Stereoselective Crystallization and Specific Interactions in Polylactides

The stereoselective annealing conditions for polylactides to crystallize as α homocrystals (T m = 180 °C) and/or higher melting temperature (T m = 230 °C) η stereocomplex crystals are reported in this paper. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies provide evidence of the isothermal annealing conditions in which equimolar mixtures of optically pure poly(l-lactide) and poly(d-lactide) take exclusively the form α, exclusively the form η, or coexist as both α and η crystal polymorphs. Fourier transformed infrared (FTIR) spectroscopy studies reveal a shift to lower wavenumbers in the CO stretching band of polylactides in the stereocomplex with regard to that in the homocrystal. This band shift is accompanied by similar displacements in the C−H spectral bands. These results are interpreted in terms of H-bonding forces causing specific CH3···OC and CαH···OC interactions between both stereoisomers of polylactide. Molecular models support that the suggested hydrogen-bonding arrangement can be accommodated into the stereocomplex crystal lattice. The strength of the interactions has been calculated from both CO and C−H spectral regions, resulting in a mean value of 1.1 kcal/mol, close to that expected according to ab initio calculations.