Reversibility between Glass and Melting Transitions of Poly(oxyethylene)

Abstract

The heat capacities, Cp, of poly(oxyethylene), POE, with molar masses from 1500 to 900 000 Da, were analyzed by differential scanning calorimetry (DSC), quasi-isothermal, temperature-modulated DSC (TMDSC), and wide-angle X-ray diffraction (WAXD). There is no change in crystal structure before melting, but the lattice parameters increase rapidly in the melting region. Perfected extended-chain and once- or twice-folded crystals of the oligomers with a molar mass above 1100 Da melt practically fully irreversibly and permit direct measurement of the thermodynamic Cp. The folded-chain crystals of high molar mass show some locally reversible melting. The reversing, apparent Cp depends on molar mass and amplitude and frequency of modulation. After separation from the latent heat effects, the reversible, thermodynamic Cp depends on the melting temperature for low molar masses and increases beyond the vibrational Cp due to conformational motion. Molar masses of 8000−20 000 have almost the same Cp. These observations permit a quantitative discussion of the thermodynamic Cp and the locally reversible melting of the globally metastable POE in the melting range. The increase in Cp between 250 K and the melting temperature is interpreted as a glass transition within the crystal.