Tiny Spherulites Research Articles

Crystallization studies on heterogeneous melts of polybutene-1

The heterogeneous melts of isotactic polybutene-1 (iPB-1) containing Form I or Form II seeds with uniform size and random distribution were prepared by selective melting of melt crystallized and transformed samples, respectively. Compared with a reference blank melt, the remnant Form I or II seeds are able to significantly reduce the half-time of crystallization under isothermal conditions. Besides, the nucleation efficiency of Form II seeds is much higher than that of Form I seeds because of the lower lateral surface energy and highly lattice match. Furthermore, a great number of tiny spherulites with submicrometer radius nucleate and grow simultaneously, offers an opportunity to follow the development of spherulites at the very early stage. The analysis of the differential scanning calorimetry (DSC) curves confirms that the mass fraction for faster primary crystallization of heterogeneous melts is lower than that of blank melt, which clearly indicates that growth rate of spherulites is decreasing before impingement. The radius of spherulites was not much larger than the diffusion length provides the foundation for the observed non-linear growth of spherulites. This quantitative comparison of the crystallization kinetics of blank melt and heterogeneous melts can therefore be applied as a method to improve our understanding of nucleation and the diffusion-controlled growth of polymer.

UCST Type Phase Boundary and Accelerated Crystallization in PTT/PET Blends.

We investigated the structure development and crystallization kinetics in the blends of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET) by polarized optical microscopy and light scattering. The crystallization of the blend was found to be faster and the size of the spherulites was much smaller than those of the neat component polymers by melt crystallization at low temperature of 180 °C. The discontinuous gap of the crystallization time with temperature was seen in the blends, suggesting phase transition at the temperature Ttr; e.g., the Ttr of the 60/40 PTT/PET was 215 °C. The crystallization was accelerated due to enhancement of the nucleation rate, and interconnected tiny spherulites were obtained at the temperature below the Ttr. The accelerated crystallization and the development of the interconnected structure might be attributed to the liquid-liquid phase separation via spinodal decomposition, due to existence of the upper critical solution temperature (UCST) type phase boundary.

Influence of Boron Nitride Nanosheets on the Crystallization and Polymorphism of Poly(l-lactide).

This work reports on the impact of delaminated boron nitride nanosheets (BNNSs) on the crystallization behavior and crystalline structure of melt-crystallized poly-L-lactic acid (PLLA). Wide-angle X-ray diffraction and scanning electron microscopy data revealed that the addition of lower loadings of BNNSs (∼0.5 wt %) resulted in the highly dispersed PLLA nanocomposites, whereas the higher loading of BNNSs (≥1 wt %) leads to the agglomerated nanocomposites. It is shown that the presence of lower loadings of the BNNSs (∼0.5 wt %) induces the formation of ordered α form when crystallizing from the melt at a cooling rate of 10 °C/min, but the mixture of α' and α forms is formed in the presence of higher loading of BNNSs (≥1 wt %). Polarized optical microscopy images revealed that the crystallization rate of PLLA was significantly enhanced in the presence of lower loading of BNNSs (∼0.5 wt %) as corroborated by the increasing number of tiny spherulites. The strong interaction between the highly dispersed BNNSs and PLLA chains induces the conformationally ordered α form, and the various experimental techniques revealed that crystallization of PLLA occurred rapidly with the narrow distribution of crystal size and degree of crystal perfection in highly dispersed nanocomposites. Furthermore, the thermal conductivity of PLLA/BNNSs nanocomposites was found to increase significantly with BNNSs loading.

The effects of crystallization condition on the microstructure and thermal stability of istactic polypropylene nucleated by β‐form nucleating agent

AbstractThis article deals with the crystallization behaviors of original (prepared in a torque rheometer), DSC crystallization and mold crystallization (quenching and slow nonisothermal crystallization) of isotactic polypropylene (iPP) mixed with β‐form nucleating agent. The microstructure and thermal stability of these samples were investigated. The wide angle X‐ray diffraction (WAXD) results indicate that fast cooling is favorable for β‐form iPP formation. With slower cooling rate and higher concentration of nucleating agent, the lamellar thickness and stability of crystal0s were enhanced. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) both showed that rapid crystallized samples gave rise to tiny spherulites, whereas under slow crystallization condition, nucleated samples could be fully developed in the form of dendritic or transcrystalline structures, depending on the nucleating agent concentration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Crystal content investigation in dried human bulbo-urethral gland (Cowper's) secretion.

The present study compiled the data furnished by 77 different specimens of human bulbo-urethral secretion obtained from 4 donors. It established that Cowper's fluid, allowed to dry at room temperature, gives rise to dendritic crystalline structures showing a remarkable pattern closely resembling fern leaves. In addition to the dendrites, which may develop in some other mucoid secretions and are known to be mainly composed of NaCl crystals, numberless tiny spherulites are also observed within interdendritic intervals, in the central part of the preparations. These spherulites consist of polycrystalline microstructures radiating from a central nucleus. It can be inferred from their structure and localization that they are composed of salts other than NaCl and do not crystallize in the cubic system. Far from being an occasional phenomenon, ferning crystallization is considered to be an unequivocal physical constant of dried human bulbo-urethral fluid. As for interdendritic crystalline spherulites that were described previously as occasional structures present only in ovulatory cervical mucus, both their abundance and range of size variation make them a constant and specific feature of dried Cowper's fluid.

