Abstract
The inter- and intramolecular contributions to the entropy and energy of fusion are calculated for several linear aliphatic polyesters and polyamides assuming the fusion process consists of two independent contributions: the volume expansion (intermolecular contribution) and the increase in the conformational freedom of each polymer chain on melting (intramolecular contribution). The intramolecular entropy and energy contributions are obtained from the configurational partition function and its temperature coefficient calculated for an isolated, unperturbed polymer chain using the rotational isomeric state approximation. Subtraction of the calculated changes in the conformational entropy and energy upon melting from the measured entropy and enthalpy of fusion, respectively, provides an estimate of the intermolecular contributions to these two quantities if the small volume change accompanying melting is ignored and the enthalpy and energy of fusion are assumed to be nearly identical. Application of this procedure to several aliphatic polyesters and polyamides leads to the following conclusions: (i) The calculated intramolecular contribution to the entropy of fusion is nearly the same for the polyesters and polyamides; (ii) the measured enthalpies of fusion are nearly the same for both classes of polymers; (iii) the entropies of fusion are smaller for the polyamides; and (iv) the lower entropies of fusion of polyamides are probably caused by disorder in the crystal resulting in their higher melting points compared to the polyesters.
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