Ringed Spherulites Research Articles

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.

Coaxial μSAXS/POM simultaneous measurement for microstructural characterization of one-dimensional and two-dimensional crystalline polymer

The microfocus beam small-angle X-ray scattering (μSAXS) provides a high spatial resolution ability, and the μSAXS/POM simultaneous measurement facilitates the acquisition of the micro-structural variations in the heterogeneous samples. Herein, at the SSRF BL19U2 beamline, a coaxial μSAXS/POM simultaneous measurement system has been established, comprising of the compound refractive lenses and an in-situ polarizing microscope. The SAXS beam size of 5.43 μm × 6.52 μm with a flux of 2.52 × 1011 photons/s has been employed. In order to obtain realistic microstructural information, the visible light path of the polarizing microscope has been set coaxially with the X-ray to eliminate the field of view deviation. The microstructure of the ringed spherulites of the polyoxymethylene/polybutylenesuccinate blends and high-density polyethene (HDPE) fibers has been simultaneously characterized. The differences in the structural information at different positions inside the banded spherulites have been evaluated. The crystallinity of the HDPE fibers increases from the skin to core, whereas the average crystalline lamellae thickness of the HDPE fibers decreases from skin to core, and the long spacing remains almost unchanged in the fibers.

Synthesis, Self-assembly, and Fluorescence Application of Bottlebrush Polyfluorene-g-Polycaprolactone with Conjugated Backbone and Crystalline Brushes.

A series of bottlebrush copolymers with conjugated backbone and crystalline branch chains, polyfluorene-g-polycaprolactone (PF-g-PCL), are synthesized by combining Suzuki cross-coupling polymerization and cationic ring-opening polymerization. The PF-g-PCLs are prepared to self-assembled in solution and thin film. Due to the J-type aggregation of the polyfluorene main chains, the self-assembly spherical micelles have been observed. Meanwhile, in film, they exhibited self-assembly ringed spherulites because of the PF microregions in the bottlebrush copolymer. As a result of the interruption of PCL side chains, the aggregation tendency of PF main chains is weakened. And both the polymer solution and solid can overcome the aggregation-caused quenching to provide more pronounced fluorescence. Especially, owing to the good processability of the PF-g-PCL, as a fluorescent ink for different substrates, they can easily be prepared as high-brightness fluorescent films that are invisible under ambient light.

Non-Isothermal Crystallization Kinetics of Poly(4-Hydroxybutyrate) Biopolymer.

The non-isothermal crystallization of the biodegradable poly(4-hydroxybutyrate) (P4HB) has been studied by means of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). In the first case, Avrami, Ozawa, Mo, Cazé, and Friedman methodologies were applied. The isoconversional approach developed by Vyazovkin allowed also the determination of a secondary nucleation parameter of 2.10 × 105 K2 and estimating a temperature close to 10 °C for the maximum crystal growth rate. Similar values (i.e., 2.22 × 105 K2 and 9 °C) were evaluated from non-isothermal Avrami parameters. All experimental data corresponded to a limited region where the polymer crystallized according to a single regime. Negative and ringed spherulites were always obtained from the non-isothermal crystallization of P4HB from the melt. The texture of spherulites was dependent on the crystallization temperature, and specifically, the interring spacing decreased with the decrease of the crystallization temperature (Tc). Synchrotron data indicated that the thickness of the constitutive lamellae varied with the cooling rate, being deduced as a lamellar insertion mechanism that became more relevant when the cooling rate increased. POM non-isothermal measurements were also consistent with a single crystallization regime and provided direct measurements of the crystallization growth rate (G). Analysis of the POM data gave a secondary nucleation constant and a bell-shaped G-Tc dependence that was in relative agreement with DSC analysis. All non-isothermal data were finally compared with information derived from previous isothermal analyses.

Crystalline Structures and Structural Transitions of Copolyamides Derived from 1,4-Diaminobutane and Different Ratios of Glutaric and Azelaic Acids.

