Design and analysis of dual-curing systems

Resumen

Dual-curing processing is a method to prepare thermoset materials through two polymerization reactions carried out simultaneously or sequentially. In these processes, a firm understanding of reaction mechanisms enables the design of catalytic systems to control reaction kinetics and to impart sequentiality to the curing reactions. Material properties at different curing stages are dictated by the choice of monomers. Furthermore, by employing click-based approaches, eco-friendly and efficient dual-curing processes can be designed. In this thesis, a number of sequential dual-curing systems were prepared and characterized. Sequentiality was either intrinsic due to the monomers used or it was achieved by employing latent catalysts. The dual-curing systems were designed with an objective of improved physical and mechanical properties of the fully cured materials. The majority of the monomers were processable through click reactions, although a few processes did not strictly fit click criteria. In terms of the characterized properties, this did not pose any shortcoming. Due to the limited number of related publications, the thiol-epoxy reaction was investigated in more detail. Accurate phenomenological and mechanistic models of reaction kinetics were developed to study reaction kinetics in and out of dual- curing context. For reactive latency, a new family of photobase generators (PBGs) were developed. As the name implies, these PBGs liberated base catalysts upon UV irradiation. The possibility of thermal initiation of some of these PBGs was also demonstrated. Storage stabilities of uncured and partially-cured (i.e. intermediate) materials were significantly improved since PBGs allowed temporal control over curing stages. In some dual-curing systems, step-wise click polymerizations such as Michael additions were combined with chain-wise homopolymerizations such as acrylate photopolymerizations. In these systems, the initial step-growth proces delivered intermediate materials with desirable properties such as polymer network homogeneity, high gel point conversion, and low polymerization shrinkage. The chain-wise process was carried out as a second curing stage, at the end of which final materials were obtained with increased crosslinking density, hardness and Tg. In all dual-curing systems presented here, final materials had significantly improved properties compared to intermediate materials, regardless of the nature of the curing processes. In one part of the project, a new set of catalyst comonomers were designed. These comonomers, which were also prepared using click-based procedures, had pendant allyl functionalities and wielded tertiary amine groups in their structure. The tertiary amines catalyzed a thiol-acrylate reaction carried out as a first curing stage in a dual-curing system. Later, as a second curing stage, the pendant allyl groups of the comonomers participated in thiol-ene polymerizations with the excess thiols initially present in the formulation, thereby getting incorporated into the final polymer network. The dual-curable materials developed here can be used in diverse applications ranging from high-performance adhesives, to rigid shape-memory materials. As a matter of fact, a preliminary demonstration of these two applications is provided. Prospectively, the materials presented here could benefit from a more detailed characterization in the context of specific applications. Without a doubt, such an effort would increase the possibility of successful commercialization of these formulations.