Melting and Crystallization of Polyethylene of Different Molar Mass by Calorimetry

Abstract

Crystallization and melting of n-paraffins of chain lengths up to C60H122 can be largely reversible, and practically no supercooling is seen in differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC). To find the changes with chain length for this effect, polyethylene fractions of oligomers of mass 560, 1150, and 2150 Da and a polymer of mass 15 520 Da were analyzed. The mode of analysis was quasi-isothermal TMDSC with an amplitude of 0.5 K about a fixed series of temperatures. For the oligomer of 560 Da, a mainly reversing melting was seen, similar to the behavior of paraffins of the same molar mass. The oligomer of 2150 Da as well as the polymer shows an irreversible crystallization with a crystallization of more than 5.0 K below the melting temperature, and the oligomer of 1150 Da is intermediate. Typical for polymers, a small amount of reversing material remains in the melting range of the polymer. The 1150 and the 2150 Da oligomers grow to extended-chain crystals, and the polymer yields folded-chain crystals. This research reveals that there is little difference in reversibility between monodisperse paraffins and fractions with similar average mass. There is no major difference in supercooling behavior for extended- and folded-chain crystals. The critical chain length for which a substantial supercooling is observed is 12.5 nm. The three types of reversible latent heat observed in macromolecules are traced to the short-chain molecules.