Crystal Lamellae of Mutually Perpendicular Orientations by Dissecting onto Interiors of Poly(ethylene adipate) Spherulites Crystallized in Bulk Form

Transcrystalline Mechanism of Banded Spherulites Development in Melt-Crystallized Semicrystalline Polymers

The decades-long paradigm of continuous and perpetual lamellar twisting constituting banded spherulites has been found to be inconsistent with several recent studies showing discontinuity regions between consecutive bands, for which, however, no explanation has been found. The present research demonstrates, in three different semicrystalline polymers (HDPE, PEG10000 and Pluronic F-127), that sequential transcrystallinity is the predominant mechanism of banded spherulite formation, heterogeneously nucleated on intermittent self-shear-oriented amorphous layers excluded during the crystals' growth. It is hereby demonstrated that a transcrystalline layer can be nucleated on amorphous self-shear-oriented polymer chains in the melt, by a local melt flow in the bulk or in contact with any interface-even in contact with the interface with air, e.g., in contact with an entrapped air bubble or at the edges of the sample-or nucleated following the multiple directions and orientations induced by a turbulent flow. The bilateral excessive local exclusion of amorphous non-crystallizable material, following a short period of initial non-banded growth, is found to be the source of dislocations leading to spirally banded spherulites, through the transcrystalline layers' nucleation thereon. The present research reveals and demonstrates the sequential transcrystalline morphology of banded spherulites and the mechanism of its formation, which may lead to new insights in the understanding and design of polymer processing for specific applications.

Crystal-by-Crystal Assembly in Two Types of Periodically Banded Aggregates of Poly(p-Dioxanone)

The exterior and interior lamellar assemblies of poly(p-dioxanone) (PPDO) crystallized at 76 °C yield the most regular ones to interpret the 3D assembly mechanisms and potential for structural coloration iridescence, which are investigated using atomic-force microscopy (AFM), and scanning electron microscopy (SEM). PPDO displays two types of ring-banded spherulites within a range of Tc with dual-type birefringent spherulites (positive and negative-type) only within a narrow range of Tcs = 70−78 °C. At Tc > 80 °C, the inter-band spacing decreases from a maximum and the crystal assembly becomes irregularly corrupted and loses the capacity for light interference. Periodic grating assemblies are probed by in-depth 3D dissection into periodically banded crystal aggregates of poly(p-dioxanone) (PPDO) to disclose such layered gratings possessing iridescence features similar to nature’s structural coloration. This work amply demonstrates that grating assembly by orderly stacked crystal layers is feasible not only for accounting for the periodic birefringent ring bands with polarized light but also the distinct iridescence by interfering with white light.

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.

In-Situ Growth of Nucleus Geometry to Dual Types of Periodically Ringed Assemblies in Poly(nonamethylene terephthalate)

Monitoring of nucleus geometry and growth into dual types of periodically ring-banded morphology in poly(nonamethylene terephthalate) (PNT), respectively, Type-1 and Type-2, are done with detailed analyses using polarized-light optical microscopy (POM) in-situ CCD recording; the periodic assembly morphologies are characterized using atomic-force microscopy (AFM) and scanning electron microscopy (SEM). Different annealing treatments (Tmax = 110, 120, 130 °C) are accomplished at a crystallization temperature of 85 °C; effects on the nucleus geometry, number (25–10%) and volume fractions (33–15%) of Type-2 among two types of banded PNT spherulites are expounded. Growth of a specific type of periodically banded PNT spherulite is initiated from either highly elongated sheaf-like or well-rounded nuclei, with the final grown lamellae being self-packed as multi-shell structures. Nucleation geometry and crystallization parameters collectively lead to development of multiple types of banded PNT spherulites of different relative fractions.

Lamellar Assembly Mechanism on Dendritic Ring-Banded Spherulites of Poly(ε-caprolactone)

The self-assembly structures of lamellae in optical ring bands have a critical effect on their optical and physical arrangements. Two different types of dendritic banded spherulites (namely ring-banded and zigzag ring-banded) are formed in poly(ε-caprolactone)/poly (phenyl methacrylate) blend at crystallization temperatures of 42 and 46 °C, respectively. The difference in optical birefringence of ring bands in two types of spherulites is resolved by means of direct morphological comparison. Banded spherulites are fractured carefully to facilitate lamellar orientation analyses of both the top surface and the interior surface. The results have revealed the existence of tree-like dendritic fractal growth lamellar assemblies in both banded spherulites. The optical ring patterns of the banded spherulites are differentiated mainly by the fractal orientation of the edge-on crystal branches in the ridge region. On the basis of detailed morphological analysis, 3D-lamellar assembly mechanisms are proposed to explain the growth of dendritic ring-banded spherulites at 42 °C and dendritic zigzag ring-banded spherulites at 46 °C.

