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Biomedical subjects

R L Clarke

Publications and source records attributed to R L Clarke.

At least 19 recordsLinked to original sources
PubMed1992

Modified Arrhenius equation for the frequency dependence of the transition temperature of polymers--a note on some further observations.

Activation energies obtained from differential mechanical thermal analysis work on polymers have been observed to relate to similar behaviour of diffusion coefficients of gases in polymers. Comparison of the two techniques showed the additional energy terms to be virtually identical and the pre-exponential factors to relate to segmental motion of a five-unit carbon atom.

Biocompatible Materials↗
PubMed1989

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.

Aziridines↗
PubMed1989

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.

Biocompatible Materials↗
PubMed1989

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.

Dental Materials↗
PubMed1989

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.

Acrylates↗
PubMed1984

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.

Absorption↗
PubMed1983

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.

Abnormalities, Radiation-Induced↗
PubMed1982

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.

Chemical Phenomena↗
PubMed1978

(2-exo-3-endo)-2-Aryltropane-3-carboxylic esters, a new class of narcotic antagonists.

Tropanes 4 bearing an exo aromatic group on carbon-2, an endo-carbomethoxy group on carbon-3, and either a methyl group or a hydrogen on the nitrogen were found to be narcotic antagonists which were devoid of demonstrable analgesic activity. The activity resided in the 1S enantiomer. Compound 4 (R = m-hydroxyphenyl) showed an AD50 of 0.37 mg/kg sc and 1.8 mg/kg po (rats) as an antagonist in the Harris-Pierson modification of the D'Amour-Smith test. The tropane esters for this study were prepared by a Grignard reaction which gave essentially complete 1,4-addition in the absence of copper salts. Nearly equal quantities of esters epimeric at carbon-3 were formed.

Acetylcholine↗
PubMed1978

(exo, exo)-2-Aryltropane-3-carboxylic esters, hypoglycemic agents with accompanying analgesic activity.

(exo, exo)-2-Aryltropane-3-carboxylic esters of types 6, 7, and 10 lower circulating blood glucose levels by 60--80%. This activity is accompanied by an analgesic activity roughly equal to that of codeine. Both of these activities reside in the 1R enantiomer and extensive structure-activity studies failed to separate them. The specific opioid antagonist nalorphine blocks the analgesic activity but does not diminish the hypoglycemic action. Conformational integrity afforded by the ethylene bridge is neccessary for the observed activities.

Administration, Oral↗
PubMed1975

Perianal Paget's disease: report of two cases.

A patient with extensive perianal extra-mammary Paget's disease was treated by wide local excision of the perianal skin. No underlying associated adenocarcinoma was identified, and the patient is clinically free of disease five years after treatment. A second patient had a small focus of Paget's disease confined to a hemorrhoid. The necessity for histologic evaluation of all anal lesions is emphasized.

Adult↗
PubMed1975

Beta-cocaine.

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Anesthetics, Local↗
PubMed1975

Analgesic activity of the epimeric tropane analogs of meperidine. A physical and pharmacological study.

Condensation of cis-N-benzyl-2,5-bis(chloromethyl)pyrrolidine (6) and phenylacetonitrile afforded a mixture of epimers 7 and 8. Compound 8 was readily converted to the meperidine analog 1 prepared earlier by Bell and Archer. Compound 7 was converted to a new tropane analog of meperidine, compound 3. The ED50 of 1 and 3 in the D'Amour-Smith "tail flick" test for narcotic type analgesia, which differs by a factor of only 3 or 4 potency, supports the accumulated data that suggest that the analgesic activity of the meperidine type is not very sensitive to the conformation of the phenyl group in 4-phenylpiperidines. A proton and 13C magnetic resonance spectral comparison of 1 and 3, as well as a reevaluation of the conformational requirements of 17-19, leads to the conclusion that the differences in conformation of 1,3,17, and 18 are due to the varying degrees of flattening of piperidine ring. The 1H NMR and 13C NMR data are not consistent with the boat conformation suggested earlier for compound 17.

Analgesics↗