Preparation and Characterization of Thiol Based Polymeric Materials

The thiol click-type reaction is one of the most efficient chemical processes that can undergo at room temperature in the benign organic solvent providing excellent opportunity in the fabrication of new materials as well as development of new manufacturing processes. Moreover, the versatility of thiol click-type reactions with many different functional groups such as ene, yne, epoxy, isocyanate, and alkyl halide is the important aspect that should be differentiated from the classic azide-alkyne click chemistry. Although, there are tremendous studies on many areas using thiol click-type reactions, the efficiency and utilities for the material applications have been barely explored. Especially, the combination of different types of thiol click reactions can provide new synthetic methodologies for the preparation of thiol based materials and much more choice in designing chemical structures and thus tenability of physical and mechanical properties. In addition to the advantages of thiol click-type reactions in the chemical reaction point of view, the uniform and homogeneous network structure resulting from the unique reaction mechanism presents an excellent platform to investigate the effects of the fundamental molecular parameters on physical and mechanical properties of thiol based networks. Thus, in this dissertation, three major thiol click-type reactions, i.e. thiol-electron rich double bonds by free radical, thiolelectron poor double bonds by nucleophile catalyzed hetero-Michael addition, and tertiary amine catalyzed thiol-isocyanate nucleophilic coupling reactions, were studied in terms of the efficiency of thiol click-type reactions for the fabrication of materials and their physical / mechanical properties. Also, sub-rg aging of thiol-ene photopolymerized network films monitored by enthalpy relaxation was investigated in terms of fundamental molecular parameters such as network density, rigidity, uniformity, and polar / non-polar side chains. In Chapter III, highly elastic segmented polythiourethanes linear polymers were synthesized through sequential thiol click-type reactions involving the phosphine catalyzed thiol-acrylate hetero-Michael addition and triethylamine catalyzed thiolisocyanate nucleophilic coupling reactions. Real-time FTIR and NMR showed that both the thiol-acrylate hetero-Michael addition and the thiol-isocyanate coupling reactions are very fast and efficient with no side products. Physical and mechanical properties investigated by DSC, DMA, and tensile strength measurements showed that polythiourethanes are highly elastic materials due to the micro-phase separated soft and hard segments. In Chapter IV, thiol-ene-isocyanate ternary networks were studied as a new approach to modify classic thiol-ene networks by incorporating thiourethane linkages through sequential and simultaneous two thiol click-type reactions. The thiol-isocyanate coupling reaction providing strong hydrogen bonding in thiol-ene networks was triggered thermally and photolytically in order to control the sequence with the thiol-ene photopolymerization. The kinetics of the ternary networks was investigated for both sequential and simultaneous processes. The relationships between the chemical composition and physical/mechanical properties of thiourethane-thiolene hybrid networks were also established. The identical thermal properties that are independent of the reaction sequence were obtained, indicating that highly uniform and dense network structure of thiol based networks was not affected by the reaction sequence change. In Chapter V-VIII, sub-Tg aging behavior of thiol-ene networks measured by the extent of enthalpy relaxation was extensively investigated. The fundamental enthalpy relaxation study on thiol-ene networks was accomplished in Chapter V. The highly dense and uniform network structure of the thiol-ene networks, exhibiting narrow glass transition temperature ranges, showed characteristic temperature and time dependency relationships for enthalpy relaxation. The extent of enthalpy relaxation was correlated with thiol-ene network density and chemical group rigidity. Mechanical property measured by pendulum hardness values for a selected thiol-ene film showed a clear change in hardness upon aging indicating sub-Tg mechanical relaxation, consistent with the related enthalpy relaxation process. In Chapter VI, the effect of chemical modification of thiol-ene networks on enthalpy relaxation was investigated as an unprecedented methodology to control sub- Tg aging process. Flexible alkyl side chains and hydrogen bonding were incorporated into thiol-ene networks without sacrificing network uniformity using the phosphine catalyzed Michael addition reaction. Overall both the rate and extent of enthalpy relaxation slightly decreased as a function of the flexible n-alkyl chain length, while hydrogen bonding resulted in enhanced enthalpy relaxation. A trifunctional acrylate (TMPTA), being capable of homopolymerization as well as thiol-acrylate copolymerization, was incorporated into a thiol-ene network structure, to investigate the effect of network uniformity on enthalpy relaxation. Inhomogeneous TMPTA homopolymer domains disrupted the uniformity of thiol-ene networks, thereby, glass transition temperature and enthalpy relaxation were significantly affected. In all cases, the clear relationships between the extent of enthalpy relaxation and chemical structural effects were established. In Chapter VII, as an extended study of the modification of thiol-ene networks in order to control sub-7g aging process described in Chapter VI, the degree of restriction effect of the rigid TMPTA homopolymer domains and gold nanoparticles on thiol-ene networks was quantitatively determined by calculating the apparent activation energy (Ah*) for enthalpy relaxation. The incorporation of TMPTA homopolymer and gold nanoparticles into the thiol-ene network increased Tg and decreased the extent of enthalpy relaxation due to molecular mobility restrictions. In addition, the apparent activation energy for enthalpy relaxation (Ah*) obtained by the differential cooling rate experiments on DSC clearly indicated significant restriction effects of the TMPTA hard domains and gold nanoparticles on the molecular mobility in the thiol-ene network. Finally, in Chapter VIII, 10- and 32-layered thiol-ene based films with different components were fabricated to investigate sub-^g aging of multi-layered thiol-ene network films. The distinctive glass transition temperatures of each component were observed at corresponding transition regions of each bulk sample. Enthalpy relaxation of each layer component occurred independently and showed overlapped unsymmetrical bell shaped enthalpy relaxation distribution having peak maximum at Tg-10 °C of each layer component, resulting in wide distribution of enthalpy relaxation over wide temperature range. Enthalpy relaxation of each layer component in the multi-layered thiol-ene films was significantly accelerated compared to that of bulk thiol-ene networks