Impact of topology on the degradation and mechanical properties of crosslinked polymer networks synthesised by radical polymerisation

Abstract

This thesis explores the relationships between the topology of crosslinked polymer networks and their physical properties such as swelling, elastic modulus and degradability. By using different radical polymerisation techniques (free radical polymerisation (FRP) and reversible deactivation polymerisation techniques (RDRP)), the gelation behaviour of various model polymer systems was investigated.
In Chapter 1, a general introduction to polymer networks and their topology is presented, as well as an overview of the radical polymerisation techniques used in this work.
In Chapter 2, a study on the effect of polymer dispersity on the gelation behaviour of a model poly (methyl acrylate) (PMA) gel network was performed, using parallel kinetic reactions of linear and crosslinked systems. By using photoinduced atom-transfer radical polymerisation (ATRP), the dispersity of primary chains could be tuned by varying the loading of Cu catalyst. Theoretical gel points were then calculated, using the measure dispersities and initiator efficiency from the non-crosslinked parallel reactions. By comparing these gel points to those calculated from a published equation derived from simulations, an empirical modification to the F-S equation for ATRP was implemented, resulting in more accurate predictions of the gel point. Furthermore, the mechanical properties of the networks, such as equilibrium swelling ratio and shear storage modulus, were studied for gels with different primary chain dispersities and crosslinking density,
In Chapter 3, chemically degradable poly (butyl acrylate) gels were produced by incorporation of a labile disulfide crosslinker into the polymerisation. By cleavage of the disulfide bond via thiol-disulfide exchange, these networks should degrade into soluble fragments allowing for further reprocessing. Reversible addition–fragmentation chain transfer (RAFT) polymerisation and conventional FRP was used to synthesise gels with contrasting topologies. Reports of degradable networks by FRP in the literature are inconsistent, so polymerisations of other monomer classes were also tested, and a link between polymerisation kinetics, topology and degradability is presented. Other methods of inducing degradation in gels made by FRP are also investigated, as well as the effect of using the degradable crosslinker on the mechanical and thermal properties of the gels.
In Chapter 4, an alternative approach to synthesising chemically degradable networks is studied. By using a cleavable comonomer, dibenzo[c,e]oxepine-5(7H)-thione (DOT), incorporation of labile thioester bonds within the polymer backbone can be achieved. The use of a cleavable comonomer rather than a cleavable crosslinker is compared, and the minimum amount of DOT required to induce degradation is also found. The structure of the degraded fragments are analyses in relation to their ability to reform solid networks by air oxidation of thiol groups.
In Chapter 5, another cleavable comonomer lipoic acid, and its ester derivative ethyl lipoate were also tested for their ability to produce degradable poly(butyl acrylate) gels. The minimum comonomer loading to induce degradation in gels synthesise by RAFT and FRP were found, as well as the effect of comonomer addition on the polymerisation kinetics and mechanical properties of the networks.
Finally, Chapter 6 presents the conclusions of this work, and discusses possible directions for further work on this topic.

Date of Award	23 Jul 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Maciek Kopec (Supervisor), Adam Squires (Supervisor), Antoine Buchard (Supervisor) & K Edler (Supervisor)