SynopsisCobaloxime, synthesized from Co" acetate and dimethylglyoxime, was used as a chain transfer agent in the free radical polymerization of methyl methacrylate. It proved to be a catalytic chain transfer agent analogous to work recently reported on cobalt porphyrins. Chain transfer constants ranged from 1W-10' and were chain length dependent. Catalytic behavior was demonstrated by calculating turnover numbers which showed that each cobaloxime molecule participated in many hundreds of chain transfer reactions.

This study focuses on the interaction between polyamide and butyl or bromobutyl rubbers blended in a high shear environment. The fact that these two normally incompatible systems can be mixed is explained by a chemical reaction that occurs between polyamide and the butyl rubber components during processing. Measurement of the melt viscosity and differential scanning calorimetry of these blends, along with analysis of the extracted soluble butyl rubber component, supports the presence of small quantities of block or graft polymers in the system, signifi-cant crosslinking during the blending process, and possibly other interactions between the blend components. The effect of electron beam radiation on interaction in these blends was briefly evaluated and was found to increase crosslinking in the blends, with some degradation of the polyamide component.

Polyamide-12 and chlorobutyl rubber were blended by dynamic vulcanization in a high shear environment using curing systems based on sulfur, dithiocarbamate/ZnO, and 4,4-methylenebiscyclohexylamine/MgO. As expected, all blends with curing agents show increased tensile strength and elongation at break in comparison to blends without curing agents. Maximum mechanical properties are obtained at relatively low levels of curing agent in all systems. Hexane extraction of the mixtures and measurement of percentage of insolubles along with the swelling index of the rubber phase confirm that a high level of cure is achieved at low levels of curing agent. Although the curatives are designed for the rubber phase, differential scanning calorimetry results indicate that both phases are affected during the dynamic vulcanization process, with polyamide-12 showing a reduced melting temperature that is indicative of molecular weight reduction, structure changes, or reaction with the rubber phase. Scanning electron microscopy results indicate that phase size is reduced with increased blending time and level of curing agent. Rheological studies indicate that blends containing curing agents exhibit non-Newtonian behavior to a greater extent than polyamide or nonvulcanized polyamide/chlorobutyl rubber blends.

ABSTRACT:The effect of an external stress on the barrier properties of natural, bromobutyl, and nitrile rubber were studied using a modified ASTM permeation method. Stress-induced changes such as a decrease in the breakthrough time with mechanical elongation was observed. Upon application of a small mechanical deformation, little change was observed in terms of steady-state permeation flux. On the other hand, a stress-enhanced diffusion was observed for most of the solvent/rubber pairs studied.

Dynamic vulcanization was used to prepare thermoplastic elastomer blends of nylon (polyamide), polypropylene (PP) and polybutylene terephthalate thermoplastics with chlorobutyl (CIIR) and nitrile (NBR) rubbers. Mechanical properties of the blends were correlated against composition. Although hardness and tensile strength increase with increasing thermoplastic content for all blends, elongation at break values initially decrease and then increase in the range of 20-40% thermoplastic. For various blend compositions, the swelling behavior was evaluated with solvents that are able to dissolve the uncured rubber portion but not the thermoplastic component of the mixtures. All five systems showed swelling index values that were substantially less than the calculated ''theoretical'' values of swelling index. This was attributed to a caging effect of the thermoplastic component on the rubber phase, which restricts access of solvent and swelling of the rubber phase. In turn, this affects the solvent resistance of the blend. Some of the blends were evaluated by differential scanning calorimetry to assess the compatibility of the components in the blend. scanning electron microscopy was also used to determine the degree of compatibility of the two phases generated in the mixing process.