Epitaxial crystallization and crystalline polymorphism of polylactides

Two crystal phases of poly(L-lactide) and that of the racemate of poly(L-la ctide) and poly(D-lactide) can be grown epitaxially on one and the same cry stalline substrate, hexamethylbenzene (HMB), which had been shown by Zwiers et al. [Polymer 1983;24:167] to form a eutectic with these polymers. The s table oc-crystal modification of the optically active polymer, based on a 1 0(3) helix conformation (for PDLA; 10(7) for PLLA), is obtained for T-c nea r 155 degrees C. A new crystal modification is produced by epitaxial crysta llization at slightly lower T-c (approximate to 140 degrees C). The crystal structure of this new form is established by electron diffraction and pack ing 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 A ngstrom. The racemate of poly(L-lactide) and poly(D-lactide) also crystalli ze epitaxially (at approximate to 165 degrees C) on HMB, which appears to b y: a very versatile substrate. (C) 2000 Elsevier Science Ltd. All rights re served.