Title page for ETD etd-10022012-005937


Type of Document Dissertation
Author Hou, Jianbo
Author's Email Address jianbo@vt.edu
URN etd-10022012-005937
Title Transport and Anisotropy inside Ionic Polymer Membranes
Degree PhD
Department Chemistry
Advisory Committee
Advisor Name Title
Madsen, Louis A. Committee Chair
Dorn, Harry C. Committee Member
Marand, Herv L. Committee Member
Valeyev, Eduard Faritovich Committee Member
Keywords
  • ionomer
  • transport
  • pulsed-field-gradient NMR
  • structural characteristic
  • molecular interactions
Date of Defense 2012-09-18
Availability restricted
Abstract
Water and ion transport critically determine the performance of many functional materials and devices, from fuel cells to lithium ion batteries to soft mechanical actuators. This dissertation aims to address some fundamental issues regarding transport and anisotropy, structural heterogeneity and molecular interactions inside ionic polymers.

I first discuss a main deficiency of a standard protocol for calibrating high pulsed-field-gradient NMR. I show that high gradient calibration using low γ nuclei is not amenable to measurements on slow diffusing high γ nuclei. Then I employ NMR diffusometry to investigate transport and anisotropy for a series of ionic polymers, from poly(arylene ether sulfone) hydrophilic-hydrophobic multi-block copolymers to polymer blends to perfluorosulfonate random copolymers.

For the multi-block copolymers, NMR diffusion measurements yield diffusion anisotropy as a function of water uptake and block lengths. 2H NMR spectroscopy on absorbed D2O probes membrane alignment modes. These measurements also provide insights into average defect distributions.

For the blend membranes, we examine the impact of compatibilizer on their transport properties. An increase in compatibilizer significantly improves the membrane phase homogeneity confirmed by SEM and transport studies. Theories of diffusion in porous media yield changes in domain size and tortuosity that correspond to drastic changes in local restrictions to water diffusion among different blend membranes. NMR

relaxometry studies yield multi-component T1 values, which further probe structural heterogeneities on smaller scales than diffusion experiments.

For the random copolymer, the exploration of ion transport reveals inter-ionic associations of ionic liquids (ILs) modulated by hydration level and ionic medium. When ILs diffuse inside ionic polymers, isolated anions diffuse faster (≥ 4X) than cations at high hydration whereas ion associations result in substantially faster cation diffusion (≤ 3X) at low hydration inside membranes, revealing prevalent anionic aggregates.

Finally, I present the strategy and analytical protocol for studying ionomer membranes using ILs. The normal cation diffusion contrasts to the anomalous anion diffusion caused by local confinement structures inside the membranes, which vary drastically with temperature and hydration level. These structures correspond to a density variation of SO_3^- groups, which define a distribution of local electrical potentials that fluctuate with temperature and nature of ionic media.

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