Photopolymerization and Characterization of Thiol-enes and Thiourethanes

Compared to conventional acrylic monomer systems, thiol-ene photopolymerization, an efficient click process leading to the formation of dense and uniform molecular networks, has several distinct advantages including relative insensitivity to oxygen inhibition, high monomer conversion and low shrinkage. Although there has been a revival of interest in thiol-enes in the past 6 years, the structure-property relationship of these networks has not been explored in detail. For future applications, thiol-enes with higher shelf-life stability and glass transition temperatures need to be used. Sulfur containing urethanes, thiourethanes and dithiourethanes are a class of widely used materials due to their distinct properties such as high refractive index. However, the structure-property relationships of thiourethanes and dithiourethanes have not been explored in detail. This research provides a fundamental study of the photopolymerization and properties of thiol-enes and the structure-property relationships of sulfur containing urethanes. The effect of chemical structure of thiol-ene monomers on physical and mechanical properties, the photopolymerization of thiol-ene free-radical/ene cationic hybrid systems, the development and characterization of thiourethane thiol-ene networks, the effect of hydrogen bonding on the physical aging of thiourethane thiol-ene networks, the investigation of hydrogen bonding behavior of thiourethanes and dithiourethanes and the structure-property relationships are all presented. The first study deals with the photopolymerization and characterization of four different types of ene monomers with both primary and secondary multifunctional thiols. The results indicate that ene structures can significantly affect the rigidity and the physical and mechanical properties of the thiol-ene networks. Network density controlled by the functionality of ene monomers was found also to be a major factor in defining network properties. Networks formed from the secondary thiol-ene systems are basically equivalent to those made from primary thiol-enes with respect to physical mechanical and optical properties. The secondary thiol monomer samples evaluated were found to have excellent storage stability and relatively low odor. The second study reports the photopolymerization kinetics of mixtures containing a trithiol and a trivinyl ether (in different molar ratios) with a cationic photoinitiator. Using the combination of real-time FTIR and rheology to follow both chemical conversion and Theological property development, a clear picture of physical property development during the complete polymerization process is obtained. This represents the first example of a thiol-ene radical/ene cationic two-step hybrid photopolymerization process in which thiol copolymerizes with vinyl ether functional groups in a rapid radical step growth process followed by vinyl ether cationic homopolymerization. The sequential thiol-vinyl ether copolymerization and the vinyl ether cationic polymerization result in crosslinked networks with thermal and mechanical properties that are combinations of each system. The third study concentrates on the development of novel thiourethane based thiol-ene (TUTE) films prepared from diisocyanates, tetrafunctional thiols and trienes. The incorporation of thiourethane linkages into the thiol-ene networks results in TUTE films with high glass transition temperatures. Increase of Tg was achieved by aging at room temperature and annealing the UV cured films at 85 °C. The aged/annealed film with thiol prepared from isophorone diisocyanate and cured with a 10,080 mJ/cm radiant exposure had the highest DMA based glass transition temperature (108 °C) and a tan 8 peak with a full width at half maximum (FWHM) of 22 °C, indicating a very uniform matrix structure. All of the initially prepared TUTE films exhibited good physical and mechanical properties based on pencil hardness, pendulum hardness, impact and bending tests. The physical aging behavior of a class of photopolymerized thiourethane thiol-ene networks were characterized by thermal and spectroscopic analysis, the results of which are directly related to changes in macroscopic physical and mechanical properties. The hydrogen bonding associated with the thiourethane chemical structure exerts at most a slight retarding effect on the enthalpy relaxation, but there is a significant increase in the glass transition temperature of the thiourethane thiol-ene networks, an important implication for application of these materials and the stabilization of their physical, mechanical and thermal transition properties. To define the difference between ordinary urethanes and thiourethanes, the hydrogen bonding behavior of a homologous family of model urethane, thiourethane and dithiourethane compounds prepared from primary isocyanates/isothiocyanates were investigated in solution, melt and solid states. The relative strengths of hydrogen bonds in these systems were evaluated, and the results compared to theoretical calculations of hydrogen bonding strength. The polyurethane and polythiourethane were found to have approximately equivalent physical and mechanical properties as a result of a similar extent of hydrogen bonding, whereas the polydithiourethane model compound, due to a lower degree of hydrogen bonding, has reduced thermal and mechanical transition temperatures as well as lower hardness values. The polythiourethane and polydithiourethane networks exhibit narrower glass transitions compared to polyurethane networks apparently the result of an efficient isocyanate/isothiocyanate-thiol reaction with little or no side products. Due to weakness of the C-S bond compared to the C-0 bond, thiourethanes and dithiourethanes have lower thermal stability than corresponding urethanes. Finally the thiourethanes and dithiourethane have higher refractive index values than their urethane counterparts. To complete the comparison study on urethane type materials, another homologous family of model urethane, thiourethane and dithiourethane prepared from both aliphatic and aromatic secondary isocyanates were comprehensively characterized by a series of spectroscopic, thermal, physical and mechanical analysis measurements to define the relative hydrogen bond strength and its correlation with properties. The polyurethane and polythiourethane systems have similar physical and mechanical properties as a result of their similar structures and hydrogen bonding behavior, whereas the polydithiourethane, due to relatively weaker hydrogen bonding has reduced physical properties. The NMR, FTIR and XRD measurements of small molecule models in solution, melt and solid states indicate the relative hydrogen bonding strength as: urethane ~ thiourethane \u3e dithiourethane. The aromatic urethane is more stable under UV irradiation than the corresponding thiourethane analogues. Due to the weaker C-S bond compared to C-0 bond, thiourethane and dithiourethane have reduced thermal stability compared to their urethane counterpart. Similar Tg values observed for polyurethane and polythiourethane systems are higher than those for the polydithiourethane, consistent with the lower hydrogen bonding in the latter