The synthesis of a MMA/MAh copolymer and properties of the copolymer-based gel polymer electrolytes

Poly(methyl methacrylate) (PMMA) is one of the matrix polymers commonly used in gel polymer electrolytes, and has been proposed for lithium battery applications. PMMA-based gel polymer electrolytes (GPE) show high ionic conductivity at ambient temperatures. However, low thermal stability and mechanical properties with hard forming free-standing film in gel systems at high organic solvents will limit its application. In order to overcome these problems, methyl methacrylate (MMA)-maleic anhydride (MAh) (P (MMA-co-MAh)) copolymer was prepared, in the copolymer the ring structure of MAh units can improve the thermal stability and mechanical properties, consequently, improving the thermal stability of P(MMA-co-MAh)-based gel polymer electrolytes. The copolymer of P(MMA-co-MAh) was synthesized through solution copolymerization with toluene as solvent, 2, 2′-azobisiobutyronitrile (AIBN) as the initiator, MMA and MAh as the monomers with the molar ratios of MMA: MAh monomer is 1:1. The reaction was carried out at 80°C under nitrogen atmosphere for 8 h. The structure and properties of the copolymer were characterized by Fourier transform infrared spectroscopy (FTIR), ¹H-nuclear magnetic resonance (¹H-NMR), X-ray diffraction (XRD), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC). It was found that the unsaturated bonds existed in both of MMA and MAh (1637 cm^-1) disappeared in the copolymer, whereas a strong absorption peak at 950 cm^-1 for O=C-O-C=O ring stretching vibration of a saturated cyclic five-membered anhydride and a peak at 1445 cm^-1 for -CO-O-CH₃ stretching vibration of MMA were observed in the FTIR spectrum, respectively. The ¹H-NMR spectrum of P(MMA-co-MAh) copolymer showed four peaks, and the peaks at δ = 2.65-3.20 and 3.5-3.8 represent the chemical shift of H for MAh groups and -COOCH ₃ groups, respectively, indicating the molecular structure of copolymer P(MMA-co-MAh). The molar ratios of MMA and MAh in the P(MMA-co-MAh) copolymer is 8:1. XRD showed that the P(MMA-co-MAh) was a typical amorphous polymer, which is beneficial in improvement of ionic conductivity of its GPE. GPC resalts show that the number average molecular weight (M_w) of the copolymer is 6.40 × 10⁴ and the mass average molecular weight (M_w) is 6.68 × 10⁴; the molecular weight distribution (M_w/M_n) is 1.04. The TGA curves show that the thermal degradation temperature of P(MMA-co-MAh) is 300A°C, which is higher than that of the pure PMMA polymer. The DSC results of the P(MMA-co-MAh) copolymer reveal that the glass transition temperature (T_g) is 121.3°C, which is higher than that of the pure PMMA. The gel polymer electrolyte was prepared from the P(MMA-co-MAh) copolymer, propylene carbonate (PC) and lithium perchlorate. It is found that the GPE can form free-standing films.The ionic conductivity is 3.0 × 10^-5 S/cm at room temperature for 45 wt% P(MMA-co-MAh) gel copolymer enlectrolyte with 55 wt% 1 mol/L LiClO₄-PC solution. The ionc conductivity for GPE of P(MMA-co-MAh) copolymer is lower than that of the gel polymer electrolytes of pure PMMA containing 50 wt% PMMA with 50 wt% 1 mol/L LiClO₄-PC solution.