DYNAMIC SWELLING AND CONDUCTIVITY OF ION-CONTAINING BLOCK COPOLYMER THIN FILMS

Open Access
Author:
Petrina, Stephanie Ann
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 30, 2010
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
Keywords:
  • swelling
  • thin film polymer
  • ionic polymer
  • ellipsometry
Abstract:
Ion-containing polymers have been extensively studied in an attempt to increase the efficiency of ion transport in fuel cells. Block copolymers with an ionic phase and a hydrophobic phase represent one promising approach to improved fuel cell materials. Despite abundant research on bulk membranes composed of ion-containing block copolymers, little attention has been given to swelling and conductivity in thin films (~10-500 nm) of these materials. Thin ion-containing polymer films play a significant role in fuel cell operation and performance by facilitating proton conduction between the catalyst layer and proton exchange membrane (PEM). By lowering the interfacial resistance between the catalyst and PEM in a fuel cell, these films become a critical component in accessing the maximum amount of reactive catalyst surface area. Typically, thin ion-containing polymer films are treated as having the same properties as measured for bulk membranes. However, in thin films, substrate and air interfaces affect material properties. The high ratio of surface area to volume in films less than 100 nm in thickness results in a large percentage of interfacial polymer with unique morphological, swelling, and conductivity characteristics compared to what is observed for bulk films. When a PEM is exposed to increased water activity, or more commonly, relative humidity, additional water is absorbed into the film. The morphological structure and polymer chain dynamics of the film may vary with thickness, which affects the rate of water absorption, the total amount of water absorbed, and therefore, the conductivity of the films. To understand thin film conduction, the following points were investigated: the morphological differences between bulk and thin (< 100 nm) film polymers; the effect of film thickness and morphology on water uptake and dynamic swelling; and the interactions between the ion-containing polymers and the substrate. The microscopic structure before swelling, the change in thickness during swelling, and ion conductivity of thin films of ion-containing block copolymers were examined. The initial morphology of the films in ambient conditions was established by atomic force microscopy (AFM). Spectroscopic ellipsometry was used to measure the change in thickness and film composition in real time to characterize the dynamic water uptake. Ion conductivity was measured using AC impedance. The aim of this research is to understand the swelling behavior of thin ion-containing block copolymer films and to determine the fundamental water sorption phenomena and morphological features that may influence the conductivity of these thin films.