Effect of comonomer type and concentration on the gelation and viscoelasticity of photocurable poly(ethylene glycol) diacrylate networks

We use rheological tools to investigate the gelation kinetics and mechanical properties of polymer networks produced by the bulk photopolymerization of mixtures of poly(ethylene glycol) diacrylate (PEGDA) ( M n = 700 g/mol) with two different comonomers, 2-Hydroxyethyl acrylate (HEA) and poly(ethylene glycol) methacrylate (PEGMA) ( M n = 360 g/mol). Irgacure 2959 at a concentration of 1 wt. % is used as a photoinitiator. The molar ratio of comonomer and PEGDA is varied between 200 and 0.5. The mixtures are loaded in a rheometer equipped with a homemade photocuring device, which allows for measuring the viscoelastic properties during the bulk polymerization process. With this method, the effect of both comonomer type and concentration on the gelation kinetics and viscoelastic properties of the resulting networks is investigated. For both HEA and PEGMA, the increase in comonomer concentration leads to a slower gelation and a decrease in elasticity of the network. The limited swelling ability of the photopolymerized materials in water suggests a high spatial heterogeneity of the crosslinking density. At a fixed mixing ratio, the incorporation of HEA in the network results in slower gelation kinetics and a softer material compared to PEGMA. The evolution of the loss modulus during the photopolymerization suggests that the addition of comonomer reduces the number density of dangling chains in the intermediate states of gelation.