Biomedical subjects
R L Clarke
Publications and source records attributed to R L Clarke.
Physician payment reform: are you watching?
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Partners cite need for consistency in reporting--discussion.
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Dynamic mechanical thermal analysis of dental polymers. II. Bis-phenol A-related resins.
Dynamic mechanical analysis of bis-phenol A-related resins for dental usage provided loss tangent maxima at their transitions and an Arrhenius-type dependence of transition temperature and frequency. A constant maximum in the loss tangent curves, over the frequency range for the unfilled resins below their glass transition temperature, was thought to be caused by their high degree of unsaturation. The calculated activation energies showed the polycarbonate denture base resin required considerably greater energy for segmental motion than the other tested materials. In contrast, the ethylene imine-based temporary crown and bridge material exhibited the lowest activation energy.
Modified Arrhenius equation for the frequency dependence of the transition temperatures of polymers.
A linear relationship was established for several polymers between the pre-exponential factor and the activation energy of a modified form of the standard Arrhenius equation. In addition, the energy term itself was modified such that the alpha and beta absorptions were related to the number of carbon atoms available for segmental motion. At large activation energies the modified Arrhenius relationship suggested upper limits of 495.5K and 321K for the alpha and beta transition temperatures, respectively.
Dynamic mechanical thermal analysis of dental polymers. I. Heat-cured poly(methyl methacrylate)-based materials.
The visco-elastic characteristics of homogeneous, heterogeneous and fibre-reinforced poly(methyl methacrylate) for use as denture base resins were measured by a dynamic mechanical thermal analyser in the frequency range 0.033-90 Hz. Dynamic Young's modulus of both polyethylene woven yarn and the polyaramid plain fabric-reinforced acrylics showed considerable improvement over the conventional denture base, whereas the carbon fibre-reinforced acrylic produced results lower than expected. From the loss tangent curves of the homogeneous and heterogeneous methacrylates their transition temperatures were recorded and the reciprocals of both the alpha and beta absorption temperatures were plotted against frequency to obtain their respective activation energies.
Dynamic mechanical thermal analysis of dental polymers. III. Heterocyclic methacrylates.
Four heterocyclic methacrylates were investigated with respect to their loss tangent and activation energy using a dynamic mechanical thermal analyser over a limited frequency range of 0.033 to 90 Hz. The linear regression lines obtained showed an Arrhenius-type relationship between transition temperature and frequency. Their low activation energies in comparison to poly(methyl methacrylate) and bis-phenol A based methacrylates was thought to be a direct consequence of their low molecular weights.
Only one thing is clear in health care's crystal ball.
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System CFOs at top of healthcare finance career ladder.
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What's on the CFO's desk and why you need to know.
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What's all the fuss about?
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Ethical choices in an ambivalent world.
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Richard Clarke: listening, questioning, and serving members.
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Cash flow slowdowns threaten provider viability.
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Water absorption characteristics of dental microfine composite filling materials. I. Proprietary materials.
Long-term water absorption characteristics have been studied for four microfine composite filling materials over a period of some three years. By measuring gravimetrically the kinetics of successive absorption/desorption cycles, equilibrium uptake, diffusion coefficients, and solubility data have been obtained. Equilibrium uptake and solubility data are best rationalized if expressed as volume/volume rather than weight/weight. Diffusion coefficients were similar to those obtained previously for conventional composites.
Temperature gradients in the microwave-irradiated egg: implications for avian teratogenesis.
Five experiments were performed on a total of 60 non-fertile eggs of Gallus gallus to determine the spatial character, persistence, and physical basis of thermal gradients after a 300-s exposure to the intense, multipath, 2.45-GHz yield of a multimode cavity (dose rates: approximately 80 to 120 mW/g). After irradiation of an intact egg that was first equilibrated to the ambient temperature, a 3-mm diameter Plexiglas rod, which was fitted with junctions of four microwire thermocouples at 10-mm intervals, was inserted to place the distal junction in the approximate center of the yolk, the most proximal junction in peripheral thin white. Temperatures measured immediately after irradiation revealed a highly reliable linear gradient of mean temperatures from central yolk to peripheral white (P less than 0.001). The gradient was also highly persistent: Mean temperatures of central yolk exceeded those of outer thin white by more than 4 degrees C 5 minutes after irradiation, and by more than 2 degrees C 60 minutes afterward. In contrast, when an egg's contents were mixed before irradiation, the gradient was effectively eliminated. A previous report of athermally induced (field-specific) teratogenesis in chick embryos is placed under an interpretive cloud by the present findings: Terata emerged from eggs that were structurally intact during microwave irradiation, but estimates of maxima of embryonic temperatures were based on thermal measurements of non-fertile eggs the contents of which had been mixed by a thermal probe before irradiation.
Photic cuing of escape by rats from an intense microwave field.
A total of 16 female hooded rats was first observed for baseline behaviors and then they received 25 2-min trials of training, five trials per day, under one of four stimulus conditions (all ns = 4): exposure to a highly intense 918-MHz field (dose rate, 60 mW/g); exposure to photic stimulation (approximately 350 1x); exposure to the field in synchrony with photic stimulation; or exposure to faradic shock (approximately 800 micro A rms). During conditioning trials, which were separated by 2-min intertrial intervals, entry by a rat into a safe area of a multimode cavity resulted in immediate and complete cessation of stimulation; exit, in resumption. Acquisition of the escape response was rapid and highly efficient for shocked animals and was less rapid and efficient but was reliably demonstrated by irradiated animals that were also signaled by light. In the absence of microwave irradiation, cessation of light did not reliably motivate escape behavior. Although there was weak evidence of escape learning by rats subjected only to microwave irradiation, their performance failed to differ reliably from those of rats in the light-only condition. These data confirm and extend those of Carroll et al, which indicate that potentially lethal, deeply penetrating, nonpulsed microwaves in a multipath field lack the sensory quality to motivate efficient aversive behavior by the rat.
Visco-elastic properties of some room-temperature polymerizing resins.
Visco-elastic properties of a number of room-temperature polymerizing resins have been determined using a torsional pendulum apparatus. The storage and dissipative components of the complex shear modulus were determined in the temperature range 20-180 degrees C, together with the mechanical loss tangent.