(163bh) Segmental Relaxation Characteristics of Rubbery Membrane Networks

The relaxation characteristics of rubbery networks based on bisphenol A ethoxylate diacrylate [BPA-EDA] crosslinker have been investigated using dynamic mechanical thermal analysis and broadband dielectric spectroscopy methods. The primary goal was to elucidate the inherent molecular processes associated with the sub-glass and glass-rubber relaxations observed in these materials and to correlate these processes with overall composition, network architecture and resultant gas transport properties.

Two sets of rubbery membrane networks based on the BPA-EDA crosslinker were prepared via UV polymerization with controlled variations in composition, backbone structure and crosslink density. Variations in crosslink density were realized by polymerizing the BPA-EDA crosslinker with different amounts of monoacrylate co-monomer. Fourier Transform Infrared Spectroscopy (FTIR) measurements were performed to confirm essentially 100% conversion of the reactive acrylate species in these membranes.

Dynamic mechanical studies were employed to establish the glassy and rubbery modulus values for the networks as well as their glass transition characteristics. A systematic variation in rubbery modulus was observed for the various films that could be related to molecular weight between crosslinks according to classical network theories. Time-temperature superposition was used to establish modulus-frequency master curves across the glass-rubber relaxation and the data were fit using the “stretched exponential” Kohlrausch-Williams-Watts (KWW) relaxation function. Relaxation breadth, and its correlation with network fragility, was examined as a function of effective crosslink density. The molecular origin of the dipolar reorientation processes observed in the corresponding dielectric spectra were analyzed using the empirical Havriliak-Negami function, with dielectric relaxation intensity determined as a function of temperature and network composition. Gas transport measurements (e.g. CO₂, H₂, CH₄) indicate lower CO₂ permeability and selectivity values for membranes based on the bulkier BPA-EDA crosslinker as compared to poly (ethylene glycol) diacrylate [i.e., PEGDA] networks studied previously.