Influence of silanization and filler fraction on aged dental composites.

The effect of silanization and filler fraction on the mechanical properties of aged dental composites was investigated. Experimental composites (75/25 Bis-GMA/TEGDMA resin reinforced with 0, 12.6, 30.0, and 56.5 vol% 8 microm silanized/unsilanized BaSiO6) were fabricated into 4.7 mm diameter x 2.2 mm thick discs and 3.5 mm diameter x 7.3 mm thick discs for diametral tensile and compressive tests, respectively. The effect of immersion in 75% ethanol at 37 degrees C for 0-30 days on the diametral tensile strength (DTS) and compressive strength (CS) of the samples was evaluated and analysed by ANOVA and Tukey LSD test. The fracture interface between filler and resin matrix was then examined by scanning electron microscope. Results and subsequent statistical evidence from DTS (18.6+/-7.6 MPa, silanized versus 11.7+/-2.6 MPa, unsilanized) and CS (85.1+/-29.7 MPa, silanized versus 56.0+/-11.3 MPa, unsilanized) strongly implies that silanization may greatly enhance the mechanical properties of the resin composites. Furthermore, it also shows that both DTS and CS increased proportionally as the filler fraction of the composites increased. However, in the unsilanized groups, DTS decreased (up to 40%) as the filler fraction increased, and CS showed no relevance to the filler fraction at all. As for the influence of aging, it was found that both DTS and CS showed a significant decrease after immersion in 75% ethanol, and silanization heavily correlated with the filler fraction of aged-resin composites. Microscopic examination of the fractured samples showed that failure primarily occurred within the resin matrix per se for silanized composites and adjacent to the filler particles for unsilanized composites. All the evidence points to the conclusion that mechanical properties of aged-resin composites can be greatly influenced by silanization and the filler fraction.

On the Behavior of Nonionic Surfactants at the N-Heptane/Silica Gel Interface: Influence of the Presence of Interfacial Water Inferred from Adsorption Isotherms and Calorimetric Data.

The behavior of two polydisperse nonionic surfactants, poly (oxyethylene) glycol alkylphenyl ether TX-35 and TX-100, at the prewetted silica gel/n-heptane and dried silica gel/n-heptane interfaces has been compared by the determination of the average adsorption isotherms of the polydisperse surfactants and of displacement enthalpies. From HPLC experiments, we could also separately quantify the adsorption of each ethyleneoxide (EO) fractions for silica gel from the polydisperse surfactant solution. The adsorption isotherms clearly indicate an incomplete preferential adsorption of the large (EO) chains over the small ones, as well on dried silica gel as on a prehydrated sample. This preferential adsorption and its driving force follow the solubility rules of the poly(oxyethylene) glycol alkylphenyl ether in an apolar solvent and support the idea of a solubility-limited adsorption: solubility in organic solvents of the smaller (EO) chains is much more significant than that of the longer ones and hence prevents adsorption of the smaller species. Consequently, it is observed that the presence of interfacial water decreases the affinity of TX-35 molecules for the hydrophilic silica surface due to the hydration of (EO) chains. In contrast, for TX-100 adsorption after the prewetting treatment the clearest trend is a drastic increase of the adsorption ascribed to the additional solubilization (and micellization) of the TX-100 molecules in the interfacial aqueous phase. The differential molar enthalpies of displacement show a change in the adsorption mechanism, depending on the presence of molecular water on the surface. In the initial part of the adsorption isotherm, a prevailing exothermic process is obtained with prehydrated silica and suggests that hydration of the polar heads of TX-35 and the solubilization of the TX-35 in interfacial water are occurring. For higher equilibrium concentrations, the enthalpies of displacement observed with the prehydrated adsorbent become slightly lower than those obtained with dry silica gel. It may be that this difference is due to the micellization phenomenon of the surfactant species with longer EO chains in interfacial water. These features emphasize the influence of interfacial water on the adsorption of EO fractions from organic solvent. Copyright 2000 Academic Press.

Laser-induced acoustic emissions in experimental dental composites.

A laser thermoacoustic technique was innovated to evaluate laser-induced acoustic emissions (AEs) in experimental dental composites aged with 75% ethanol solution. Experimental composite systems of 75/25 BisGMA/TEGDMA resin filled with 0, 12.6, 30.0, and 56.5 vol% of 8-microm silanized and unsilanized BaSiO6 were analyzed. The sample size was 4.65 mm (diameter) x 0.5 mm (thick). Aging effects of immersing in 75% ethanol for up to 14 h on AEs were then evaluated. A continuous-wave CO2 laser was used to heat the samples. Acoustic emissions were collected as a function of filler fraction, laser power, silanization, and immersion time. Onset of burst-pattern acoustic signals characteristic of fracturing occurred at different laser powers for different tested groups. Acoustic emissions generally increased with laser power, in which lower laser powers produced low-amplitude (45-50 dB) signals; the amplitude distribution (50-85 dB) became more extensive as laser powers increased. After immersion, the lower laser powers could produce the same phenomenon. The higher the filler fraction, the fewer AEs generated. A large percentage AE reduction due to silanization was noted as a function of filler fraction. Unsilanized specimens showed more thermal damages than did silanized ones.

