The drawing behavior of linear polyethylene. I. Rate of drawing as a function of polymer molecular weight and initial thermal treatment

TLDR

A molecular network model that incorporates both crystalline and amorphous regions better explains polyethylene drawing across all molecular weights. Polyethylene sheets were produced by quenching molten polymer or by slow cooling followed by quenching, and their structure was characterized by optical microscopy, SAXS, and Raman spectroscopy to relate features to molecular weight and distribution. Drawing experiments on linear polyethylene (M̄ w 67,800–3.5 M) revealed that low‑M̄ w polymers are highly sensitive to initial thermal treatment, high‑M̄ w polymers are not, yet high‑M̄ w samples can achieve draw ratios above 17 and very high Young’s moduli.

Abstract

Abstract The drawing behavior of a series of linear polyethylene homopolymers with weight‐average molecular weight (M̄ w ) ranging from 67,800 to ∼3,500,000 and variable distribution (M̄ w /M̄ n = 5.1−20.9) has been studied. Sheets were prepared by two distinct routes: either by quenching the molten polymer into cold water or by slow cooling below the crystallization temperature (∼120°C) followed by quenching into cold water. When the samples (2 cm long) were drawn in air at 75°C using a crosshead speed of 10 cm/min it was found that for low M̄ w polymers the initial thermal treatment has a dramatic effect on the rate at which the local deformation proceeds in the necked region. At high M̄ w such effects are negligible. An important result was that comparatively high draw ratios (λ > 17) and correspondingly high Young's moduli could be obtained for a polymer with M̄ w as high as 312,000. It is shown how some of the structural features of the initial materials (mainly studied by optical microscopy, small‐angle x‐ray scattering and low‐frequency laser Raman spectroscopy) can be interpreted in terms of the molecular weight and molecular weight distribution of the polymers. Although crystallization and morphology can be important at low M̄ w , it suggested that the concept of a molecular network which embraces both crystalline and noncrystalline material is more helpful in understanding the drawing behavior over the whole range of molecular weights.