research papers
Crystallization in confined environments usually induces polymers showing complicated crystallization kinetics and unusual crystalline structure. Beyond the typical confined polymer systems, pre-existing crystals can also exert confinement effects on the subsequent crystallization of polymorphic or multi-component polymers; this, however, is not well understood at present. Herein, poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA, abbreviated as L/D) asymmetric blends with various PDLA fractions (fD = 0.02–0.5) are chosen as a model system and the effects of pre-existing stereocomplexes (SCs) on the crystallization kinetics and polymorphic structure are investigated. It is found that unusual β-form homocrystals (HCs) of poly(lactic acid) can be formed in an asymmetric L/D blend, which are strongly influenced by the molecular weights (MWs) of the used polymers, L/D mixing ratio, thermal treatment temperature (Tmax) and crystallization temperature (Tc). The formation of β-HCs is preferred in asymmetric L/D blends with low and medium MWs, medium fD (0.1–0.2), medium Tmax (170–200°C), and low Tc (70–110°C). The metastable β-HCs reorganize into the more stable α-HCs via melt recrystallization in the heating process. It is proposed that the β-HC formation stems from the constraining effects of pre-existing SCs; this constraining effect is governed by the content of pre-existing unmelted SCs in the thermally treated samples.
Keywords: stereocomplex crystallization; poly(lactic acid); polymorphic structures; phase transitions; homocrystals.
Supporting information
Portable Document Format (PDF) file https://doi.org/10.1107/S160057672001078X/fs5186sup1.pdf |