Crosslinked network synthesis by radical chain photopolymerization: Natural and directed evolution of macromolecular structure.

摘要

Fundamental understanding of natural structural evolution and strategies to direct bulk and surface macromolecular properties of highly crosslinked networks will facilitate application of multifunctional monomer photopolymerizations to emerging and novel technologies. The initial, naturally-forming, macromolecular structure impacts final network properties; therefore, an experimental methodology, a simulation approach, and selected results are presented from investigations of network structural evolution during multifunctional monomer polymerizations at low conversion. In particular, intramolecularly crosslinked macromolecules (ICMs), formed from individual initiation events, are characterized with atomic force microscopy, size exclusion chromatography, dynamic light scattering, and NMR. Furthermore, an off-lattice kinetic gelation model is introduced and applied to exploration of network structural evolution. Specifically, simulations of ICM formation (i.e., propagation of a single radical within reaction volume containing ca. 100,000 divinyl monomer spheres) provide analogous characterization information to the experimental techniques (e.g., ICM size, kinetic chain lengths, and intramolecular crosslink density). Most importantly, the combination of simulations and experiments provides insight into natural structural evolution from complementary and/or validating characterizations of network formation at low conversion during polymerizations of various multifunctional monomers.; In addition to advances derived from better understanding of natural structural evolution, applications of multifunctional monomer polymerizations will benefit from development of strategies to direct surface and bulk macromolecular properties. For example, a novel technology for fabricating three-dimensional polymeric microdevices from multifunctional monomers is introduced. To facilitate spatially-resolved surface modification or functionalization of bulk networks within these microdevices, fundamental studies into the integration and activation of living radical initiators on the surfaces or throughout the bulk of crosslinked networks are presented. In particular, photoiniferter moieties are incorporated within highly crosslinked networks formed from multifunctional monomer polymerizations. Subsequent activation (i.e., by UV exposure) in the presence of additional monomers, facilitates synthesis of grafted films and/or covalently bound crosslinked layers. Furthermore, thermally-activated living radical initiators are integrated within loosely crosslinked networks. Upon swelling in additional monomer and subsequent activation, these systems produce highly specific, functional gels for chemical or biological reactions, detection schemes, and/or separations.