Iridescent Features Correlating with Periodic Assemblies in Custom-Crystallized Arylate Polyesters.

In this study, five different aryl polyesters, i.e., poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), poly(octamethylene terephthalate) (POT), poly(nonamethylene terephthalate) (PNT), and poly(decamethylene terephthalate) (PDT), upon crystallization at a suitable temperature range, all exhibit ring-banded spherulites with universal characteristics. Previous research has revealed some fundamental mechanisms underlying the formation of periodic hierarchical structures. Additionally, this study further explored correlations among micro/nanocrystal assemblies in the top surface and internal grating architectures and the structural iridescent properties. The interior lamellar assembly of arylate polyesters' banded spherulites is shown to exhibit periodic birefringence patterns that are highly reminiscent of those found in a variety of biological structures, with the capacity for iridescence from light interference. A laser diffraction analysis was also used to support confirmation of this condition, which could result in an arc diffraction pattern indicative of the presence of ringed spherulites. Among the five arylate polyesters, only PET is incapable of regularly producing ring-banded morphology, and thus cannot produce any iridescent color.

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 Tc . This work first discovers that ring bands present in HDPE are crystallized in a quite wide Tc 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.

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. Tc = 30 °C) serves as ideal comparisons. Each interior arc-shape shell is composed of tangential and radial lamellae mutually intersecting at 90o 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.

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.

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.

Perpendicularly oriented lamellae in poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) blended with an amorphous polymer: ultra-thin to thick films.

The oppositely oriented lamellae in ridge and valley bands of ring-banded spherulites in biodegradable poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) interacting with amorphous poly(vinyl acetate) (PVAc) were examined using polarized-light optical, scanning electron, and atomic-force microscopy techniques (POM, SEM, AFM). Solvent-etching and fracturing were utilized for probing the interior morphology of the large-size ring-banded spherulites in PHBV/PVAc (70/30) blend [T(c) = 110 °C] films or thick bulk of various thicknesses. SEM analysis revealed that dual ridges of two opposite-oriented lamellae correspond to two-color bands (yellow and blue) of successive rings in POM micrographs. Fracture of thick blend samples further exposed that interior 3D spherulites were composed of sheath-kebab (similar to corrugated board) lamellae of two mutually perpendicular orientations.

Probing the Nano-Assembly Leading to Periodic Gratings in Poly(p-dioxanone).

This study used scanning electron microscopy via 3D dissection coupled with synchrotron radiation with microfocal beams of both small-angle X-ray scattering and wide-angle X-ray diffraction to analyze the periodic crystal aggregates of unusual poly(p-dioxanone) (PPDO) dendritic cactus-arm-like ring bands upon crystallization with a diluent poly(vinyl alcohol) (PVA) that is capable of hydrogen bonding interactions with PPDO. Three-dimensional microscopy interior dissection clearly expounds that the banded periodic architectures are packed by alternately normal-oriented flat-on crystals underneath the valley, periodically interfaced/branched with horizontal-oriented edge-on fibrils underneath the ridge. The oblique angles between the valley's flat-on crystals with the branches are ca. 25-45° (depending on gradient inclines and bending), which is also proved by the azimuthal angle in microbeam X-ray diffraction. The grating-like strut-rib assembly in the PPDO cactus-arm-like ring bands is further proved by novel iridescence tests.

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.

Microbeam X-ray Reanalysis on Periodically Assembled Poly(ß-Hydroxybutyric acid-Co-ß-hydroxyvaleric acid) Tailored with Diluents.

Self-assembly of 3D interiors and iridescence properties of poly(ß-hydroxybutyric acid-co-ß-hydroxyvaleric acid) (PHBV) periodic crystals are examined using microcopy techniques and microbeam X-ray diffraction. Morphology of PHBV can be tailored by crystallizing in presence of poly(vinyl acetate) (PVAc) or poly(trimethylene adipate) (PTA) for displaying desired periodicity patterns. The regular alternate-layered lamellae of banded PHBV crystal aggregates, resembling the structures the natural mineral moonstone or nacre, are examined to elaborate the origin of light interference and formation mechanisms of periodic lamellar aggregation of PHBV spherulites. By using PHBV as a convenient model and the crystal diffraction data, this continuing work demonstrates unique methodology for effectively studying the periodic assembly in widely varying polymers with similar aggregates. Grating structures in periodically assembled polymer crystals can be tailored for microstructure with orderly periodicity.

