Single Spherulites Research Articles

Bio-Inspired Crystalline Core-Shell Guanine Spherulites.

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture is discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent ( ) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, the synthesis of core-shell spherulites composed of guanine crystal platelets (( ) is presented in a two-step emulsification process in which a water/oil/water emulsion and induced pH changes are used to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, β-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, it is found that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenicparticles.

Planerite with aheylite and faustite from the Chala deposit, Spahievo Ore Field

Planerite, aheylite and faustite are minerals of the turquoise group. They were found in samples from the Chala deposit, Spahievo Ore Field. The samples consist of a small crystalline mass of the investigated mineral phases among quartz. Single spherulites up to 2 mm in size and aggregates of spherulites with a yellowish-greenish color are observed in small cavities. Transparent, colorless wavellite crystals and aggregates are crystallized on spherulites. For the first time in Bulgaria, the presence of aheylite was established.

Real-Time Imaging of Stress in Single Spherulites and Its Relaxation at the Single-Particle Level in Semicrystalline Polymers.

Crystallization-induced microscopic stress and its relaxation play a vital role in understanding crystallization behavior and mechanism. However, the real-time measurements for stress and its relaxation seem to be an unachievable task due to difficulties in simultaneous labeling, spatiotemporal discrimination, and continuous quantification. We designed a micron-sized fluorescent probe, whose fluorescence can respond to stress-induced environmental rigidity and whose three-dimensional (3D) flow can respond to stress relaxation. Using the as-prepared fluorescent probe, we established a versatile strategy to realize the real-time 3D imaging of stress and its relaxation in the crystallization process. The rigidity-responsive fluorescence clearly indicated the stress, while the 3D flow movement could quantify the stress relaxation. It is revealed that stress in spherulites increased dramatically as a result of the suppression of stress relaxation in polymer melts. The developed method provides a novel avenue to simultaneously detect stress and its relaxation in various semicrystalline polymers at the single-particle level. This success would achieve the microscopic ways to guide the development of advanced crystallization-dependent materials.

Orientation-Independent Charge Transport in Single Spherulites from Solution-Processed Organic Semiconductors

Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction.

Growth regimes in phase ordering transformations

Growth, shape and texturing dynamics of single 2D spherulites are analyzed using the Landau-de Gennes (LdG) liquid crystal model of isotropic-nematic phase ordering. Direct numerical simulation shows that non-circular nucleation due to anisotropy in the interfacial tension results in non-circular shapes in the early stages of growth. However, interfacial heterogeneities in growth and structure then lead to interfacial defect nucleation and shedding and a reshaping of the interface into a circular shape. The formulated dynamics show that a growing spherulite in the early stage of phase ordering also acquires topological higher charges than expected from the well-known Kibble mechanism. In agreement with experiments under strong quenching, the predicted growth dynamics of a spherulite of characteristic radius R is linear: $R $~$ t$. To better understand these computational results, a dynamic shape equation is obtained from the LdG model and is shown to have the same form as the growth and shape equations of crystal growth. The shape equation is used to reveal the mechanisms involved in shape transformations, interfacial defect shedding, and growth dynamics computed by direct numerical simulation of the bulk LdG equations.

Formation of lamellar structure by competition in crystallization of both components for crystalline–crystalline block copolymers

Crystallization and structure formation of poly(ethylene oxide)–poly(ɛ-caprolactone) block copolymers (PEG–PCL) in which the melting temperatures of the components are close to each other were elucidated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The diblock copolymers with 33, 46 and 59 wt% of PEG composition formed ordinary single spherulites similar to those of PCL homopolymers, while concentric double-circled spherulites appeared for the PCL–PEG–PCL triblock copolymer with 66 wt% PEG composition as observed previously. For the diblock copolymers, despite of the ordinary appearance of the single spherulites, the DSC thermograms and the WAXD patterns indicated the crystallization of PEG as well as PCL. The time-resolved SAXS profiles for the diblock copolymers showed that long spacings of the crystal lamellae decreased stepwise in the crystallization process. Synthesizing these results for the single spherulites, it was concluded that PCL crystallized first followed by the crystallization of PEG with preservation of the PCL crystal lamellar structure. This means that PEG must crystallize within confined space between the formerly formed PCL crystal lamellae. Such confined crystallization of PEG caused the suppressed melting temperature, crystallinity and crystallization rate especially in the smaller PEG compositions. In the melting process of the diblock copolymers, it was observed that the PEG component first melted with a stepwise increase in the long spacing.

