Crystal Polymorphism and Morphology of Polylactides

The crystal structures of poly(l-lactic acid) (PLLA), its stereocomplexes, and some features of its crystallization process are reviewed. PLLA exists in different crystal modifications that are based on two different left-handed helix geometries having three units in one turn (31) or ten units in three turns (103). The stable α-phase with a 103 helix exists in two variants: an ordered α-phase produced at high crystallization temperature and a less ordered α′-phase produced at low crystallization temperature. Two structures are based on the 31 helical conformation: an elusive γ-phase, obtained so far only by epitaxial crystallization, and a frustrated trigonal β-phase. The β-phase, first obtained in stretched fibers, could be an intermediate or precursor phase of the low crystallization temperature α′-PLLA variant, which would explain a number of its unusual crystallization features (increased growth rate, thicker lamellae, structural disorder). Stereocomplexes of PLLA and poly(d-lactic acid) (PDLA) are formed through intimate association of left- and right-handed threefold helices. They have remarkably higher melting temperatures than the homopolymers, which is explained by the presence of a dense network of weak CH…OC hydrogen bonds. Single crystals of PLLA are obtained both from solution and thin film growth. Spherulites of chiral polylactides are frequently made of twisted lamellae with a large pitch, with the sense of twist depending on the polylactide chirality.