From Nano-Crystals to Periodically Aggregated Assembly in Arylate Polyesters-Continuous Helicoid or Discrete Cross-Hatch Grating?

This work used several model arylate polymers with the number of methylene segment n = 3, 9, 10, and 12, which all crystallized to display similar types of periodically banded spherulites at various Tc and kinetic factors. Universal mechanisms of nano- to microscale crystal-by-crystal self-assembly to final periodic aggregates showing alternate birefringence rings were probed via 3D dissection. The fractured interiors of the birefringent-banded poly(decamethylene terephthalate) (PDT) spherulites at Tc = 90 °C revealed multi-shell spheroid bands composed of perpendicularly intersecting lamellae bundles, where each shell (measuring 4 µm) was composed of the interior tangential and radial lamellae, as revealed in the SEM results, and its shell thickness was equal to the optical inter-band spacing (4 µm). The radial-oriented lamellae were at a roughly 90° angle perpendicularly intersecting with the tangential ones; therefore, the top-surface valley band region appeared to be a submerged "U-shape", where the interior radial lamellae were located directly underneath. Furthermore, the universal self-assembly was proved by collective analyses on the three arylate polymers.

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.

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.

Surface and interior views on origins of two types of banded spherulites in poly(nonamethylene terephthalate).

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.

Unique Optical Periodicity Assembly of Discrete Dendritic Lamellae and Pyramidal Single Crystals in Poly(ε-caprolactone).

A unique zig-zag banded morphology poly(ε-caprolactone) (PCL), crystallized at high Tc = 46-47 °C and confined in thin films (<1 µm), is found to be assembled of parallel cogrowth of two totally crystal entities: edge-on dendritic lamellar protrusion from surfaces and flat-on pyramidal single crystals. The alternating PCL optical bands are assembled as flat-on single crystals aligned as straight dendrites as a valley band in series with edge-on branches as a ridge band, leading to a unique assembly mechanism of periodic optical rings where these two crystal pieces are assembled in series. Detailed assembly mechanisms are proposed to explain how the composite bands in the PCL aggregates are correlated to optical birefringence periodicity. By techniques of melt crystallization via periodic species drainage, PCL single crystals could be prepared and tailor-made to a variety of nanopatterns as templates for applications.

Microscopic Fourier Transform Infrared Characterization on Two Types of Spherulite with Polymorphic Crystals in Poly(heptamethylene terephthalate).

FTIR microspectrometry with in situ temperature variation and IR-peak-mapping capability, and POM characterization were used to study the crystal distribution in dual spherulites in poly(heptamethylene terephthalate). By tracing the crystalline IR bands of the α-crystal and ß-crystal to get the crystal distribution, the techniques resolve that the ringed and ringless spherulites comprise α- and ß-crystals, respectively. In addition, temperature-dependent IR analyses on the spots related to the two crystals also reveal the α- and ß-crystals melt at 98 and 104 °C, respectively. The ringed and ringless spherulites were proven to be correlated with the α- and ß-crystal forms, respectively.

Periodic Fractal-Growth Branching to Nano-Structured Grating Aggregation in Phthalic Acid.

Small-molecule phthalic acid (PA), confined in micrometer thin films, was crystallized in the presence of strongly interacting tannic acid (TA) to investigate crystal assembly and correlation between banded patterns and branching structures. Several compositions of the mixture of ethanol/water solutions and evaporation temperatures were also manipulated to investigate the kinetic effects on the morphology of PA crystals. With increasing evaporation rate, the morphology of PA crystals systematically changes from circular-banded spherulites to highly ordered grating-banded patterns. A unique periodic fractal-branch pattern with contrasted birefringent bands exists at intermediate evaporation rate, and this unique grating architecture has never been found in other banded crystals. Crystal assembly of these three periodic morphologies was analyzed by utilizing atomic-force microscopy (AFM) and scanning electron microscopy (SEM) to reveal the mechanisms of formation of hierarchical structures of PA. The detailed growth mechanisms of the novel fractal-branching assembly into circular- or grating-banded patterns are analyzed in this work.

