Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Jeffrey W. Stansbury

Second Advisor

Christopher Bowman

Abstract

Multifunctional acrylic and methacrylic monomers have been widely applied in many photopolymerization applications to produce crosslinked polymers with advantages such as rapid curing, broad choices of commercially available monomers and desirable physical and mechanical properties. However, there still remain critical challenges for these materials during polymerization including limited conversion and early onset of gelation as well as the generation of significant polymerization shrinkage and stress. This thesis explores the effects of network property modification through the addition of polymeric nanoparticles or nanogels. In order to understand the relationship between nanogel structure and composite material properties, nanogels with different architectures and functionalities were studied during polymerization in terms of kinetics, shrinkage and stress reduction, mechanical performance and reaction mechanisms.

Nanogel composite formulations were evaluated to understand the interaction between nanogel structure with the resin matrix during polymerization through adjustment of nanogel branching densities and reactivity of polymer chain ends. It was found that both the chemical crosslinking from reactive chain ends and physical entanglements of high branching density nanogels with the resin matrix dramatically could improve final material mechanical strength. The reductions in overall volumetric shrinkage and shrinkage stress were found to follow at least proportional behavior with respect to nanogel loading concentration while maintaining similar final conversion and modulus results compared with the control resin.

Nanogels containing unique functionalities were designed in order to modify reaction mechanism during secondary polymerization. A nanogel containing an integrated photoinitiator and active chain-end RAFT groups was able to initiate secondary polymerization from the nanogel phase so that localized polymerization was achieved from the beginning of the reaction process to prevent early bulk gelation. A large amount of stress was dissipated before gelation to yield materials with low residual stress. With the incorporation of a photochromic functionality, another nanogel was found to be able to change dimensions under UV irradiation due to the change of solubility parameter after isomerization. It was observed that the final conversion of the resin matrix increased significantly with the addition of only small amounts of this nanogel albeit with somewhat reduced rates of polymerization. A delay of vitrification was also noticed for these nanogel systems with dramatic stress reduction achieved with minimal nanogel additive levels.

Finally, due to the non-controlled nature of nanogel synthesis from solution polymerization, whether free radical or RAFT controlled radical based processes, uniform nanogel structure formation was studied through a block copolymer self-assembly method. Core-shell micelles were formed through the assembly of an amphiphilic block copolymer in hydrophilic environment. The crosslinking in the core region generated well-controlled, internally crosslinked nanogel particles with 30 nm dimension in aqueous solution. The uniform nanogel particles were further applied to understand particle-particle interspacing by dispersing in an inert solvent at different concentrations followed by macrogel formation tests with interparticle reaction.

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