Water hydrogen bonding in proton exchange and neutral polymer membranes

Open Access
Author:
Smedley, Sarah Black
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2015
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • John B Asbury, Committee Chair
  • Mark Maroncelli, Committee Member
  • Michael John Janik, Special Member
Keywords:
  • hydrogen bonding
  • proton exchange membrane
  • FTIR
  • vibrational spectroscopy
  • superacid
  • water treatment membrane
Abstract:
Understanding the dynamics of water sorbed into polymer films is critical to reveal structure-property relationships in membranes for energy and water treatment applications, where membranes must interact with water to facilitate or inhibit the transport of ions. The chemical structure of the polymer has drastic effects on the transport properties of the membrane due to the morphological structure of the polymer and how water is interacting with the functional groups on the polymer backbone. Therefore studying the dynamics of water adsorbed into a membrane will give insight into how water-polymer interactions influence transport properties of the film. With a better understanding of how to design materials to have specific properties, we can accelerate development of smarter materials for both energy and water treatment applications to increase efficiency and create high-flux materials and processes. The goal of this dissertation is to investigate the water-polymer interactions in proton exchange and uncharged membranes and make correlations to their charge densities and transport properties. A linear Fourier Transform Infrared (FTIR) spectroscopic method for measuring the hydrogen bonding distribution of water sorbed in proton exchange membranes is described in this thesis. The information on the distribution of the microenvironments of water in an ionic polymer is critical to understanding the effects of different acidic groups on the proton conductivity of proton exchange membranes at low relative humidity. The OD stretch of dilute HOD in H2O is a single, well-defined vibrational band. When HOD in dilute H2O is sorbed into a proton exchange membrane, the OD stretch peak shifts based on the microenvironment that water encounters within the nanophase separated structure of the material. This peak shift is a signature of different hydrogen bonding populations within the membrane, which can be deconvoluted rigorously for dilute HOD in H2O compared to only qualitative observations that can be made with pure D2O or H2O. The theory and experimental practice of determining the hydrogen bonding distribution of water in a range of proton exchange membranes bearing aromatic sulfonate and perfluorosulfonate groups using this OD stretch technique is discussed. The OD stretch of dilute HOD in H2O absorbed in a series of sulfonated syndiotactic poly(styrene) and sulfonated poly(sulfone) membranes was studied using FTIR spectroscopy to measure how the character of the sulfonate headgroup and the backbone polarity influenced the water-membrane interactions. Using a three-state model, the OD stretch yielded information about the populations of absorbed water participating in hydrogen bonds with polymer-tethered sulfonate groups, water in an intermediate state, or water hydrogen bonding with other water molecules. The perflouroalkyl sulfonate moiety, which behaves as a superacid, consistently displayed the largest fraction of headgroup-associated water due to its strong acidic character. Measurements of the OD stretch gave insight to the strength of the hydrogen bonds formed between water and the sulfonate groups. Water associated with the superacid displayed an OD stretch peak position that was blueshifted by 39 cm-1 compared to the aryl sulfonate associated water with an OD stretching frequency that was centered at 2547 cm-1. The polarity of the polymer backbone also affected the OD stretch peak position. As hydration increased, the OD peak stretching frequency in poly(styrene)-based membranes displayed a redshift from 2566 cm-1 to 2553 cm-1, whereas there was no OD peak maxima shift in poly(sulfone)-based membranes due to the greater amount of intermediate water in the more polar poly(sulfone) backbone system. To further understand how the acidity of the sulfonate can be altered and how the acidity affects the hydrogen bonding network of water in a polymer membrane, various polymers with small chemical differences in the perfluorosulfonate sidechain were studied. In addition to the vibrational spectroscopy measurements using HOD as a probe, the partial charges of the sulfonate groups were calculating using DMol3 DFT calculations. The calculations and the experimentally determined peak position of the OD stretch both correlated to give a ranking of acidity for the various sidechains. It was found that having a thioether linkage instead of an ether linkage (typical linkage for perflurosulfonates) increased the acidity of the sulfonate group due to the capability of sulfur to expand its octet and more readily accept additional electron density. Through DFT geometry optimization, it was discovered that the thioether linkage prefers a kinked configuration while the ether linkage gives a more linear sidechain structure. This structural configuration correlated to experimental findings allowing more water to interact with the sulfonate group containing the ether linkage than the thioether linkage due to the sulfonate group being more easily accessible, even though the thioether sidechain is more acidic. Three sulfonated poly(arylene sulfone) based polymers were studied using FTIR and DFT calculations to better understand how the acidity of the sulfonate groups were affected by the placement on the backbone. By increasing the number of sulfone groups, which have electron withdrawing properties, flanking the sulfonated aromatic ring, the acidity was increased. The charge density of a sulfonate group flanked by two sulfone groups was -1.626 (in units of fundamental charge), while the charge density of a sulfonate group flanked by one sulfone group increased to -1.703. Additionally, if the subsequent ring was unsulfonated, the charge density further increased to 1.737, indicating that some stability is gained by both available rings being sulfonated. The differences in charge density are reflected in the water uptake and conductivity measurements, where the samples with the lowest charge density had the highest water uptake and conductivity. The deconvoluted OD peak revealed that the sample with two sulfone groups flanking the sulfonated aromatic ring contains the highest amount of bulk-like water, which led to the increased conductivity. The polyamide active layer of commercially available reverse osmosis membranes was studied at various relative humilities to better understand how the structure of the active layer changes when hydrated. The fingerprint region was used to analyze changes in the vibrational signature of specific functional groups and to understand how different chemical moieties interact with water. Using the difference spectrum, the water-polymer interactions could be quantified and correlated to transport properties of the membrane. Increasing the amount of free carboxylic acid groups on the backbone will lead to an active layer that is less crosslinked and contains a greater number of larger pores, which results in a higher flux. Active layers that contained a smaller concentration of free carboxylic acids were more highly crosslinked and had a higher amount of smaller pores, resulting in a lower flux. In summary, by studying the water hydrogen bonding network in various proton exchange membranes and neutral polyamide membranes, a new understanding of structure-property relationships has been developed. This will lead to a greater understanding of transport properties and conductivity in various polymer membranes. Expanding this fundamental knowledge will lead to the development of smarter materials for energy and reverse osmosis applications, and the ideas developed here can be extended to new types of materials used for various needs.