Stacks Of Lamellar Crystals Research Articles

Crystallization behavior and secondary nucleation model of PP/GN nanocomposites under supercritical N2

Supercritical N2, as a physical blowing agent, is commonly used in microcellular foam injection molding to improve the crystallization kinetics and foaming performance of polymer-based composites. Herein, the crystal morphology and growth behavior of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N2 were investigated using a self made in-situ high pressure microscope system. Due to the heterogeneous nucleation effect of GN, more irregular lamellar crystals inside the spherulites were generated and the stacking of lamellar crystals was more disordered. The intrinsic mechanisms of GN content on the growth rate of spherulites were investigated on the basis of the interfacial free energy induced by GN. The calculated results of the secondary nucleation model are in line with the results obtained from the isothermal crystallization experiments, indicating that the model can effectively predict the grain growth rate of PP/GN nanocomposites under supercritical N2. In addition, based on the crystallization kinetics of PP/GN composites, the action mechanism of GN on the foaming behavior of PP was explored. As expected, more cell nuclei were induced due to the refinement of grains and the increased of GN content.

Crystallization and melting of inulin crystals. A small angle X-ray scattering approach (SAXS)

Summary — Bulk crystallization of the biopolymer inulin from concentrated solutions results in the same macroscopic and microscopic morphologies as those found for synthetic polymers. Optical and electron microscopy confirm the lamellar nature of the crystals and their arrangement into stacks of lamellar crystals in superstructures like axialites and spherulites. There is evidence for the occurrence of folded chain lamellar crystals. These findings justify the attempt to explore the crystallization and melting of inulin by investigating the morphology of inulin crystals by time-resolved small angle X-ray scattering. The thickness of the lamellar crystals observed by SAXS varies with the degree of supercooling — dynamically or isothermally — as observed for synthetic polymers crystallized from the solution. The complex melting behaviour of polydisperse inulin, crystallized from the concentrated solution, as observed by DSC, and previously desribe by the present authors, can be explained by the occurrence of thin lamellae with lower thermal stability (endotherms I and II) together with thicker lamellae composed of the longer chains with a higher melting temperature (endotherms III and IV), the latter lamellae being able to thicken upon annealing by recrystallization or solid state reorganisation. The broad range of molar masses proper to the samples, the different crystallization conditions (temperature, isothermal and dynamic cooling and melting) account for fractionation and the presence of partially overlapping populations of lamellar stacks with different lc‘s and hence different thermal stabilities. The reported experiments are a basis for the exploration of the semi crystalline nature of inulin gels.

Morphological and structural evolution of tensile deformed high density polyethylene during melting: in situ synchrotron small-angle X-ray scattering study

The morphological and structural evolution of quenched high density polyethylene specimens subjected to different strains during the heating process was explored via synchrotron small-angle X-ray scattering technique. On the basis of one-dimensional scattering intensity distribution profile analysis, a general scheme for the melting of polyethylene was proposed that surface melting occurs below the crystallisation temperature, followed by melting and recrystallisation and/or lamellar thickening process via sequential melting and stack melting at the ultimate stages of melting. It was shown that in the case of undeformed specimen, the crystallisation process has finished at the temperature of ∼90°C during quenching, while the crystallisation temperature is equivalent to 70°C for the highly oriented sample, which was attributed to the thermal effects in the necked region due to high strain rate. A qualitative argument that the isotropic material possesses a wide distribution of lamellar (or amorphous domain) thicknesses with respect to the highly oriented one can be made in terms of the temperature dependent one-dimensional correlation function. In addition, it could be evidenced that moderate deformation ratio does not affect the intrinsic property of existing stacks of lamellar crystals by a direct comparison of the evolution of structural parameters of their lamellar structure during heating.

Kinetic study of the II–I phase transition of isotactic polybutene-1

Kinetics of the solid–solid II–I phase transition of isotactic polybutene-1 was investigated. The fraction WI of phase I as a function of time ttr during the phase transition was measured by X-ray diffraction at various temperatures Ttr. The Avrami indices n of the WI–ttr plots are approximately unity for Ttr>288K. A bell-shaped temperature dependence of the transition rate V with the maximum transition rate at 285K was obtained. The V–Ttr curve and the Avrami index n=1 suggest that the rate-determining process is primary nucleation. The dependence of V on Ttr for Ttr<283K is described by the William–Landel–Ferry (WLF) equation, which shows that the glass transition affects the transition rate. The Avrami index decreases to n<1 for Ttr<283K, indicating a broadened distribution of the transition rate caused by the spatial heterogeneity of the amorphous state at low temperatures near the glass transition. Those evidences at low temperature clearly suggest that the solid–solid phase transition is influenced by the mobility of chain folding, tie chains and cilia in the amorphous between the stacks of lamellar crystals.

Microphase separation in poly(isoprene-b-ethylene oxide) diblock copolymer melts. I. Phase state and kinetics of the order-to-order transitions