Crystallization improvement of poly(L‐lactide) induced by functionalized multiwalled carbon nanotubes

AbstractIn the present work, the crystalline structures and the melting behaviors of poly(L‐lactide) (PLLA) obtained after being annealed at different conditions have been investigated through differential scanning calorimetry and wide‐angle X‐ray diffraction, respectively. To improve the crystallization of PLLA, functionalized multiwalled carbon nanotubes (f‐MWCNTs) are introduced into PLLA. Our results show that by prolonging the annealing duration or enhancing the annealing temperature, the degree of crystallinity of PLLA gradually increases. Very important, the addition of f‐MWCNTs promotes the cold‐crystallization of PLLA dramatically even at relatively lower annealing temperature or in shorter annealing duration. Further results show that, whether in neat PLLA or in PLLA/f‐MWCNTs nanocomposite, only α form crystal forms during the annealing process. The glass transition temperature shifts to high temperatures because of the increase of crystallinity. F‐MWCNTs exhibit great heterogeneous nucleation effect for PLLA crystallization through enhancing the nucleation density, leading to homogeneous and tiny spherulites formation in a very short time. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 326–339, 2009

Crystallization behavior and morphology of PE-g-LCP copolymers

This study presents DSC and optical microscopy investigations on copolymers of semiflexible liquid crystalline polymer SBH 112 grafted to functionalized low molecular mass polyethylene (PEox) obtained by melt polycondensation or reactive blending procedures. The crystallization behavior of the PE-g-SBH copolymers has been studied under non-isothermal measurement conditions carried out at different cooling rates. The crystallization temperature (Tcr) of the PE component of the copolymers decreases steadily upon increasing the concentration of the SBH grafts. It was found that the copolymers prepared by reactive blending crystallize at slightly higher Tcr than those prepared by polycondensation and with a higher rate, confirmed by the determination of the crystallization rate coefficients (CRC). The results have been interpreted by the fact that the PE crystallizable segments and SBH grafts of the copolymers obtained by reactive blending are longer than those of the copolymers prepared by polycondensation. The overall nonisothermal crystallization kinetics has been studied by the Harnisch and Muschik equation. The results show that the mechanism of the crystallization of the PE phase changes only when the SBH content overruns ca.50%, due to the decrease of both nucleation and crystal growth rates. The morphology of the copolymers crystallized nonisothermally from melt has been examined by polarization microscopy. Fairly homogeneous morphology with tiny PE spherulites is observed for PE-g-SBH copolymers prepared by polycondensation with SBH as the minor phase. No sign of the dispersed LCP domains can be recognized. On the contrary, the morphology of the copolymers prepared by reactive blending is distinctly biphasic. The allegedly longer PE segments crystallize into tiny spherulites too, but the LC domains formed by the long SBH branches present in this type of copolymers appear clearly in the micrographs at room temperature. It is concluded that the copolymers prepared by reactive blending would be more effective as compatibilizers for PE/SBH blends than those prepared by polycondensation.

Spherulites in Calcrete Laminar Crusts: Biogenic CACO3 Precipitation as a Major Contributor to Crust Formation

ABSTRACT Spherulites (calcitic fibro-radial spherulitic polycrystals) are a major component of calcite layers in Quaternary calcrete laminar crusts. To explain the formation of spherulites, petrographic and chemical studies were performed on Pleistocene calcrete laminar crusts, Holocene biological crusts, and laboratory (in vitro) cultures of cyanobacterial strains. Spherulites were found to be formed of acicular and radiating crystals, either smooth and regular-edged, or twisted and irregular-edged. Spherulites were composed of low-Mg calcite, the concentration of Mg in the spherulites increasing from nucleus to periphery. The shape and chemistry of the in vitro spherulites and laminar crust spherulites were identical. The structure, elementary crystal shape, and size of spherul tes can be used to differentiate them from Microcodium, with which they are commonly confused in the literature. Spherulite genesis can be linked to cyanobacterial activity involving photosynthetic uptake of HCO-3, leading to CaCO3 precipitation, the bicarbonate causing release of equivalent amounts of OH- in the mucilaginous sheath. The main steps in cyanobacteria biomineralization producing spherulites are: (1) uptake of CO2 and/or HCO-3 from the medium as the inorganic carbon source for photosynthesis, (2) release of OH- in the sheath, (3) carbonate' ion formation resulting from the reaction of OH- and HCO-3, followed by (4) CaCO3 precipitation. This process involves two steps in desert environments: during dry spells, the cyanobacterial mat calcifies, leading to spherulite production and the fo mation of thin, clear microsparitic layers. During wet periods, a thin sedimentary deposit forms, mixing micrite, detrital particles, and inherited or new tiny spherulites with the mucilaginous mat of cyanobacteria, which traps all of this material. An organo-micritic layer forms. During the following dry spell, the process is repeated and the laminar crust is formed. In conclusion, the presence of spherulites in the laminar crust is indicative of subaerial exposure, the crust being a terrestrial stromatolite.