Rubbery networks were prepared by UV polymerization of diacrylate crosslinkers based on polyethylene glycol or polypropylene glycol: i.e., poly(ethylene glycol) diacrylate, poly(propylene glycol) diacrylate, and bisphenol A ethoxylate diacrylate. The resulting rubbers, which rely primarily on solubility selectivity as the basis for gas separation, exhibit high permeability and favorable selectivity for the transport of polar or quadrupolar species (e.g. CO2) over light gases (H2, N2). Crosslink density and corresponding fractional free volume in the films was controlled by copolymerization with selected monofunctional acrylates; e.g. poly(ethylene glycol) methyl ether acrylate or poly(propylene glycol) methyl ether acrylate. Copolymerization results in the introduction of fixed-length pendant groups into the network structure and a progressive reduction in crosslink density with increasing co-monomer content. Variations in the character of the crosslinker, pendant branch length and pendant terminal group were explored as means by which to modify the static and dynamic characteristics of the networks and thereby tailor their gas separation properties.
Dynamic mechanical studies indicated systematic variations in rubbery modulus for the various network series that could be related to the molecular weight between crosslinks. Time-temperature superposition was used to establish modulus-frequency master curves across the glass transition, which allowed an objective determination of relaxation breadth, intermolecular cooperativity, and corresponding dynamic fragility as a function of network structure. Dielectric spectropscopy measurements provided detailed information as to the intensity and motional origin of the sub-glass relaxations operative in these materials and the influence of network constraint on local chain mobility. Glass transition temperature, network fragility, and sub-glass relaxation characteristics were all sensitive to network structure as governed by effective crosslink density, and length and character of the non-reactive pendants. The observed relaxation properties of the polymers were correlated with measurements of static and dynamic free volume, as well as permeability and solubility of selected gases such as CO2, H2, N2, and CH4.
In addition to the controlled network formulations discussed above, the incorporation of non-porous metal oxide nanoparticles was studied as a further strategy to enhance the gas separation performance of the rubbery polymers. The introduction of up to 20 wt% MgO particles (2.5 nm nominal diameter) in crosslinked PEO membranes has been observed to produce a nearly one-order of magnitude increase in light gas permeability with little change in selectivity as compared to the unfilled network. A key factor when seeking to optimize these materials is the influence of the nanoparticles on the surrounding polymer matrix; i.e., polymer-particle interactions, physical confinement effects, and the influence of the particles on molecular-level packing and local polymer chain mobility. In PEGDA/MgO nanocomposites, dynamic mechanical studies indicate the emergence of a distinct population of polymer chains constrained by their proximity to the polymer-particle interface. The fraction of chain segments influenced by the particles, as well as their conformation, has important implications for the gas transport properties that are ultimately obtained in these membranes.