Mechanical and optical anisotropy in low-density polyethylene

Abstract

Polymers develop optical and mechanical anisotropy on drawing. The measurement of the extensional modulus of drawn polymer films in directions O, 45, and 90† to the initial draw direction (called E₀, E₄₅, and E₉₀, respectively) gives an indication of the extent of mechanical anisotropy. On the basis of studies on fibers and films it has previously been shown that nylon, polyethylene terephthalate, polypropylene, and high-density polyethylene give a “conventional” pattern of anisotropy; i.e., E₀ increases steadily with increasing draw ratio, and at high orientations E₀ is much higher than E₉₀ and E₄₅. Low-density polyethylene is known to give an anomalous pattern in that E₀ shows a large minimum at low draw ratios, whereas at high orientations E₀ is fairly close to E₉₀ and both are much larger than E₄₅. The present study was undertaken to throw some light on the unique behavior shown by low-density polyethylene. A series of oriented low-density polyethylene films was made by drawing isotropic, melt-pressed sheets to a wide range of draw ratios. For each draw ratio, E₀, E₄₅, and E₉₀ were measured over the temperature range -125 to 60†C. The pattern of anisotropy changes markedly with temperature, and the following features emerge. First, the minimum in E₀ which can be observed at low draw ratios is largest at 20†C and is absent within experimental error at —125†C. Second, considering only the most highly oriented sample, at —125†C E₄₅ and E₉₀ are comparable and about one-third as great as E₀., whereas at room temperature E₄₅ is an order of magnitude less than E₀ or E₉₀. The pattern of anisotropy observed at room temperature has been explained by a theoretical model in which the polymer is regarded as an aggregate of anisotropic units whose properties are those of the most highly oriented film. The unusual pattern observed on this model is attributed to the very low shear modulus of highly oriented low-density polyethylene. It is shown here that the aggregate model gives reasonable agreement with the experimental data over the whole range of temperatures. The absence of a minimum in E₀ at the low temperature is consistent with the relatively larger shear modulus of the highly orierited polyethylene at that temperature. Reasons for the discrepancies between the experimental data and the predictions of the aggregate model are discussed. In addition, some tentative molecular implications of the behavior are suggested, based on the correlation between the low shear modulus and the presence of branch points in the structure.