Reversible melting and crystallization of short and long flexible chain molecules by Temperature-modulated calorimetry

Melting and crystallization of linear, flexible molecules of different lengths was studied by temperature-modulated differential scanning calorimetry, TMDSC. Various techniques for TMDSC with single and multifrequency modulations have been analyzed to optimize the conditions for the present study. The finally chosen method involved a quasi-isothermal mode with a temperature amplitude of 0.5 K and a period of 60 s (frequency = 0.167 Hz). The interpretation of the reversible and irreversible melting was developed by comparison of a variety of different modes of analysis (sinusoidal, sawtooth, and complex sawtooth). The analyzed molecules ranged from n-paraffins, oligomeric fractions of polyethylene and poly(oxyethylene) to macromolecules of polyethylene. The most important discovery was that there is a critical chain length for reversible melting and crystallization of small, flexible molecules at 10 nm or about 75 backbone chain atoms. Below this chain length, melting and crystallization is reversible under the given conditions of analysis and in the presence of primary crystal nuclei. Above this chain length, the crystallization requires a degree of supercooling which becomes constant for 200 chain atoms or more at a value of 6.0 !10 K. This critical chain length sets a lower limit for the need of supercooling, a characteristic property of flexible polymers. This result was then applied to resolving the problem of the existence of a small amount of reversible melting in polymers, discovered about five years ago. The following was shown: chain segments with melting temperatures equal to oligomers of less than the critical chain length can crystallize and melt reversibly, even when contained within the metastable structure of semicrystalline polymers. Above this chain length, longer segments can only show reversible melting when a molecular nucleus remains on the crystal surface after partial melting. The short-chain segments have been seen in linear-low-density polyethylene. The longer segments in the main melting peak of polyethylene. These observations are combined with the knowledge derived from the ATHAS data bank to propose three reversible and three irreversible calorimetric contributions to the heat capacity of flexible macromolecules.