Effects of morphology and molecular structure on tensile impact behavior of linear polyethylene

An instrumented tensile impact test which allows a load time curve to be obtained coincident with the energy to rupture values has been applied to the examination of a series of linear polyethylene homopolymers and copolymers of varying molecular weight and thermal history. The effect of test specimen geometry was also investigated. A morphological examination of these resins indicated a strong dependence of spherulite size on molecular weight, and to a much lesser degree on molding conditions; it appeared to be independent of comonomer. The overall crystallinity, as determined by both x‐ray and density methods, and crystallite perfection was increased by a slower crystallization rate, but was independent of molecular weight. The tensile impact strength (energy to rupture) was found to increase with molecular weight, and decrease on the introduction of comonomer. But the effect of thermal history gave rise to results which bore no clear‐cut relation to molecular and morphological parameters. The value of the instrumented tensile impact test is shown by the fact that these apparent anomalies can now be resolved by separating the elastic and plastic portions of the deformation. An increased level and perfection of crystallinity results in higher load bearing properties accompanied by a reduction in duration of impact. It is primarily the duration, rather than the peak load, which is affected by molecular weight. The main effect produced by changing from a type L (long) tensile impact specimen to a type S (short) was to reduce considerably the duration of the plastic region while simultaneously raising the maximum elastic load; this is thought to be associated with a more localized deformation and therefore a higher rate of strain.