Stress–strain curves at various temperatures and secant modulus vs. temperature curves were used to investigate the differences that exist between peroxide crosslinked polyethylene, and vulcanized rubber. A series of hypotheses in terms of crosslinking density and crystallinity is advanced to help visualize the differences between crosslinked polyethylene and cured rubber. The same techniques were used to investigate the nature of crosslinked ethylene–ethyl acrylate copolymers. These copolymers, being more flexible than polyethylene, appear similar to “mechanical rubber,” particularly after crosslinking. Recent experiments, however, indicate that profound differences exist. The performance of crosslinked polyethylene and of ethylene copolymers at elevated temperatures depends on crosslinking density, while at lower temperatures it is dominated by polyethylene crystalline morphology. This latter factor makes crosslinked polyethylene at ambient temperature a completely different product from vulcanized rubber. It also means that a crosslinked polyethylene will have lower crosslinking density than most rubber compositions of equivalent room temperature stiffness. A good indication of crosslinking density of crystalline polyolefins may be obtained by measuring mechanical properties such as modulus at elevated temperatures. Use of reactive co-agents is effective in raising crosslinking density to the point where fairly good elevated temperature properties are obtained without increasing the room temperature rigidity.
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