M.Sc Thesis


M.Sc StudentKeren Dahiana
SubjectEmulsion-Templated, Simultaneously-Synthesized,
Interpenetrating Polymers with Degradable
Components for Hierarchical Porosity
DepartmentDepartment of Materials Science and Engineering
Supervisor PROF. Michael Silverstein
Full Thesis textFull thesis text - English Version


Abstract

PolyHIPEs (PHs) are emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs), emulsions containing over 74% internal phase. PHs are usually synthesized through free radical polymerization (FRP), but other polymerization mechanisms can be used, such as step-growth polymerization (SGP). A PH can consist of a homopolymer or several polymers, such as interpenetrating polymer networks (IPNs) combining two chemically distinct polymer networks that are physically intertwined at the molecular level but are not covalently connected. In semi-IPNs, just one polymer network is formed while the other polymer is not crosslinked. IPNs and semi-IPNs can be synthesized by the sequential polymerization of the polymers or simultaneously by two mutually exclusive polymerization mechanisms. Templating within water-in-oil HIPEs has been used to generate IPN-PHs consisting of crosslinked polystyrene combined with crosslinked, degradable poly(urethane urea) (PUU). Hierarchically porous polymers are of interest for many applications, including contaminant adsorption, and oil spill clean-up. It should be possible to generate polyHIPEs with hierarchical porosities combining micropores (<2 nm), mesopores (2-50 nm), and/or macropores (>50 nm) from IPN-PHs by hypercrosslinking the polystyrene and/or by degrading the PUU. The amount of micro/mesoporosity will be reflected in the SBET, the specific surface area measured using the BET (Brunauer-Emmett-Teller) method.

The objective of this research was to synthesize hierarchically porous structures through the simultaneous synthesis of IPN-PHs containing both a crosslinked network from the free radical copolymerization of vinylbenzyl chloride (VBC) and divinylbenzene (DVB) and a crosslinked PUU network based on the step-growth polymerization of an oligomeric polycaprolactone triol (PCL-T) and hexamethylene diisocyanate (HDI). The P(VBC-co-DVB) was to undergo hypercrosslinking through a Friedel Crafts alkylation with FeCl3, while the PCL was to undergo hydrolytic degradation in 3 M NaOH. The resulting macromolecular structures, porous structures, and properties of the resulting PHs were to be characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, dynamic mechanical thermal analysis, thermogravimetric analysis, uniaxial compression tests, nitrogen porosimetry, and mercury intrusion porosimetry.

All the PHs exhibited relatively low densities. The PUU and the P(VBC-co-DVB) reference PHs differ in their porous structures, and these differences were evident in the PHs combining the two. Unexpectedly, the PUU reference PH completely dissolved in dimethylformamide (DMF), indicating that it was not crosslinked and that the resulting PHs were not IPNs but rather semi-IPNs. The semi-IPN PH with a high P(VBC-co-DVB) content exhibited two broads, overlapping tan δ peaks, reflecting a heterogeneous material. These peaks merged with increasing PUU content, reflecting a more homogeneous material. Hypercrosslinking reduced the tan δ peak height with the reduction increasing with increasing P(VBC-co-DVB) content. Most of the PHs exhibited stress-strain curves that are typical for PHs, with the exception of the elastomeric stress-strain curves of the PHs with the highest PUU contents. The hypercrosslinked semi-IPN-PHs exhibited SBET up to 445 m2/g that were associated with the generation of micropores between 0.2 and 1.0 nm. The increase in the porosity following hypercrosslinking and etching was associated with the removal of the PUU and the generation of relatively small macropores of ~2 μm.