Photopolymerization and characterization of thiol-enes

Thiol-ene monomers are a unique class of UV curable materials due to a step-growth free radical polymerization mechanism. Photopolymerization of thiol-ene monomers exhibits several distinct advantages over traditional acrylates including fast curing rate with high monomer conversion, uniform crosslinked network formation, and low oxygen inhibition with low shrinkage rates. Although there is increased research interest in thiol-ene photopolymerizations, structure-property relationships of these monomers has not been explored in detail. This research is a fundamental study of the mechanical properties of thiol-ene photopolymers such as glass transition temperatures, impact absorption, crosslink density and fracture toughness. The dissertation includes several chapters dealing with the effect of low molecular weight liquid diluent (liquid crystals) on the glass transition temperature and the electro-optic properties of thiol-ene based holographic polymer dispersed liquid crystals, the development of a three component visible photoinitiator, the investigation of thermal and mechanical properties of thiol-ene/acrylate based photopolymers, and the optimization of impact absorption and fracture toughness of thiol-ene polymers.;The first fundamental study deals with a special application area of thiol-ene polymers; holographic polymer dispersed liquid crystals (H-PDLC). Traditional trifunctional thiol-ene monomers are polymerized in the presence of low molecular weight liquid crystal molecules and polymerization kinetics are monitored to elucidate the effect of liquid crystal (LC) molecules. Also, the effect of the glass transition temperature (Tg) of thiol-ene networks on the electro-optic properties of H-PDLC films is studied as a function of liquid crystal concentration. Results demonstrate the effect of the liquid crystal concentration and the chemical structure of the ene monomer on the glass transition temperature and thus the electro-optic properties of the H-PDLC films formed from thiol-ene monomers.;The second study reports the photopolymerization efficiency of a visible initiator package with thiol-ene and acrylate monomers. This work is an extension of the three component ketocoumarin/maleimide/amine UV initiator system into the visible region. It is shown that a two component ketocoumarin/tertiary amine photoinitiator can be used to photopolymerize thiol-ene monomers with blue and cyan LED light sources.;Thiol-ene polymers are known to form very homogenous crosslinked network structures with low Tg due to the uniformity of crosslinks and the flexibility of the thioether structures in the polymer backbones. In order to increase the Tg of thiol-ene matrices, acrylate monomers with different concentrations are added into thiol-ene mixtures. The third study is devoted to the characterization of thermal and mechanical properties of ternary thiol-ene/acrylate systems. The functionality and chemical structure of the acrylate monomer are found to be the controlling parameter in properties of the final network structures such as crosslink density, free volume, glass transition temperature, impact absorption and fracture toughness. It is shown that bisphenol-A based difunctional acrylate monomers provide improved impact absorption at room temperature. Results from this work are used to improve fracture toughness and impact absorption properties of thiol-ene networks.;Thermoset polymers tend to suffer from brittle fracture under applied impact forces due to the high crosslinking densities and tightly knit network structures which act as stress concentrators in the network structure. Likewise, thiol-ene based polymers produced with off the shelf chemicals exhibit low elongations at break, insufficient fracture toughness, and poor tear strengths. By utilizing urethane chemistry, bisphenol-A structure is incorporated to the multifunctional ene monomers. These monomers are subsequently photopolymerized with a trifunctional thiol monomer. The thiol-urethane ene based thin films demonstrate fast curing rates with excellent coating properties, whereas the thick plates exhibit improved fracture toughness, high elongation at break, and high impact absorption at room temperature. The improved properties of thiolurethane ene networks are attributed to the advantages of urethane chemistry such as the hydrogen bonding ability of the monomers.