Stereoselective Crystallization and Specific Interactions in Polylactides

Abstract

The stereoselective annealing conditions for polylactides to crystallize as α homocrystals (Tm = 180 °C) and/or higher melting temperature (Tm = 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 CO 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···OC and CαH···OC 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 CO and C−H spectral regions, resulting in a mean value of 1.1 kcal/mol, close to that expected according to ab initio calculations.