Investigation of Porous Polymer Particle Shells Synthesised by RAFT-PISA with pH- and Thermo- Response

Microporous organic polymers (MOPs) with high specific surface areas, low skeletal density, good chemical & thermal stability and ease of functionality are of interest for a range of applications such as gas storage, capture and separations, energy storage, catalysis as well as drug delivery, and chemosensing. However, one of the greatest challenges for the industrial use of MOPs is their lack of solubility limiting their applications to the solid state. This is primarily due to the highly rigid and cross-linked structure required to induce porosity. One class of MOP has however found commercial success due to their solubility in a range of organic solvents - PIMs which achieve the combination of high surface areas and solubility by using rigid and twisted linear polymer chains. There are however a limited number of monomers which can be used to synthesise PIMs. To overcome this limitation, a number of strategies have been used to make network MOPs both highly porous and dispersible. Based on previous research in our group, the metal-free reversible addition fragmentation transfer polymerisation induced self-assembly (RAFT-PISA) was used as a strategy to form core-shell structures to improve the solubility of the MOPs. To date only PEG based solubilising chains have been reported, and this thesis therefore investigates the use of different solubilising shell polymer chains including poly acrylic acid (PAA), poly(diethylaminoethyl methacrylate) (PDEAEMA), poly(4-vinyl pyridine) (P4-VP) and poly(N-Isopropylacrylamide).
In Chapter 3, PAA is first investigated as the hydrophilic shell to determine the best reaction conditions to maximise the BET surface area. BET surface areas of up to 322 m2 g-1 and obtained for PAA based particles. Chapter 4 looks at the use of pH- and thermo-responsive PDEAEMA and P(4-VP) based particles with BET surface areas up to 101 m2 g-1 and 133 m2 g 1 respectively. It is shown that these particles exhibit switchable pH-responsive properties. At low pH small particle sizes (~200 nm) are observed while at high pH the particles increase to ~800 nm due to aggregation. Chapter 5 discusses the thermo-response of the particles including above particles and PNIPAm based particles with surface areas of 193 m2 g-1. At low temperatures the particle sizes are big and form aggregation and precipitated out at high temperatures as water is expelled from the particles. Finally, methods to improve the BET surface areas are investigated in Chapter 6. These include the mixing of chain lengths, the addition of anionic shells to cationic particles and the use of non-ionic higher surface area inducing shells. The surface areas of P4 VP are able to be increased by up to 5 times by the addition of PEG shells while still maintaining both pH and thermo-responsive behaviour.
In a summary, the reaction conditions of core-shell structure dispersible porous polymer particles are mapped out, which the particles with a small core and short shell length have a higher BET surface area. Moreover, the stimuli responsive shell chains with pH- and thermos- responsive, which are successfully introduced into the particles, which have small particle sizes at acid condition or low temperature, and large particle size at base condition or high temperature. The adding of anionic shells to cationic particles and the use of non-ionic shells can indeed increase surface area of single shell-chain particles.