Reversibility between Glass and Melting Transitions of Poly(oxyethylene)

The heat capacities, C p, 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 C p . The folded-chain crystals of high molar mass show some locally reversible melting. The reversing, apparent C p depends on molar mass and amplitude and frequency of modulation. After separation from the latent heat effects, the reversible, thermodynamic C p depends on the melting temperature for low molar masses and increases beyond the vibrational C p due to conformational motion. Molar masses of 8000−20 000 have almost the same C p . These observations permit a quantitative discussion of the thermodynamic C p and the locally reversible melting of the globally metastable POE in the melting range. The increase in C p between 250 K and the melting temperature is interpreted as a glass transition within the crystal.