Cracks in Polymer Spherulites: Phenomenological Mechanisms in Correlation with Ring Bands

This article reviews possible mechanisms of various crack forms and their likely correlations with interior crystal lamellae and discontinuous interfaces in spherulites. Complex yet periodically repetitive patterns of cracks in spherulites are beyond attributions via differences in thermal expansion coefficients, which would cause random and irregular cracks in the contract direction only. Cracks in brittle polymers such as poly(l-lactic acid) (PLLA), or poly(4-hydroxyl butyrate) (PHB), or more ductile polymers such as poly(trimethylene terephthalate) (PTT) are examined and illustrated, although for focus and demonstration, more discussions are spent on PLLA. The cracks can take many shapes that bear extremely striking similarity to the ring-band or lamellar patterns in the same spherulites. Crack patterns may differ significantly between the ring-banded and ringless spherulites, suggesting that the cracks may be partially shaped and governed by interfaces of lamellae and how the lamellar crystals assemble themselves in spherulites. Similarly, with some exceptions, most of the cracks patterns in PHB or PTT are also highly guided by the lamellar assembly in either ring-banded spherulites or ringless spherulites. Some exceptions of cracks in spherulites deviating from the apparent crystal birefringence patterns do exist; nevertheless, discontinuous interfaces in the initial lamellae neat the nuclei center might be hidden by top crystal over-layers of the spherulites, which might govern crack propagation

Grating Assembly Dissected in Periodic Bands of Poly (Butylene Adipate) Modulated with Poly (Ethylene Oxide)

Polarized optical microscopy (POM), scanning electron microscopy (SEM), and synchrotron microbeam wide-angle X-ray diffraction (WAXD) were used to investigate the mechanisms of periodic assemblies leading to ring-banded crystal aggregates with light-grating capacity for iridescence in poly (1,4-butylene adipate) (PBA) modulated with poly (ethylene oxide) (PEO). A critical finding is that the PBA crystal assembly on the top surface and in the interior constitutes a grating architecture, with a cross-bar pitch equaling the inter-band spacing. The inner lamellae are arranged perpendicularly to the substrate under the ridge region, where they scroll, bend, and twist 90° to branch out newly spawned lamellae to form the parallel lamellae under the valley region. The cross-hatch grating with a fixed inter-spacing in the PBA aggregated crystals is proved in this work to perfectly act as light-interference entities capable of performing iridescence functions, which can be compared to those widely seen in many of nature’s organic bio-species or inorganic minerals such as opals. This is a novel breakthrough finding for PBA or similar polymers, such as photonic crystals, especially when the crystalline morphology could be custom-made and modulated with a second constituent

Morphology Modulation in Self-Assembly of Chiral 2-Hydroxy-2-Phenylacetic Acids in Polymeric Diluents

This study focused on the chirality effects that control the lamellar bending sense in self-assembled crystals of chiral 2-hydroxy-2-phenylacetic acids. 2-Hydroxy-2-phenylacetic acid or mandelic acid (MA) was crystallized in the presence of poly(4-vinyl phenol) (PVPh), and its crystalline structures and morphologies were assessed using polarized optical microscopy (POM) and scanning electron microscopy (SEM). MA of two opposite chiral forms (S- and R-) was crystallized with PVPh as the morphology modulator; with adjustment of the PVPh content, the morphology of MA crystals transforms from ring-banded spherulites to highly dendritic spherulites. For MA/PVPh (50/50 wt./wt.) blend and neat MA at same Tc, the dendritic spherulites are packed with single crystals where the lamellae bend at a specific direction varying with Tc and chirality. Contrary to conventional thought, the bending senses of the MA lamellae in the dendritic spherulites are not solely governed by the MA molecular chirality (S or R), but also by Tc. Only at high Tc (>65 °C) is the lamellar bending direction in dendritic spherulites of (S)-MA or (R)-MA blended with PVPh dictated by the chirality, i.e., displaying counterclockwise and clockwise bending direction for (S)-MA/PVPh and (R)-MA/PVPh, respectively. Nevertheless, at low Tc (45 °C), the bending sense of dendritic spherulites displays an opposite direction from those at the higher Tc, which is to say that the chirality alone does not control the lamellar bending direction

Explosive Fibonacci-sequence growth into unusual sector-face morphology in poly(l-lactic acid) crystallized with polymeric diluents

