]
1475 [38]
1778 [34]
–
2165
2105 [41]
2165 [41]
2241
2353
2545
–
2876–2426
2241 [41]
2353 [41]
2545 [36]
2986–2678 [38]
3708–2682 [36]
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
263
amorphous regions due to the addition of KI [33]. The characteristic
bands of KI salt at 766, 1103, 1369, 1599, and 2348 cm−1 disappeared
in the spectra of complexed polymer blend electrolytes. This may be
due to the disruption of the initial order of the polymer blend by the
salt [45].
3.3. Morphology studies
SEM micrographs of pristine PVC/PEO and PVC/PEO:KI (47.5:47.5:5),
(42.5:42.5:15) polymer blend electrolyte films are displayed in Fig. 3
and 3(a) shows smooth morphology of PVC/PEO, but with 5 wt.% KI,
and Fig. 3(b) displays an irregular wavelike uneven appearance owing
to the polymer-salt complex formation. The smooth morphology is
closely related to the amorphous nature of the polymer blend electrolyte complex. Fig. 3(c) depicts the SEM photograph for the blend
polymer of PVC/PEO:KI (42.5:42.5:15). This film also exhibits a uniform and homogeneous surface. It is found that there is no phase
separation for any PVC/PEO blend polymer films, and all samples appear translucent. The smooth surface morphology indicates that the
salt is completely dissolved in the polymer blend matrix. In addition,
as can be seen from the DSC result in the following, there is only one
Tg indicating that the PVC polymer blended with PEO forms a homogenous membrane.
Fig. 4. DSC curves of (a) PVC/PEO, (b) PVC/PEO:KI (47.5:47.5:5), and (c) PVC/PEO:KI
(42.5:42.5:15).
was calculated from their DSC curves based on the following equation,
assuming that the pristine blend is 100% crystalline [49–51],
Relative crystallinity ¼ ΔHm =ΔH m 100
ð1Þ
3.4. Thermal stability
To understand the thermal nature and stability of present polymer
blend electrolyte samples, DSC and TGA measurements were carried
out. The DSC thermograms of pristine PVC/PEO and PVC/PEO:KI complexes of various compositions in the temperature range of 20–100 °C
(Fig. 4) display a strong endothermic effect superimposed on heat
flow shift in the region between 43 and 75 °C, which can be attributed
to the melting of PEO crystallites [46,47]. The glass transition temperature (Tg), percentage of relative crystallinity (%χc), and melting temperature (Tm) of polymer blend electrolytes were measured from the DSC
thermograms and their variations with salt concentration are given in
Table 2.
DSC thermograms show a glass transition temperature at around
54 °C, which is often indicated as DSC Tg (onset) value [48]. Although literature results confirm that the glass transition temperature of pure
PEO lies circa −57.6 °C [46] and for PVC about 85 °C, present PVC/PEO
blend samples show a single Tg that lies between those of the individual
components, indicating that the present polymer blend is miscible.
Furthermore, the glass transition temperature shifts towards lower
temperatures with the addition of various salt concentrations. This effect is the result of the reduction in cohesive forces of attraction
among polymer chains due to the penetration of the salt ions into the
polymer matrix. It establishes polar attractive forces between ions and
chain segments, thereby increasing the segmental mobility. Consequently, the amorphous phase becomes more flexible. The percentage
of the relative crystallinity of complexed polymer blend electrolytes
where ΔH°m is the crystalline melting enthalpy i.e. the energy in the
form of heat absorbed per unit weight of the polymeric sample obtained
from the crystalline endothermic melting curve of the pristine blend
(found to be 81 J/g in the present work) and ΔΗm is the melting enthalpy estimated from the crystalline melting peaks of complexed PVC/PEO
blend films. The intensity of melting endotherm decreases and shifts
slightly to lower temperatures with increasing salt concentration in
the polymer blend electrolyte samples. The decrease in the enthalpy
of melting (ΔΗm) apparently indicates a reduction in the crystallinity
of the polymer electrolyte films with increasing salt concentration.
