Evidence of a dual network/spherulitic crystalline morphology in PLA stereocomplexes

Stereocomplex formation between poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) in the melt state was investigated. Blends with up to 5% PDLA were prepared via melt-blending and characterized by rheological, calorimetric, X-ray diffraction (XRD) and optical microscopy techniques. Kinetic studies suggest that the stereocomplex formation half-time (t1/2) is in the range of 5–7 min. The nucleating role of the stereocomplex for PLA homocrystallization was demonstrated in XRD experiment and in optical microscopy observations through an increase in the spherulite density as well as the growth of a transcrystalline layer on stereocomplex spherulites. More interestingly, a dual crystalline morphology for PLA stereocomplex comprising a spherulitic and a network structure was revealed by optical microscopy observations. This complex morphology was associated to a double stereocomplex melting peak behavior observed after isothermal crystallization in DSC as well as to a huge viscosity increase observed in the presence of a small concentration of stereocomplex in a PLLA melt.