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

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.

Annular Multi-Shelled Spherulites in Interiors of Bulk-Form Poly(nonamethylene terephthalate).

Morphology of crystallized spherulites in poly(nonamethylene terephthalate) (PNT) in bulk forms, instead of thin-film forms, was investigated using scanning electron microscopy (SEM), polarized-optical microscopy (POM), and transmission electron microscopy (TEM). Ring textures were found to exist on the fractured surfaces or microtomed films from bulk PNT samples. By further SEM and TEM examinations, the fracture surfaces and microtomed films from the interior of the bulk display similar ring-banded patterns as those in thin films cast on glass slides. Although the cast PNT thin films in two-dimensional growth are known to display both Type-1 ring bands as majority and Type-2 ring bands as minority, this study further shows that the interior of PNT bulk exhibits mainly Type-1 (single rings with narrower spacing than Type-2) ring bands, and Type-2 spherulites exist sporadically in three-dimensional forms. From these analyses on the ring bands in the interiors of bulk-form PNT crystallized at several specific temperatures, it can be proposed that the growth of spherulites in three dimensions is layer-by-layer, packing into a multi-shells structure in three-dimensional ring-banded spheres.

Effects of Amphiphilic Chitosan on Stereocomplexation and Properties of Poly(lactic acid) Nano-biocomposite.

This article demonstrates an elegant approach for the fabrication of high heat-stable PLA using an industrially viable technique i.e. melt extrusion, which is quite challenging due to the higher viscosity of poly(lactic acid) melt. scPLA has been fabricated by melt extrusion of PLLA/PDLA using nano-amphiphilic chitosan (modified chitosan, MCH) which has been synthesized by grafting chitosan with oligomeric PLA via insitu polycondensation of L-lactic acid that possibly increases the molecular surface area and transforms it into nano-amphiphilic morphology and in turn lead to the formation of stereocomplex crystallites. The effect of MCH loading on the structural, morphological, mechanical and thermal properties of PLLA/PDLA have been investigated. The modification of chitosan and formation of stereocomplexation in PLA has been confirmed by FTIR and XRD techniques, respectively. Heat treatment has also laid a significant effect on the stereocomplexation and the degree of crystallinity of stereocomplex crystallites has been increased to ~70% for 1.5 wt % MCH content with the absence of homocrystals. The heat deflection temperature is found to be more than 140 °C for the biocomposite with 1.5% MCH in comparison to ~70 °C for pristine scPLA. The biocomposites display significant improvement in UTS and Young's modulus.

Lamellae Assembly in Dendritic Spherulites of Poly(l-lactic Acid) Crystallized with Poly(p-Vinyl Phenol).

Lamellar assembly with fractal-patterned growth into dendritic and ringed spherulites of crystallized poly(l-lactic acid) (PLLA), of two molecular weight (MW) grades and crystallized at (temperature of crystallization) Tc = 120 and 130 °C, respectively, are evaluated using optical and atomic-force microscopies. The results of surface-relief patterns in correlation with interior microscopy analyses in this work strongly indicate that the observed birefringence changes in PLLA polymer dendritic or ringed spherulites (from blue to orange, or to optical extinction) need not be definitely associated with the continuous helix twisting of lamellae; they can be caused by sudden and discontinuous lamellae branching at intersected angles with respect to the original main lamellae, as proven in the case of dendritic and zig-zag rough-ringed spherulites. Intersection angles between the main stalks and branches tend to be governed by polymer crystal lattices; for PLLA, the orthorhombic lattice (α-form) usually gives a 60° angle of branching and hexagonal growth. The branching lamellae then further bend to convex or concave shapes and finally make a 60⁻90° angle with respect to the main stalks. Such mechanisms are proven to exist in the straight dendritic/striped high-molecular weight (HMW)-PLLA spherulites (Tc = 120 °C); similar mechanisms also work in circularly ringed (Tc = 130 °C) HMW-PLLA spherulites.