Are phase definitions and the gibbs phase rule applicable to crystalline polymers?

Three issues that challenge the usefulness of Gibbs's phase rule in polymer science are discussed. First, the extent to which linear chains and cycles form different crystal lattices and, consequently, different phases is examined. Second, the question of whether semicrystalline polymers containing monodisperse extended-ring crystals (ERCs) represent a triphasic system is addressed. Third, the question of whether extended-chain crystallites (ECCs) and/or ERCs may form in the solid state under thermodynamic control is addressed. The theoretical framework is presented in the first chapter, followed by a review of the experimental evidence recently reported by the authors in the second chapter. Experimental data obtained by annealing polylactide, polyglycolide, and poly(butylene terephthalate) in the presence of transesterification catalysts provides evidence that transesterification favors the separate crystallization of linear chains and cycles in the solid state up to a degree of polymerization (DP) corresponding to the crystallites' thickness. ERCs are defined as the thermodynamic optimum of low-molecular-weight polyesters. However, it is difficult to answer the question of whether these ERCs can be considered a new phase. Defining them as a different phase relative to crystallites of folded linear chains would contradict the Gibbs phase rule. Furthermore, the formation and characteristics of polylactides consisting of ERCs, extended-chain crystals (ECC), and folded-chain crystals are discussed. Finally, the formation of monodisperse ERCs by thermodynamically controlled transesterification in the solid state is described.