Blends of poly(meth)acrylates with 2-Oxo-(1,3)dioxolane side chains and lithium salts as lithium ion conductors

A novel approach is described to obtain polymers which show significant lithium ion conductivity when blended with lithium salts. Polymerization of acrylates and methacrylates with 2-oxo-1,3-dioxolane (cyclic carbonate) containing side chains gives polymers which contain the structural element of the prototypical solvent propylene carbonate (PC) firmly attached to the polymer host. (2-Oxo-1,3-dioxolane-4-yl)methyI methacrylate (DOMA) and the corresponding acrylate (DOA) were obtained by reaction of the (meth)acryloyl chloride with glycerol carbonate while (2-oxo-1,3-dioxolane-4-yl)butyl methacrylate (DOBMA) and the respective acrylate (DOBA) were obtained from the omega-hexenyl esters by first epoxidation followed by CO2-insertion into the oxirane ring. Free radical polymerization in DMF gave the desired polymers. All homopolymers were thermally stable at least up to 200 degrees C; the glass transition temperature was found for PDOMA at 93 degrees C, for PDOA at 50 degrees C, for PDOBMA at 16 degrees C, and for PDOBA at 11 degrees C. Free radical copolymerization of DOMA with butyl methacrylate (BMA) (r(1) = 1.24, r(2) = 0.80) gave copolymers with T-g dependent on the BMA content. Blends of the homopolymers with lithium bis(trifluoro)methane sulfonimide (LiTFSI) gave appreciable lithium ion conductivities, particularly for blends of PDOA (3.7 x 10(-6) S cm(-1) at 40 degrees C) which could be substantially increased by further blending with small amounts of propylene carbonate. These blends of honey-like consistency showed conductivities of 1.8 x 10(-3) S cm(-1) (40 degrees C for PDOA and 5.1 X 10(-4) S cm(-1) (40 degrees C) for PDOBMA each blended with LiTFSI and PC at equimolar amounts. The temperature dependence of lithium ion conductivity follows a WLF-type behavior in all cases however, the reference temperatures do not bear any correspondence to the observed glass transition temperatures but are estimated from modified WLF-plots to be positioned in a temperature regime typical for local side chain relaxations in common polymers.