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

Kristi Anseth

Third Advisor

Christopher N. Bowman

Fourth Advisor

Yifu Ding

Fifth Advisor

Carmem Pfeifer

Abstract

The design of heterogeneous polymer networks has been used to combine the properties of different constituents in a single material. Heterogeneous networks with enhanced mechanical integrity, processing, and defined covalent interactions have been developed utilizing methods such as blending, block copolymer synthesis, and phase separation. However, these approaches have not yet been exploited to tailor heterogeneity in densely cross-linked networks formed under ambient photopolymerization conditions. This limits the utility of heterogeneous networks in many biomaterials, coatings, adhesive, and lithographic applications, as they often require in situ, ambient processing. To successfully engineer these materials, this thesis studies polymerization-induced phase separation (PIPS) as an approach to design network heterogeneity. Furthermore, this work identifies control parameters that can be utilized to tailor the development of phase morphology during polymerization.

A model system composed of a dimethacrylate homopolymer modified with thermoplastic, linear prepolymers was studied. Phase separation was detected at very early stages of polymerization, resulting in the formation of two phases, one enriched in homopolymer, and another composed of both homopolymer and prepolymer. The efficacy of this model system at reducing polymerization stress was probed. Significant stress reduction was observed when PIPS delayed the onset of macrogelation. The delay in macrogelation permits the formation of co-continuous network structure, providing maximum interfacial area for internal rearrangement that compensates for volumetric shrinkage.

The influence of thermoplastic prepolymer properties on PIPS were studied, specifically chain-length and glass transition temperature (Tg). With decreasing chain-length, the system free energy decreases due to entropic changes, thus decreasing the thermodynamic driving force for PIPS. However, with increasing chain-length physical limitations become more significant, and PIPS can be suppressed under rapid polymerization conditions. The prepolymer Tg was found to influence stress and modulus development. When the prepolymer Tg was significantly lower than that of the bulk matrix, the development of stress was delayed during polymerization as the lower Tg domains can flow readily to compensate for volume changes. The difference in relative reaction rate between phases formed was probed through analysis of Tg during cure. In the model system studied here, polymerization is preferred and accelerated in the homopolymer-enriched domains at the start of polymerization.

Finally, the influence of bulk matrix structure on phase separation was evaluated. Introducing structurally similar mono-vinyl monomers was found to enhance the period for phase separation via diffusion. This extended period permitted the use of incident UV-irradiation to tune the resulting size and morphology of heterogeneous domains formed. Inert particulate filler was introduced into the bulk matrix to probe the efficacy of PIPS in spatially constricted domains. Physical suppression of PIPS, due to decreased interparticle spacing and increased solution viscosity, was observed at a sufficiently high loading of filler into the matrix.

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