Characterization of Blended Polymer Electrolyte Thin Films Based on PVDF + PEG Doped with Nano SiO2

Introduction

The wide spread usage of polymers virtually all materials to made advances in science and technology. The nature of crystallinity and thermal stability of these polymer electrolytes for applications are of current interest [1-3].  Blending of polymers is outstanding method to get require properties such as increasing thermal stability, mechanical strength and conductivity, polymers have been doped with some salts (LiClO₄, NaClO₄, some nano fillers) and futher these are used in electrochemical devices such as batteries, Sensors, electric vechiles; Smartphone’s and laptops . Characterization of PVDF solid polymer electrolyte mixed with LiClO₄ using XRD, DSC, SEM, FTIRandUVwas reported by us[4].Some studies on polyvinylidene fluoride (PVDF and polyethylene glycol (PEG) complexation of lithium perchlotate (LiClO₄) are recently reported [5].In this paper, we present the preparation and characterization of a new solid polymer electrolyte system consisting of PVDF + PEG +nano silicon dioxide (SiO₂). The prepared samples are characterized using the techniques DSC, XRD, SEM, FTIR and UV.  The results are compared and discussed to explain the crystalline nature, thermal stability and optical absorption of the polymer electrolyte samples PDVF+PEG + Nano SiO₂.

Experimental

Solid polymer blend films based on PVDF + PEG)+ nano SiO₂   have been prepared by using solution casting procedure. Pure PVDF (320000MW from Merck), PEG(6000MW from Merck) and nano SiO₂   are added with various percentages i.e., (70:30), (70:30:2), (70:30:4), (70:30:6), (70:30:8), (70:30:10)by wt% ratio by Solution Casting Technique. The blend polymer electrolytes are dissolved in DMF by using ultrasonicator[6] the homogeneous solutions are obtained by stirring the solutions about 10-12 hours These solutions are allowed to get evaporated in dishes and after 48 hours, thin films are obtained. These films are separated from the dish surface and are stored in Desiccators. 

XRD patterns of the above polymers are recorded using PHILIPS PW 3710. Fourier Transform Infrared (FTIR) spectroscopic studies are carried out using JASCOFTIR-5300 Spectrometer. Surface properties of these polymers are studied using Scanning Electron Microscope (SEM). Optical absorption spectra are recorded at room temperature in the range of 200-1000nm using UV Optical Spectrometer. Thermal properties are studied using Differential Scanning Calorimetry (DSC) in the temperature range 50⁰C – 250⁰C.

Results and Discussion

DSC

As shown in Fig. 1, in DSC plots, there is a small endothermic dip at 174.34⁰C, for zero concentration of nano SiO₂ in PVDF + PEG [Fig 1 (A)].  As in Fig. 1(B) the endothermic peak is found at 188.4⁰C for 2% [Fig. 1 (B). For 4%, as in Fig, 1(C),the endothermic peak is found around187⁰C.Increase of concentration of nano SiO₂ as in Figs. 1(C) to 1(F), the endothermic peak is not prominent.

Figure 1: DSC thermo grams (A). PVDF+PEG (70:30), (B). PVDF+PEG+nanoSiO2 (70:30:2), (C). PVDF+PEG+nanoSiO2 (70:30:4). Click here to View figure

The disappearance of a small dip at higher concentrations of DSC plots indicates the decrease in the crystallinity. It is also observed in 1(D)&1(E)shifting of glass transition temperature(Tg) towards lower Temperature indicates Involved of PEG, nano SiO₂ [7].

XRD

From Fig.2, it is clear that there is a sharp peak around 2θ=20.2⁰ (110),(200) indicates β-phase PVDF[8,9,10]with small peaks18.59⁰  (020) indicates α-Phase crystal of pvdf from the literature[8,9]which is the Characteristic Peak of Pure PVDF, present in all the concentrations of nano SiO₂ in PVDF+PEG [Figs.2(A) to 2(F)]  which represents a crystalline phase in the amorphous matrix of the samples. From Figs. 2(A) to 2(F), the sharp peak persists continuously where the small peaks disappear with increase of concentration. In Fig.2, it is also clear that the magnitude of crystallinity decreases with increase the concentration of nano SiO₂ in PVDF+PEG. In the pure PVDF+PEG sample, the small peaks in addition to the sharp peak indicate the partial crystalline nature of the sample [Fig.2(A)].

