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. to form a eutectic with these polymers. The stable α-crystal modification of the optically active polymer, based on a 10₃ helix conformation (for PDLA; 10₇ for PLLA), is obtained for T_c near 155 °C. A new crystal modification is produced by epitaxial crystallization at slightly lower T_c (≈140 °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 angstroms, b = 6.25 angstroms and c = 8.8 angstroms. The racemate of poly(L-lactide) and poly(D-lactide) also crystallize epitaxially (at ≈165 °C) on HMB, which appears to be a very versatile substrate.