Stereoselective crystallization and specific interactions in polylactides

The stereoselective annealing conditions for polylactides to crystallize as a homocrystals (T-m = 180 degrees C) and/or higher melting temperature (T-m = 230 degrees C)eta 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 a, exclusively the form eta, or coexist as both a and 77 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 (CH3O)-O-...=C and (C alpha HO)-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.