Copolyamides derived from even 1,4-butanediamine and different mixtures of odd dicarboxylic acids with a great difference in the number of methylene groups (i.e., glutaric and azelaic acids with 3 and 7 groups, respectively) have been synthesized, characterized and structurally studied. Calorimetric analyses revealed a complex behavior with multiple melting peaks associated to lamellar reordering and the presence of defective crystals. Equilibrium melting temperatures were evaluated and showed a eutectic behavior with composition. Copolymers were able to crystallize even for samples with comonomer percentages close to 50%. Negative and ringed spherulites from the melt state and small lath-like lamellar crystals from dilute solution crystallizations were attained. Furthermore, calorimetric data pointed out the exclusion of the less abundant monomer from the lattice of the predominant structure. All samples at room temperature showed a similar crystalline structure (form I) defined by two predominant reflections at spacings close to 0.430 and 0.380 nm, which has been related for even-odd nylons with a two-hydrogen bonded structure. Real time synchrotron experiments showed that melt crystallized samples have two polymorphic transitions on heating, which were practically reversible and consequently were also detected during cooling from the melt state. Interestingly, a different behavior was detected among solution crystallized samples and specifically the transition to the intermediate structure (form II) was not detected during heating for samples enriched on the azelate component or more precisely when they were exclusively crystallized in the form I.

Systematic probing into periodic lamellar assembly via induced cracks in crystallized polyesters

Using poly(3-hydroxybutyrate) (PHB) of two molecular weights and various other polymeric or blend systems, this study has shed a critical clue that cracks are inclined to be guided by interior lamellae via interior inter-lamellae discontinuity and interfaces that exist between the successive ring bands or radial/tangential lamellae. Advanced interior dissection analysis on the aggregated spherulites of several polymers further proves that the cracks occur at the interface between the valley and ridge bands of the ringed spherulites, where interfaces with discontinuous lamellae exist with either impingement or intersection. Propagation of the cracks follows the spiral or circular lamellae assembly into similarly spiral or circular concentric patterns, Such updated views via interior lamellae dissection and analysis on PHB's or other polymers' banded spherulites provide avenues for cracks to develop upon cooling contraction. The mechanism of spiral/circular cracks' formation is well proven in this work, which is related to the inner arrangement/assembly of the periodic spiral lamellae into banded types; by contrast, without such lamellae periodicity, cracks tends to be randomly irregular.

Anatomy into Interior Lamellar Assembly in Nuclei-Dependent Diversified Morphologies of Poly(l-lactic acid)

Sophisticated dissection into the interiors of the three different birefringent types of poly(l-lactic acid) (PLLA) spherulites via delicate fracturing across the thickness sections, coupled with proper etching techniques, was adopted to reveal the correlations between the diversified birefringence patterns and interior lamellae assembly. The three types of spherulites are circularly ringed spherulite (type 1), hexagon-shaped axialite (type 2), and circularly core–stripe dendrites (type 3). Such morphological diversification originates from different nuclei geometries. For all three different types (circularly ringed, hexagonal, and core–stripe) of PLLA aggregation into spherulites or dendrites/axialites, the dissected inner lamellar arrangement shares universal commonality of intersecting at a 60° angle in the intersection between different lamellar species, with distinct discontinuity in the grating structures being characteristic of all three types of aggregated PLLA spherulites. This kind of grating a...

Mapping the Mechanical Properties of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Banded Spherulites by Nanoindentation.

Nanoindentation provides clear evidence that spherulite banding can be associated with a continuous modulation of mechanical properties from the more compliant peaks to the stiffer valleys. The structural arrangement in polymer-banded spherulites has intrigued scientists for many decades, and the debate has been recently intensified with the advent of new experimental evidence. The present paper approaches this issue by exploring the local mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-ringed spherulites via nanoindentation and discussing the confidence of the results. It was found that storage modulus and hardness across the banding morphology can be described as a sequence of regular oscillations with a periodicity that exactly matches the one observed using optical and atomic force microscopy. Results are consistent with the model of regular twisting of the lamellae, with flat-on arrangement in the low regions and edge-on lamellae in the crests.

