Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1123 membranes were measured as functions of temperature between 30 and 90°C. Water permeation increased with temperature. Water permeation from liquid water increased with the water activity difference across the membrane. Water permeation from humidified gas into dry nitrogen went through a maximum with the water activity difference across the membrane. These results suggested that the membrane was less swollen in the presence of water vapor and that a thin skin formed on the dry side of the membrane that reduced permeability to water. Permeation was only weakly dependent on membrane thickness; results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance. The diffusivity of water in Nafion deduced from water sorption into a dry Nafion film was almost two orders of magnitude slower than the diffusivity determined from permeation experiments. The rate of water sorption did not scale with the membrane thickness as predicted by a Fickian diffusion analysis. The results indicated that water sorption was limited by the rate of swelling of the Nafion. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Water desorption appeared to be limited by the rate of interfacial transport across the membrane/gas interface. The analysis of water permeation and sorption data identifies different regimes of water transport and sorption in Nafion membranes corresponding to diffusion through the membrane, interfacial transport across the membrane–gas interface and swelling of the polymer to accommodate water.
† Department of Chemistry and Beckman Institute University of Illinois at Urbana-Champaign Urbana, IL 61801 ‡ Department of Chemical Engineering University of Texas at Austin Austin, TX 78712 § Department of Materials Science and Engineering University of Illinois at Urbana-Champaign Urbana, IL 61801 ׀׀ Department of Aerospace Engineering University of Illinois at Urbana-Champaign Urbana, IL 61801
i,ii three-arm star, 2 six-arm star, iii and comb iv architectures by atom transfer radical polymerization (ATRP) in order to determine why SCLCPs synthesized by non-living polymerizations often exhibit extremely broad phase transitions, and to find an ideal architecture for generating SCLCPs with more fluid mesophases. However, the most branched analog of poly[11-(4'-cyanophenyl-4-phenoxy)undecyl acrylate] would be a hyperbranched polymer generated by homopolymerization of an inimer (initiator monomer), which would also enable us to determine if high degrees of branching hinders the ability of the polymer to form a liquid crystalline phase. (Although numerous dendrimers and hyperbranched polymers have been synthesized with mesogens attached only at their periphery, or within the main-chain of the polymer, none have been synthesized with the mesogen attached as a side chain throughout the branched structure.) Hyperbranched polyacrylates have been synthesized previously by self-condensing vinyl polymerizations (SCVPs) 5 of various inimers using controlled radical techniques such as ATRP; 6,7 however, all have used the alkyl ester substituent to incorporate the initiating site, which leaves no possibility for functionalizing the inimer with an ester substituent corresponding to any known linear polyacrylate. We therefore designed the inimer shown in Scheme 1, which minimizes the chemical variation between the linear and resulting hyperbranched polyacrylate; i.e. the inimer incorporates a free alkyl ester in addition to the requisite acrylate and α-halo functional groups, and the resulting hyperbranched polymer has an ester group attached to every other carbon atom along the polymer backbone, with a nonfunctionalized alkyl ester attached as a free side chain, both of which are features of linear polyacrylates. This talk will compare different synthetic routes for making these inimers, polymerization results, as well as structural characterization of the resulting hyperbranched polymers and their properties.
