Hierarchical Macroporous Polymers: Synthesis and Characterisation

Abstract

This thesis describes the production of two types of hierarchical macroporous polymers using the emulsion templating technique. The first are those with a hierarchy of macroporous, defined as the efficient packing of pores with multi-modal pore size distributions. The second are macroporous polymers containing a hierarchical particulate network, defined by an interconnected particle network within the polymer matrix. In the first section of the thesis, macroporous hierarchy was achieved using 3 different methods. In the first method, the properties of surfactant stabilised high internal phase emulsion (HIPE) were optimised by varying selected emulsification parameters such as the surfactant concentration and stirring rate. The Finite Element Method (FEM) was subsequently used to qualitatively compare and validate the effect of pore hierarchy on the Young’s modulus of macroporous polymers. It was believed that the hierarchical arrangement of macropores facilitated the load transfer during compression, which improved its mechanical properties. The second method involved the use of a mixed surfactant and particle emulsifier system to prepare w/o HIPEs. The mixture of surfactants and particles in the emulsion produced synergistic effects which resulted in a hierarchical macroporous arrangement after polymerisation. The hierarchical porous materials prepared using this method showed high gas permeabilities while maintaining high crush strengths and Young’s moduli compared to ‘conventional’ poly(merised)HIPEs. The improvement in mechanical strength despite the high interconnectivity was attributed to the efficient packing of macropores in a hierarchical configuration. The third approach was to mechanically-froth viscous air in w/o emulsion templates. A bio-based monomer, acrylated epoxidised soyabean oil (AESO) was chosen as a component of the continuous phase of the emulsion for its high viscosity and ability to trap air bubbles during mechanical frothing. Medium Internal Phase Emulsions (MIPEs) containing varying concentrations of AESO were mechanically frothed to incorporate air bubbles, prior to polymerisation. This was found to generate a multi-modal distribution of droplets and air bubbles which polymerised into hierarchical foams with high porosities of up to 81%. In the second section of the thesis, a hierarchical particulate network within the polymer matrix of a porous material was produced using Pickering HIPEs stabilised by varying the concentrations of thermally reduced graphene oxide (rGO) flakes. Macroporous nanocomposites containing 0.006 vol.% of rGO had a conductivity of 1.2 10-5 Sm-1, demonstrating the presence of an interconnected, conducting rGO network within the polymer matrix. The rGO-network created an additional level of hierarchy in these macroporous materials which also improved the overall mechanical properties (viscoelastic properties, Young’s modulus and crush strength).