Each polymer blend electrolyte exhibits a relatively sharp crystalline
melting endotherm (dip) circa 62.5 °C. The melting temperature (Tm) is
taken at the apex of melting endotherm as displayed in Fig. 4, at which
the polymer blend was completely melted. When salt was added, the
melting temperature (Tm) of pristine PVC/PEO was shifted to lower
temperatures, such a phenomenon is quite common [47], and has
been related to the decrease in spherulite sizes [39] and their surface
free energy, due to the suppression of crystallites, thereby increasing
amorphous phase content in the polymer matrix. As a result of more
flexible amorphous environment getting trapped in or adjacent to the
crystalline matrix, the suppressed crystalline portion of the PVC/PEO
blend complexes melt probably at lower temperatures. In the present
investigations, the value of Tg was observed to be more sensitive than
that of Tm to the salt concentration. It is due to the fact that the concentrations of KI salt in the crystalline and amorphous phases are different
due to the change in the relative amounts of the two phases present.
Fig. 3. SEM images of (a) PVC/PEO, (b) PVC/PEO:KI (47.5:47.5:5), and (c) PVC/PEO:KI (42.5:42.5:15).
264
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
Table 2
Glass transition temperatures, melting enthalpies, relative crystallinity and melting temperatures of pristine, and complexed polymer blend electrolytes.
No.
1
2
3
Polymer
electrolyte
Composition
Tg (onset)
(°C)
ΔHm
(J/g)
χc (%)
Tm
(°C)
PVC/PEO
PVC/PEO:KI
PVC/PEO:KI
50:50
47.5:47.5:5
42.5:42.5:15
59.22
57.15
56.87
81
62.5
57
100
31.79
29.23
62.53
61.65
60.43
maximum shift in the position of weight loss curve with addition of
salt is only about 200 °C on temperature axis. It indicates that the inclusion of salt does not greatly affect the thermal stability. The plateau region between 50 and 190 °C indicates that samples are stable in this
range.
3.5. Impedance spectroscopy studies
The electrical conductivity of the solid polymer blend electrolyte (PVC/PEO:KI) system was determined from the complex impedance plots. The plots of the polymer blend electrolyte PVC/PEO:KI
(47.5:47.5:5) for various temperatures are shown in Fig. 6. The complex
impedance diagram displays two well-defined regions. The high frequency semicircle is due to bulk effect of the electrolyte, and the arc region in the low frequency range is attributed to the effect of the blocking
electrodes. In the ideal case, the complexed impedance plots at low frequencies should be a straight line parallel to the imaginary axis, but the
double layer at the blocking electrodes causes the curvature [54]. The
ion migration and bulk polarization are physically parallel, therefore,
the semi-circle at high frequency can be observed in all samples. The
bulk resistance decrease in the polymer electrolytes due to the enhancement of the ionic mobility and the number of charge carrier ions
along with increasing concentration of dopant [54–56]. The bulk resistance was measured from the high frequency intercept on the real
axis. The ionic conductivity of electrolytes was calculated using the
equation:
However, both Tg and Tm show a decreasing trend as both are controlled
principally by main chain stiffness. The smaller crystallites, due to the
presence of salt, are less stiff to give a low Tm and consequently the
amorphous portions will be in or adjacent to less stiff environment
with decreasing crystallite size, hence the lower Tg values are obtained.
Thermal stability is represented by the weight loss of the sample
after heating over the temperature range of 30–500 °C. Typical thermogravimetric analysis (TGA) results of pristine and KI complexed PVC/
PEO polymer blend electrolytes are depicted in Fig. 5. The initial 0–1%
weight loss between 30 °C and 50 °C is mainly due to the evaporation
of moisture absorbed by the samples during the process of sample loading [36]. The weight loss curves show degradation of samples above
165 °C in multistep. This multistep trend proves that the present samples are blend polymers [52]. Besides, the thermal degradation of polymer samples involves an additional step when salt is added [53]. The
first major degradation step and minor second step (poorly resolved)
in the temperature range of 165–400 °C are due to degradation of polymer blend host, and the other steps in the range of 400–500 °C may be
due to the degradation of salt, which is apparent in complexed polymer
blend samples [53]. A polymer is thermally stable until the decomposition process starts. Two types of thermal decomposition processes are
usually recognized in polymers, chain de-polymerization and random
decomposition. Chain de-polymerization is the release of monomer
units from a chain end or at a weak link; it is essentially the reverse process of polymerization. It is often called de-propagation or unzipping.
