Crystal structure and disorder in Poly(l-lactic acid) δ form (α′ form) and the phase transition mechanism to the ordered α form

Crystal structure of poly(l-lactic acid) (PLLA) α′ form (or the newly-termed “δ” form) has been analyzed on the basis of the X-ray diffraction data. The oriented δ form was obtained by stretching and annealing the melt-quenched sample at ca. 100 °C. The unit cell parameters were a = 10.80 Å, b = 6.20 Å, c (fiber axis) = 28.80 Å and α = β = γ = 90°. As seen from the lack of even-numbered 00l reflections (002, 004, ..) except the 0010 reflection, the chain conformation of the δ form was found to be more remarkably disordered than that of the regular α form. As already reported, the α form takes the (10/3) helix with approximately 21 screw symmetry although, more strictly speaking, this screw symmetry is not actually existent. The δ form does not possess any symmetry along the chain axis although the local torsional angles are not very much different from each other. As for the chain packing structure, the two different possibilities were assumed in the structural refinement procedure: (i) the corner and center chains direct upward and downward along the chain axis, respectively, just like the case of regular α form (alternate packing mode) and (ii) the upward and downward chains are located at the same lattice site at 50% probability (statistically-disordered packing mode). The structure analysis using the Bragg reflections suggested the preferability of the model (i). The diffuse and streaky lines were observed along the layer lines, originating from the relative-height disorder between the neighboring chains in the δ form. The X-ray diffuse scatterings coming from the various types of the disordered structure have been simulated, among which the disordered multi-domain structure model was found to be the most plausible model, where the domains constructed by the alternate arrangement of upward and downward chains are gathered together with the mismatch in the relative height between the neighboring domains. In this way the disorder-to-order phase transition from the δ to α form was found to occur through the conformational ordering of the chains accompanied with the chain packing regularization as well as the matching of the neighboring domain height to give a large single domain of the α form.