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Control of polymer network structure and degradation




Yaghoubi Rad, Ima, author
Stansbury, Jeffrey W., advisor
Kipper, Matthew J., advisor
James, Susan P., committee member
Bailey, Travis S., committee member

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This thesis presents work performed evaluating controlled degradation in polymeric networks via incorporation of nanogels either as precursor or as a component of pseudo-interpenetrating polymeric networks. These polymeric crosslinked nanoparticles have applications in drug/gene delivery, cell imaging, and inert or functional prepolymer nano-fillers, therefore controlling their molecular weight (size) and structural properties are mandatory requirements. In addition to the primary effects of reactant selection on the nanogel formation, the solvent used and the agitation rate can provide additional parameters to control nanogel size. This work also develops a practical understanding of polymer characteristics and degradation kinetics of networks constructed from reactive nanogels with regiospecifically degradable linkages. Analogous non-degradable control structures are also prepared for each experimental condition. Clear, monolithic photopolymers are prepared from 50 wt% solvent-based dispersions of the reactive nanogels. The results of equilibrium swelling, mass loss, and compressive modulus (dry/swollen) demonstrate interplay between hydrophilic/hydrophobic effects, labile linkage location, and the crosslinking density that appears to dominate many of predicted property trends. The introduction of hydrolytically degradable linkages (PLA) into the internal crosslink structure of the nanogel promotes greater hydrophobic character compared to PLA placement in external reactive side chains. Consequently, nanogel-based networks with shorter hydrophilic crosslinker and lower crosslinker concentration show lower mass swelling rate, higher Tg, and lower compressive modulus reduction. Nanogels unlock an immense potential in designing superior alternatives for accepted materials with significantly reduced network heterogeneity compared to conventional hydrogels, which ultimately appoint them as novel candidates for controlled drug delivery and tissue engineering applications. Another part of this work demonstrates the effects of nano-scale pre-crosslinked hydrophobic particles as additive to model labile monomer on hydrolytic degradation. The modification of hydrolytically vulnerable polymers through the intimate integration of secondary networks based on styrenic nanogel structures is intended to reduce or even eliminate hydrolytic degradation potential. Nanogel addition at any level produces reduction in network swelling and mass loss proportional to nanogel content. The flexural modulus and ultimate transverse strength of nanogel-loaded resin monomer (TEGDMA) does not change compared to neat resin homopolymer as one control material in addition to the homopolymer of PEG2000PLADMA, which includes polylactic acid segments in the crosslinks. The use of a monomer-swollen highly crosslinked hydrophobic nanogel offers a versatile platform from which hydrolytic and potentially enzymatic degradation can be suppressed in a variety of applications such as polymer-based dental restoratives while retaining resin formulation, handling and mechanical properties. Some of the most important challenges in processing high performance materials are their high viscosity and limited solubility as a result of high molecular weight, intermolecular interactions, and rigid monomeric structure. Alternatively, high strength thermoset materials formed by ambient photopolymerization are limited in their performance by incomplete, vitrification-limited conversion, and relatively low glass transition temperature. In non-biological applications, significant effort has been focused on improving processing techniques and advanced machinery, and notably trivial attention has been paid to upgrade molecular structure of the resins. On the other hand, in biological applications photocuring is the perfect choice since applying high temperatures is practically impossible. As a result, another objective of this work was to develop an alternative photocurable material with enhanced processability, yet retaining thermal and mechanical properties of conventional resins. The average diameter of these polymeric particles is less than 20 nm with glass transition temperatures greater than 200 °C. These paradoxical properties trace back to molecular-level rearrangements of the same monomeric building blocks used in current thermoplastic/thermoset resins.


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