## Doctoral Dissertations

#### Title

Tailoring the interface in S-glass fiber polycarbonate composites for dental applications

January 1997

#### Keywords

Chemistry, Polymer|Health Sciences, Dentistry|Engineering, Materials Science|Plastics Technology

Ph.D.

#### Abstract

Continuous S-Glass fiber reinforced polycarbonate composites have been evaluated clinically for orthodontic applications. It was found that the stability of the fiber/matrix interfaces in the oral environment was essential for satisfactory performance. To achieve maximum hydrolytic stability, polycarbonate oligomers were chemically grafted onto the glass fiber surface through use of a silicon tetrachloride intermediary. The interfacial shear strength, fracture toughness and hydrolytic stability of the resulting interphase was measured and compared to those of two commercial sizings and ozone cleaned surfaces. Evaluation was accomplished by measuring the stress transmission across the interface, $\tau$, using an embedded single fiber fragmentation test and by using computer simulations and a finite element analysis to calculate the strain energy release rate, G, of the observed fiber-matrix debonding at the interface accompanying the first fiber fracture. The oligomer-grafted interphase exhibited improved stress transmissibility and toughness, particularly after 24 hours in boiling water. The tenacity of the tightly bound oligomers was confirmed via DRIFT, TGA and GC/MS experiments on Soxhlet extracted fibers.^ High resolution solid state $\sp{13}$C and $\sp{29}$Si CP/MAS NMR has been used to investigate the grafting mechanism, morphology and interfacial mobility of polycarbonate oligomer and bisphenol A grafted onto silica surfaces. The NMR spectra demonstrate differences between the neat and grafted PC oligomer that suggest strong bonding. A model compound, bisphenol A, was used to resolve signal overlaps due to repeat units and to verify the formation of primary bonding at the silica surface by the existence of a downfield shift of the C$\sb4$ resonance peak and other changes in the spectrum. Proton spin-lattice relaxation times in the rotating frame offer secondary evidence of the formation of Si-O-C bonds on the silica surface. The proton spin-lattice relaxations of the grafted molecules were characterized by a bimodal distribution of relaxation times, while unreacted molecules were represented by a single relaxation time. Temperature dependent studies show that the oligomer loses mobility as a result of grafting, and that the transition responses of the material are lost. The grafted material is visualized as a low density monomolecular layer of covalently bonded material. ^

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