Abstract Lamellar assembly in unusual sector-face PLLA spherulites from crystallization of poly(l-lactic acid) (PLLA) diluted with amorphous poly(methyl methacrylate) (PMMA). The growth and morphology of the crystalline structures is studied using polarized optical microscopy (POM), atomic-force and scanning electron microscopies (AFM, SEM). Crystals are also analyzed using differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). The two alternate sectored faces differ dramatically in their optical birefringence and top-surface and interior lamellar assembly. By originating from the nucleus center, an explosive fan-like sector of high-birefringence lamellae is packed by fractal growth from an initial single stalk into hundreds of branches upon reaching the periphery, with the number of stalks increasing roughly by the Fibonacci sequence along the radial distance. The exploded pattern resembles a cross-hatch grating structure, and displays a cauliflower-like fractal-branching of optical birefringence blue/orange stripes. This finding suggests that growth with periodic branching is one of the main mechanisms to fill the ever-expanding space in the spherulitic 3D aggregates

Unique Periodic Rings Composed of Fractal-Growth Dendritic Branching in Poly(p-dioxanone)

Amorphous poly(p-vinyl phenol) (PVPh) was added into semicrystalline poly(p-dioxanone) (PPDO) to induce a uniquely novel dendritic/ringed morphology. Polarized-light optical, atomic-force and scanning electron microscopy (POM, AFM, and SEM) techniques were used to observe the crystal arrangement of a uniquely peculiar cactus-like dendritic PPDO spherulite, with periodic ring bands not continuingly circular such as those conventional types reported in the literature, but discrete and detached to self-assemble on each of the branches of the lobs. Correlations and responsible mechanisms for the formation of this peculiar banded-dendritic structure were analyzed. The periodic bands on the top surface and interior of each of the cactus-like lobs were discussed. The banded pattern was composed of feather-like lamellae in random fractals alternately varying their orientations from the radial direction to the tangential one. The tail ends of lamellae at the growth front spawned nucleation cites for new branches; in cycles, the feather-like lamellae self-divided into multiple branches following the Fibonacci sequence to fill the ever-expanding space with the increase of the radius. The branching fractals in the sequence and the periodic ring-banded assembly on each of the segregated lobs of cactus-like dendrites were the key characteristics leading to the formation of this unique dendritic/ringed PPDO spherulite

Interior Lamellar Assembly in Correlation to Top-Surface Banding in Crystallized Poly(ethylene adipate)

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

Interior Lamellar Assembly and Optical Birefringence in Poly(trimethylene terephthalate) Spherulites: Mechanisms from Past to Present

Poly(trimethylene terephthalate) (PTT) with its unique spherulitic morphologies, highly birefringent features, and crystal stability serves as a good candidate to study polymer crystallization and assembly. This review compiles the main findings on crystallization in PTT, including birefringence and morphology, thermal behavior, as well as the interior structure of PTT banded spherulites, in order to elucidate the origin and formation mechanism of banded spherulites. Interior observation through the inner anatomy of crystal assembly in banded spherulites hidden under the top surface is necessary to provide a complete picture for the unsettled arguments about formation mechanism. Careful attention should be taken when selecting the etching agent for exposing the lamellar structure of polymer spherulite, otherwise, misinterpretation could resul

Interior Dissection on Domain-Dependent Birefringence Types of Poly(3-hydroxybutyrate) Spherulites in Blends

The mechanisms and correlations between the rotation of Maltese cross axis, optical birefringence types, and lamellar reassembly induced by phase-separation domains, in poly­(3-hydroxybutyrate) (PHB)/poly­(1,4-butylene adipate) (PBA). In this work, a novel approach via interior dissection into spherulites in correlations with the top-surface morphology of the PHB/PBA (50/50) blend crystallized at a series of <i>T</i>_<i>c</i>’s with various domain sizes. The PHB/PBA blend is used as the model system in comparison to alternative blends of PHB with other polyesters of various extents of phase-separated domains. Domains of optimal sizes were found to be essential in lamellar reassembly, which strongly affects the birefringence types of spherulites. Via the proven approaches, this work expounds how domain sizes and lamellar reorientation in partially miscible blends cause changes in the birefringence types of PHB spherulites, while, in contrast, complete miscibility or extreme phase separation with large domains does not