Crystallization kinetics of compatibilized blends of liquid crystalline polymer with PEEK

A fully aromatic thermotropic liquid crystalline polyester (TLCP) has been blended with poly(ether ether ketone) (PEEK). Multiblock copolymer (BCP) was used as the compatibilizer in the concentration at 2 phr. The isothermal crystallization kinetics and morphology of compatibilized blends were studied using differential scanning calorimetry (DSC) and polar light micrograph (PLM). TLCP acted as a heterogenous nucleation sites to accelerate the crystallization for PEEK. However, PEEK crystallization rates decreased with increasing TLCP fraction. Isothermal crystallization exotherms showed that the addition of BCP retarded crystallization of PEEK in PEEK/TLCP blend, which was probably resulted from the constraint effect of BCP as well as the size reduction of PEEK spherulite domain. The equilibrium melting temperature of PEEK for blends was below that of pure PEEK. After adding BCP, it decreased further. Morphological analysis showed that it was difficult to discern the single PEEK spherulites when BCP was added. POLYM. COMPOS., 27:642–650, 2006. © 2006 Society of Plastics Engineers

Investigation of secondary crystallization of polymers by means of microbeam X-ray scattering

The kinetics of secondary crystallization during spherulite growth of isotactic poly(propylene) (iPP) and poly(vinylidene flouride) (PVF 2) is studied using a novel technique that employs a micron size X-ray beam. The data are combined with separate conventional simultaneous on-line SAXS/WAXS measurements and optical microscopy studies. In our experiments, crystallization takes place at low undercooling so that slowly growing large single spherulites are obtained. The data reveal that the main mechanism of secondary crystallization is the growth of new lamellae stacks within remaining amorphous regions. It is shown that a substantial amount of crystallites form as a result of secondary crystallization while the spherulite is growing. Furthermore, secondary crystallization is strongest directly behind the boundary of the spherulite and is independent of its size or growth state. A separate, off-line microfocus study on a quenched spherulite sample confirms this observation; the crystallinity is higher in the main body of the spherulite and lower near the boundary, where crystallization progressed to a lesser degree.

On the validity of crystallization kinetics parameters derived from the depolarized light intensity (DLI) technique: An experimental study of polyvinylidene fluoride

Although the depolarized light intensity technique has been known for some decades, the time dependence of light intensity does not yet seem to be well understood. In this article, devoted to the crystallization kinetics of polyvinylidene fluoride (PVDF), we present some of the problems associated with quantitative analysis. Parameters such as incident light intensity—in combination with the detection system—and specimen thickness are shown to dramatically affect the apparent kinetics. Experiments on single spherulites grown in very thin films can help interpret the intensity–time curve. Our results demonstrate that care should be taken when comparison is made between DLI and differential scanning calorimetry. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 997–1008, 1999

Influence of coagulation process on solid‐phase structure of poly(1,3,4‐oxadiazole‐2,5‐diyl‐1,4‐phenylene): non‐solvent induced crystallization

AbstractA preliminary investigation on the influence of the coagulation process on the solid‐phase structure of poly(1,3,4‐oxadiazole‐2,5‐diyl‐1,4‐phenylene) (1) is reported. Interesting correlations between the method of coagulation of the polymer from sulfuric acid solution and the structure which develops, as observed by thermal analysis, X‐ray diffraction, infrared spectroscopy, and optical microscopy are found. A new method for the preparation of highly crystalline spherulites of 1 is described. This method allows to grow a relatively large number of “single spherulites”, because coagulation and crystallization induced by a non‐solvent may be stopped before impingements occur.

Electron microscopy of polymer blends

Polymer blends have come to play an increasingly important role in materials science as the need for polymeric materials with wider ranging properties has developed. Data will be presented in this paper on two polymer blend systems that have benefited from the use of electron microscopy in understanding the complex morphologies than can occur in polymer systems.Blends of Poly(vinylidene fluoride) and poly(ethyl acrylate)Poly(vinylidene fluoride) (PVF2) and poly(ethyl acrylate) (PEA) form a miscible, single phase melt over the composition range of 50% PVF2 (by weight) and less. The melt exhibits phase separation upon heating to 10-50°C above the observed melting point of PVF2, depending on the composition, PVF2 is a semicrystalline polymer, while PEA is an amorphous polymer. The fact that PVF2 is compatible in the melt with PEA has allowed for detailed morphological studies of single crystal lamellae and spherulites of the α crystal form of PVF2 grown at temperatures very close to those found in conventional melt crystallization.

