Chinese Chemical Letters  2015, Vol.26 Issue (09): 1191-1196   PDF    
Effects of ceramic filler in poly(vinyl chloride)/poly(ethyl methacrylate) based polymer blend electrolytes
P. Pradeepa, S. Edwinraj, M. Ramesh Prabhu     
School of Physics, Alagappa University, Karaikudi 630003, India
Abstract: Effects of nano-ceramic filler titanium oxide (TiO2) have been investigated on the ionic conductance of polymeric complexes consisting of poly(vinyl chloride) (PVC)/poly(ethyl methacrylate) (PEMA), and lithium perchlorate (LiClO4). The composite polymer blend electrolytes were prepared by solvent casting technique. The TiO2 nanofillers were homogeneously dispersed in the polymer electrolyte matrix and exhibited excellent interconnection with PVC/PEMA/PC/LiClO4 polymer electrolyte. The addition of TiO2 nanofillers improved the ionic conductivity of the polymer electrolyte to some extent when the content of TiO2 is 15 wt%. The addition of TiO2 also enhanced the thermal stability of the electrolyte. The changes in the structural and complex formation properties of the materials are studied by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques. The scanning electronmicroscope image of nano-composite polymer electrolyte membrane confirms that the TiO2 nanoparticles were distributed uniformly in the polymer matrix.
Key words: Poly(vinyl chloride)     Poly(ethyl methacrylate)     Polymer composite electrolytes     Nanofillers     Ionic conductivity    
1. Introduction

The incorporation of inorganic filler into polymers is a common practice in polymer processing to enhance physical and mechanical properties. The high aspect ratio of the nanofiller gives rise to a high degree of polymer surface interaction resulting in much improved mechanical and barrier properties [1, 2]. Polymers containing metal chloride have attracted much attention because of their useful technical applications. They may serve as optical components in a wide variety of opto-electronic devices. Therefore, the field of use of these polymeric materials, especially poly(ethyl methacrylate) (PEMA) as a polymer waveguide and as optical and/or electronic components, has become important, especially given that the properties and structure of the polymers can easily be tuned upon small additions of metal halide [3, 4, 5, 6]. Nanoscale TiO2 in the polymer electrolyte systems was confirmed to play some useful roles in forming particle networks within the polymer bulk (particle dispersion), inhibiting the crystallization and reorganization of polymer chains and interactions with lithium ion species. These features eventually resulted in the improvements of polymer electrolyte properties such as mechanical strength, ionic conductivity, electrochemical stability, cation transference number, lowering of interfacial resistance, and so on [7]. TiO2 may partially attach into the polymer chains or resides in the amorphous/crystalline boundaries and diffuse preferentially through the amorphous regions, forming charge transfer complexes, or it may exist in the form of molecule aggregates between the polymer chains. In the case of composite polymer electrolytes, the particle size and filler concentrations play an important role because the addition of a small amount of inert filler will collapse the chain organization of the polymers which in turn facilitates higher ionic conduction [8].

In this present work, to overcome the drawbacks of plasticized polymer blend electrolyte, PEMA/PVC based nanocomposite polymer electrolytes have been prepared with various weight ratios of TiO2 ceramic filler to improve their ionic conductivity. PVC is commonly used as a general commodity plastic because of its excellent electrical and corrosion resistance, self extinguishing characteristics, low cost, and recoverability. However, its low impact strength and poor thermal stability limit its applications. The inherent problems of processing rigid PVC are also quite well known and are overcome by the use of certain plasticizers. Such cases run the risk of compromising the mechanical properties of rigid PVC. PEMA is chosen for blending with PVC because it has better miscibility and compatibility in low or high molecular weight and also offers good mechanical strength due to the lone electron pair in the chlorine atom, which in turn stiffens the backbone of the polymer [9]. Further, the problem of poor mechanical strength can be circumvented by blending PEMA with a polymer such as PVC, because of its poor solubility in the plasticizer medium results in a phase separated morphology that provides a rather rigid framework in the polymer electrolyte film.

