The frustrated structure of poly(L-lactide)

The crystal structure formed upon stretching or stroking of poly(L-lactide) is determined by electron diffraction and conformational energy analysis. It rests on a frustrated packing of three three-fold helices in a trigonal unit-cell of parameters a = b = 1.052 nm, c = 0.88 nm, space group P3₂. The frustrated packing is of the type described as North-South-South (NSS). This structure appears to be formed to accommodate the random up-down orientation of neighbor chains associated with rapid crystallization conditions. This randomness introduces structural disorder (c-axis shifts and azimuthal setting of neighbor helices). The resultant streaking of the diffraction pattern is modeled. Frustrated packings observed in polymeric systems that depart from three-fold symmetry, and in pseudo-racemates of low molecular weight compounds are discussed. (C) 2000 Elsevier Science Ltd.