The role of chain length dependent kinetics on observed non-classical multivinyl photopolymerization behavior.

摘要

Multivinyl monomers photopolymerize to produce highly crosslinked networks with exceptional material properties that afford increased strength, toughness, and chemical resistance over linear polymers. The photopolymerization reaction occurs under ambient conditions, does not require a solvent, is rapid when compared to thermal polymerizations, and provides spatial and temporal control. Thus, photopolymer networks are used to fabricate materials for numerous applications including microelectronics, contact lenses, dental restorations, adhesives, biomaterials, and coatings. However, the complex photopolymerization reaction hinders the complete exploitation of flexible monomer chemistry, monomer functionality, and curing conditions to tailor the resulting polymer material properties for emerging applications.;Under typical photopolymerization conditions, crosslinking polymerizations exhibit numerous non-classical phenomena (autoacceleration, autodeceleration, diffusion limitations, reaction diffusion controlled termination, incomplete conversion, and chain length dependent termination (CLDT)). These non-classical behaviors render network property prediction over various polymerization conditions challenging.;A kinetic model is developed that accounts for CLDT, chain transfer to either polymer (CTP) or a unimolecular species, the accumulation of persistent radicals, and the probability that a radical chain is tethered to the gel of infinite molecular weight. The model predicts qualitatively the aforementioned non-classical phenomena that typify crosslinking polymerizations and reveals that CLDT contributes to the experimentally observed non-classical polymerization rate dependence on the initiation rate (alpha 1/2) at low to moderate conversions. Additionally, model predictions and gel permeation chromatography (GPC) results for a multivinyl monomer that reacts to form highly crosslinked, biodegradable network support the importance of CLDT during network formation.;Furthermore, the different polymerization kinetics exhibited by rubbery and glassy network forming polymers are examined. Electron paramagnetic resonance (EPR) spectroscopy observations and model predictions provide evidence that multivinyl monomers exhibit chain length dependent kinetics at high double bond conversions. Also, the rubbery system, poly(ethylene glycol-600) dimethacrylate, simultaneously exhibits CLDT and reaction diffusion controlled termination at high conversions. The effects of crosslinking density, glass transition temperature, solvent quality, and non-uniform irradiation profiles on CLDT are investigated.