Encapsulation of glass fillers by UHMWPE for modification of the glass-resin interface

Dental composites are complex, tooth-colored filling materials composed of synthetic polymers, particulate ceramic reinforcing fillers, molecules which promote or modify the polymerization reaction that produces the cross-linked polymer matrix from the dimethacrylate resin monomers, and silane coupling agents which bond the reinforcing fillers to the polymer matrix. Each component is critical to the success of the final mechanical properties and lifetime of the composite.; In all filled composites, an interface exists, which can provide sites for failure as well a place at which energy can be adsorbed and dissipated, thus improving fracture toughness. A key to understanding failure of filled composites thus lies in understanding the nature and extent of the filler-polymer interface. The ultimate goal of this research was to engineer a ductile resin-filler interface with high toughness and to study the effects of interfacial bonding of the interface on the composite mechanical properties.; The tough ductile interface was engineered using ultra high molecular weight polyethylene (UHMWPE) which has high yield strength and modulus (when oriented or drawn) but is chemically inert and non polar. To improve the chemical bonding between UHMWPE and the resin, the surface of UHMWPE was functionalized using various silane coupling agents via a swelling technique and interfacial strengths were measured. UHMWPE in the powder form was surface treated using octadecyl trimethoxy silane (OMS)/3-methacryloxypropyltrichlorosilane (Cl-MPS), OMS/Acetoxyethyl trichlorosilane (AES) and used as a filler in the resin matrix at various volume fractions. A possible correlation between the surface treatments and the composite mechanical properties was investigated. Glass beads were surface functionalized with a low molecular weight polyethylene silane (PE-silane) or OMS and an adsorbed layer of UHMWPE (MW = 3-6 million) was applied to the surface of glass beads using a novel solution technique. The effect of time and temperature on the amount of PE-silane/UHMWPE adsorbed was investigated. The surface modified inorganic-organic hybrid glass fillers were used to prepare composites at different volume fractions and mechanical properties were measured. The thickness of the ductile interface was varied and the effects of encapsulated glass particles were compared with composites prepared with equivalent amounts of glass beads and surface treated submicron size UHMWPE particles. Fracture surfaces of composite samples were studied for modes of failure using SEM.; The interfacial shear strengths were measured using microbond shear strength tests showed that silane treatments for UHMWPE fibers improved interfacial strength as compared to nonsilanated specimens. Fracture toughness was improved with increasing filler volume fraction and wetting of UHMWPE powder with Cl-AES was found to have a marginally higher effect on fracture toughness. Composites based on UHMWPE encapsulated glass beads showed improved toughness but associated decrease in modulus and strength due addition of lower modulus UHMWPE. The increase in layer thickness of ductile UHMWPE followed a similar trend of improved fracture toughness but reduced modulus and strength. The addition of submicron size UHMWPE particles as cofiller also improved toughness but showed a higher decrease in modulus and strength when compared to encapsulated beads at similar volume fractions of UHMWPE in the composite.