MORPHOLOGY AND TRANSPORT PROPERTIES OF ION-CONTAINING TRIBLOCK COPOLYMERS

Open Access
Author:
Moore, Hunter David
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 04, 2011
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
Keywords:
  • ion-containing polymers
  • block copolymers
Abstract:
Ion-containing polymers are an interesting subset of polymeric materials with applications in structural materials, water treatment, energy storage, and energy conversion. Within this class of polymers, ion-containing block copolymers hold promise in advancing our understanding of the structure-property relationships of water-absorbing, ion-containing membranes through their unique ability to form tunable ionic domain morphologies. Non-ionic block copolymers have been studied extensively and the theory of how their nanophase structure is formed and the experimental methods for probing their phase behavior are well-documented in the literature. However, the addition of ions to these systems may change the traditional descriptors of phase formation in block copolymers by introducing strong electrostatic or hydrogen bonding interactions into one of the blocks. Therefore, much attention has recently been given to the study of how the chemical composition and ion content of ion-containing block polymers affect their morphology. These studies of phase formation, combined with work on how the microphase separated morphology affects transport properties, e.g. ion conductivity, will help to advance the design of next-generation, ion-containing block copolymer membranes for ion and water transport applications. The focus of this thesis is to elucidate the phase behavior and ion transport properties of sulfonated and quaternary ammonium-containing triblock copolymers. The polymers studied in this work are sulfonated poly(hexyl methacrylate)-b¬-poly(styrene)-b-poly(hexyl methacrylate) [PHMA-SPS-PHMA], sulfonated poly(perfluorooctyl methacrylate)-b¬-poly(styrene)-b-poly(perfluorooctyl methacrylate) [PFMA-SPS-PFMA], and quaternary ammonium functionalized poly(styrene)-b¬-poly(ethylene/butylene)-b-poly(styrene) [QA-SEBS]. In particular, the work presented here describes the investigation of the effect of functionalizing the midblock of the sulfonated triblock copolymers with the idea that the hydrophobic endblocks will help to increase mechanical stability of the film when the polymer is introduced to humidity. This study also investigated how the morphology of end-block functionalized quaternary-ammonium containing triblock copolymers change when introduced to varying levels of humidity. Small angle x-ray scattering and transmission electron micrography were used to quantitatively probe the morphology of the samples. Water uptake and impedance spectroscopy provided information on the transport properties of these samples. Comparisons are drawn between samples with different levels of functionalization and some comments are made on the basic mechanisms of phase formation in sulfonated and quaternary-ammonium containing block copolymers. In the study of midblock functionalization, two different endblocks, poly(hexyl methacrylate) and poly(perfluorooctyl methacrylate), were chosen to observe how the hydrophobic portion of the polymer affected the morphology and conductivity of the membranes. Fluorinated PFMA endblocks produced materials with less order that materials with PHMA triblocks in unannealed solvent cast films, likely due to solubility and kinetic trapping issues, and these less-ordered samples showed lower proton conductivity. Better ordering of the non-fluorous PHMA-containing block copolymers was due to their low Tg and good solubility during membrane casting. Poly(hexyl methacrylate)-b¬-sulfonated poly(styrene)-b-poly(hexyl methacrylate) transitioned from lamellar to disordered to hexagonally packed cylindrical morphologies with increasing functionalization while poly(perfluorooctyl methacrylate)-b¬-sulfonated poly(styrene)-b-poly(perfluorooctyl methacrylate) remained disordered throughout all tested levels of functionalization. Quaternary ammonium functionalized SEBS block copolymers showed no change in morphological order with increased functionalization or increased humidity. The ionic domains swelled during water uptake, and the change in interdomain spacing with relative humidity depended on the percent functionalization of the styrene domain. High conductivity with low water uptake was obtained for a QA-SEBS sample with high unfunctionalized styrene content which had very low swelling upon humidification. These results demonstrate that different trends may be observed for sulfonated and quaternary ammonium triblock copolymers as dictated by the ion clustering in each system and the chemical details of the backbone.