Specific heat of synthetic high polymers. VIII. Low pressure polyethylene

TLDR

Specific heat of granular and annealed Marlex 50 was measured from −25 °C to 150 °C, and the resulting heat‑temperature curve was successfully modeled using Flory’s copolymer crystallization theory by treating branch points and chain ends as noncrystallizing units. Crystallinity was found to be 82.6 % for granular and 93.8 % for annealed Marlex 50; its crystalline specific heat at room temperature is slightly below predictions for infinite‑chain paraffinic hydrocarbons, its maximum melting temperature is 134.8 °C (≈1.7 °C below polymethylene), 90 % of the solid melts within a 15 °C range, and unexplained negative temperature drifts just below the melting point and low specific heat just above were observed.

Abstract

Abstract Specific heat measurements have been carried out on granular and annealed Marlex 50, a polyethylene polymerized at relatively low pressures, over the temperature range −25 to 150°C. Calculation of the crystallinity yields maximum values of 82.6 and 93.8% for the granular and annealed Marlex 50, respectively. The specific heat of crystalline Marlex 50 at room temperature is slightly less than that previously predicted for a crystalline paraffinic hydrocarbon of infinite chain length. The maximum melting temperature, 134.8°, is about 1.7° less than that of polymethylene and close to the predicted value for a long chain solid hydrocarbon. 90% of the solid Marlex 50 melts over a 15° range of temperature. The shape of the specific heat‐temperature curve can be well reproduced by application of Flory's theory of crystallization in copolymers by considering branch points and chain ends to be the noncrystallizing copolymer units. Negative temperature drifts just below the melting point and low specific heat values just above have not been explained.