synopsisPolytetrafluoroethylene surfaces have been treated to reduce thrombogenicity in order to make them suitable for use in prosthetic devices that come in contact with blood. This was done by first etching the surface with potassium in liquid ammonia to produce double bonds and then using these double bonds as sites for grafting on polyacids or as sites for chemical reactions. Tubes 80 treated were tested for thrombogenetic activity by implantation in the thoracic aorta or inferior vena cava of dogs. These tests showed that the thrombogenicity of a polytetrafluoroethylene surface can be reduced by attaching negatively charged groups provided the surface concentration of these groups is not too high (order of 1-2 X 10-equivalent per cm* geometric area) and provided the distribution of these groups is uniform. Sulfonic acid groups obtained by chlorosulfonation and carboxyl groups attached by grafting krt-butyl crotonate and hydrolyzing to crotonic acid were effective. Long chains of poly(scrylic acid), poly-(ethylenesulfonic acid), and poly(viny1 alcohol) sulfate were less effective.
Most germane: Hexacoordinate germanium(IV) species exhibit unprecedented activities, yet controlled behavior, as initiators for the ring-opening polymerization of rac-lactide to form polylactide polymers.
ABSTRACT:The thermal properties and the isothermal cure of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4Ј-diamino-3,3Ј-dimethyldicyclohexylmethane (3DCM) were analyzed by differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The quasi-isothermal TMDSC scans were performed at curing temperatures between 40 and 140°C for different periods of time, and the modulation conditions were amplitude of 0.5K and a period of 60 s. The heating rates used on the DSC scans were between 2.5 and 20 Kmin
Ϫ1. The heat of curing measured nonisothermally increases when the heating rate decreases. An average value of 440 Jg Ϫ1 was estimated. The glass transition of the unreacted system measured by DSC was Ϫ40.4°C. The final glass transition temperature of the resin depends on the heating rate of cure and postcure conditions. Values between 143°C (measured by DSC) and 154°C (measured on the total heat flow by TMDSC) were obtained after isothermal curing and postcure at 10 and 1 Kmin Ϫ1 , respectively. The vitrification was analyzed by the modulus of the complex heat capacity ͉C* p ͉, which decay gives the interval and the time of vitrification. These properties were used to build the time-temperature-transformation cure diagram. The intensity of the vitrification was measured by the change in ͉C* p ͉, which decreases quasi-linearly with the curing temperature. The phase angle of the heat flow shows a peak in the vitrification region, which agrees with the vitrification time determined by ͉C* p ͉. . The analysis of the diffusion controlled step is studied using a mobility factor determined from the normalised variation of ͉C* p ͉ during the vitrification. The simulated overall reaction rate, which is obtained from the product of the chemical kinetics and the mobility factor, agrees with the experimental reaction rate.
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