| Abstract: | Isoprene–urea and butadiene–urea block polymers have been prepared by reaction of isocyanate-terminated prepolymers with diamines and diisocyanates. It was found that the per cent of blocked urea in these copolymers is dependent on the isocyanate–diamine stoichiometry. Stress–strain data were obtained on sulfur-cured copolymers. Stress levels at any given extension were directly dependent on the urea content, however, ultimate strength was maximized at about 35 wt-% urea. Slight variations in urea structure produced rather dramatic changes in the modulus of these cured elastomers. Diamine–diisocyanate stoichiometry did not affect the physical properties of these cured copolymers at equal or excessive diamine concentrations. However, at excess diisocyanate levels, elastomeric properties deteriorated rapidly. Finally, stress–strain properties were affected by polymerization solvent. Copolymers prepared in hexane gave higher values of stress at low strains than corresponding elastomers prepared in toluene. These latter results are explained in terms of the nonaqueous emulsion in which the urea blocks form. |
