Phase-separation-induced single-crystal morphology in poly(l-lactic acid) blended with poly(1,4-butylene adipate) at specific composition

The single-crystal morphology of poly(l-lactic acid) (PLLA) in blending with poly(butylene adipate) (PBA) in PLLA/PBA blends was for the first time reported in melt crystallization. At crystallization temperature (T_c) = 110 °C, by adding 30 wt % PBA into PLLA, the lamellae exhibit six-stalk dendrites with single-crystal packing. Phase separation and crystallization took place simultaneously at T_c = 110 °C in PLLA/PBA (70/30) blend, leading to discrete PBA domains and continuous PLLA domains. For PLLA/PBA (70/30) blend, all PBA were rejected from the growth front of PLLA crystals, expelled, and crystallized at ambient temperature as ring-banded PBA spherulites inside the discrete domains only, resulting in a favorable environment for formation of PLLA single crystals in the continuous domain. Atomic force microscopy (AFM) observation on individual crystallites reveals that lozenge-shaped single crystals were packed with a clockwise spiral pattern, stacked in 1-3 layers, and these lozenge-shaped crystals are aligned six hexasected directions into hexastalk dendrites with occasional side branches that are also aligned at 60° to main branches. The monolamellar thickness of lozenge-shaped single crystals was measured to be about 13-34 nm, and the dimension is about 0.8-3 μm along the short axis and 1.6-5 μm along the long axis. Typically, three layers of single crystals are stacked one on another; the lozenge crystals on the bottom layer are about twice as large as those on the top layer, forming a pyramid shape in the depth direction. Formation mechanisms of single crystals in melt-crystallized PLLA/PBA blend from 700 nm film thickness are discussed in correlation with exact phase separation at 30 wt % PBA.