Reversible changes induced by temperature in the spherulitic birefringence of nylon 6 9

Spherulitic morphologies of nylon 6 9 as an example of an even-odd nylon were studied by optical microscopy. A well-defined dependence on crystallization temperature was found. In particular, positively birefringent spherulites were characteristic for temperatures higher than 232 °C, low birefringence spherulites developed between 232 and 225 °C, positive ringed spherulites were found between 225 and 220 °C, and finally negative fibrillar spherulites were formed at temperatures lower than 220 °C. These optical properties were clearly different from those observed with even–even nylons (e.g., negative and positive birefringence for high and low temperatures, respectively), and may derive from the peculiar crystalline structures determined for even-odd nylons. Furthermore, low birefringence spherulites were characterized by a flat-on lamellar disposition and reversibility of the birefringence sign in the 80–120 °C temperature interval (positive and negative values at high and low temperatures, respectively).Real time WAXD profiles taken during heating and cooling processes demonstrated that different crystalline structures (named I, II and III) developed depending on the temperature and crystallization procedure of samples (e.g., from solution or from the melt state). Crystalline structures were characterized by the formation of hydrogen bonds along two crystalline directions in all cases, a peculiar arrangement that may account for the development of positive and negative spherulites in a more simple way than formulated for conventional polyamides having a single hydrogen bonding direction. DSC and FTIR data also showed a complex structural behavior with structural transitions in the 80–120 °C range, a region that corresponds to birefringence sign reversibility.

Tuning Radial Lamellar Packing and Orientation into Diverse Ring-Banded Spherulites: Effects of Structural Feature and Crystallization Condition

Spherulite morphologies of a highly asymmetrical double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) diblock copolymer in solution-cast films were explored from a unified standpoint of tuning radial lamellar organization via controlled evaporation. Besides Maltese cross spherulites, three kinds of ring-banded spherulites that display non- and half-birefringent concentric ringed features as well as extinction banding were first encountered in the same polymer. Structural analyses based on atomic force microscopy, transmission electron microscopy, and grazing incidence X-ray technique revealed that concentric ringed spherulites possess the nature of a rhythmic variation of the radial lamellar packing and extinction banded spherulites have the origin of a periodic change of the radial lamellar orientation. Morphological transitions among different kinds of spherulites were achieved by altering the drying condition. PEO segment crystallized at low temperatures even if it is being confined by PCL lamellae. Combined with poly(e-caprolactone) and poly(ethylene adipate), the influences of structural feature and crystallization condition on radial lamellar organization of polymer spherulites were discussed. These present findings are encouraged to enhance our understanding and then governing generation of expected polymer crystal morphology for special material performance.

Poly(butylene azelate-co-butylene succinate) copolymers: Crystalline morphologies and degradation

Random copolymers derived from 1,4-butanediol and two dicarboxylic units differing in the parity of the number of methylene groups and length of the polymethylene sequence (i.e. succinic and azelaic acids) were studied in terms of thermal properties, crystalline structure and morphology, crystallization kinetics and biodegradability. All samples were semicrystalline and their thermal properties varied in a wide temperature range. Copolymers crystallized according to the monoclinic α-form of polybutylene succinate and the orthorhombic structure postulated for polybutylene azelate depending on the predominant dicarboxylate unit. The behavior of the copolymer with an intermediate composition was complex due to strong dependence of the predominant crystalline form on crystallization and processing conditions. Interestingly, crystallization into the azelate structure was favored when samples were rapidly cooled from the melt, resulting in an unexpected increase in the degree of crystallinity. Spherulitic morphologies were clearly different (i.e. ringed spherulites and axialites) depending on the preferential crystalline structure. Enzymatic degradability of the two homopolyesters was highly different and could be enhanced by incorporation of comonomer units. Preferential enzymatic attack on amorphous regions highlighted the spherulitic morphologies of copolymers having well developed, distinctive ringed structures.

Coupling between crystallization and evaporation dynamics: Periodically nonlinear growth into concentric ringed spherulites

Despite the extensive study of periodic precipitation and rhythmic crystal growth into ringed patterns, the detail of the evolution process remains unclear, and thus the explanation is rather elusive. Herein, we focus on monitoring the detailed growth process and dynamic, elucidating the underlying mechanism, and exploring key factors for the generation of poly(ε-caprolactone) (PCL) concentric ringed spherulites in evaporating droplets. In situ observation exhibits that accompanying the rhythmic evolution of the crystal, the region ahead the growth face changes periodically and the radial growth within each period is non-linear. It shows that having an evaporation-driven convection that carries liquid to the growth face drives the periodic dimple generation and rupture that leads to the rhythmic growth into discrete ringed spherulites. The non-linear growth is attributed to the coupling of evaporation and crystallization. We find that there are two key factors in the drying process that ensure the occurrence of the evaporation-driven flow and then the periodic crystal pattern. The structure formation reveals a complex interplay among solvent extraction, solution flow, solute diffusion, and crystal growth.

