Extensive development of precursory helical pairs prior to formation of stereocomplex crystals in racemic polylactide melt mixture

Melt crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 °C/min from 270 °C) was studied via a combination of wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (3 ₂/3 ₁) helical pairs below 220 °C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 °C; the intensity of this mesophase peak started to decrease at 150 °C, with concomitant emergence of WAXS- or DSC-discernible formation of stereocomplex (β _c) crystals. Isothermal measurements at 200 vs 170 °C revealed the presence of low vs high populations of helical pairs; β _c crystals were observed to develop only at 170 °C but not at 200 °C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of β _c nuclei. The clear change in the melt structure well before the formation of incipient β _c crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of β _c crystals.