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  • 1
    Electronic Resource
    Electronic Resource
    Effect of salivary viscosity on frictional coefficients of orthodontic archwire/bracket couples (1995)
    Springer
    Journal of materials science 6 (1995), S. 390-395 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Stimulated whole saliva samples were collected from 30 healthy individuals and analysed with a cone and plate viscometer. On the basis of these dynamic viscosity measurements saliva from patients, who represented the mean value and two standard deviations above and below the mean value, were selected for frictional force testing. Four archwire/bracket couples (SS/SS, SS/PCA, β-Ti/SS, and β-Ti/PCA) were each tested in these three salivas as well as in the dry state a total of five times, each at five different normal loads. With two exceptions no significant differences were found between any of the three wet states for any couple studied. When the dry state was compared to any of the wet states, the SS archwire couples showed a singificant increase in frictional forces, while the β-Ti archwire couples showed a slight decrease in frictional forces, which was not statistically significant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00120279
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  • 2
    Electronic Resource
    Electronic Resource
    Rheology of stimulated whole saliva in a typical pre-orthodontic sample population (1995)
    Springer
    Journal of materials science 6 (1995), S. 385-389 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: The dynamic viscosity (μ) of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s-1 from T=20°C to T=40°C. Data from the μ-T plots showed a negative dependence of the form, μ=a−bT, over a range of μ from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean μ of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in μ occurred during the 3 h period. The μ-T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in μ as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00120278
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  • 3
    Electronic Resource
    Electronic Resource
    Ionic control of the rupture of fetal membranes (1995)
    Springer
    Journal of materials science 6 (1995), S. 569-576 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Preterm premature rupture of the chorioamniotic membrane frequently leads to preterm birth and perinatal mortality. To ascertain whether the rupture of these membranes is influenced by variations in environmental pH and sodium concentration, we punctured 180 specimens from 9 membranes that were equilibrated in solutions of different pH, and 196 specimens from 10 membranes that were equilibrated in solutions with different sodium concentrations. Whole-membrane mechanical characteristics—strength, stiffness, toughness and ductility—were measured. These characteristics were defined based on a variant of the weakest-link theory: from a pool of three to five specimens, the specimen that had the lowest values of strength and concomitant values of stiffness, toughness, and ductility represents the mechanical characteristics of the entire membrane section. Strength-related mechanical characteristics—strength, stiffness and toughness—correlated negatively with pH (p〈0.001, p〈0.001, and p〈0.02, respectively), while ductility did not correlate significantly with pH. Membrane hydration and thickness correlated positively with pH (p〈0.001). The greatest increase in hydration accompanied by the greatest drop in strength, stiffness and toughness was observed between pH values of 3.68 and 5.58, suggesting that insufficient quantities of vaginal H+ could cause—as well as mark—premature rupture of the chorioamniotic membrane. No whole—membrane mechanical characteristics correlated significantly with changes in sodium concentration. Membrane hydration and thickness had slight positive correlations with sodium concentration (p〈0.05). Changes solely in the sodium concentration of the amniotic fluid during normal pregnancy are not sufficient to facilitate rupture at term.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00121280
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  • 4
    Electronic Resource
    Electronic Resource
    Structural degradation of polyurethane-based elastomeric modules (1995)
    Springer
    Journal of materials science 6 (1995), S. 377-384 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Degradative characteristics were studied using specimens of three polyurethane-based elastomeric modules that were treated in solutions of varying acidity, oxygen content and temperature. After periods of 10 and 100 days, molecular weight distribution changes, mechanical property changes and elemental chemistry changes were studied using gel permeation chromatography (GPC), stress-relaxation testing and X-ray photoelectron spectroscopy (XPS), respectively. As determined through analyses of the molecular, mechanical, and elemental changes after treatment, the degradative mechanism was influenced by whether the polyurethane was polyester-based or polyether-based, and the degree of structural degradation was influenced by the type and duration of conditioning treatment. Degradation of the polyester-based materials after treatment was dominated by a chain scission mechanism; conversely, the degradation of the polyether-based material was dominated by a crosslinking mechanism. Among the conditioning treatments, the combined effects of an increase in acidity, oxygen content and temperature most influenced the degree of degradation with time, with the increase in temperature having the greatest effect of any of the single variables investigated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00120277