Tear strength of polyethylene

The fracture energy of moulded sheets of polyethylene has been found to depend strongly upon the thickness of the sheet, increasing linearly over the range 0.05 to 1.0 mm. This variation is attributed to a dependence of the volume of the plastic zone at the crack tip upont2, wheret is the torn thickness. By extrapolation, threshold values of fracture energy were determined at zero thickness. These represent the strength in the absence of large-scale plastic yielding. The fracture energies of both thin and thick sheets of high-density polyethylene (HDPE) were found to vary strongly with rate of tearing and test temperature, passing through maxima at particular rates and temperatures. This behaviour is attributed to corresponding changes in local ductility. A general correlation was found between the dependence of fracture energy upon tear rate and the dependence of loss modulusE″ upon the frequency ω of small oscillatory deformations. It is concluded that the work of fracture is mainly expended in local yielding at the tear tip, on the scale of single spherulites, about 4μm for HDPE. Annealing HDPE at 120° C reduced the fracture energy of thick sheets to about 10 kJ m−2 but did not greatly alter the threshold strength. In contrast, annealing sheets of LDPE had relatively little effect on the fracture energy at any thickness.

Amorphous linear polyethylene: Annealing effects

Abstract The effect of annealing amorphous linear polyethylene films prepared by an improved ultraquenching technique at temperatures just below and above a dynamic mechanical relaxation peak (torsion braid) observed at ∽190K has been characterized by electron microscopy and torsion braid analysis. Based on the results described, this peak is believed related to the lower glass transition temperature Tg(L), the Tg of wholly amorphous linear polyethylene, whereas the β peak at 260K is Tg (upper). Annealing just below Tg (L) results in a growth in size of the nodules observed in the as-quenched samples, whereas annealing above Tg (L) can result in the growth of single crystal-like structures, spherulites, and shish-kebobs. Storage of the crystallized samples at room temperatures results in a decrease in size of the relaxation peak during subsequent torsion braid spectroscopy measurements. The results indicate significant amounts of molecular motion can occur during crystallization even at Tg.

Structural studies of single spherulites and partially spherulitic films of poly(methyl‐D‐glutamate)

AbstractIsolated spherulites of poly(methyl D‐glutamate) (PMDG) were nucleated in bromoform. The morphology of these spherulites was investigated by SEM and photographic small‐angle light scattering (SALS). From the SALS patterns, the development and growth of the spherulites could be noted. Films of PMDG cast from bromoform solution were found to be partially spherulitic. The mechanical properties of these films, prepared by different procedures, were compared with the general behavior of PMDG films cast from chloroform, the latter of which does not contain spherulitic texture.

Improved gas-discharge etching techniques in the electron microscope study of polyamide structure

ACCORDING to establ ished views, fibrils, spherulites and single crystals are typica l supermolecular structures of polyamides. Fibr i l lar structures mainly characterize oriented systems [1, 2]. Xray diffraction a t low angles reveals a fairly strict al ternation of ordered and disordered regions along the fibrils [3], which is confirmed by an electron microscopic method of negative contrast [4] and are known as long periods. For polyamides they are within the range of 70-700 •, according to the chemical nature and t rea tment conditions of samples. Polyamide spherulites are large (up to several microns) formations with fibrils growing in a radial direction from the spherulite centre. They normally characterize crystallized, non-oriented systems. Up to recently single crystals were.considered as unlikely structural forms of polyamides since they can only be formed with unbranched polyamides and under certain conditions of crystall ization. For the same reason i t is difficult to imagine the simultaneous coexistence in the polymer of two crystalline morphological forms--single crystals and spherulites. The aim of this s tudy was to use etching with active oxygen (EAO) obtained in a field of a.c., linear, electrodeless, high-frequency discharge to examine the supermolecular structure of polyamide materials. Wi th most methods of electron microscope investigation used up to now, model polymer films have been used obtained under special conditions. We sought to observe s t ructural detai ls in industr ial polymer samples using EAO combined with the replica method. The use of chemical etching to reveal the supermolecular structure of polyamides does not enable the rate of etching to be controlled to the required extent and almost inevi tably causes s tructural variat ions in the surface layers of the polymer. The use of cathode sputtering (ion bombardment in plasma of high-voltage d.c. discharge) results in considerable heating of the polymer surface layer by absorption of par t of the electronic plasma component mainly consisting of fast electrons and recombination of electrons and ions in the polymer. The use of EAO based both on the effect of ionic polymer bombardment and the action of act ivated oxygen, apparent ly consisting of neutral atoms and gas molecules, in burning

Spherulitic crystallization and cracking during heat aging of polypropylene

AbstractThe spherulitic crystallizations of polypropylene were investigated by a polarizing microscope. At high temperature, crystallization proceeds through the process of dendritic mechanism. Long needles grow rapidly at first from the nucleus to the supercooled melt, and then they fan out around each needle by branching. The rates of these two processes depend on the polymer and the condition of crystallization. In large well‐grown spherulites, cracks are formed both between radial fibers and at boundaries during heat aging. The cracks are void and single spherulites are separated. The boundary cracks are also caused by thermal decomposition of molecules and the radial cracks by stresses due to the rearrangement of molecules or the perfection of spherulites.

Single Spherulites Differ from Arrays

Single Spherulites Differ from Arrays