2. Experimental

PVC (average Mw ~ 534,000) and PEMA (average Mw ~ 515,000) were purchased from Aldrich and dried under vacuum at 80 ℃ for 24 h. Reagent grade anhydrous lithium perchlorate (LiClO4) was used after drying in vacuum at 110 ℃ for 24 h. The plasticizer propylene carbonate (PC) (from Aldrich) was used as supplied. The nanosized TiO2 (<100 nm particle size) was purchased from Aldrich and used as a ceramic filler. All the electrolytes have been prepared by the solvent casting technique. Appropriate quantities of PVC, PEMA, and LiClO4 are dissolved by adding them in sequence to tetrahydrofuran (THF) and stirred for 24 h. The resulting solution is poured onto a glass plate, and the THF is allowed to evaporate in air at room temperature for several hours. The films are further dried at 60 ℃ for 24 h in vacuum to remove any traces of THF. Thin films thus obtained were subjected to XRD and FTIR studies to investigate the complexation behavior and the nature of crystallinity of the polymer electrolytes using a Bruker (D8 Advance) diffractometer and a Perkin-Elmer (Paragon 500 grating) IR spectrophotometer, respectively. Thermal stability of the film was also characterized by TG/DTA. TGA measurements were carried out under nitrogen atmosphere at a heating rate of 10 ℃/min from room temperature to 830 ℃. Nitrogen was used as the carrier gas with flow rate of 25 mL/min. The electrical conductivity of polymer complexes was measured from impedance plots at different temperatures using a Keithley 3330 LCZ meter. The impedance measurement was recorded in the frequency range 40 Hz-100 kHz with signal amplitude of 10 mV. The SEM images were recorded using HITACHI S-3000 H Scanning electron microscope.

3. Results and discussion 3.1. X-ray diffraction

X-ray diffraction patterns of pure PVC, PEMA, LiClO4, and PVC(5)-PEMA(20)-PC(67)-LiClO4(8) polymer electrolyte with X% of TiO2 (where X = 0, 5, 10, 15, 20) are shown in Fig. 1. It is clear that both polymers are amorphous in nature because no sharp crystalline peaks are observed [10]. The diffraction peaks appearing at 2θ values 18° and 23° indicate the crystalline phase of LiClO4 in Fig. 1(c). The peaks corresponding to LiClO4 were not observed in Fig. 1(e)-(i), indicating that the LiClO4 does not remain as a separate phase in the polymer electrolyte system which confirms the complete dissociation of LiClO4 in the polymer matrix [11]. The characteristic peaks at 2θ = 13° for PVC and 18.6° for PEMA are revealed from Fig. 1(a) and (b), respectively. The diffraction peak of PEMA is markedly reduced in all the complexes. The shift and decrease in the relative intensity of the peaks suggest that complexation has occurred between the salt and the polymers. The analysis shows that the nature of diffraction patterns of pure samples significantly changed due to the disturbances in the ordered arrangement of polymer side chains when they are blended. This indicates that the salt, LiClO4, most likely blends with PVC/PEMA polymer blend at the molecular level and gives a clear indication of complexation of the salt in the polymer blend system. The obtained amorphous phase of polymer blend in the electrolyte membrane enhances higher ionic conduction, meanwhile the crystalline phase of the filler provides strong mechanical support in the polymer electrolyte. We can observe that the position of the sharp peak coincided with a peak at 2θ = 25.2° and 48° from the Xray scans of pure TiO2 in Fig. 1(d). This confirms the presence of TiO2 crystallites within the polymer matrix. On the other hand, the spectrum of PVC/PEMA films containing TiO2 showed a sharp peak at 2θ = 25.2°, showing W ≥ 5 wt% can cause structural variations in the polymeric network. The nano-sized TiO2 dispersed emulsion can penetrate the space between the polymer chains, and consequently the homogeneously dispersed ceramic filler in the matrix prevents or retards crystallization of the polymers due to its large surface area. The intensity of the peaks abruptly decreases in the composite electrolytes. No peaks appeared corresponding to the salt in the complexes, which confirms the amorphous nature of the electrolytes.