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  • 5
    Electronic Resource
    Electronic Resource
    Frictional characteristics of composite orthodontic archwires against stainless steel and ceramic brackets in the passive and active configurations (1998)
    Springer
    Journal of materials science 9 (1998), S. 611-620 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The frictional characteristics of prototype composite archwires were investigated. The resistance to sliding was measured in the dry state for wires with three different volume fractions of fiber reinforcement against stainless steel, polycrystalline alumina, and single crystal alumina orthodontic brackets. Each archwire and bracket combination was tested at 34 °C with twelve different normal forces (from 0–400 g) and six different angulations (from 0°–12.5°). The kinetic coefficients of friction were determined from the slopes of linear regressions through plots of the resistance to sliding versus normal force data. The y-intercepts of these regressions were also evaluated as indicators of the binding magnitude. The tested archwire samples were examined for wear using a scanning electron microscope. A fully factorial model analysis-of-variance showed no significant differences in the frictional coefficients for changes in bracket material, reinforcement level, or angulation. Highly significant differences were observed in the y-intercepts for changes in the reinforcement level and angulation. Highly significant, positive, and linear correlations between the y-intercepts and angulations were also established. Abrasive wear of the composite surface was observed at the archwire–bracket interface, particularly at higher normal forces and angulations. Relative to other frictional studies of metallic archwire materials, the composite archwires had higher kinetic coefficients of friction than stainless steel but lower coefficients than either nickel titanium or beta-titanium archwires against all bracket materials tested. © 1998 Kluwer Academic Publishers
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008977106648
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  • 6
    Electronic Resource
    Electronic Resource
    Behaviour of photopolymerized silicate glass fibre-reinforced dimethacrylate composites subjected to hydrothermal ageing Part II Hydrolytic stability of mechanical properties (1998)
    Springer
    Journal of materials science 9 (1998), S. 651-660 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The flexural properties and failure morphologies of dimethacrylate-copolymer composites reinforced with either S2-glass® or quartz fibres (≈33–66 vol%) were examined after hydrothermal ageing (0–3 mon at 37 °C). Initially the S2-glass® composites were generally stiffer and stronger than comparably reinforced quartz composites, but within 1 wk the properties of S2-glass® composites decreased by 12%–26%. The properties ofquartz composites were relatively stable, except for those of composites with the least reinforcement (35 vol%), which decreased by roughly 15%. Scanning electron microscopy revealed that in all composites buckling had occurred at the site of load application. Evidence of good fibre–matrix adhesion was observed for both types of composites under all conditions. Modelling of degradation between 1 wk and 3 mon revealed that: (1) the only temporal change was a slight increase in the stiffness of S2-glass® composites; and (2) higher reinforcement levels reduced the retention of strength in S2-glass® composites but had the opposite effect (on both properties) for quartz composites (p〈0.05). For the most highly reinforced S2-glass® composites, susceptibility to degradation was offset by high initial properties; and after ageing (elastic modulus ≈50 GPa, strength ≈1.2 GPa), these composites were still, on average, approximately 25% stiffer and 50% stronger than the more hydrostable quartz counterparts. © 1998 Kluwer Academic Publishers
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008983509374
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  • 7
    Electronic Resource
    Electronic Resource
    Behaviour of photopolymerized silicate-glass-fibre-reinforced dimethacrylate composites subjected to hydrothermal ageing: Part I Steady-state sorption characteristics (1998)
    Springer
    Journal of materials science 9 (1998), S. 243-248 
    Details
    ISSN: 1573-4838
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Steady-state water sorption due to hydrothermal ageing (37 °C) was determined via gravimetric methods for both an unreinforced dimethacrylate copolymer network and for composites of the copolymer reinforced with two different silicate yarn products – S2-glass® and quartz. The copolymer sorbed 4.9±0.3 wt%, and the composites sorbed 0.5–2.5 wt%. For the two types of composites, two different positive linear correlations were established between the sorption gain and the copolymer content in the composites. In the case of the S2-glass® composites, the sorption gain was fully explained in terms of sorption in the copolymer; and normalization of these data by the mass fraction of copolymer yielded a constant sorption value matching that of the unreinforced copolymer. After normalizing the data for the quartz composites, an additional positive correlation was established between the water gain (per unit weight of copolymer) and the linear density of interfacial surface area. For both types of composites the sorption response was stable over a three-month ageing period. © 1998 Chapman & Hall
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008896425841
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  • 8
    Electronic Resource
    Electronic Resource
