Probing Dynamics of Photo-Gelation in Polymeric Systems Using Rheology

Light-mediated crosslinked polymers and gels are being considered for a wide range of applications including 3D printing, coatings, photolithography and drug development. Understanding the mechanism of gelation, when and if the system forms a critical gel, 'dark' curing and competition between polymerization and crosslinking are crucial to the design of photo-cured polymer materials. In this presentation, we elucidate such characteristics using three systems of interest, acrylate-based free-radical crosslinking polymers, photo-active ion-containing biopolymers and photo-polymerizing coordinated ionic liquids. The critical gel point, as envisaged by the Winter-Chambon criteria is clearly demarcated in the acrylate system, but not so much in the ion-containing biopolymer. An alternative approach based on reduction in linear strain limit as the sample approaches gel point is proposed in this regard. We find that all these systems exhibit 'dark' curing, i.e., changes in elastic (G'') and viscous (G'') moduli after cessation of UV radiation. The traditional acrylate system exhibits such behavior only past the gel point with trapped free radicals furthering network formation and modulus increase only when exposed to additional deformation. In contrast, the ion-containing polymer system shows dark curing at all states as the diffusion of the ions which leads to gelation, occurs with and without radiation. Finally, the coordinated ionic liquid system containing vinyl monomers and salts simultaneously undergoes both polymerization and network formation. The ratio of monomer to salt dictates its rheological behavior - so one could potentially obtain systems with high G' that could either be a gel or a viscoelastic fluid. These examples illustrate that while UV crosslinkable systems have some commonality (e.g., modulus enhancement, network formation, dark cure), each is unique with its own mechanism, making them such exciting systems to probe.