Segmental relaxation characteristics of cross-linked poly(ethylene oxide) copolymer networks

The segmental relaxation characteristics of UV cross-linked networks based on poly(ethylene glycol) diacrylate [PEGDA] copolymerized with either poly(ethylene glycol) methyl ether acrylate [PEGMEA] or poly(ethylene glycol) acrylate [PEGA] were investigated using dynamic mechanical analysis. The molecular weights of the PEGDA cross-linker and acrylate comonomers were selected to obtain rubbery, amorphous membrane materials with constant ethylene oxide content. The introduction of PEGMEA or PEGA in the reaction mixture was used to control cross-link density and led to the insertion of flexible oligomeric branches within the resulting networks. For both copolymer series, the introduction of acrylate comonomer led to a decrease in glass transition temperature (T _g) and a systematic reduction in cross-link density; the downward shift in T _g was much more pronounced for the PEGDA/PEGMEA copolymers. Time-temperature superposition was used to construct modulus master curves across the glass transition, and these could be satisfactorily fit using the Kohlrausch-Williams-Watts (KWW) function. The KWW curve fits indicated a narrowing of the glass-rubber relaxation with reduced cross-link density that correlated with a decrease in fragility for the networks. Gas transport measurements revealed a strong sensitivity to copolymer composition for the PEGDA/PEGMEA networks (-OCH ₃ branch end group) as compared to the PEGDA/PEGA networks (-OH end group), with CO ₂ permeability and CO ₂/H ₂ selectivity increasing with increased branch content in the PEGDA/PEGMEA membranes. Both the observed glass transition and transport behavior correlated with measured variations in fractional free volume for these networks.