|M.Sc Student||Niv Sharon|
|Subject||Elastomeric Emulsion-Templated Polymers|
|Department||Department of Materials Science and Engineering||Supervisor||PROF. Michael Silverstein|
|Full Thesis text|
PolyHIPEs are porous polymers that are typically synthesized within the external phases of water-in-oil (w/o) high internal phase emulsions (HIPEs), emulsions with over 74 % internal phase. Removing the HIPE's internal phase generates the porous structure which, for surfactant-stabilized HIPEs, are usually highly interconnected. More closed-cell-like structures can be generated through synthesis within Pickering HIPEs, HIPEs stabilized through the spontaneous assembly of amphiphilic nanoparticles (NPs) at the oil-water interface. Recent work has shown that the HIPE-stabilizing NPs can also be used to initiate the polymerization and to crosslink the polymer. Liquid droplet elastomers (LDEs) are elastomeric monoliths containing around 85 % water in the form of individually encapsulated micrometer-scale droplets. The original closed-cell LDEs were polyHIPEs based on 2-ethylhexyl acrylate (EHA) synthesized using interfacially initiated free radical polymerization (FRP) within HIPEs stabilized using crosslinking NPs. However, the lack of elastomeric polyHIPEs in the literature reflects the challenges involved in such syntheses.
The objectives of this research were to expand the elastomer-based polyHIPE family and to characterize the resulting properties. Four innovative elastomer-based systems (Systems 1 to 4) were investigated: (1) an EHA-based system aimed at the production of LDEs using different concepts of crosslinking and stabilizing (FRP); (2) copolymerization of EHA and oligomeric 1,2-polybutadiene (PB) (FRP); (3) a linear poly(amide-urea) (PAU) from the step-growth polymerization (SGP) of an oligomeric carboxyl-terminated 1,4-polybutadiene (PBDC) with hexamethylene diisocyanate (HDI) that produces an amide group; (4) linear poly(urethane-urea)s (PUUs) from the SGP of oligomeric hydroxyl-terminated polydimethylsiloxanes (PDMSs) with HDI that produces a urethane group. The reaction of water with the HDI produces urea. The macromolecular structures, densities, morphologies, thermal properties, mechanical properties, and water storage behavior of the resulting polyHIPEs were investigated. The oligomers in the external phase in Systems 2 through 4 resulted in the formation of extremely viscous, and therefore relatively unstable, HIPEs. Thus, producing stable, oligomer-containing HIPEs was a significant challenge.
The breakthrough that enabled HIPE formation in Systems 2 through 4 was the introduction of a polysaccharide into the internal phase, such that its viscosity would be closer to that of the external phase. For System 1, the morphology and mechanical properties were significantly affected by the emulsification strategy, by the crosslinking strategy, and by the degree of crosslinking. The elastomeric nature of the polyHIPE was reduced when a crosslinking comonomer was used. For System 2, the porous structure, thermal properties, mechanical properties, and water retention were significantly affected by the locus of initiation (organic phase or interface), the crosslinker content, and the emulsification strategy (surfactant or NPs). Interfacial initiation produced closed-cell structures and relatively elastomeric polyHIPEs (moduli of around 30 kPa) with enhanced water retention. For Systems 3 and 4, the morphologies, densities, and mechanical properties were strongly affected by the oligomer/HDI mass ratio and by the PDMS molecular weight. Higher moduli were obtained at higher HDI contents, reflecting both the reduction in the amount of elastomer in the HIPE and the increase in the amount of stiff urea groups.