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Fig. 1.X-ray diffraction patterns of (a) pure PEMA, (b) pure PVC, (c) pure LiClO4, (d) pure TiO2, (e) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(0), (f) PVC(5)–PEMA(20)– LiClO4(8)–PC(67)–TiO2(5), (g) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(10), (h) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(15), and (i) PVC(5)–PEMA(20)– LiClO4(8)–PC(67)–TiO2(20).
3.2. FTIR analysis

Polymer complexes with ionic salt and ceramic filler have been characterized by FTIR spectroscopy. This technique provides a powerful tool to characterize organic and inorganic components and their composition. FTIR spectra of composite polymer complexes are shown in Fig. 2. The peak at 1725 cm-1 represents the carbonyl stretching vibration of PEMA. In PEMA, the peaks at 2982, 2939, and 2910 cm-1 are due to the methylene (C) CH3 and ethylene (O) C2H5 groups which overlap [12]. Peaks at 2963 and 1329 cm-1 are assigned respectively to asymmetric C-H methylene group vibration and in-plane CH deformation of PVC. Further, the peaks at 954 and 630 cm-1 are assigned to trans CH rocking and cis-CH wagging of PVC. The peak at 1777 cm-1 represents the CH3-C- vibration of propylene carbonate. The frequency of C=O at 1785 cm-1 indicates the interaction of the plasticizer with LiClO4. The weak intensity peak appearing at 507 cm-1 is assigned to the stretching mode of ClO4 - [13]. The vibrational peaks at 1776, 1254, 959, 847, and 693 cm-1 of pure PVC, 1473, 1250, 1157, 1032, and 777 cm-1 of pure PEMA, 2924 and 1378 cm-1 of pure LiClO4, and 1650 cm-1 of pure TiO2 shifted to 1789, 1266, 972, 861, 713, 1482, 1266, 1173, 1025, 792, 2936, 1389, and 1660 cm-1, respectively. It is also found that some of the peaks appearing in the pure polymers and salts disappeared in the complexes, such as 2910, 1807, 1400, 1087, 923, and 632 cm-1. In addition to this, a few new peaks are observed at 1532, 1142, and 1173 cm-1 in polymer complexes. The shifting of peaks and formation of new peaks in electrolyte systems suggests the polymer-salt interaction in PVC/PEMA blend based composite polymer electrolytes.

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Fig. 2.FTIR spectra of (a) pure PVC, (b) pure PEMA, (c) LiClO4, (d) PVC(5)–PEMA(20)– LiClO4(8)–PC(67)–TiO2(0), (e) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(5), (f) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(10), (g) PVC(5)–PEMA(20)–LiClO4(8)– PC(67)–TiO2(15), and (h) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(20).
3.3. Conductivity studies

Fig. 3 shows the complex impedance plot of PVC(5)-PEMA(20)-PC(67)-LiClO4(8) with 15 wt% of TiO2 at various temperatures. This plot shows linear spikes. The disappearance of the high frequency semicircular portion in the polymer complex impedance plot indicates that the majority of the current carriers in the electrolyte medium are ions and that the total conductivity is mainly due to ion conduction [14]. The bulk resistance of the electrolyte was measured by extrapolating the intercept of this plot on the real axis. The electrical conductivity of the electrolyte was calculated for the known values of bulk resistance (Rb), area (A), and the thickness (l) of the film using the formula σ = l/RbA. The conductivity values obtained for the polymer electrolyte increases with temperature and ceramic concentration. This may be due to the availability of ions throughout the ceramic-rich phase which entraps the residual solvents, increasing ion mobility. The conductivity is not a linear function of filler concentration. The increase in conductivity has been attributed to the ceramic particles acting as nucleation centers for the formation of minute crystallites of the ceramic particles in the formation of amorphous phase in the polymer electrolyte and to the formation of a new kinetic path via polymer-ceramic boundaries. Hence, ceramic plays a dual role, i.e. enhancement of ionic conductivity up to a particular concentration, above which it acts as the hindering agent to conductivity [15]. From Table 1, it is seen that the addition of inorganic fillers leads to an increase in the ambient temperature conductivity up to 15 wt% TiO2, and then the ionic conductivity decreases with increasing concentration of ceramic fillers. At low concentration levels, the dilution effect which tends to depress the conductivity is efficiently countered by the specific interactions of the ceramic surface, which promotes fast ion transport. Thus, the net result is a progressive enhancement of the conductivity. At higher filler content, the dilution effect predominates and the conductivity decays [16]. It is found that the PVC-PEMA-LiClO4-PC complex with 15 wt% TiO2 has the maximum room temperature conductivity of 7.179 × 10-3 S cm-1 which is higher compared to the system bereft of ceramic addition. Irrespective of the reasoning, it can safely be assumed that as the Tg decreases, the less ordered amorphous phase region becomes flexible, resulting in increased segmental motion of the polymer chains as reflected by enhanced conductivity. However the conductivity is found to decrease after an optimum concentration is attained; on further addition of filler a continuous non-conductive phase built up by a large amount of filler blocks up lithium-ion transport, resulting in an increase in total resistance of the composite polymer electrolyte. It is reported for many composite polymer electrolytes formed by the addition of filler that ion conductivity increases to a maximum value at 5-15 wt% of filler. The result reveals that the addition of small particle size ceramic powders enhances the degree of amorphicity of the polymer electrolyte. Further, the particle size and content of the ceramic additive appear to be a critical factor. It is also seen that a reasonably high concentration of the filler is also necessary to affect the recrystallization rate of the polymer host. The temperature dependence of the ionic conductivity of PVC(5)-PEMA(20)-PC(67)-LiClO4(8)-X% of TiO2 (where X = 0, 5, 10, 15, 20) in the total polymer electrolyte is shown in Fig. 4. From the plot it has been observed that as temperature increases the conductivity values also increase for all the compositions. The non-linearity in Arrhenius plots indicates that ion transport in polymer electrolytes is dependent on polymer segmental motion. The curved behavior of the plots suggests that the data can be better described by the Vogel-Tamman-Fulcher (VTF) relation [17], which describes the transport properties in a viscous matrix. This supports the idea that the ion moves through the plasticizer-rich phase.

