Crystal density and heat of fusion for a stereo-complex of poly(L-lactic acid) and poly(D-lactic acid)

Measured densities of films, consisting of only stereo-complex (sc) crystals of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA), are sometimes significantly higher than the density of the sc-crystal calculated on the basis of the previously proposed triclinic and trigonal unit cells. In this work, we reexamined the crystal density and determined the heat of fusion for the sc-crystal. Twenty wide-angle X-ray reflections from a highly oriented sc-film could be well indexed on the basis of a trigonal unit cell previously proposed by Cartier et al. The refined parameters of the trigonal cell are a = b = 1.50 nm, c = 0.823 nm, α = β = 90°, and γ = 120°. The c-axis determined from the WAXD fiber pattern was significantly shorter than those (0.87-0.93 nm) previously reported. The crystal density of the modified cell was 1.342 ± 0.002 g/cm³, which was significantly higher than those previously reported (1.21-1.27 g/cm³). The heat of fusion for the sc-crystal, extrapolated from the linear increase in the observed heat of fusion with decreasing the specific volume for the samples consisting of only sc-crystals, was 155 ± 5 J/g. This value was significantly higher than those of α-form and β-form homo-crystals of PLLA suggesting stronger interactions between the 3₂ helices of PLLA and 3₁ helices of PDLA in the sc-crystal. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2632-2639, 2007