Photo-reactive nanogels as a means to tune properties during polymer network formation

Photo-reactive nanogels with an integrated photoinitiator-based functionality were synthesized via a Reversible Addition-Fragmentation chain Transfer (RAFT) process. Without additional free initiators, these nanogels are capable of radical generation and initiating polymerization of a secondary monomer (i.e. dimethacrylate) that infiltrates and disperses the nanogel particles. Due to the presence of a RAFT functionality and the fact that all initiating sites are initially located within the nanogel structure, gelation can be delayed by sequencing the polymerization from the nanogel to the bulk matrix. During polymerization of a nanogel-filled resin system, a progressive delay of gelation conversion from about 2% for conventional chain growth polymerization to 18% for the same monomer containing 20 wt% nanogel additive was achieved. A significant delay of stress development was also observed with much lower final stress achieved with the nanogel-modified systems due to the change in network formation mechanics. Compared with the nanogel-free dimethacrylate control, which contained a uniformly distributed free initiator, the flexural modulus and mechanical strength results were maintained for the photopolymers with nanogel contents greater than 10 wt%. There appears to be a critical interparticle spacing for the photo-reactive nanogel that provides effective photopolymerization while providing delayed gelation and substantial stress reduction.