Transport and Surface Properties of Ion-containing Polymer Membranes

Open Access
Author:
Xie, He
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 19, 2012
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Michael Anthony Hickner, Committee Chair
  • Paul C Painter, Committee Member
  • James Hansell Adair, Committee Member
  • John Michael Regan, Committee Member
Keywords:
  • Desalination
  • Ionic Polymer Membranes
  • Transport Properties
  • Water Binding
  • Zeta Potential
  • Layer-by-Layer Assembly
  • Antifouling
Abstract:
Ion-containing polymer membranes have wide applications in desalination, electrochemical devices, and other technologies that require semipermeable membranes with specific transport selectivity properties. Therefore, a great deal of research has focused on the transport of ions and water in these materials and the correlation of these properties with polymer backbone chemical structure, the type of tethered ionic group to the polymer backbone, and polymer membrane morphology. The way that water is bound within the polymer membrane also attracts great interest because species like protons, methanol, and salt ions transport through the hydrophilic domain of the phase separated polymer membrane and thus are affected by the water dynamics. In addition to the bulk properties of these materials, surface properties are also important for practical applications and have been the focus of significant efforts in new membranes. Surface fouling presents severe problems in the applications of ionic polymer membranes, especially in water treatment. New tools to control fouling and understanding the fundamental interactions in these materials that contribute to fouling are important to improve material antifouling performance and guide future material design. It is the goal of this dissertation to investigate the water binding in ion-containing polymer membranes and make correlations to their transport properties, as well as study the use of layer-by-layer assembly as a surface antifouling modification for desalination membranes. The uptake and dynamics of water in polymer membranes have been found to influence their transport properties. Using Fourier transform infrared spectroscopy (FTIR), the water-polymer interactions in sulfonated and aminated poly(phenylsulfone) membranes were investigated and compared. The OD stretch region in D2O-doped water absorbed in the polymer membrane gives information on the water-water hydrogen bonding dynamics and the interaction of water with the ionic functional groups. The fingerprint region spectra representing water-polymer backbone interactions were analyzed as well. It was found that anion exchange membranes with tethered quaternary ammonium functional groups exhibited more free water content and less water-polymer interactions than cation exchange membranes with tethered sulfonate groups. Measures of membrane water flux and rejection were correlated to these observations and it was found that membranes with quaternary ammonium functional groups displayed faster water and salt transport due to their greater free water content. Highly selective proton conductive networks were prepared based on crosslinking of chain-end functionalized polymers with perfluorosulfonate side groups. A study was carried out regarding the water binding in these crosslinked membranes and the water binding results were tied to their superior electrochemical selectivity. It was found that for these membranes the content of bound water increased with increasing relative humidity or increasing IEC. The lack of bulk-like water explained their low methanol permeability and the presence of bound water still facilitated proton conductivity due to these polymers’ high water uptake. Polystyrene-based homopolymers and block copolymers with quaternary ammonium or imidazolium functional groups were investigated to elucidate the influence of membrane morphology as well as ionic groups on water binding. Based on observations of the OD stretch vibration of water absorbed in the polymer membranes, it was found that homopolymers displayed more bulk-like water than copolymers due to their higher degree of functionalization. From the OD stretch peak, water interacted more strongly with the imidazolium group than with the quaternary ammonium group and imidazolium functionalized polymers exhibited higher bound water content than quaternary ammonium functionalized polymers. Membrane surface charge has a significant influence on membrane retention and fouling performance. As a key parameter describing the surface charge properties of membranes used in aqueous applications, zeta potential measurements on membranes of various types have been used as a standard evaluation method to assess the possible fouling of the material’s surface. In electrokinetic streaming measurements, it was found that the measured streaming current unexpectedly varied with the thickness of the sample and decreased with increasing ion exchange capacity of ion-conductive sulfonated poly(phenylsulfone) membranes. It was determined that membrane bulk conductance influenced the streaming current in the microfluidic channel and produced unanticipated results given the materials’ composition. Extrapolating the measured streaming current to a membrane thickness of zero has proven to be a feasible method for eliminating the error and obtaining correct surface charge information for ion conductive polymer membranes. A linear resistance model relating measured streaming current to membrane bulk conductance is proposed based on the hypothesis that ion conductive membranes reduced the observed streaming current by allowing ions to flow back through the membrane bulk. To better control the surface properties and improve the antifouling performance of polymer surfaces, layer by layer (LBL) assembly of anionic and cationic PEO-based polyurethanes were used to modify surfaces. Surface charge and dynamic protein and polysaccharide adsorption behaviors were measured and correlated to the chemical composition of the assemblies. The PEO-based polyurethanes LBL helped reduce protein fouling on sulfonated and aminated poly(phenylsulfone) membrane surfaces due to the hydrophilicity of PEO. After salt annealing, intermixing between LBL assembly layers was promoted. Salt ions were absorbed into the assembly and increased the hydration of the layers by breaking some of the electrostatic crosslinks between anionic and cationic PEO-poly(urethane)s. By comparing the surface charge as well as protein and polysaccharide adsorption behavior of the LBL assembly surfaces before and after salt annealing, it was discovered that bovine serum albumin adsorption was governed by surface charge while sodium alginate adsorption was influenced by electrostatic interaction with the surface besides surface distribution of charged groups.