Interphases for Tougher Composites

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DESCRIPTION (provided by applicant): Epidemiological studies indicate that dental composite restorations fail for two main reasons: because of recurrent decay caused by polymerization shrinkage, and because of material fracture. The problem of shrinkage is being addressed primarily by changing the matrix polymer systems. Fracture continues to be a concern, as the dental composites currently available for clinical use are brittle, and literature surveys indicate that fracture toughness values have not changed (increased) significantly over the past 20+ years; KIc remains below approximately 1.75 MPa.m1/2. Brittle materials are prone to catastrophic failure, and increasing toughness remains a major goal of materials research. Studies of naturally-occurring bio-composites have identified several toughening strategies which have the potential to significantly increase composite lifetimes and also the indications for their use, but which are not adequately exploited in synthetic composites. Of particular interest to us is the energy absorption exhibited by nacre proteins by virtue of the sacrificial bond mechanism, which contributes to nacre toughness. We propose the following specific aims: Aim 1: Synthesize 2- peptides analogous to nacre proteins that will covalently bridge inorganic and organic composite components. Aim 2: Compound spherical particle- and fiber-reinforced model composites to test the hypothesis that energy absorbing interphases can toughen composites even if the filler morphology and arrangement does not exactly simulate natural composite architectures. In any case, numerous studies of non-nacreous composites have shown ductile interphases to be effective in toughening. If the strategy proves successful, the scheme can be applied to a variety of filler and matrix polymer compositions simply by changing the reactive en groups of the 2-peptide chain. PUBLIC HEALTH RELEVANCE: The proposed research is intended to lead to the development of tougher and longer-lasting composites that could be used to restore missing teeth or tooth structure. The strategies that will be pursued here is to synthesize 2-amino acid polymers that mimic proteins found in nacre, then use these peptide-like polymers as coupling agents to bond matrix polymers to reinforcing filler particles. The expectation is that these 2-peptides will possess the energy-absorbing and toughening properties of nacre proteins. Studies proposed here will test the hypothesis that tougher composites will derive from 2-peptides covalently and elastically spanning the filler-matrix interface, attached with silane and methacrylic ends. Variables that will be controlled include chain length and attachment density, and filler morphology; fiber reinforcement will be included as an approximation of the stacked architecture of nacre. If the hypothesis is proven true, this research will provide a significant advance in coupling agent design that will be applicable to a wide range of composite matrix materials and filler compositions.