Miscibility, Crystallization, and Morphology of the Double-Crystalline Blends of Insulating Polyethylene and Semiconducting Poly(3-Butylthiophene)

A series of crystalline semiconducting poly(3-butylthiophene) (P3BT)/crystalline insulating polyethylene (PE) blends were prepared and the miscibility, crystallization, and structure/morphology were investigated. Even though phase separation was observed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), several pieces of evidence indicated that limited miscibility should be present in PE/P3BT blends: small changes in both Tm and crystallinity of PE phase and a small portion of PE being dissolved in P3BT. The study of PE isothermal crystallization kinetics revealed that the introduction of P3BT significantly influenced the nucleation mechanism and growth geometry, i.e., PE was transformed from three-dimensional (3D) spherulitic to two-dimensional (2D) disc crystals. A striking reduction of nucleation density and an obvious ringed morphology of PE spherulites (2D) in PE/P3BT blends were also observed by polarized optical microscopy; it is proposed that the limited miscibility between PE and crystalline P3BT favors the formation of ringed PE spherulite in the blends. Additionally, preferred orientation of PE lamellae, with their b-axis largely constrained to the thin film plane, was observed by X-ray diffraction in PE/P3BT blend films. It is evidenced that the PE orientation was due to the b-axis being the crystal growth direction, which can only be in film plane.

FRACTAL GROWTH AND MORPHOLOGICAL TRANSITIONS DURING CRYSTALLIZATION OF AMINO ACIDS IN PRESENCE OF GLUCOSE

Two-dimensional fractal and spherulitic patterns have been developed on microslides during the crystallization of amino acids in the absence and presence of glucose and agar-agar from their aqueous solutions. Lysine crystallized uniformly in the form of ringed spherulite, but in the presence of glucose it crystallized in the form of branched morphologies. Dependence of glucose concentrations on morphology of glutamic acid has also been studied. Morphology of glutamic acid was found to depend on glucose concentration. Due to interaction between glutamic acid and glucose, following morphological transitions were observed depending on glucose concentrations: spherulite → ringed spherulite → Fractal geometry → DLA-like pattern (D ≈ 1.725). Growth morphologies were characterized by measuring the location of bands (xn) as a function of band number n obeying the relation xn = mn + c, where m and n are slope and intercept respectively, and calculating the fractal dimension at different conditions. Influence of agar-agar on the morphology was also studied. Results showed that branched morphologies were more dominant in the presence of agar-agar. In case of glycine, irregular growth patterns were observed. Interaction between amino acids and glucose was studied by viscosity measurements. It has been inferred that interaction is maximum in case of interaction of glucose with glutamic acid (acidic) and minimum in case of glycine (neutral).

Crystallization behavior of poly(L‐lactic acid)/montmorillonite nanocomposites

AbstractIn this study, we modified montmorillonite (MMT) with dilauryl dimethyl ammonium bromide (DDAB) and then exfoliated the structures in a poly(L‐lactic acid) (PLLA) matrix. We used polar optical microscopy and X‐ray diffraction (XRD) to examine the morphologies of the resulting composites, differential scanning calorimetry to study the melting and crystallization behavior, and Fourier transform infrared (FTIR) and Raman spectroscopy to measure the influence of the intermolecular interactions between PLLA and MMT on the isothermal crystallization temperature. We found that the DDAB‐modified MMT was distributed uniformly in the PLLA matrix. At temperatures ranging from 130 to 140°C, the crystalline morphology resembled smaller Maltese cross‐patterned crystallites; at temperatures from 150 to 170°C, however, the number of crystallites decreased, their sizes increased, and they possessed ringed spherulite structures. In the XRD spectra, the intensity of the diffraction peaks of the 200/110 and 203 facets of the PLLA/MMT nanocomposites decreased as the crystallization temperature increased. In the FTIR spectra, the absorption peak of the CO groups split into two signals at 1748 and 1755 cm−1 when the isothermal crystallization temperature was higher than 140°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Ringed spherulitic and undulated textures in the nematic phase of a mixture of trans-4-hexylcyclohexanecarboxilic and benzoic acids

We present a polarizing optical microscopy study of a liquid-crystal mixture of trans-4-hexylcyclohexanecarboxilic (C6) and benzoic acids. Both materials have carboxylic groups that can form dimers through hydrogen bonding. The mixture is nematic at room temperature and has the clearing point at 88°C. The nematic phase shows ringed spherulites, sometimes looking like spiral structures, and undulated textures, which remain visible when heating the sample till 71°C. This is the temperature of a texture transition inside the nematic phase.