Figure 2: XRD spectra of (A). PVDF+PEG(70:30), (B). PVDF+PEG+nanoSiO2 (70:30:2),  (C). PVDF+PEG+nanoSiO2(70:30:4), Click here to View figure

SEM

The surface morphology of PVDF+PEG and PVDF+PEG+nano SiO₂ polymer systems is observed using SEM. Thus, Fig. 3 shows the surface structure of PVDF + PEG and PVDF+PEG+ nanoSiO₂polymer systems with different concentrations of nano SiO₂ in PVDF+PEG. From Fig.3, it is clear that the surface of PVDF+PEG film appears to be crystalline for 0% nano SiO₂which is also confirmed by XRD studies on these films. As the concentration increases, the crystalline nature decreases due to disappearance of crystalline phases and increase of amorphous nature. This occurs due random positioning   of molecules.  For 10% concentration, the amorphous nature is more which also increases the thermal stability of the material.  Fig. 3 shows the SEM pictures of PVDF + PEG membranes with increasing nano SiO₂ concentration representing the decrease crystallinity from Fig. 1(A) to Fig. (F). At lower nano SiO₂ concentration, nano   SiO₂ particles homogeneously dispersed in PVDF + PEG matrix and led to an improvement of mechanical and thermal properties.

Figure 3: SEM images of  (A) PVDF+PEG (70:30),(B) PVDF+PEG+nanoSiO2 (70:30:2), ( C) PVDF+PEG+nanoSiO2(70:30:4). Click here to View figure

Fig. 4 shows   strong growth in the CF₂ stretching in FTIR with increasing concentratration of nano SiO₂ in PVDF+PEG.In Fig.4, the intensity of the aliphatic C-H scattering irrational bands are observed in FTIR spectra of PVDF+PEG and PVDF+PEG+nano SiO₂. Using these polymer blend electrolytes   electrochemical cells can be fabricated and their discharge characteristics can be studied. The FTIR pattern in Fig. 4 remains almost same with minor changes in the positions and widths of the peaks with increase of the concentrationnanoSiO2   in PVDF+PEG The peaks observed  at 840 and 1174 cm1 are indicates β-phase crystals, similarly the bands exist  at874, 960, 1074, and 1403 cm1 indicates the α-phase crystals of the PVDF[8,9,10,11].

Figure 4: FTIR spectra of of  (A) PVDF+PEG (70:30),(B) PVDF+PEG+nanoSiO2 (70:30:2), ( C) PVDF+PEG+nanoSiO2(70:30:4). Click here to View figure

Optical Properties

Figure 5: UV absorption spectra of membranes of PVDF+PEG (70:30)with different concentrations of  nano SiO2 Click here to View figure

Figure 6: UV spectra of (A) PVDF+PEG (70:30),(B) PVDF+PEG+nanoSiO2 (70:30:2), (C) PVDF+PEG+nanoSiO2(70:30:4). Click here to View figure

Figure 5 shows the optical absorption spectra recorded at room temperature in the range 150 nm – 1200 nm using Optical Spectrometer. Near fundamental band edge, direct band transitions may occur[12,13].Thus, considering α as absorption coefficient, h as Planck’s constant, υ as frequency of incident light and hυ as the photon energy, (αhυ)² versus hυ can be plotted as shown in Fig. 6. The direct band gap values of PVDF+PEG, PVDF+PEG+different concentrations of nano SiO₂are tabulated in Table1.From Table 1, the band gap of pure PVDF+PEG polymer electrolyte is2.420eV. The direct optical band gap values have been modified by the addition of nano SiO₂ to PVDF+PEG blend thin film which supports the conductivity levels of  PVDF+PEG+nano SiO₂ thin films.Thus, the direct band gap values of PVDF+PEG+ nano SiO₂ polymer electrolyte indicates the influence of nano SiO₂ on PVDF+PEG for better conducting properties. Table1 also shows that the direct band gap of 10% of nano SiO₂ in PVDF+PEG is 3.290 eV.

Table 1: The direct band gap values of PVDF+PEG+ nano SiO₂ electrolytes for different concentrations.

Sample code	Wt%			Direct band gap(eV)
	PVDF	PEG	Nano SiO2 salt
(a)	70	30	0	2.42
(b)	70	30	2	2.846
(c)	70	30	4	2.736
(d)	70	30	6	2.570
(e)	70	30	8	2.610
(f)	70	30	10	3.290

 

Conclusions

DSC studies on nano SiO₂ doped on PVDF+PEG blend films revealed that the decrease in the crystallinity which leads good thermal stability and the same also supported with XRD and SEM results. FTIR results show strong growth in the CF₂ stretching with increasing concentration of nano SiO₂ in PVDF+PEG and also confirmed that, intensity of the aliphatic C-H scattering vibrational bands decreases. The increase in direct band gap values of PVDF + PEG + nano SiO₂ polymer electrolyte indicated that the influence of nano SiO₂ on PVDF+PEG for better conducting properties.

Acknowledgement

This research work was encouraged and supported by Maturi Venkata Subba Rao Engineering College and JNTU-Hyderabad the authors are express gratitude to these organizations for the same.

Conflict of Interest

There is no conflict of interest.

Funding Sources

There is no funding Source.

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