Photopolymerization and Characterization of Crosslinked Thiol-ene Networks and POSS/Thiol-ene Nanocomposites
Interests in the area of thiol-ene photopolymerization are rapidly expanding due to the numerous advantages over the polymers produced by traditional solvent based polymerizations. Although current research of photoinitiated thiol-ene polymerization is diverse, numerous opportunities are available for investigating structure/property and structure/reactivity relationships and novel material applications. This research in this dissertation includes a fundamental study of the effect of monomeric thiol functionality on thiol-ene polymerization kinetics and formation of the thiol-ene network structure, and an investigation of the development of novel silicate based thiol-ene nanocomposites. The first fundamental study investigates the effect of thiol functionality on the kinetics and ensuing network structure. More specifically, the influence of thiol functionality on the polymerization rate and the thermal and mechanical behavior is described. Novel multifunctional thiol monomers having functionalities, f = 2, 5.6, 8.1, and 11, were synthesized via an amine-catalyzed thiol Michael addition reaction. High conversions of functional groups and marginal changes in thermal and mechanical properties for highly functional thiol monomers (f > 6) suggest that delayed gelation occurs, resulting in a polymer network with reduced effective crosslinked density. Also, thiol functionality has a marginal effect on polymerization rates. The development of a novel silicate based thiol-ene nanocomposite involves an investigation of the changes in the network structure that occurs upon the inclusion of organically modified silicate nanoparticles. As for all nanocomposite materials, the prevention of aggregation is a challenge and is addressed by improving compatibility and optimizing concentration of the silicate particle within the polymer matrix. A fundamental study examines the effect of compatibility and method of incorporation (physical or chemical) of the silicate particle on the morphology and subsequent thermal, mechanical, and physical behavior by varying the type of organic substituents on the caged silicate particle and the molar concentration of silicate particle within the thiol-ene matrix. In all cases, POSS whether incorporated chemically or physically in the network reduces flame spread. Results show that compatibilization of the silicate particles has a great influence on the thermal and physical properties of the network. The influence of silicate particle inclusion is examined by analyzing thermal, mechanical, and physical properties, including enthalpic relaxation. When incorporated chemically into the network with no aggregation, POSS does not alter the thermal transitions, physical properties or mechanical transitions significantly. If hydrogen bonding chemical groups are attached to POSS, an increase in thermal and mechanical transitions as well as modulus in the rubbery region occurs. The interaction of incorporated POSS within the crosslinked thiol-ene polymer was also characterized by the changes in free volume within the network structure. This investigation includes direct analysis of free volume by positron annihilation lifetime spectroscopy (PALS) and oxygen flux measurements. Results indicate that if POSS can be connected chemically into the polymer matrix with little or no aggregation the free volume is unaffected. In addition, the oxygen permeability is unaffected by the presence of POSS whether or not it is incorporated into the thiol-ene network.