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- Title
Crystal Polymorphism and Morphology of Polylactides.
- Authors
Lotz, Bernard
- Abstract
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 ß-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.
- Subjects
POLYLACTIC acid; CRYSTALLIZATION; CRYSTAL structure; CRYSTALLINE polymers; SPHERULITES (Polymers)
- Publication
Advances in Polymer Science, 2018, Vol 279, p167
- ISSN
0065-3195
- Publication type
Article
- DOI
10.1007/12_2016_15