Synchrotron X-Ray Analysis and Morphology Evidence for Stereo-Assemblies of Periodic Aggregates in Poly(3-hydroxybutyrate) with Unusual Photonic Iridescence

3D morphology of poly(3-hydroxybutyrate) (PHB), crystallized in the presence of diluents of poly(1,3-trimethylene adipate) and poly(ethylene oxide), is probed using a novel approach coupled with selective etching. For interpreting the mechanisms of crystal periodic aggregation, various microscopic techniques and synchrotron microbeam X-ray analysis are used to observe the top surface in connection with the 3D crystal assemblies. Periodic grating architectures, with the cross-bar pitch exactly matching with the optical band spacing, are proved in banded PHB. The crystals under the ridge branch out to spawn finer crystals orienting/bending horizontally underneath the valley band, repeating till species drainage or impingement. The grating structure in the banded PHB resembles many nature's iridescence crystals and is further proved by photonic reflection results as a critical breakthrough novel finding.

Microstructural Periodic Arrays in Poly(Butylene Adipate) Featured with Photonic Crystal Aggregates

Poly(butylene adipate) (PBA) self-aggregation into unique periodicity correlating to its interfacial photonic properties is probed in detail. Investigations on the unique periodic morphology and top-surface and interior architectures in specifically crystallized PBA are focused on its novel photonic patterns with periodic gratings. Detailed analysis of the interior lamellae from ringless to periodically ordered aggregates (crystallized at 33-35 °C vs. T_c = 30 °C) serves as ideal comparisons. Each interior arc-shape shell is composed of tangential and radial lamellae mutually intersecting at 90^o angle. The interior layer thickness in SEM-revealed arc-shape shish-kebab shell is exactly equal to the optical inter-band spacing (≈6 µm). A 3D assembly mechanism of periodically banded PBA crystals is proposed, where the orderly arrays on top surfaces as well as the interior microstructures of strut-rib alternate-layered assembly resemble nature's photonic crystals and collectively account for the interfacial photonic properties in the ring-banded PBA crystal that is novel and has potential applications in future.

Periodic Assembly of Polyethylene Spherulites Re-Investigated by Breakthrough Interior Dissection

A completely novel 3D dissection approach is taken to re-investigate high-density polyethylene (HDPE) crystallized into periodic architectures in a wide range of T_c . This work first discovers that ring bands present in HDPE are crystallized in a quite wide T_c range (90-120 °C) all within regime-III growth. With further detailed analyses of the top-surface-relief patterns and 3D architectures of HDPE spherulites, this work has fully clarified the periodic morphology packed with alternate ways of single-crystal aggregates in correlation with the optical banding patterns. The proposed assembly mechanism sheds light that the periodic bands are actually composed of a cross-hatch grating structure in that the alternately perpendicular orientations from the ridge to valley bands being related to the interior radial to tangential lamellae. Such grating architectures in the interiors of HDPE can be viewed as a mimicry resembling shish-kebab lamellae self-aligned by Archimedean spiral-spins from the nucleus center.

Sophisticated dual-discontinuity periodic bands of poly(nonamethylene terephthalate)

Crystallized poly(nonamethylene terephthalate) (PNT) displays mirror-image and Fermat's-spiral ring-banded spherulites, respectively.

Sluggish growth of poly(ε-caprolactone) leads to petal-shaped aggregates packed with thick-stack lamellar bundles

Kinetically sluggish growth of poly(ε-caprolactone) leads to peculiar camellia-petal-like bands on top, where a stereo-dissection demonstrates interior self-assembled lamellae being periodically grating-structured.

Crystal aggregation into periodically grating-banded assemblies in phthalic acid modulated by molten poly(ethylene oxide)

A small-molecule compound, phthalic acid (PA), crystallized in the presence of poly(ethylene oxide) (PEO) with various compositions was utilized as a model to investigate the morphology and crystal assembly of periodically ordered structures in banded spherulites.

Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites of Semifluorinated Alkanes Coexistent with Close-Packed Self-Assembled Surface Nanodomains

