Development of photopolymerizable clay nanocomposites utilizing reactive dispersants

Nanocomposites hold tremendous promise for expanding the utility of polymeric materials. However, accessing particulate sizes in the nanoscale domain continues to be a scientific challenge, especially in highly cross-linked photopolymerizable systems. In this study, photopolymerizable nanocomposites utilizing clay nanoparticles and reactive dispersants have been developed. The influence of particle size, dispersant-clay interactions, and surfactant concentration on photopolymerization behavior and nanoparticle dispersion has been elucidated. Clay particles serve as templates upon which surfactants aggregate during photopolymerization. This results in higher photopolymerization rates with addition of increasing concentrations of polymerizable surfactants. Furthermore, polymerizable surfactants induce faster photopolymerization rates compared to non-polymerizable analogues in systems that have ionically-bound dispersants on the particle surface. Utilizing reactive organoclays induces significant changes to the photopolymerization behavior depending on the choice of reactive functionality employed. Faster acrylate photopolymerization rates occur in photopolymer systems containing thiol-modified clays, while much slower rates occur for nonpolymerizable organoclay systems.;In addition, chemical compatibility between monomer and clay dispersant (based on chemical similarity or polarity) allows enhancement of exfoliation in photopolymerizable formulations. With polymerizable dispersants, exfoliation is readily achieved in various multifunctional acrylate systems. The degree of exfoliation depends on the position of the reactive group relative to the surfactant's cationic site and the type of functionality. Thiolated organoclays exfoliate during polymerization, while methacrylated clays show substantially less dependence on polymerization behavior. Interestingly, changes in the physical properties of the resulting nanocomposite are independent of the degree of exfoliation in polymerizable organoclay systems. The polymer cross-link density dictates the magnitude of change in both modulus and glass transition temperature of the nanocomposite. Substantial increases in modulus and Tg occur in elastomeric and low cross-link density polymers, while decreases occur in the modulus and Tg of highly cross-linked polymer networks.;Finally, these parameters have formed a basis for developing nanocomposites with higher moduli and lower volumetric shrinkage. The photopolymerization rates of these systems are controllable and increase substantially with addition of polymerizable organoclays. Such properties occur in traditional multifunctional acrylate photopolymer systems as well as new binary thiol-(meth)acrylate and ternary thiol-ene-(meth)acrylate photopolymers.