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Fig. 3.Impedance diagram for PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(15) wt% at various temperatures.

Table 1
Ionic conductivity values for PVC(5)–PEMA(20)–PC(67)–LiClO4(8)–TiO2(X wt%) (where X = 0, 5, 10, 15, 20) system.

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Fig. 4.Arrhenius plot of PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(X wt%) (where X = 0, 5, 10, 15, 20) composites.
3.4. TG/DTA analysis

The thermal stability of the polymer electrolyte has been studied for its use in practical applications because thermal stability is an important parameter for guaranteeing acceptable performance during high temperature operating which is related to safety concerns [18]. In this work, the thermal stability of the polymer electrolytes was observed using thermo gravimetric analysis. TG-DTA traces of all the prepared polymer electrolyte samples are shown in Fig. 5(a)-(e). In Fig. 5(a), the DTA curve shows a small endothermic peak at 70 ℃, indicating the melting of the polymer film, and it exhibits a linear trend toward its decomposition temperature at 291 ℃. This peak is concurrent with the TG curve. The first and second decompositions of the film take place between 60-70 ℃ and 200-230 ℃ (Table 2), respectively, and the corresponding weight losses were 8% and 16% (Fig. 5(a)) at 100 ℃ and 200 ℃, respectively.

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Fig. 5.(a) TG/DTA curves of PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(0 wt%), (b) TG/DTA curves of PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(5 wt%), (c) TG/DTA curves of PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(10 wt%), (d) TG/DTA curves of PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(15 wt%), and (e) TG/DTA curves of PVC(5)–PEM (20)–LiClO4(8)–PC(67)–TiO2(20 wt%).

Table 2
TG/DTA results of the prepared samples.

In the DTA curve (Fig. 5(b)), the melting of the polymer film is observed at 55-65 ℃, which is indicated by a small endothermic peak. Followed by it, there is no other peak until 280 ℃. This trend is accompanied by a rapid weight loss of the film E2. The thermal stability of the polymer film is found to be poor. The exothermic peak at 280 ℃ corresponds to the decomposition of the polymer electrolyte. The first and second decompositions of the film take place in between 50-60 ℃ and 220-300 ℃, respectively, and the corresponding weight losses were 16% and 30% at 100 ℃ and 200 ℃, respectively.