    Effect of methacrylic acid:Methyl methacrylate monomer ratios on polymerization rates and properties of polymethyl methacrylates (1997)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 36 (1997), S. 190-199 
    Details
    ISSN: 0021-9304
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: Five binary formulations were prepared from methyl methacrylate (MMA) and methacrylic acid (MAA) monomers, and six ternary formulations were prepared from polysols of 30% wt polymethyl methacrylate (PMMA)/MMA and MAA. Using thermal analyses (DSC and TGA) the polymerization kinetics, condition of postcuring, relative amount of residual monomers, and glass transition temperature (Tg) were determined. From bar-shaped samples, 25 × 5 × 0.9 mm in dimensions, mechanical properties [flexural moduli (E) and maximum strengths (σ)] were measured in three-point bending. Polymerization kinetics of binary formulations improved over pure PMMA (from 15 to 4 min) as a result of over a 60-fold increase in propagation-to-termination constants (Kp/Kt) of MAA/MMA. The further addition of PMMA increased the viscosity, slowed down termination, and, consequently, improved the polymerization kinetics twofold. These enhancements occurred without a substantive change in the Tg of the ternary system (ca. 107°C) over pure PMMA (ca. 112°C). Moreover, the Es of the four ternary formulations averaged 2.94 GPa, which was comparable with many values reported in the literature. In contrast the σs of these same formulations averaged 97 MPa, which was about 25% better than earlier investigations of pure acrylic. When a thermoplastic material is required for pultruding profiles that cure fast and have good thermal-mechanical properties, ternaries of PMMA/MMA/MAA should be considered. © 1997 John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Investigation of dual-staged polymerization and secondary forming of photopultruded, fiber-reinforced, methacrylate-copolymer composites (1998)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 41 (1998), S. 549-559 
    Details
    ISSN: 0021-9304
    Keywords: photopultrusion ; composites ; fiber-reinforced polymers ; photopolymerization ; methacrylates ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: To develop a dual-curing monomer system for the photopultrusion of reformable (soft) composites, a microhardness assay showed that in a blend with 2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (Bis-GMA), the substitution of methyl methacrylate (MMA) for triethylene glycol dimethacrylate (TEGDMA) delayed the onset of gelation during photopolymerization. Adding lauroyl peroxide permitted the completion of polymerization thermally. This system was used to form silicate-glass-fiber-reinforced composites, with varying degrees of conversion, by photopultruding over a range of pulling speeds. Sol-gel extractions demonstrated both fully soluble and insoluble matrices. For the soluble material, gel permeation chromatography elucidated a trimodal distribution of molecular weights that corresponded to MMA, Bis-GMA, and polymeric molecules with molecular weights in the tens of thousands. Composites with matrix solubilities above about 10% wt could be swaged after photopultrusion to change the cross section from circular to rectangular before thermal processing. The effect on the final elastic modulus was small (∼44GPa, as measured in flexure for 57% vol-reinforced composites); but the final flexure strength was reduced by approximately 25% to a constant of about 1.2 GPa. Morphological characteristics that were seen in the circular-sectioned precursors were observed in the swaged rectangular products as well, including flaws when swaging was conducted at matrix solubilities above about 75%. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 41, 549-559, 1998.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Plasticizer-level study of poly(vinyl chloride) ion-selective membranes (1996)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 30 (1996), S. 313-320 
    Details
    ISSN: 0021-9304
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: Highly plasticized poly(vinyl chloride) (PVC) membranes (200 per hundred resin [phr]) form the basis of one class of ion-selective electrodes (ISEs). In previous work on the mechanical properties of membranes, the optimal ratio of plasticizer level employed to minimal level required for complete plasticization (phrexp/phrmin) was found to be 2.0. The current study was designed to determine whether this ratio is necessary or sufficient for proper ISE function. Dynamic mechanical analysis (DMA) was used to examine the effects of five plasticizers on the dynamic mechanical properties of membranes at three frequencies (110, 11.0, and 1.1 Hz) as a function of temperature (-100°C to +100°C): dicotyl sebacate (DOS), epoxidized propylene glycol dioleate (PGDO), ortho-nitrophenyl octyl ether (o-NPOE), epoxidized soybean oil (ESO), and expoxidized linseed oil (ELO). The glass transition temperature of PVC, which was found to be +77.1°C at 11.0 Hz, was depressed by the addition of 200 phr of each plasticizer from a high of -1.4°C (PGDO at 110 Hz) to a low of -70.2°C (DOS at 1.1 Hz). DMA and electromotive-force (EMF) measurements on membranes plasticized with o-NPOE through a range of phrexp/phrmin from 0.5 to 9.3 indicated that a “transition window” occurs between phrexp/phrmin of 2.0 and 3.3 in which the membranes change from minimally plasticized polymer films to predictable ion-selective membranes, coinciding with the optimal mechanical properties observed previously. Based on dynamic mechanical properties and EMF response data, the optimal phrmin/phrmin ratios for membranes as a function of plasticizer were proposed: 0.8 for ESO and ELO, 1.3 for PGDO, 1.7 for DOS, and 3.0 for o-NPOE.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jbm.820300302
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