Epitaxial crystallization and crystalline polymorphism of polylactides

Two crystal phases of poly(L-lactide) and that of the racemate of poly(L-lactide) and poly(D-lactide) can be grown epitaxially on one and the same crystalline substrate, hexamethylbenzene (HMB), which had been shown by Zwiers et al. [Polymer 1983;24:167] to form a eutectic with these polymers. The stable oc-crystal modification of the optically active polymer, based on a 10(3) helix conformation (for PDLA; 10(7) for PLLA), is obtained for T-c near 155 degrees C. A new crystal modification is produced by epitaxial crystallization at slightly lower T-c (approximate to 140 degrees C). The crystal structure of this new form is established by electron diffraction and packing energy analysis. Two antiparallel helices are packed in an orthorhombic unit-cell of parameters a = 9.95 Angstrom, b = 6.25 Angstrom and c = 8.8 Angstrom. The racemate of poly(L-lactide) and poly(D-lactide) also crystallize epitaxially (at approximate to 165 degrees C) on HMB, which appears to by: a very versatile substrate. (C) 2000 Elsevier Science Ltd. All rights reserved.