THIN FILM DYNAMICS OF ION-CONTAINING POLYMERS UNDER CONFINEMENT

Open Access
Author:
Kushner, Douglas Ian
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 03, 2016
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Michael Anthony Hickner, Committee Chair
  • Ralph H Colby, Committee Member
  • Evangelos Manias, Committee Member
  • Enrique Daniel Gomez, Outside Member
Keywords:
  • Thin Polymer Films
  • Nafion
  • Water Sorption
  • Fuel Cells
  • Spectroscopic Ellipsometry
  • Quartz Crystal Microbalance
Abstract:
Ion-containing polymers that are useful in electrochemical devices such as fuel cells, batteries, and electrolysers were investigated to determine the structure-property relationships of thin films in a confined geometry. Comb-shaped polymer, random copolymer, and block copolymer proton exchange membranes (PEMs), such as Nafion, and anion exchange membranes (AEMs), such as poly(hexyl methacrylate–block–quaternized vinyl benzyl chloride) (PHMA-b-QAPVBC) and comb-shaped quaternized poly(phenylene oxide), were investigated in order to elucidate the effect of confinement on material properties and morphology in these systems. Ion-containing polymers rely on water to facilitate ion transport in electrochemical systems resulting in a large portion of this work studying the in situ effects of humidity on the polymer structure and the relationship to water uptake. The morphology of the polymer systems was primarily examined using grazing incidence small angle X-ray scattering (GISAXS) while spectroscopic ellipsometry (SE) and quartz crystal microbalance (QCM) were employed to study the in situ water uptake. By studying a number of different ion-containing polymer systems using multiple techniques and a deep analysis of structure-property relationships, a more complete illustration of the confinement effect in these polymer systems was developed. Nafion has remained the standard PEM material, driving decades of research; however, in the past 7-10 years, operational inefficiencies have been associated with confinement of Nafion in the catalyst layer of PEM fuel cells, surging investigations into confinement. The water uptake, molecular orientation, and structural relaxations were characterized in order to elucidate the substrate effect on the structure-property relationship. Substrates utilized include gold, platinum, carbon, and native- and sputtered SiO2. The strength of the interactions dictated the molecular orientation, where strong polymer/substrate interactions resulted in highly ordered molecular orientations that were oriented parallel to the substrate. Weakly interacting systems formed parallel orientations while moderate interactions resulted in Nafion self-assembling into an isotropic structure. The molecular orientation was found to have an impact on the swelling where perpendicular structures exhibited a greater swelling compared to parallel structures. To further analyze the polymer/substrate interactions, the physical aging of unannealed Nafion was measured on gold, carbon, and native-SiO2. The polymer/substrate interactions were found to dictate the structural relaxation resulting in two cases: (1) structural relaxations induced by increases in humidity as a result of strong interfacial interactions or (2) no observable structural relaxations as a result of weak interfacial interactions. In order to examine the effect of confinement, the aging rate was measured as a function of thickness for films fabricated on gold substrates. As the thickness decreased the rate of aging increased, an indication of the increased configurational entropy with decreasing thickness. To further examine substrate confinement influences on the morphology and water uptake of ion-containing thin films, the diblock copolymer PHMA-b-QAPVBC was investigated. Water plasticization occurs during water uptake resulting in chain mobility which can have a profound impact on the stability of the morphology. PHMA-b-QAPVBC thin films demonstrated a morphological transition from random cylinders to parallel lamellae using atomic force microscopy and GISAXS techniques. In situ measurements were performed on the diblock copolymer by increasing the relative humidity (RH) in steps of 1% RH in order to monitor the dynamic morphological transition. By simultaneously measuring the thickness (SE) and mass (QCM), the density could be computed where observable inflections in density corresponded to this dynamic transition. By cycling the samples twice in a relative humidity environment, the glass transition temperature due to water plasticization was pinpointed, indicated by changes in data. The water content was input into the Kelley-Bueche glass transition temperature equation in order to estimate the temperature departure from a bulk membrane. It was found that the water content in thinner films was lower before the dynamic morphological transition occurred, indicating that thin film confinement reduced the glass transition temperature of QAPVBC block and ultimately destabilized the as-cast morphology. To continue the investigation of confinement on the properties of polymer films, three comb-shaped polymer AEMs were examined with a control polymer that had no side chain. The comb-shaped polymers were synthesized with 6, 10, and 16 carbon alkyl side chains with the purpose of increasing mechanical strength through side chain crystallization, inducing phase separation, and limiting chemical degradation. Using cantilever curvature, the mechanical properties of thin films could be measured. The comb-shaped polymer synthesized with a 16 carbon side chain was the only sample to show increased mechanical properties while the other three polymers had relatively similar moduli. The phase separation and crystallinity were studied using grazing X-ray scattering techniques and showed that phases observed in membranes were also present in the thin film configuration however the scattering was not nearly as strong, indicative of weaker phase separation. As the side chain length increased, scattering features at the same q-value as hexagonally packed polyethylene appeared from the amorphous halo indicating that the side chains were crystallizing although there was no mechanical reinforcement in the 6 and 10 carbon samples. Confinement disrupted the crystallinity and domain formation compared to the bulk membranes resulting in increased water uptake in the thin films, most notably the 10 and 16 carbon samples. By disrupting the domain and crystallinity formation, the swelling was not as restricted compared to the membranes, resulting in the greater water uptake. In order to provide accurate results, a full understanding of the characterizing techniques was necessary, especially working with thin films where 0.5 nm could represent a 3% increase in swelling. During these studies it was found that the sputter-coated SiO2 layer on the QCM crystals was porous which resulted in erroneous water uptake due to the hydrophilic nature of the silanol groups that line these pores. This has gone largely unnoticed in literature because the measured mass and thickness on these crystals coincided resulting in erroneous data being interpreted as a thin film phenomenon. As water penetrates the porous network, the refractive index of porous SiO2 layer was increasing as a result of water replacing void space. The optical model used to describe the polymer film accounted for the extra water associated with the SiO2 layer leading to incorrect thickness values. A simple model was developed to account for the water in the SiO2 layer by determining the porosity and thickness using a dry and humidified substrate. The humidity-dependent refractive index was applied to the in situ thickness measurement resulting in a corrected thickness measurement that was in agreement with the same polymer measured on a silicon wafer. Similarly, the QCM recognized that mass was being added to the system and was being accounted for in the water uptake of the polymer film. In summary, by studying the morphology and water uptake characteristics of these confined polymer thin films under controlled humidity, structure-property relationships were developed. This will lead to a greater understanding of how confinement will influence the future generation of PEMS and AEMS. Expanding this knowledge will lead to the development of materials that increase the performance of fuel cell devices by providing a more intimate image of how these polymers will perform in the catalyst layer, and the ideas developed here can be extended to new types of materials used in various applications.