Influence of Molecular Transport on the Structure-Property Relationships of Amphiphilic Block Copolymer Membranes

Polymers are essential to modern life. Yet there still remains a wealth of knowledge to discover regarding novel polymeric materials, processing techniques, and applications. The synthesis and application of nanostructured amphiphilic block copolymers have attracted significant interests in the last decade. Depending on the structure, composition, and architecture; a wide range of applications have been reported in various fields of research. The need to accurately access the fundamental structure-property relationships in polymers are of paramount importance to the performance in various applications. Some of these applications include the use of polymer membranes for CO2 capture from flue gas, for water purification and wastewater treatment, as well as polymer electrolytes for lithium batteries. This dissertation therefore, focuses on contributing fundamental knowledge on the structure-property relationships of amphiphilic block copolymer membranes to improve its performance. With the use of poly(styrene–block–ethylene oxide), (SEO) and high molecular weight poly(ethylene oxide) membranes and polystyrenes of various molecular weight and tacticity, experimental work has been conducted with relevant consideration in the aforementioned application areas. The fundamental study of the effects of molecular transport (e.g. water vapor) in an amphiphilic block copolymer membrane consisting of hydrophilic blocks of poly(ethylene oxide) and hydrophobic blocks of polystyrene (PS-b-PEO) was investigated. The influence of water solubility and diffusivity on the block copolymer morphology were examined at various temperatures and water concentrations. A comprehensive study conducted using Fourier-transform infrared spectroscopy (FTIR) to investigate the effect of water activity on PEO crystallinity, and how the PEO crystallinity in turn affects water sorption and diffusion was investigated. Also, isothermal vapor-sorption equilibria and diffusion coefficients of water in different architectures of block copolymer membranes will be discussed. This fundamental study is important for applications that rely on PEO-containing materials, as PEO crystallite melting dramatically impacts transport and mechanical properties. For lithium battery application, the study of ion association effects and the ion-polymer interactions in high molecular weight poly(styrene–ethylene oxide) block copolymer (SEO) and complexes with lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI) as polymer electrolyte using FTIR-ATR spectroscopy were conducted. The dissolution of the lithium salt in the PEO phase as it influences the structure of the ion conducting phase of the polymer (PEO) was investigated. The infrared bands observed in the polymer–salt complexes as a function of salt concentration and temperature show different solvation and degree of ion association behavior. An understanding of the relationship between ionic conductivity and degree of solvation of lithium salts as a function of ion concentration was explained from the FTIR-ATR results. Also, Structural and stress relaxations have been measured with x-ray photon correlation spectroscopy (XPCS) and rheology, respectively, as a function of salt concentration and temperature. Results from XPCS experiments showed hyperdiffusive motion for various lithium salt concentrations and at varying temperatures, which is indicative of soft glassy materials. This behavior is attributed to cooperative dynamics. The decay time was a weak, non-monotonic function of salt concentration and decreased with increasing temperature, in an Arrhenius fashion. In contrast, the shear modulus decreased with increasing salt concentration and increasing temperature. The entanglement relaxation from rheological measurements followed Vogel-Fulcher-Tammann behavior. The structural decay time was slower than the entanglement relaxation time at temperatures above the glass transition temperature, but they were approximately equal at Tg regardless of salt concentration. This may indicate a fundamental connection between cooperative structural motion and polymer chain motion in this material.