The phase state and the kinetics of the order-to-order transitions have been studied in a series of poly(isoprene-b-ethylene oxide) (PI-PEO) diblock copolymers with a PI volume fraction in the range 0.25&amp;lt;fPI&amp;lt;0.92, using small angle x-ray scattering (SAXS), and rheology. The mean-field theory (MFT) structure factor is used to describe the SAXS profiles in the disordered phase and to extract the temperature dependence of the interaction parameter χ(T). In general, an agreement is found with the phase diagram proposed by an extended MFT, except at fPI=0.61 where the following sequence of phases was found: Lc→Hex→Gyroid→Dis (Lc is the crystalline lamellar phase, Hex signifies hexagonally packed cylinders, Gyroid is the bicontinuous cubic network with the Ia3̄d symmetry, and Dis is the disordered phase). We found that crystallization disrupts the amorphous ordered morphologies and imposes a layered structure (Lc). The study of the kinetics of the Hex to Lc and the Hex to Gyroid transitions is facilitated by the different viscoelastic contrast and the distinctly different scattering patterns of the three phases involved (Lc, Hex, Gyroid). Our studies show that it is possible to undercool and overheat ordered phases just as we can undercool the disordered phase. The transformation from the Hex to the Lc phase proceeds via a heterogeneous nucleation and growth process and results in the formation of a spherulitic superstructure composed from stacks of lamellar crystals. The transformation of the Hex to the Gyroid phase involves two steps. The first step—which is too fast to be picked up by rheology—involves fluctuations of the hexagonal phase. The second “slow” step involves a nucleation and growth process of elongated objects. The transformation proceeds nearly epitaxially and has an activation energy of 47 kcal/mol which is typical for a collective process.

Time-Resolved SAXS, WAXS, and DSC Study of the Annealing of Poly(aryl ether ether ketone) (PEEK) from the Glassy State

The annealing of poly(aryl ether ether ketone) (PEEK) from the glassy state was investigated by time-resolved simultaneous small-angle (SAXS) and wide-angle (WAXS) X-ray scattering using synchrotron radiation and by differential scanning calorimetry (DSC). Each experiment consisted of a heating ramp from 100 °C to the annealing temperature (280, 300, 310, 315, 340, and 350 °C) at 10 °C/min, followed by a 30 min isotherm at this temperature and a cooling ramp down to 100 °C at −10 °C/min. The long spacing L and the invariant Q were estimated from the Lorentz-corrected SAXS intensity curves. The crystalline (Lc) and amorphous (La) thicknesses and the linear degree of crystallinity (vclin) within the stacks of lamellar crystals were obtained from the correlation function γ(r). The annealing temperature range can be divided into two regions based on the final morphology of the PEEK films. Samples annealed at temperatures up to 315 °C contain lamellar crystals that have crystallized during the isotherm. For annealing temperatures above 315 °C, thick lamellar crystals grow during the isotherm. However, a large fraction of the material remains molten at the end of the isotherm. The magnitude of this molten fraction increases with the annealing temperature. This molten fraction nonisothermally crystallizes during cooling as revealed by calorimetry and X-ray diffraction. The linear degree of crystallinity of PEEK crystallized from the glassy state at temperatures between 280 and 340 °C is always below 0.5, implying that the crystalline thickness of the lamellae corresponds to the smallest value of the two partial thicknesses obtained from the correlation function. For the last experiment, the polymer is fully molten at 350 °C and nonisothermally crystallizes during the cooling as revealed by calorimetry. In contrast, the analysis of the corrected invariant and the integrated WAXS intensity for this last sample indicates that the latter consists of lamellar stacks with a linear degree of crystallinity of up to 0.75.

Organizing for better synthesis

To sieve out one molecular species from another, reach for a zeolite; or now that they are available, choose a microporous inorganic oxide tailor-made for the job by taking a miniature 'plaster cast' of assemblies of organic molecules. On page 430 of this issue, Archibald and Mann show how this method can be stretched to produce microtubules or stacks of lamellar crystals.

Topotactic crystallization of isolated poly(β-hydroxybutyrate) granules fromAlcaligenes eutrophus

Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), wide-angle X-ray powder diffractometry (WAXD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the crystallization behavior of PHB granules from Alcaligenes eutrophus isolated by enzymatic purification. TEM examination of freeze dried granules after mortar and pestle grinding at liquid nitrogen temperature revealed that the dry granules have a non-crystalline core/crystalline shell morphology. TEM micrographs sections of PHB granules showed that upon annealing, the non-crystalline molecules in the core transform into stacks of lamellar crystals with a thickness of ∼ 100 A. The FTIR results revealed the presence of bound water in a sample of freeze dried granules and WAXD of the same sample showed an increase in crystallinity after removal of this water by vacuum drying. The WAXD diffractograms showed an increase in crystallinity of PHB granules when going from the in vivo to the dry state. In spite of the possibility of deforming them at very low temperatures (liquid nitrogen temperature) the glass transition temperature (Tg) of nascent PHB granules, as revealed by the DSC thermograms, was in the range −0.5−4°C. These results suggest that water is responsible for keeping the core of nascent PHB granules in a non-crystalline state. A model for biosynthesis where emerging PHB chains in an extended conformation are simultaneously hydrogen bonded to water molecules is proposed.

Topotactic crystallization of isolated poly(β-hydroxybutyrate) granules from Alcaligenes eutrophus

Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), wide-angle X-ray powder diffractometry (WAXD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the crystallization behavior of PHB granules from Alcaligenes eutrophus isolated by enzymatic purification. TEM examination of freeze dried granules after mortar and pestle grinding at liquid nitrogen temperature revealed that the dry granules have a non-crystalline core/crystalline shell morphology. TEM micrographs sections of PHB granules showed that upon annealing, the non-crystalline molecules in the core transform into stacks of lamellar crystals with a thickness of ∼ 100 A ̊ . The FTIR results revealed the presence of bound water in a sample of freeze dried granules and WAXD of the same sample showed an increase in crystallinity after removal of this water by vacuum drying. The WAXD diffractograms showed an increase in crystallinity of PHB granules when going from the in vivo to the dry state. In spite of the possibility of deforming them at very low temperatures (liquid nitrogen temperature) the glass transition temperature ( T g ) of nascent PHB granules, as revealed by the DSC thermograms, was in the range −0.5−4°C. These results suggest that water is responsible for keeping the core of nascent PHB granules in a non-crystalline state. A model for biosynthesis where emerging PHB chains in an extended conformation are simultaneously hydrogen bonded to water molecules is proposed.