Photopolymerization is one of the most rapidly expanding processes for materials production and is employed over a wide range of fields. Currently, the field of free radical photopolymerization is dominated by acrylic and methacrylic systems. However, the applicability and utility of these traditional systems is severely limited due to several disadvantages, including, inhibition by the presence of oxygen, significant volume shrinkage due to polymerization induced densification, large stresses that arise from shrinkage, presence of residual unreacted monomer, formation of heterogeneous networks, and the use of added toxic photoinitiator molecules. Thiol-vinyl photopolymerizations, which are free radical polymerization reactions between thiol (-SH) containing monomers and vinyl (-C=C) containing monomer, represents a fundamental shift in the mechanism of polymerization in which the reaction proceeds primarily by chain transfer and propagation reactions. The unique reaction mechanism of thiol-vinyl systems along with its chemical versatility potentially alleviates all the current limitations and drawbacks of traditional chain growth photopolymerization systems.
In this work, we exploited the unique properties, superior polymerization kinetics, and controlled network evolution of thiol-vinyl systems to form materials for several applications: formation of degradable networks with tunable degradation characteristics, design of biomaterial scaffolds for tissue engineering applications, formation of reactive micro and nano patterned structures and their surface modification, and fabrication of complex polymer derived ceramics.
Biomaterials: For applications ranging from drug delivery to tissue engineering, tunable biomaterials are needed. These materials should be readily tailored to exhibit desired mechanical properties, degradation rates, and transport of entrapped molecules. The ability of thiol-vinyl systems to form biocompatible degradable networks with defined mechanical properties will be presented. The network properties of these biomaterial scaffolds are manipulated by changing the monomer chemistry, their polymerization mechanism (from step-growth to mixed step-chain growth), and the monomer functionality. The network control aspects of these biomaterial scaffolds will also be utilized to investigate and regulate the impact of mechanical and biomolecular stimuli on tissue development.
Lithographically controlled materials and Surface Modification: Binary and Ternary Thiol-vinyl systems with decreased shrinkage stresses and high glass transition temperatures were designed based on the understanding of network evolution aspects of these systems. The low shrinkage stresses in these systems allowed us to rapidly form micro (conventional photolithography) and nano (step and flash imprint lithography) structured materials with high aspect ratios and superior material properties. Polymerization of these unique systems in presence of dithiocarbamate iniferters further facilitated the formation of reactive structures that are readily surface modified. A wide range of surface modifications, from hydrophilic to hydrophobic, was obtained utilizing the iniferter mediated living radical polymerization. The chemical versatility and superior cure kinetics of thiol-vinyl systems were also utilized to develop a new technology for forming polymer derived ceramics. Here, the thiol monomers were reacted with vinyl silazane monomers to form polymeric precursors for polymer derived ceramics. The high degree of reaction and the low stress resulting from this reaction enabled thicker ceramic devices to be formed without warping or cracking as compared to the standard polymerizations in the absence of thiol.