Structure and mechanical properties of poly(L-lactic acid) crystals and fibers

The elastic constants of poly(L-lactic acid) (PLLA) crystals are reported on the basis of a commercial software package and the published crystal structure of the alpha form. A chain modulus of 36 GPa and a shear modulus of 3 GPa have been obtained for cylindrically symmetric aggregates of perfectly oriented crystals. The helical conformation of the PLLA molecule reduces the stiffness in the chain axis direction because bond rotation plays a significant role in the deformation. X-ray crystal strain measurements suggest that shear of the alpha crystal parallel to the helix axis is the easiest mode of deformation, in agreement with the expectations obtained from the low shear modulus of 3 GPa obtained from the theoretical calculations. A combination of small- and wide-angle X-ray scatteririg, differential scanning calorimetry, dynamic mechanical thermal analysis, and shrinkage measurements has been used to characterize the structure that develops and the crystal transformation that occurs during fiber processing. The structure that develops during processing very much depends on the crystal transformation, and a structural model is proposed for fibers at different degrees of plastic deformation. The transformation of the alpha crystal into the beta form and vice versa is governed primarily by shear along the helix axis because the chains must shear past each other during the crystal transformation, disrupting the lamellar packing. (c) 2007 Wiley Periodicals, Inc.