In the DTA curve (Fig. 5(c)), the polymer film exhibits a linear trend beyond 55 ℃ at which an endothermic peak is observed which clearly indicates the melting of the polymer film. A large exothermic peak is observed in the temperature range 225-295 ℃, indicating the final decomposition of the polymer film which is supported by the TG curve wherein the weight loss of the film is gradually decreasing. From the DTA and TG curves, it can be seen that, the thermal stability of the film is 236 ℃. The first and second decompositions take place at 50 ℃ and 236 ℃, respectively, and the corresponding weight losses were 12% and 13% at 100 ℃ and 200 ℃, respectively.

In the DTA curve (film E4), the polymer film exhibits a linear trend beyond 40 ℃ at which an endothermic peak is observed. This peak indicates the presence of moisture at the samples. A sharp exothermic peak observed at 292 ℃ indicates the final decomposition of the polymer film, which is accompanied by the TG curve wherein the weight loss of the film is continuously decreasing. From the DTA and TG curves, it can be seen that, with the increase in the temperature, the film E4 loses its weight continuously, showing poor thermal stability. The first and second decompositions of the film take place in between 40-60 ℃ and 240 ℃, respectively, and the corresponding weight losses were 13% and 30% at 100 ℃ and 200 ℃, respectively. In the DTA curve (Fig. 5(e)), an endothermic peak is observed in the temperature range 55-70 ℃, indicating the melting of the polymer film. A clear, larger exothermic peak is noticed in the temperature range 330-390 ℃ confirming the decomposition of the polymer film. The TG curve indicates no appreciable weight loss until 250 ℃. The thermal stability of the polymer film is found to be at 250 ℃ in the TG curve. The first and second decompositions take place at 60 ℃ and 250 ℃, respectively, and the corresponding weight losses were 9 and 25% at 100 ℃ and 200 ℃, respectively. The conductivity value has been found to be maximum (7.179 × 10-3 S cm-1) for the film E4 compared to other films. Hence, the film E4 is found to be superior among the other films on the basis of both thermal stability and conductivity.

3.5. SEM analysis

The scanning electron micrographs of 0, 15, and 20 wt% TiO2 based PVC(5)-PEMA(20)-PC(67)-LiClO4(8) polymer electrolyte systems are shown in Fig. 6(a)-(c). No spherullitic structure due to crystalline phase are observed in Fig. 6(a) and (b). This confirms the amorphous nature of the developed electrolytes. As the content of TiO2 increases, in Fig. 6(c) the film surfaces become rough. Also, it is found that in Fig. 6(c) the grain size increases, with a reduction in the number of grain aggregates that tend to restrict the ionic movement. Consequently, the conductivity decreases. It is evident from the image of the surface of the film that the ceramic had not undergone any chemical reaction with the polymer. The 15 wt% TiO2 based complex shows that the ceramic particles present in the samples are more dispersed when compared with others.

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Fig. 6.SEM images of (a) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(0 wt%), (b) PVC(5)–PEMA(20)–LiClO4(8)–PC(67)–TiO2(15 wt%), and (c) PVC(5)–PEMA(20)– LiClO4(8)–PC(67)–TiO2(20 wt%).
4. Conclusion

The addition of nanofillers TiO2 into the polymer blend matrix greatly enhances the amorphous region, which in turn improves the overall conductivity. The composite membrane containing 15 wt% TiO2 nanofiller showed better ionic conductivity (7.179 × 10-3 S cm-1), and the membrane is found to be thermally stable up to a temperature of 260 ℃. The addition of plasticizer propylene carbonate (PC) enhances the amorphous phase as well as the charge carrier dissolution in the matrix. The XRD patterns revealed the dissolution of the lithium salt and it was found that excess filler content (above 15 wt%) builds a crystalline phase which causes a reduction in the ionic conductivity of the membranes. The complex formation has been confirmed from FTIR spectral studies. The salt-in-polymer electrolytes prepared from the PVC/PEMA show a strong enhancement of the ionic conductivity by the addition of TiO2 nanoparticles. The ionic conductivity of the resulting composite polymer electrolyte is better than the blend polymer electrolyte. The temperature dependent ionic conductivity plot of the composite films seems to obey the VTF relation. The porous natures have been identified using scanning electron microscopy. The optimized polymer electrolyte can be used as an electrolyte in the fabrication of Li batteries.

Acknowledgment

The corresponding author has gratefully acknowledged the UGC, New Delhi, India for providing financial support to carry out this work.

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