Photopolymerizations of multifunctional monomers: Reaction mechanisms and polymer structural evolution.

摘要

Photopolymerizations of multifunctional monomers provide an excellent method for the fast curing of liquid monomers at room temperature into highly crosslinked, glassy polymer networks. The high crosslinking density of the polymer combined with the rapid curing rate and spatial control of the photopolymerization has led to a wide array of applications. However, the polymerization behavior and the structure of the resulting polymer network are extremely complex and difficult to characterize. The reaction kinetics and mechanisms of multifunctional monomer polymerizations were investigated with differential scanning calorimetry and infrared spectroscopy. In particular, reaction diffusion controlled termination was examined and quantified in terms of a reaction diffusion constant. The influence of reaction temperature, solvent concentration, and monomer type and functionality on the reaction diffusion parameter was also investigated.;In addition to these kinetic studies, several structural features of the polymer network and its evolution were characterized. In this regard, a photochromic technique was developed to measure the free volume distribution throughout the polymerization. With this technique, the presence of microgels at the early stage of the polymerization was clearly identified, and the influence of the polymerization rate or volume relaxation on the system's free volume was characterized. Electron spin resonance spectroscopy was also used to determine the concentration and environment of radicals during the polymerization. Radial species were identified as trapped or free, and the importance of radical trapping as a unimolecular termination mechanism was quantified.;Finally, the polymerization reaction and the structural evolution of the network were modeled using two different approaches. A kinetic model was developed to predict the effects of reaction conditions on the polymerization kinetics and the maximum attainable double bond conversion. A kinetic gelation model was modified to predict features of the polymer structural evolution such as crosslinking versus cyclization tendencies, relative reactivity of pendant functional groups, trapped radical fractions, and species aggregation.