A series of semifluorinated alkanes (C _nF_{2 n+1}C _mH_{2 m+1} diblocks, F n H m, n = 6, 8, 10; m = 16, 18, 20), when cast as films onto solid substrates, were found to form ring-banded or radial spherulites when heated above their isotropic temperature and subsequently cooled down to room temperature, demonstrating that the formation of two-dimensional (2D) spherulites is a general feature of molecular fluorocarbon-hydrocarbon diblocks. These spherulites are not birefringent, a seldom encountered feature for such structures (never, so far, for spherulites made of small molecules). They also provide examples of fluorinated 2D spherulites. Film morphology was analyzed by optical microscopy, interferometric profilometry, atomic force microscopy (AFM), and scanning electron microscopy. Increasing the length of the Fn segment favors the formation of ring-banded spherulites, whereas short Fn segments tend to favor extended radial stripes. Variation of the cooling rate provides control over the size and morphology of the spherulites: slow cooling promotes fibers and radial spherulites, whereas fast cooling fosters ring-banded spherulites. The AFM studies of F10 H16 films revealed that the latter consist of stacks of regularly spaced lamellae. We also observed that, remarkably, stacked lamellae (repeating distance ∼6 nm) can coexist with a layer of close-packed monodisperse circular self-assembled surface nanodomains of Fn Hm diblocks (∼30 nm in diameter); the latter are known to form from such diblocks at interfaces at room temperature. Substrates partially covered with F10 H16 contain incomplete ring-banded spherulites and smaller objects in which the lamellae and circular nanodomains coexist.

Three-dimensional interior analyses on periodically banded spherulites of poly(dodecamethylene terephthalate)

Polarized optical, atomic force, and scanning electron microscopy images showing the correlations of three-dimensional crystal arrangements with optical properties exhibited by poly(dodecamethylene terephthalate).

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.

Time-Resolved Imaging of the Phase Transition in the Melt-Grown Spherulites of Isotactic Polybutene-1 as Detected by the Two-Dimensional Polarized IR Imaging Technique

In order to visualize the 2D spatial distribution of the structural change in the phase transition from crystal form II to I of isotactic polybutene-1 spherulite grown from the melt, the time-dependent measurement of the 2D polarized FTIR spectra has been performed. At a melt-isothermal crystallization temperature of 103 °C, the square-shape spherulite appeared from the melt and grew with time. When the isothermal crystallization occurred at 98 °C, the round-shaped spherulite was observed. In both cases, after the temperature jump to an ambient temperature, the 2D images changed clearly in the process of the phase transition from form II to form I, but the spherulite morphology itself was not changed detectably. The polarized IR imaging has revealed the preferential orientation of the crystallites in the spherulite. In the case of the spherulite grown at 103 °C, the ab plane is oriented in parallel to the spherulite plane and the molecular chains stand along the normal to the surface. On the other hand, in the spherulite grown at 98 °C, the chains were found to lie on the spherulite plane preferentially. Such a difference in the crystal orientation in the spherulite is related intimately with the outer shape of the spherulite and also with the growth mechanism of the spherulite. In this way, the polarized 2D IR imaging was found to be quite useful for the in situ detection of the time-dependently changing 2D spatial distribution of the crystallites in the spherulite.

Analysis of crystal assembly in banded spherulites of phthalic acid upon solvent evaporation

Differences are seen in the mechanism of lamellar assembly of two alternating banded regions (valley and ridge) of phthalic acid spherulites solvent-evaporation crystallized at either higher (80 °C) or ambient (28 °C) temperature.

Intertwining lamellar assembly in porous spherulites composed of two ring-banded poly(ethylene adipate) and poly(butylene adipate)

Poly(1,4-butylene adipate) (PBA) and poly(ethylene adipate) (PEA), each with the ability to form ring-banded morphologies at same Tc, were simultaneously crystallized from mixtures of various compositions. Investigations on morphology, phase and thermal behavior were conducted in order to reveal lamellar packing and spherulitic structures in this binary system. As PBA is faster-crystallizing and dominates the crystallization process, it is relatively easy to maintain its ordered ring-banded pattern in a PBA/PEA blend when there is a moderate amount of PBA in the composition (40 wt% or greater). On the other hand, PEA is much slower crystallizing and it has to be in extreme majority (PEA > 95 wt%) in the PBA/PEA mixtures in order to crystallize into ring-banded spherulites of PEA pattern. When PBA composition is between 10 and 40 wt% in the PBA/PEA blend, simultaneous crystallization of PBA and PEA leads to an interpenetrating morphology with an interwoven bird-nest pattern. Porous structures with crevices, owing to the interpenetrating PBA and PEA lamellae, resulted in simultaneous crystallization of these two biodegradable polyesters.

Synthesis and properties of novel poly(ethylene succinate-co-decamethylene succinate) copolymers

The tensile properties of poly(ethylene succinate-co-decamethylene succinate) copolymers may be well regulated by the decamethylene succinate contents.

Unusual crystallization behavior of biodegradable poly(ethylene adipate) based nanocomposites induced by graphene oxide

GO increases both the nucleation density and the growth rate of PEA spherulites in nanocomposites.

Composite banded core and non-banded shell transition patterns in stereocomplexed poly(lactide acid) induced by strongly interacting poly(p-vinyl phenol)

Composite ring-banded core and non-banded outer peripheral patterns were found in sc-PLA interacting with amorphous PVPh at several T_cs.