Three-dimensional dissecting onto interior lamellar structures of spherulites in poly(ethylene adipate) (PEA) in bulk forms crystallized at 28 ± 3°C were studied in correlation with thin-film forms. Interior morphology in bulk PEA samples revealed onion-like alternating shells. Interior spherulites clearly exposes that the lamellae within the shell interior are composed of stacked plate lamellae in radial direction while the shell surface region are arranged by tangential lamellae. Shell interior and surface correspond to the valley and ridge of ring-banded on the top surface, respectively. Morphology characterization on fracturing patterns further evidence that the shells could be cleaved apart along the shell surface, indicating weak physical connection between these two perpendicularly oriented shells. Further thermal analysis and X-ray results also supported existence of highly ordered lamellar packing into two mutually perpendicular chain orientations in the 28°C-crystallized PEA bulks, which differed significantly from the randomly oriented lamellae in either 2 or 33°C-crystallized PEA bulks.

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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.

Unconventional concentric up-and-down bands, differing from the conventional brightextinction or blue/orange bands in other polymers, in poly( L -lactic acid) are found via controlled thin fi lms. Non-birefringent and birefringent concentric banded spherulites are obtained from 350-400 nm and 1800-2000 nm fi lm thicknesses, respectively. Non-birefringent concentric ring-banded spherulites are arranged only by fl at-on lamellae; in contrast, complex combinations of fl at-on, edge-on, and round-shaped lamellae in four transitional zones are present within a single band in the thicker fi lms of birefringent, concentric, ring-banded spherulites. Interband is an optically extinction crevice that segregates two neighboring bands. Unique lamellar arrangements at specifi c T c and thickness confi nement are two factors for the unconventional concentric bands.

The crystalline/crystalline poly(L-lactic acid)/ poly(1,4-butylene adipate) (PLLA/PBA) blend system exhibits upper critical solution temperature (UCST) behavior below the melting temperature of PLLA. When the PLLA/ PBA (50/50) blend is cooled to crystallize below UCST, phase-separated domains appear prior to crystallization of PLLA, resulting in PLLA spherulites being overlapped with the phase-separated domains. However, phase-separated domains only appear in blends when they are crystallized after being subjected to maximum melting temperature (T max ) for a short melting time (Δt max < 3 min); oppositely, the phase domains in blends become invisible when held for longer times (>5 min). When PLLA/PBA (50/50) blend is melted for short Δt max , the chain entanglement density is less; thus, the PBA chains are easy to be mutually expelled from the PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBA-rich separate domains. Apparently, phase separation in the PLLA/PBA blends is initiated by the crystallization of PLLA below UCST. Fractured surfaces of bulk blend samples show crystals of a polygonal shape, which is due to impingement between neighboring spherulites growing in 3D interior. For short Δt max , the polygonal crystal is porous with two different sizes. Small and large pore sizes correspond to the PLLA-rich and PBA-rich domains, respectively. The formation of porous-structure lamellae in the PBA-rich domains is due to combined factors: (a) preformed PLLA spherulites acting as a growth template of PBA, leading to preferential oriention of PBA along PLLA lamellae, and (2) densification of PBA upon crystallization at ambient temperature. Detailed analyses were focused on the lamellar assembly leading to various pore sizes in the phase domains and dependence of the assembly patterns and pore sizes on the parameters governing the phase separation.

Front Cover: By controlling the film thickness, non‐birefringent and birefringent concentric ring‐banded spherulites can be obtained in poly(L‐lactic acid) (PLLA), melt‐crystallized at 112 °C, for film thicknesses of 350 nm up to 2000 nm, respectively. The nanoscale morphology in these unconventional ring bands is influenced further by the different lamellar arrangements across the bands. Further details can be found in the article by S. Nurkhamidah and E. M. Woo* .

The lamellar morphology, cracks, and ring-banded spherulites were analyzed by allowing crystallization of poly(3hydroxybutyrate) (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 shortsegmental 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.

Top-surface and three-dimensional views of Type-1 and Type-2 of ring-banded spherulites in poly(nonamethylene terephthalate) (PNT) in thicker bulk crystallized on a nucleating potassium bromide (KBr) substrate were examined using various microscopy techniques: scanning electron microscopy (SEM), polarized-optical microscopy (POM), and atomic-force microscopy (AFM). In PNT crystallized at higher crystallization temperature (T(c)) with heterogeneous nucleating substrate, typically two types of ring-banded spherulites are present that differ significantly in patterns and ring spacings: Type-1 Type-2 (single- and double-ring-banded spherulites). Three-dimensional view on fractured spherulites in bulk PNT samples reveals that the single-ring-banded spherulite (Type-1) tends to be well-rounded spheres as they are nucleated homogeneously from bulk; the double-ring-banded spherulite (Type-2) is concentric hemisphere or truncated sphere shells owing to be nucleated from bottom. With confined thickness of films, the 3-D hemispheres in PNT may become truncated into multi-shell annular cones or arcs when thickness or growth is restricted. Based on the top-surface vs. interior views of banded lamellar assembly, origins and inner structures of dual types of ring bands in PNT were examined in greater details.