Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Jeffrey W. Stansbury

Second Advisor

Chrisopher N. Bowman

Third Advisor

Kristi Anseth

Fourth Advisor

Daniel Schwartz

Fifth Advisor

Virginia Ferguson

Abstract

The demand and use of polymer-based dental restorative materials continues to rise as advances in materials research continue to improve their performance. In spite of the many advances, there remain some shortcomings in their behavior. Polymerization shrinkage and the associated shrinkage stress continue to be detrimental to the long term performance of these materials. A majority of the current research is focused on developing low shrinkage resin systems to alleviate the problem of high volumetric shrinkage. There is relatively less focus on developing novel fillers and surface modification techniques to improve material properties. This thesis focuses on studying the effect of fillers on composite properties, mainly shrinkage stress development, and the development of a novel technique to modify the surface of the fillers to control shrinkage stress and optical properties of the composites.

The effect of monomer conversion and filler content was studied on the development of shrinkage, modulus and shrinkage stress. Fillers and monomer conversion were found to affect all the properties. Shrinkage stress was found to have a complex relationship with both shrinkage and modulus, with shrinkage apparently dominating the modulus at high values of monomer conversion.

A novel surface modification technique was developed, utilizing the concept of polymer brushes, to alleviate shrinkage stress in composites. Oligomers, both commercial and synthesized in the lab, were covalently attached to the surface of fillers and mixed with a model dental resin to form composite pastes. These pastes, when photopolymerized in a shrinkage stress measuring apparatus, showed that the composites with the experimental surface modified fillers outperformed conventional filler-based composites by achieving a reduced final stress. The final stress was found to depend on various characteristics of the oligomers. In general, the higher the molecular weight of the brush, the lower was the shrinkage stress. Increased in reactive group concentration on the oligomer backbone had the effect of increasing the stress. Flexibility of the oligomers appeared to play a less significant role, with low flexibility oligomers showing a slightly lower stress than flexible oligomers.

Modern dental composites utilize different filler sizes to optimize material properties. To explore this practical scenario, composites were made with two different sizes of filler, with varying ratios of nano-fillers and submicron-sized fillers. Each filler type was treated with either a conventional silane treatment or with the experimental oligomer. It was found that composites with the experimental oligomer had reduced shrinkage stress and also better light transmission properties. The nano-fillers affected both the properties to a more significant effect than the larger fillers.

Stress development in polymerizing composites is a combination of stress increase and stress reduction. Stress reduction occurs by relaxation processes in the composite, even as the composite is polymerizing. There is a potential to increase the magnitude of the stress relaxation leading to a lower final stress. Stress relaxation was measured as a function of monomer conversion and also filler surface treatment, using a standard stress relaxation experiment. The experimental composites exhibited faster stress relaxation as compared to conventional composites, showing potential for utilizing the novel surface treatment in commercial composites.

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