No abstract

Novel approaches and schematic models have been used to provide insights in viewing lamellar assembly in ring-banded spherulites of a model polymer poly­(ethylene adipate) (PEA) via correlations between the outer-surface and three-dimensional interior morphology. When crystallized at 28 °C, PEA in thick bulk forms exhibits a ring-banded top surface with bowl-like and dome-like height profile centering on its nucleus. The three-dimensional periodical assembly leads to not only the noticeable ring bands on exterior surfaces but also corrugated-board-like multishell structures in the interior of the spherulites. The corrugated-board structure has been found to be composed of plate-like lamellae, which first grow in a tangential direction and then turn to a radial direction; the radial plates taper to form thinner cilia-like lamellae due to the polymer chain concentration gradient periodically precipitated during the growth. Alternating sequences of plate-like lamellae in two perpendicular orientations (bending from tangential to radial) in PEA ring-banded spherulites during growth in a radial direction account for the spherulites confined in thin films to display two contrast circumferential rings with alternating interference colors. Scanning electron microscopy dissection graphs clearly revealed interior corrugated layer thickness in bulk forms, which matches well with the inter-ring spacing in thin films.

Three-dimensional interior structures of poly­(lactic acid) (PLA) spherulites have been examined in crystallization with poly­(ethylene oxide). A high concentration of water-soluble PEO is used as diluent to obtain spherulitic morphologies of PLA in thick enough films (20–50 μm) for interior observation. Two different but closely related spherulite patterns (dendritic and spiral banded) of both PLA enantiomers have been studied in details, after removal of PEO by water etching. The interior of both curved dendrites and spiral banded spherulites are not connected by continuous twisting, as conventional models proposed; instead, the ridge band is composed of multiple polycrystals in parallel alignment and the valley band is also composed of polycrystalline branching lamellae. At increased film thickness, the molecular chirality may not be entirely responsible for the lamellar bending sense (i.e., clockwise or counterclockwise) in all hierarchical crystal structures. Film thickness also contributes to curvature sense and banding. The approaches taken and results presented here are ground breaking for interpreting the mechanisms of periodic patterns that can be observed in spherulites.

Poly(trimethylene terephthalate) (PTT), a highly birefringent aromatic polyester, has been utilized to understand the mechanisms of crystal assembly into diversified types of banded spherulites. PTT exhibits three main types of banded spherulites (i.e., concentric, single‐spiral, and double‐spiral) co‐existing in sample films melt‐crystallized at 165 °C, regardless of sample thickness. The three types differ in their banding structures, interference color distributions, and nuclei geometries (S‐shape, Z‐shape, or dot‐shape). Core diameter, band spacing ratio, and height difference (Δz) around the core are the three key parameters of different banding patterns in PTT spherulites. Formation mechanism for three types of banded spherulites has been interpreted, and found to be highly correlated with the initial geometry shapes of their nuclei. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1207–1216

PolyIJtrimethylene terephthalate) (PTT), with large, well-defined, and strongly birefringent spherulites upon crystallization, has been chosen as a model to study the correlation between the interior lamellae assembly and the top surface banding structures. Approaches to investigate the bulk interior of the banded spherulites are needed to shed new light on the crystallization mechanism of polymers. Two etching methods (with two different etchants: methylamine and permanganate) were applied to two identical samples to enhance the contrast of crystal arrangements in three-dimensional perspectives. Methylamine, under mild conditions, was found to etch out lamellae bundles on ridges and leave discrete crevice-separated circumferential crystal layers. On the other hand, the oxidizing potassium permanganate was found to dissolve the circumferential crystal layers between the successive bands, exposing circular patterns. These two etching results mutually support the discontinuity of radial growth of PTT spherulites, providing valuable hints as to the actual crystal assembly of PTT.