Thermoresponsive Behaviour of PEG-Based Methacrylate Polymers: Influence of Linear and Star Architectures

Thermoresponsive polymers based on poly(ethylene glycol) (PEG) have attracted significant attention in various applications, especially in biomedical fields, because of their excellent biocompatibility and high tunability which can be controlled by varying the molecular mass (MM), architecture, and monomer composition. In this study, the monomers oligo(ethylene glycol) methyl ether methacrylate (OEGMA300, average MM = 300 g mol-1) and di(ethylene glycol) methyl ether methacrylate (DEGMA) were selected. OEGMA300, with its longer EG side chains, is more hydrophilic than the relatively hydrophobic DEGMA. Linear copolymers based on these monomers have been investigated, whereas star-shaped polymers have not, and more importantly, a direct comparison of linear and star-shaped polymers as thermogels has not previously been reported. Thus, in this investigation, a series of linear and star polymers composed of OEGMA300 and DEGMA were synthesised via group transfer polymerisation (GTP) with the concentration of the OEGMA300 content ranging from 65 wt% to 35 wt %. Star polymers of different architectures and monomer sequences were synthesised, and their thermal behaviour was investigated. Gel permeation chromatography (GPC) and proton nuclear magnetic resonance (NMR) spectroscopy were used to determine the MMs and compositions of the obtained polymers. Two important transition temperatures, cloud point temperature (Tcp) and gelation temperature (Tgel), were identified through characterisation techniques, including turbidimetry, dynamic light scattering (DLS), visual tests, and rheology. The star polymer which exhibited the best gelation behaviour was the heteroarm star containing 35 wt% OEGMA300 and 65 wt% DEGMA. VT-1H NMR was also performed to investigate the dual-transition behaviour observed in DEGMA-first diblock linear polymers.