Random degradation occurs by chain rupture at random points along
the chain, giving rise to the disperse mixture of fragments. Both processes cause sample mass losses at certain high temperatures.
It is observed that the position of weight loss multistep curve shifts
towards lower temperatures with increasing salt concentration
(Fig. 5). This is due to the high flexibility acquired by polymer samples
with high salt concentration. Polymers with highly flexible chains
need less energy, and hence show low thermal resistance and decompose at lower temperatures. Thus the presence of salt contributes to
decrease in the thermal stability of polymer system [42]. However, the
where t is the thickness of the polymer blend electrolyte (cm), A the
area of blocking electrode (cm2) and Rb the bulk resistance of polymer
blend electrolyte. The variation of the AC conductivity (σ) as a function
of concentration of KI is depicted in Fig. 7. From the figure, it is observed
that the conductivity of PVC/PEO is about 10−6 S cm−1 at 303 K, and it
increases to 10−4 S cm−1 in the complex with 15 wt.% KI. The conductivity gradually increases as the wt.% of KI is increased; the conductivity
increases can be attributed to a reduction of the crystallinity of the polymer blend electrolyte and also the increase in the number of mobile
charge carriers. The co-ordination interactions, either ether oxygen
atoms of PEO or the chlorine atoms of PVC or both with K+ cations in
the PVC/PEO:KI system, are responsible for the increase of ionic conductivity. The maximum conductivity is obtained due to the effective interaction between oxygen atoms and K+ cations in the electrolyte system
[57].
Fig. 5. TGA plots of (a) PVC/PEO, (b) PVC/PEO:KI (47.5:47.5:5), (c) PVC/PEO:KI (45:45:10),
and (d) PVC/PEO:KI (42.5:42.5:15).
Fig. 6. Complex impedance plots for PVC/PEO:KI (47.5:47.5:5) at different temperatures.
σ ¼ t =ðRb AÞ
ð2Þ
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
Fig. 7. Conductivity versus composition plots of PVC/PEO:KI polymer blend electrolytes at
different temperatures.
A reduction of crystallinity in PVC/PEO electrolytes can also be seen
from the XRD analysis, which shows a decrement in the intensity of the
sharp crystalline peaks with the addition of KI salt, resulting in a dominant amorphous phase in the electrolytes. The polymer chain in an
amorphous phase is more flexible to segmental motion of the polymer,
which facilitates high ionic mobility [58,59]. Salt dissociation is an additional factor to the enhancement of the ionic conductivity. The dissociation of salt will promote more free K+ ions transfer into the electrolyte.
The variation of conductivity as a function of temperature for pristine PVC/PEO and different (PVC/PEO:KI) polymer blend electrolytes
over the temperature range of 303–363 K is depicted in Fig. 8. The conductivity increases with temperature in the pristine PVC/PEO and in all
the (PVC/PEO:KI) polymer blend electrolytes. At ambient temperature,
the highest ionic conductivity value is 3.66 × 10− 4 S cm−1 for the
PVC/PEO:KI (42.5:42.5:15) polymer blend electrolyte. The variation of
the electrical conductivity (σ) as a function of temperature (T) in the
entire temperature range can be fitted to the relation
σ ¼ σ ο expð–Ea =kT Þ
ð3Þ
where σο is a constant, k the Boltzmann constant, Ea the activation energy, and T the absolute temperature. The magnitude of conductivity is
265
found to increase with increasing temperature in all the compositions
of the polymer blend electrolyte system including pristine (PVC/PEO)
film. The Arrhenius behavior is observed for log (σ) vs. 1000/T plots,
being in agreement with the theory established by Tareev [60]. This is
rationalized by considering the free volume model [61]. The enhancement of the ionic mobility can be interpreted by the ionic transport
mechanism involving co-ordinating sites, local structural relaxations
and segmental motion of the polymer chain in the free volume model.
The increase in conductivity with temperature may be due to the decrease in viscosity and hence increase in the chain flexibility. When
the temperature increases, the vibration energy of a segment of the
polymer blend chain becomes sufficient to push against the hydrostatic
pressure imposed by its neighboring atoms, and creates a small amount
of space surrounding its own volume in which the vibrational motion
can occur [62]. Therefore the free volume around the polymer chain
causes to the increase of the conductivity. The amorphous nature also
provides a bigger free volume in the polymer blend electrolyte system
with increasing temperature [63–65].
