Interface effects on mechanical properties of particle-reinforced composites.

OBJECTIVES: Effective bonding between the filler and matrix components typically improves the mechanical properties of polymer composites containing inorganic fillers. The aim of this study was to test the hypothesis that composite flexural modulus, flexure strength, and toughness are directly proportional to filler-matrix interfacial shear strength. METHODS: The resin matrix component of the experimental composite consisted of a 60:40 blend of BisGMA:TEGDMA. Two levels of photoinitiator components were used: 0.15, and 0.5%. Raman spectroscopy was used to determine degree of cure, and thermogravimetry (TGA) was used to quantify the degree of silane, rubber, or polymer attachment to silica and glass particles. Filler-matrix interfacial shear strengths were measured using a microbond test. Composites containing glass particles with various surface treatments were prepared and the modulus, flexure strength, and fracture toughness of these materials obtained using standard methods. Mechanical properties were measured on dry and soaked specimens. RESULTS: The interfacial strength was greatest for the 5% MPS treated silica, and it increased for polymers prepared with 0.5% initiator compared with 0.15% initiator concentrations. For the mechanical properties measured, the authors found that: (1) the flexural modulus was independent of the type of filler surface treatment, though flexural strength and toughness were highest for the silanated glass; (2) rubber at the interface, whether bonded to the filler and matrix or not, did not improve toughness; (3) less grafting of resin to silanated filler particles was observed when the initiator concentration decreased. SIGNIFICANCE: These findings suggest that increasing the strength of the bond between filler and matrix will not result in improvements in the mechanical properties of particulate-reinforced composites in contrast to fiber-reinforced composites. Also, contraction stresses in the 0.5 vs 0.15% initiator concentration composites may be responsible for increases in interfacial shear strengths, moduli, and flexural strengths.

Filler-coupling agent-matrix interactions in silica/polymethylmethacrylate composites.

The interactions of the silane coupling agent methacryloxypropyltrimethoxysilane (MPS) with both fumed silica and a polymethylmethacrylate (PMMA) resin matrix were investigated using thermogravimetric analysis and Fourier transform infrared spectroscopy. OX 50 fumed silica was silanated with MPS at concentrations of 1% and 5% in aqueous ethanol (95%), acetone, and anhydrous toluene. Methyl methacrylate was polymerized with the silanated fumed silica (5% wt/wt) to form composites. The amount of MPS adsorption on the fumed silica and the amount of PMMA attached to the silanated fumed silica were determined by thermogravimetric analysis. MPS could be removed from the fumed silica after washing with methanol, but not after it underwent a drying process at 25 degrees C under vacuum. After vacuum drying at 25 degrees C, two types of adsorbed silane were found, i.e., firmly adsorbed and loosely adsorbed silane. The loosely adsorbed silane could desorb from silica and be incorporated into the polymer matrix through copolymerization with monomeric methyl methacrylate, resulting in crosslinking of the matrix. When the silanated silica was dried at 110 degrees C for 2 h, the loosely adsorbed silane was removed and the amount of firmly adsorbed silane increased. There was a positive correlation between the amount of firmly adsorbed MPS and the amount of PMMA attachment. The highest efficiency for PMMA attachment was found when MPS was adsorbed as a monolayer, because the loosely adsorbed silane did not contribute to the bonding of PMMA, and this suggested that not all of the double bonds of the MPS were accessible for reaction with the methacrylate monomer. Drying at 110 degrees C may also decrease the number of unsaturated double bonds of MPS.

Submicron-size particles of ultrahigh molecular weight polyethylene produced via nonsolvent and temperature-induced crystallization.

Submicrometer size particles of ultrahigh molecular weight polyethylene (UHMWPE) were produced by crystallization from dilute (0.1-1.0 wt % of UHMWPE) solvent/nonsolvent emulsions. The procedure consisted of mixing a hot solution of UHMWPE in decalin or decane with a nonsolvent (tetraglyme) at approximately 160 degrees C, followed by rapid cooling of the mixture to zero or subzero temperatures. The rapid cooling causes microphase separation between the two liquids, resulting in the formation of an emulsion, which consists of microdroplets of the supercooled UHMWPE solution dispersed in tetraglyme. The consequent crystallization of the polymer in the microdroplets produces a suspension of fine crystals of UHMWPE, which can easily be isolated. The particles were characterized using scanning electron microscopy, differential scanning calorimetry, and Raman spectroscopy. Their degree of crystallinity is between that of GUR 1050 original (powder) and processed (molded) polymer. By changing the polymer concentration, solvent to nonsolvent ratio, and temperature, the size (from 0.1-1.0 microm) and shape (spheroids or rods) of the particles can be controlled. These particles may be used for immunochemical investigations and the study of the influence of UHMWPE wear debris on cell response.

Determination of the degree of cure of dental resins using Raman and FT-Raman spectroscopy.

FT-IR spectroscopy has traditionally been used to determine the degree of conversion of dental resins. FT-Raman scattering provided an alternate method of obtaining degrees of conversion for these systems and was particularly useful for measuring spectra of materials without any sample preparation. Raman and FT-Raman spectroscopy gave identical results, but the latter technique was preferred for the highly fluorescent samples often encountered in commercial composites. Linear calibration curves were obtained for the aromatic mixtures Bis-GMA/TEGDMA and Bisphenol-A/TEGDMA using C = C/phi, and for the wholly aliphatic mixture EGDMA/EGDA using C = C/C = O, over a wide range of mole ratios. If both the mole and intensity ratios [C = C/phi or C = C/C = O] were known for an uncured dental resin, then the degrees of conversion could be obtained for the cured materials using Raman spectroscopy. However, if the mole ratios for the uncured resin were unknown, then the degree of conversion depended on the calibration curve, since the Raman scattering cross section of the vibrational modes depended on the molecules to which they were attached.