Solid-State Block Polyelectrolytes

Abstract

While polyelectrolytes are, in general, hydrophilic and soluble in water, there are many applications that benefit from immobilized solid-state charged materials, including membrane separations and batteries. One convenient method to immobilize polyelectrolytes in a solid-state configuration is using block polymer materials self-assembled to contain charged polyelectrolyte domains immobilized by neutral supporting domains. We used this strategy to work towards charge mosaic materials, a proposed design for a piezodialysis-based water desalination system. In a charge mosaic membrane, there are both positively and negatively charged polymer domains that are spatially separated and independently cross the thickness of the material. In Chapter 2, we first developed a new technique to integrate this design into thin films. Through the synthesis of neutral ABC triblock polymers, we casted thin films with three microphase separated domains. We then demonstrated the functionalization of these materials in a mild, 2-in-1 postpolymerization modification that converted the A domain (poly(n-propyl styrene sulfonic ester)) to a negatively charged polyanion and the C domain (poly(vinylbenzyl chloride)) to a positively charged polycation in a mild, single step vapor exposure. While these materials demonstrated successful microphase separation with a simple functionalization that maintained morphology, they had poor long-range order and suffered from brittle mechanical properties that prevented their effective use as active layers in membrane separations. During the synthesis of an ABC triblock polymer for charge mosaic applications, we found a previously unreported miscibility between polystyrene and poly(vinylbenzyl chloride). Although both polymers had been used together in a number of previous applications, their solid-state structure had never been adequately explored. In Chapter 4, we attempted to characterize the Flory-Huggins interaction parameter between these two polymers using small-angle X-ray scattering of homogeneous polymer blends. We then synthesized a vinylbenzyl chloride derivative, vinylbenzyl nitrate, that demonstrated both microphase separation from polystyrene as well as facile postpolymerization modification and explosive properties. Chapter 3 attempted to solve the mechanical problems associated with the triblock polymers by integrating poly(styrene sulfonic ester) and poly(vinylbenzyl chloride) into a system that undergoes polymerization-induced microphase separation (PIMS). PIMS monoliths were made through a simple radical polymerization initiated in a homogeneous mixture of a macroinitiator dissolved in mono- and di-functional monomers. The PIMS technique results in strong materials that contain a bicontinuous structure comprising a percolating macroinitiator domain crossing the thickness of the crosslinked matrix. We used PIMS to produce solid monoliths using the neutral polyelectrolyte precursors previously used in the ABC triblock polymers. The macroinitiator domain was then functionalized to yield either a positively charged polycation material or a negatively charged polyanion material, confirmed using oppositely charged dyes and infrared characterization. The integration of both positive and negative charges into a PIMS system is approached in Appendix A. Simply mixing macroinitiators, a procedure based on previous literature, showed unexpected macrophase separation in the monolith. The use of block polymer macroinitiators to overcome solubility differences between the segments is presented as a potential solution and the synthesis of the first block polymer PIMS is demonstrated. Finally, we introduced the potential of using a polyelectrolyte as the matrix domain. Appendix B presents a proposed model for controlling swelling in the PIMS polyelectrolyte domain. By adding a poly(lactide) block to the poly(styrene sulfonic ester) macroinitiator, a degradable domain is introduced that can be selectively removed to free swelling space for the polyelectrolyte. We hypothesize that control over the swelling will provide a model system for the systematic variation of ion conductivity and provide insights into the fundamental effects of ion density, water content, and morphology. A slightly different project is explored in Chapter 5, where we develop a poly(lactide) based foam for use in floral foam applications. We formulate a mix of surfactants that make the hydrophobic, low melt-strength poly(lactide) into a rigid, low density (<0.02 g·cm–3), hydrophilic foam that readily absorbs water and supplies it to inserted flowers. The foam is compostable and made from renewable materials, making it a significant improvement over the petroleum based, non-degradable materials that are currently commercially available.