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synopsisStressstrain curves at various temperatures and secant modulus vs. temperature curves were used to investigate the differences that exist between peroxide orosslinked 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 a t elevated temperatures depends on crosslinking density, while at lower temperatures it is dominated by polyethylene crystalline morphology. This latter factor makes crosslinked polyet.hylene 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 stiff ness. A good indication of crosslinking density of crystalline polyolefins may be obtained by measuring mechanical properties such as modulus a t 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. INTRODUCTIONIn recent years, the difference between certain plastic products and vulcanized rubber has become increasingly difficult to define. This has been particularly true with the advent of crosslinked low-density polyethylene and some rubberlike thermoplastic products such as the flexible ethylene copolymers.It seemed advisable a t this time to look more closely into some of the basic properties of crosslinked crystalline polyolefins and to see how they compare to a rubber product.Since, for the purpose of this work, a profile of each material was sufficient, only one representative resin from each type was used. Carbonreinforced vulcanized styrene-butadiene rubber controls were chosen because it would have been unrealistic to compare crosslinked rubbery poly- olefins to relatively weak, seldom used, nonreinforced rubber. On the other hand, the polyolefins could not be filled to the same degree as the rubber and retain any degree of rubberlike properties. As a result of these opposing factors, it seemed reasonable to work with crosslinked unfilled polyolefins and to compare these with carbon-reinforced rubber conpositions. The data on the rubber controls can then be viewed as outside points of reference but are not comparable in every respect. For instance, the calculations of crosslinking density or the per cent gel make interesting reading, but the values do not mean the same thing as for the unfilled olefin systems. DISCUSSION Crosslinked Low-Density PolyethylenePeroxide-crosslinked po...