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Doctor of Philosophy (PhD)
Polymer Science and Engineering
Year Degree Awarded
Month Degree Awarded
E. Bryan Coughlin
Polymer and Organic Materials
One key challenge for alkaline anion exchange membrane fuel cells is the lack of alkaline stable polycations. The synthesis of random, crosslinked and block copolymers based on the cobaltocenium phenylene norbornene (NPC) monomer is described. The polymers were synthesized from ring-opening metathesis polymerization (ROMP) of the NPC monomer showed excellent thermo-alkaline and thermo-oxidative stability. Random copolymers, crosslinked networks and amphiphilic diblock copolymers were prepared by copolymerizing NPC with different hydrophobic monomers: norbornene for random copolymers, dicyclopentadiene for crosslinked networks and a norbornene dibenzyl ether monomer for amphiphilic diblock copolymers. Mechanical robust membranes were prepared from all these copolymers.
Polymers with different architectures exhibited different morphologies. Random copolymers showed disordered interconnected cobaltocenium domains with ion clusters present; crosslinked networks showed homogenous distribution of ions; the amphiphilic diblock copolymers showed cylindrical microphase separation with the cationic domains being the continuous phase even though they constituted the minor volume component.
The morphologies of the membranes were found to have little effect on the water uptake of the membranes, but significantly influenced the ionic conductivity. The crosslinked membranes showed lower conductivity compared to the random copolymer membranes at the same composition. However, higher IECs can be achieved by crosslinking with concomitant improved mechanical integrity relative to their random copolymer analogs, ultimately allows for reaching higher ion conductivity values. For the diblock copolymer, formation of a conducting ion channel and elimination of the presence of ion clusters allowed for significantly higher ionic conductivity than the random copolymer or the crosslinked networks at the same composition.
Poly(vinyl acetate)-b-polybutadiene-b-poly(vinyl acetate) triblock copolymer was synthesized for water/alcohol pervaporation separation membrane. A difunctional chain transfer agent (dCTA) with both reversible addition–fragmentation chain-transfer (RAFT) and ROMP functionality was synthesized. The triblock copolymers were obtained by synthesizing narrow-dispersed poly(vinyl acetate) by RAFT and subsequently inserting a polybutadiene block by ROMP. The polymers were cast as thin membranes, and the polybutadiene was crosslinked by UV radiation for mechanical robustness. Solid state hydrolysis afforded poly(vinyl alcohol)-b-polybutadiene-b-poly(vinyl alcohol) membranes. Different compositions of the polymers resulted in different morphologies. A longer polybutadiene block or a shorter poly(vinyl alcohol) block contributed to stronger phase separation. This study invented a new methodology to construct amphiphilic triblock copolymers with well-defined morphologies.
Yuan, Haomiao, "SYNTHESIS AND STRUCTURE-PROPERTY RELATIONSHIPS OF POLYMERIC MEMBRANES FOR SMALL MOLECULE TRANSPORT" (2017). Doctoral Dissertations. 1069.