Segmental Relaxation Characteristics of Cross-Linked Poly(ethylene oxide) Copolymer Networks

摘要

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 crosslink 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.