Polymorphism in Poly L-lactic acid

Similar to other polyesters, PLA displays crystal polymorphism and four different crystal modifications have been identified so far, named a-, P-, y-, and e-forms. The a-form of PLA grows upon melt- or cold-crystallization, as well as from solution [18-25]. Hot-drawn, melt-spun, or solution-spun PLA fibers of a high-draw ratio show the P-form [21-23]. The y-form is obtained via epitaxial crystallization on hexamethylbenzene substrate [24] and the s-modification is a crystalline complex formed below room temperature in the presence of specific organic solvents such as tetrahydrofuran and Af,Af-dimethylformamide [25]. Besides these four main crystal polymorphs, two disordered modifications of the a-form, named a and a , were recently proposed for PLA. The a -crystals grow via melt- or cold-crystallization below 110 °C [21, 26-29], whereas the a -form develops upon crystallization under special processing conditions, below the glass transition temperature and in the presence of carbon dioxide [30]. 

Further, the thermal properties of PL A vary with crystal polymorphism. Similar to other semicrystalline polymers, PLA exhibits a three-phase structure, consisting of the crystal phase and two amorphous fractions which vitrify/devitrify in different temperature ranges [45]. Chain segments in the bulk amorphous phase 

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