The activation energy values of different polymer blend electrolytes
were calculated from the slopes of linear fit of the Arrhenius plots of different polymer blend electrolytes and these are displayed in Table 3. The
activation energy is a combination of energy of charge carrier creation
(defect formation) and the energy of ion migration [38,66]. The conductivity does not show any abrupt change with temperature (Fig. 8). The
activation energy is found to decrease gradually with increasing KI
concentration.
3.6. Evaluation of ionic transference number
To determine the nature of species responsible for the conductivity
in the present electrolyte system, the ionic transference number
(which gives a quantitative assessment of the extent of the ionic and
electronic contributions to the total conductivity) were measured by
applying a constant direct current (DC) potential of 1.5 V across the
cell in the configuration of K|(PVC/PEO/KI)|C at room temperature; in
such a cell, the carbon electrode is blocking to K+ ions. Fig. 9 represents
the polarization current versus time plots. At the very beginning of polarization, the current (IT) decays immediately and asymptotically approaches steady state after a long time of polarization. The mechanism
by which current initially flows under the influence of an applied voltage can be visualized as follows: initially, after applying DC potential,
the total current across the cell, due to the migration of ions and electrons (IT = Iion + Iele), is proportional to the applied electric field. At
the blocking electrode, there is neither a source nor a sink for mobile
ions. The migration of ions therefore leads to an accumulation of mobile
species in the region of the electrolyte adjacent to the blocking electrode
and the depletion of mobile species near the opposite electrode. Then
the ionic motion is opposed by a chemical potential gradient, after a
time period, it has increased sufficiently to counterbalance the electric
field, and then the ion migration stops. Hence the current decreases
with time as the drift of ions is continuously balanced by increasing concentration gradient induced by the electrode that blocks the ions, but
still active towards electrons and hence the cell gets polarized [67]. As
a result the ionic current is blocked, the polarization is exclusively
Table 3
Activation energies, optical band gaps and transference numbers of pristine, and complexed polymer blend electrolytes.
No.
Fig. 8. Arrhenius plots for PVC/PEO:KI polymer blend electrolytes at different weight
percentage ratios.
1
2
3
4
Polymer
electrolyte
Composition
Activation
energy (eV)
Optical band
gap (eV)
Transference
number
Direct
Indirect
tion
tele
PVC/PEO
PVC/PEO:KI
PVC/PEO:KI
PVC/PEO:KI
50:50
47.5:47.5:5
45:45:10
42.5:42.5:15
0.66
0.54
0.39
0.28
4.10
3.46
3.24
2.91
4.15
3.33
3.22
3.11
–
0.95
0.96
0.98
–
0.05
0.04
0.02
266
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
co-efficient α for amorphous materials are related to the energy of
the incident photon as follows [68,69],
ahυ ¼ C hυ–Eg
ahυ ¼ 0
r
for hυNEg
for hυ b Eg
ð6Þ
ð7Þ
where C is a constant, i.e. C = 4πσ / μcΔE, here σ0 is the dc conductivity,
ΔE the measure of the extent of band tailing, μ the refractive index, hυ
the photon energy, Eg the optical energy band gap, and r the exponent,
which can take the values of 1, 2, 3, 1/2, and 3/2 depending on the nature of the electron transitions, direct or indirect and allowed or forbidden, responsible for the optical absorption.
For direct and indirect electron transitions, the exponent r takes
values 1/2 and 2, respectively. When the direct and indirect band gap
exists, the absorption coefficient has the following dependence on the
energy of the incident photon,
Fig. 9. Polarization current versus time plots of PVC/PEO:KI polymer blend electrolytes.
carried by electrons, and hence the final current is due to electronic current only (If = Iele).
The ionic transference numbers of present electrolyte system, as determined from Fig. 9, are in the range of 0.95–0.98 and given in Table 3.
It suggests that the charge transport in these polymer blend electrolytes
is predominantly due to ions with a negligible contribution (0.05–0.02)
of electrons. The ionic transference number increases with the increase
of salt concentration, due to the enhanced ionic concentration (both cationic and anionic). The ionic transference number reaches a high value
(0.98) for the 15 wt.% KI complexed electrolyte system, which may be
sufficient to fulfill the requirement of solid state electrochemical cells
[35,57].
