Abstract
The incorporation of plasticizer to a solid polymer electrolyte have shown high conductivity, good thermal and chemical stabilities, long life and reduced costs. The improved conductivity is mainly contributed by higher ion diffusion coefficient and better ion mobility. However, little is known about the possible molecular interactions between plasticizer, polymer host and salt, particularly in explaining the mechanism pertaining to ion transport in the gel polymer electrolytes (GPEs). A flexible GPE based on poly(acrylamide-co-acrylic acid) (PAAm-PAA), ethylene carbonate (EC) and water-soluble sodium sulfide (Na2S) was developed. PAAm-PAA and Na2S are incorporated as the polymer backbone and the source of charge carriers, respectively, and EC acts as the plasticizer of the system. When 0.4 wt% of EC was added to PAAm-PAA-Na2S GPE, the ionic conductivity of polymer electrolytes increased from 5.11 × 10−2 S cm−1 to 6.92 × 10−2 S cm−1. The possible molecular interactions between PAAm-PAA, Na2S and EC and their correlation with ionic conductivity were investigated by Fourier transform infrared (FTIR) spectroscopy. Infrared spectroscopy showed that the intensities of the amide (CO, C–N), carboxylate (COO−) and sulfate (SO42−) bands increased with the addition of EC, and the position of the asymmetric bending (v4) of SO42− band down shifted from 677 cm−1 to 667 cm−1. These findings, the changes in shape, intensity, and position of the PAAm-PAA-Na2S-EC GPE, suggest a dipole-dipole interaction between (i) PAAm-PAA and distilled water, (ii) PAAm-PAA and EC and (iii) EC and distilled water. On the other hand, ion-dipoles interactions may occur between (i) Na+ cation and distilled water, (ii) Na+ cation and PAAm-PAA and (iii) Na+ cation and EC. It can be concluded that EC interacts with both PAAm-PAA and Na2S, and EC-Na+ complexes also appear in PAAm-PAA-Na2S-EC GPE. The increase in the conductivity of GPE is attributed to the high ion diffusion coefficient and mobility, which is attributed to the presence of EC.
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