The frustrated structure of poly(l-lactide)

Abstract

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=1.052nm,c=0.88nm, 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.