Synthesis and Characterization of Block Copolymer Anion Exchange Membranes and Superimide Proton Exchange Membranes

Open Access
Author:
Wang, Lizhu
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 10, 2014
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Ralph H Colby, Committee Member
  • Tze Chiang Chung, Committee Member
  • Enrique Daniel Gomez, Committee Member
Keywords:
  • block copolymer anion exchange membranes
Abstract:
Anion exchange membranes are of great interest due to their potential to enable low-cost solid polymer alkaline electrochemical technology. However, a significant problem for AEMs is long-lasting performance and robust materials stability under fuel cell and electrolyzer conditions. Block copolymers based on styrene having pendent trimethyl styrenylbutyl ammonium (containing 4 methylene groups between the ionic moieties and the aromatic rings) or benzyltrimethyl ammonium groups were synthesized by reversible addition-fragmentation radical (RAFT) polymerization. The stability under basic conditions of the block copolymers were evaluated under severe, accelerated conditions. The block copolymer with C4 side chain trimethyl styrenylbutyl ammonium motifs displayed improved stability compared to the benzyltrimethyl ammonium-based AEM at 80°C. The AEMs obtained from polystyrene based block copolymers are brittle yet highly swollen in liquid water, thus hampering their study in fuel cell devices. However, crosslinked copolymer AEMs based on poly(vinylbenzyl chloride)-b-poly(butenylstyrene) copolymers using a low temperature olefin metathesis crosslinking route were self-supporting robust membranes. The covalent crosslinking afforded robust mechanical properties in dry materials and low water uptake and small dimensional changes for the hydrated membranes. The crosslinked membranes showed good ionic conductivity when in contact with liquid water, but had lower conductivity than samples of non-crosslinked AEMs at any given relative humidity hydration condition. The morphologies of crosslinked AEMs as investigated by SAXS correlated with their conductivity values. The ionic conductivity of crosslinked samples decreased due to the disruption of ionic domains during the crosslinking process. A detailed study regarding morphology, ion conductivity and water uptake with controllable hydrophobic block composition in AEMs was conducted. In this work I described how the properties of block copolymer anion exchange membranes (AEMs) vary with the hydrophobic block composition and ion content. Under the similar ion content (IEC = ~ 2.0 meq g-1), the block copolymers with methacrylate and styrenic hydrophobic blocks showed similar ionic conductivity. The water uptake of the materials slightly increased when the ion content of the samples was above 2.0 meq g-1 below which the hydration number almost remained constant (~7 water molecules per ionic moiety).The stearyl methacrylate containing sample was mechanically robust in liquid water due to the introduction of semicrystalline blocks. In particular, the factors conducive to the conductivity will be discussed in detail. The ionic polymers with long-range ordered microstrtures and high ion content (IEC = ~ 3.0 meq g-1) had the highest conductivity. Sulfonimide-containing random copolymers based on poly(arylene ether sulfone) (PAES), poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and poly(styrene) (PS) were synthesized by low cost post-polymerization modification under mild conditions. These synthetic routes provided universal scale-up approaches for industry production due to the absence of expensive monomers purification. The Radel samples in sulfonimide or sulfonic acid form displayed higher proton conductivity than PPO analogues. These sulfonimide copolymers showed proton conductivity comparable to sulfonic analogs but lower water uptake. The thermal stability of sulfonimide copolymers was stable up to 250 °C under nitrogen atmosphere. The sulfonimide samples are applicable to high temperature PEMs due to its higher ionic moieties degradation temperature.