Cracks and ring bands of poly(3-hydroxybutyrate) on precrystallized poly(l -lactic acid) template

The lamellar morphology, cracks, and ring-banded spherulites were analyzed by allowing crystallization of poly(3-hydroxybutyrate) (PHB) on previously crystallized lamellar templates of poly(l-lactic acid) (PLLA) in mixtures of two semicrystalline polymers, PHB/PLLA (50/50) blend. PHB/PLLA blends were two-step crystallized first at crystallization temperature (T_c = 115-120°C) and then cooled to ambient temperature. At T_c, only PLLA crystallized while PHB was an amorphous liquid; then at ambient temperature PHB crystallized and followed the preformed PLLA lamellar crystals. In addition, the amorphous PHB in PHB/PLLA blends induces the formation of ring-banded spherulites of PLLA at lower T_c. The radial short-segmental cracks aggregated on the circumferential bright bands occur in crystallized PHB/PLLA blends, which cannot be adequately interpreted by the temperature-induced coefficient of thermal expansion (CTE) upon cooling, but more likely they are related to interlamellar structuring of PLLA crystals at primary stage that are to be superimposed by the PHB crystals at later stage. The solvent etching of blends not only creates a valley-ridge depth profile in outer lamellar plates, but also exposes the internal branched crystals underneath the bright band (ridge after etching), which are housed underneath the concave portion of lamellae with lenticular-shaped radial cracks. PHB and PLLA in crystallized blends are intermixed in the dark bands, and these two lamellae also are interwound as radial bundles in the bright bands.