synopsisThe anomalous freezing point depression, AT, of benzene-swollen vulcanizates has previously been attributed to the limitation of (benzene) crystallite size by the polymer network. This study was initiated to determine the benzene crystallite size in a number of rubber and benzene systems. A special low-temperature specimen holder was designed and constructed in the Cambridge Laboratories for running diffraction patterns at temperatures near -30°C. X-ray line broadening techniques were used to study a series of filled and unfilled vulcanizates of varying crosslink density. The results indicate that crystallite size is not depressed to the degree predicted by freezing-point measurements. Benzene crystallite sizes were similar in all rubber benzene systems, regardless of degree of crosslinking or benzene fraction, although carbon black loading appears to increase crystallite size. This effect may be attributed to lesser depth of penetration of the x-rays due to greater density as carbon black loading increases. Additional studies measnring the AT for solutions and similar vulcanizates of NR and SBR over a wide range of rubber concentrations showed that at the same rubber in benzene fraction, crosslinking increases AT but the addition of carbon black reduces AT. An explanation for the observed phenomena is advanced.
A detailed study has been made of the swelling of vulcanizates containing “adhering” and “non-adhering” fillers. The restriction of the swelling of vulcanizates containing “adhering” fillers compared to the swelling of the corresponding pure gum vulcanizate does not take place in solvents with solvent power below a critical value. The solvent power is indicated by the volume fraction of rubber (vr0) in the swollen pure gum vulcanizate. It appears that the critical value, vrc, is not very dependent on the type of black and slightly dependent on the crosslink density. Graphitized (2700° C) blacks differ from all other fillers in that they cause no restriction in swelling for any solvent, nor do they increase the swelling of the apparent matrix as is the case with non-adhering fillers. A semi-quantitative explanation for the observed phenomena is proposed.
Tobramycin concentrations have been determined in serum from capillary, venous and arterial blood samples taken from 16 patients during and after surgery. In 73 paired samples the concentrations in capillary samples were not significantly different from those measured in venous samples. The small concentration differences were neither dependent upon sampling time nor core‐peripheral temperature differences. In 26 paired samples, concentrations in capillary samples were not significantly different from those determined in arterial samples. We conclude that concentrations in capillary samples are precise and unbiased estimators of venous concentrations and may be used in the adjustment of tobramycin dosage regimens.
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