Suppression of Phase Separation for Exclusive Stereocomplex Crystallization of a High-Molecular-Weight Racemic Poly(L-lactide)/Poly(D-lactide) Blend from the Glassy State

Despite considerable efforts, however, the exclusive stereocomplex (SC) formation in neat high-molecular-weight (high-MW) poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blends remains challenging. This challenge in part arises from the incomplete understanding of the influence of processing conditions. Here, the cold crystallization behavior from the glassy state of high-MW racemic PLLA/PDLA blends prepared via spin-coating and solution-casting methods was investigated mainly by infrared spectroscopy, calorimetry, and atomic force microscopy (AFM). It was found that the rate of solvent evaporation greatly affected the crystallization and stereocomplexation behavior of glassy PLLA/PDLA blends. Particularly, upon cold crystallization at temperatures well below the melting temperature of homocrystals, it was demonstrated that the exclusive SC crystallization occurred from the glassy state that was established from dilute solution by sufficiently fast spin-coating, whereas the homocrystallization was prohibited even at a low-temperature range close to the glass transition temperature . AFM observations indicated that sufficiently rapid solvent evaporation resulted in the homogeneous mixing state, which is a crucial prerequisite for the exclusive stereocomplexation, whereas slow solvent evaporation led to the phase-separated structure that was probably dominated by spinodal decomposition and thus to the predominant homocrystallization. On the basis of the Flory-Huggins theory, the results were reasonably explained by the suppression of phase separation, which was achieved via rapid solvent evaporation. The present findings not only provide a new approach toward exclusive SC formation in neat high-MW PLLA/PDLA blends but also have important implications for further mechanistic understanding of the SC crystallization of polymers.