Non-equilibrium growth patterns in a reaction between thiamine and glucose

Reaction between glucose and thiamine has been studied in aqueous solution containing gel matrix. Growth patterns of the reactants and the product have been studied. Glucose crystallizes in the form of fractal with fractal dimension 1.74 and thiamine as spherulitic when observed under polarized light microscope. Glucose-thiamine reaction product crystallized in different morphologies. Following transitions were observed on increasing the concentration. Spherulite → Ringed spherulite → Revert spaced concentric rings Besides change in the morphology of the reaction product, XRD, IR and DSC studies also revealed the formation of a new product. During the reaction glucose was continuously consumed on addition of thiamine, obeying the relation [G] - 1 = m log V + c, where m and c are slope and intercept respectively. Heat of fusion ΔH of the product was calculated using the expression ΔH.m = KA, where m is the mass of the substance, K and A are the calibration constant and area under the curve respectively. The value was found to be 1.36 kcal/mol, which was lower than those of the individual reactants.

Thermal properties of poly(alkylene dicarboxylate)s derived from 1,12‐dodecanedioic acid and even aliphatic diols

AbstractA series of new aliphatic polyesters derived from 1,12‐dodecanedioic acid and different diols with an even number of methylene units have been studied to assess the effect of the chemical structure on the final thermal properties of the materials. The polyesters have high thermal stability and are fast crystallizing polymers, with crystallization rate similar to that of polyethylene (PE). This behavior is connected to the fact that long aliphatic chains assume conformational characteristics very similar to that of PE. However, the polyester prepared from ethanediol shows a peculiar behavior (for example, double melting peak, melting and crystallization temperatures, which do not fit the trend of those of the other samples and ringed spherulites) owing to a probable different conformation, deviating from the all‐trans planar typical of PE. In the isothermal crystallization studies, a bell‐shape trend has been found for the crystallization rate as a function of the number of (CH2) units in the diol. The high crystallization rate of the sample with long (CH2) sequences has been attributed to the high chain flexibility and, thus, high mobility in the molten state and ease of chain folding. By reducing the aliphatic sequence length, instead, implications of the structural characteristics of the samples are probably involved. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1053–1067, 2007

Breakup of spiral and concentric ringed spherulites in polymer crystallization

Recently, the breakup of spiral and concentric ringed spherulites has been observed experimentally in isothermal crystallization of poly(vinylidene fluoride) in its blends with ethylene vinyl acetate copolymer. To elucidate the phenomenon of spiral breakup in polymer spherulites, a theoretical simulation has been undertaken in the framework of the phase field model of solidification based on time-dependent Ginzburg-Landau equations (TDGL model C) pertaining to the conserved concentration and nonconserved phase field order parameters. The concentration order parameter was described in terms of the Landau-type potential, while the crystal order parameter was expressed in terms of an asymmetric double-well potential to account for the nonequilibrium nature (i.e., metastability) of polymer crystallization. After inserting these potentials into the TDGL model C, the resulting coupled time-evolution equations show the emergence of the target and spiral patterns that undergo fragmentation at a very shallow supercooling. It is concluded that the depth of supercooling is the major contributor to the formation of the spirals and subsequent breakup of the spiral arms.

Multiple order light scattering by ringed spherulites

The transmittivity [Point JJ. Bull Clas Sci Acad Roy Belg 1953;39:455; 1955;41:982. [1]] of light intensity through spherulitic films of poly(ethylene adipate) (PEA) is not uniform. These films may actually be used as multiple diffraction gratings. The intensity of the first order scattering depends reversibly on the temperature T of the film (between 10 and 59 °C). For sufficient film thickness (e.g. 30 μm), this intensity is an increasing function of T. It is, however, a decreasing function of T when the films are sufficiently thin (e.g. 2 μm). These, and other effects are explained by geometrical optics and by the electromagnetic theory of light. Evidently, these effects are not specific to PEA but are characteristic properties of ringed spherulites of polymers and other materials.