Changes in the masticatory function of complete denture wearers after relining the mandibular denture with a soft denture liner.

PURPOSE: The purpose of this study was to examine changes in the masticatory function of complete denture wearers after relining the mandibular denture with a soft liner. MATERIALS AND METHODS: Conventional complete dentures were fabricated for 6 completely edentulous patients. One month after completing adjustments of the dentures, maximum biting force, masticatory performance, and electromyography of the masseter muscle during mastication were recorded. Chewing strokes, chewing time, muscular activity, and masticatory rhythm were calculated using the recorded electromyography. The mandibular dentures were then relined with a soft liner. One month after finishing adjustments of the relined dentures, the same tests were performed. These values were compared using a paired t test (alpha = 0.05). RESULTS: When using the soft liner, masticatory performance and maximum biting force were significantly greater (P = 0.019 and P = 0.023, respectively). In addition, the number of chewing strokes was significantly lower (P = 0.020), and chewing time was reduced (P = 0.010). A more stable masticatory rhythm was also found in the initial chewing stage. The muscular activity tended to decrease after the insertion of the lined denture, but no significant difference was found between before and after relining. CONCLUSION: It was shown that applying a soft lining material to the mandibular dentures of 6 edentulous patients improved masticatory function with no adverse effect on the muscular task.

Surfactant-Mediated Transfer of Colloidal Silica Particles from Water into an Immiscible Weakly Polar Solvent.

Spontaneous transfer of discrete colloidal silica particles from water into 2,6-dimethyl-4-heptanone (DH) is obtained upon preliminary adsorption of dodecylbenzyldimethylammonium bromide (DBDAB) from the aqueous phase onto the solid. Partitioning of the surfactant between the silica surface and both liquid phases is evaluated through the determination of the isotherm in the aqueous phase, taking into account their mutual solubility. The organosol particles are found to be covered by a statistical monolayer of surfactant molecules presumably hydrated to the extent of 20% on a mole ratio basis. Monodispersion of this stable suspension is ascertained by dynamic light scattering. Evidence of DBDAB association in the monolayer emerges from the conductimetric behavior of the amphiphile in DH and from the reversal of the silica native charge in the organosol, as shown by electrophoretic mobility determinations. Disorganization of the water layer contacting silica upon adsorption of DBDAB is argumented to originate from the presence of sparsely distributed dodecyl chains. Copyright 1998 Academic Press.

Transfers of Colloidal Silica from Water into Organic Solvents of Intermediate Polarities

Dispersions of discrete metal-oxide submicroparticles in organic solvents of medium polarities are uneasy to generate and weakly documented. We address this topic along two general methods focusing on silica. Successive transfers of colloidal particles from water into n-propanol and then into 1,2-dichloroethane by azeotropic distillation yield a stable organosol. The particles are found to be propanol-coated by surface esterification to the extent of 0.40 nm2 per molecule. Alternatively, centrifugation-redispersion cycles make it possible to obtain stable suspensions of unaltered silica in methanol and acetonitrile starting from an aqueous silicasol. Particles are characterized by various methods including nitrogen adsorption, transmission electron microscopy, dynamic light scattering, and electrophoresis. The stabilities of these suspensions in various organic solvents are investigated with special concern for the role of residual water. Stabilization of silica in methanol is inconspicuously related to solvent permittivity and prominently dependent on the presence of adsorbed water. In contrast, the acetonitrile silicasol, which is unaffected by residual water, displays electrophoretic behavior compatible with electrostatic stabilization. Copyright 1998 Academic Press. Copyright 1998Academic Press

Cryoenzymology in aqueous media: Micellar solubilized water clusters.

Amphiphilic compounds dissolved in nonpolar organic solvents form inverted micelles whose aqueous centers can solubilize relatively large amounts of enzymes. The solutions are homogeneous and optically transparent and have low viscosity and freezing points; they provide unique and favorable systems to perform the main cryoenzymologic studies previously carried out in mixed solvents. The preparation and properties of such micelles are described. The absorbance spectra of cytochrome c at various temperatures from -38 degrees C to 20 degrees C are given. The pH dependence of trypsin-catalyzed hydrolysis of benzoylarginine ethyl ester in inverted micelles, as well as kinetic properties and activation energy, is described. Some problems are analyzed.