Crystal Structure, Conformation, and Morphology of Solution-Spun Poly(L-lactide) Fibers

Depending on the spinning and drawing conditions, two crystal structures for solution-spun poly(L-lactide) are obtained. The pseudoorthorhombic α structure (a = 10.6, b = 6.1, and c = 28.8 Å) contains two chains in the unit cell and is found at relatively low drawing temperatures and/or low hot-draw ratios. At higher drawing temperatures and/or higher hot-draw ratios a second so-called β structure appears. For this structure an orthorhombic unit cell is proposed (a = 10.31, b = 18.21, and c = 9.00 Å) containing six chains. The chain conformations of the α and β structure are left-handed 10/3 and 3/1 helices, respectively. Calculations show that both conformations have approximately the same energy. Therefore, the preference for one of the two structures is determined by packing considerations. In fibers containing a mixture of α and β structure, the latter seems to bear most of the load during stress-strain experiments. Meridional small-angle X-ray scattering experiments yield a maximum for fibers containing only α structure pointing to a lamellar folded-chain morphology. The β structure on the other hand seems to correspond to a fibrillar morphology. Differential scanning calorimetry on unconstrained fibers shows that the β structure melts at a lower temperature than the α structure. The large shift of the peak melting temperature to higher temperatures in melting experiments on constrained fibers indicates that both lamellae and fibrils contribute to the strength of the fibers. This points to a considerable amount of interconnections/ entanglements between adjacent lamellae.