Inducing Stereocomplex Crystals by Template Effect of Residual Stereocomplex Crystals during Thermal Annealing of Injection-Molded Polylactide

In the present work, the injection-molded poly(l-lactide)/poly(d-lactide) parts were thermally treated at nearly below the melting point of stereocomplex crystals (SCs) (∼210 °C) to prepare polylactides (PLAs) with good heat resistance, and it is found that exclusive formation of SC is effectively achieved. It is worth mentioning that injection-molded PLAs used in thermal treatment were first prepared by employing a high mold temperature (120 °C) to ensure proper crystallinity and SC content, which makes sure that no warpage of PLA was noticed during thermal treatment. Consequently, superbly heat resistant injection-molded parts with Vicat softening temperature of ∼200 °C were successfully achieved. Meanwhile, the template effect of residual SCs, as evidenced by in situ X-ray diffraction results, was proposed to explain the high efficiency of thermal treatment. Specifically, the residual SC in PLA melts with perfect surface and well-defined lattice parameters can act as template to guide formation of new SCs.