Superheating of Linear High-Polymer Polyethylene Crystals

TLDR

The study examined how melting times vary among different polyethylene crystal morphologies. The authors developed new DTA and calorimetry methods, including a microscope hot stage capable of heating up to 3000 °C/min, to measure melting points. Extended‑chain polyethylene crystals exhibit pronounced superheating, whereas smaller or folded‑chain crystals show reduced superheating, and less perfect crystals with large surfaces reorganize, indicating a zero‑entropy‑production melting region between slow‑rate reorganization and fast‑rate superheating.

Abstract

The time dependence of melting of morphologically different crystals of polyethylene was investigated. New DTA and calorimeter techniques were developed. A microscope hot stage allowing heating rates of up to 3000°C/min was built for melting-point determinations. The superheating of extended-chain crystals was clearly demonstrated by qualitative DTA and isothermal quantitative calorimetry. Smaller crystals and folded-chain crystals showed decreasingly less superheating. Crystals which are less perfect and have large surfaces show increasing reorganization. A region of approximately zero entropy production melting seems to lie between reorganization at slow heating rates and superheating at faster heating rates.