3.7. UV-VIS absorption
The optical absorption study gives detailed information about the
band structure of the solid materials. In the absorption process an electron is excited from a lower energy state to a higher energy state by absorbing a photon of known energy in the transmitting radiation. The
changes in the transmitted radiation can decide the types of plausible
electron transitions. The absorption co-efficient α can be calculated
from the absorbance by
I ¼ I 0 exp ð‐αt Þ
Hence a ¼
2:303
2:303
logðI0 =IÞ ¼
B
t
t
ahυ ¼ C 1
hυ–Egd
ahυ ¼ C 2
hυ–Egi
1=2
2
ð8Þ
ð9Þ
where Egd is the direct band gap, Egi the indirect band gap, and C1 and C2
are constants.
Figs. 10 and 11 represent the (αhυ)2 and (αhυ)1/2 versus photon energy (hυ) for various PVC/PEO:KI complexed polymer blend electrolytes. The direct and indirect band gap values may be obtained by
extrapolating the linear portions of the curves (αhυ)2 and (αhυ)1/2 versus photon energy (hυ) to zero absorption value. The evaluated band
gap values are displayed in Table 3; both the direct and indirect energy
band gap values decrease with increasing KI concentration. The KI salt
concentration increases the disorder of the polymer structure, which
results in an increase in the optical band gap value. Furthermore, the decreasing trend of energy band gap with salt content is similar to that of
the activation energy obtained from conductivity studies. The activation
energies are pretty small in comparison with the optical band gaps. This
is due to fact that the natures of these energies are different. The activation energy corresponds to the energy required for conduction of ions
from one site to another, whereas the optical band gap corresponds to
inter-band transition of electrons [70].
ð4Þ
ð5Þ
where I0 and I are the intensities of incident and transmitted radiation, respectively, t is the thickness of the film and B corresponds to
log (I0/I).
Usually in insulators/semiconductors, the electronic transition between the valence and the conduction bands can be direct or indirect.
In the direct band gap materials, both the top of the valence band and
the bottom of the conduction band are positioned at the same zero crystal momentum (wave vector). If the bottom of the conduction band
does not correspond to the zero crystal momentum, then it is called
indirect band gap semiconductor. In both cases it can be allowed as
permitted by the transition probability (r) or forbidden where no
such probability exists. The transitions probability r and absorption
Fig. 10. (αhυ)2 versus hυ plots of PVC/PEO:KI complexed polymer blend electrolytes.
R. Nadimicherla et al. / Solid State Ionics 278 (2015) 260–267
Fig. 11. (αhυ)1/2 versus hυ plots of PVC/PEO:KI complexed polymer blend electrolytes.
4. Conclusions
Solid polymer blend electrolytes based on polyvinyl chloride and
polyethylene oxide complexed with potassium iodide (KI) were prepared in the form of membrane using solution-casting technique. XRD
patterns of PVC/PEO:KI electrolytes suggest an increase in the amorphous
phase with increasing KI concentration in the polymer blend matrix. FTIR
analysis suggests the changes in the environment of the functional
groups upon addition of salt, which confirms the co-ordination of cations
and interaction of anions with the polar groups of pristine polymer blend
during the formation of complexes. The glass transition temperature (Tg)
and melting temperature (Tm) show similar trend as the structure becomes more flexible at high salt concentrations. Optical energy band
gaps (both direct and indirect) show a decreasing trend with increasing
KI concentration as well. The ionic conductivity was found to increase
with increasing temperature as well as KI concentration. The highest
ionic conductivity (3.66 × 10−4 S cm−1) and lowest activation energy
(0.28 eV) were obtained for 15 wt.% KI doped polymer blend electrolyte
at ambient temperature, and the ionic transference number was determined to be 0.98.
Acknowledgments
Dr. NRP is thankful to Dr. G. Hemachandra, Department of Physics,
Vellore Institute of Technology, Vellore, India, for helping in carrying
out impedance measurements and to Dr. M. Kesava Reddy, Research
Scholar Central Leather Research Institute, Chennai, India, for providing
thermal characterization data.
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