Transmission performance and free volume of gel electrolyte membranes based on comb-like methyl methacrylate copolymers

Three comb-like methyl methacrylate copolymer matrixes for gel polymer electrolyte (GPE) were synthesized by reacting methyl methacrylate-maleic anhydride copolymer (P(MMA-co-MAh)) with poly (ethylene glycol) monomethylether (PEGME) of different molecular weight (350, 600, and 750), respectively. Three kinds of GPE membranes made from comb-like copolymers as matrix, shorted for CL350, CL600, CL750, propylene carbonate (PC) as plasticizer and LiClO₄ as salt, have been prepared by solution casting technique. By studying the ion transporting properties of the GPE, it is found that the conduction mechanism of the GPE accords with VTF (Vogel-Tamman-Fulcher) equation, which is derived from free volume theory. Positron annihilation lifetime spectroscopy (PALS) was employed to analyze the properties of free volume in GPE systems and the effect of microstructure on conductivity of GPE membranes. It is noted that increasing the side chain length of polymer matrixes will increase the free volume existed in system. When the content of polymer matrix is 45 wt%, free volume in CL750 based GPE membrane is 21% larger than that in CL350 based GPE membrane. Correspondingly, compared with CL350 system, the ionic conductivity of CL750 system grows by 200%. It is apparent that the ionic conductivity of GPE membranes is affected by the structure of polymer matrix to a great extent.