Crystalline Entities Research Articles

Study of the microstructure of amorphous aluminosilicate gel before and after its hydrothermal treatment

Solid phase (gel) separated from freshly prepared sodium aluminosilicate hydrogel as well as during its hydrothermal treatment at 80 °C was analyzed by different methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal gravimetry (DTG), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Analysis of the obtained results have shown that the freshly prepared gel is mainly composed of disc-shaped primary particles, but also partially or even fully crystalline entities were observed by AFM analysis. AFM analysis of the solids separated from the hydrogel at various stages of its hydrothermal treatment (heating at 80 °C) indicates that the particles of the partially and/or fully crystalline phase are nuclei for further crystallization of zeolite. Although it is assumed that the nuclei have structure of faujasite rather than zeolite A, further growth of zeolite A was readily explained by the fact that regardless of the “structure” of nuclei, the type of zeolite to be formed is determined by the concentrations of silicon and aluminum in the liquid phase of the crystallizing system.

Co-deposition of silver nanoclusters and sputtered alumina for sensor devices

Heterogeneous thin films may be beneficial for sensoring devices. The electrical conductivity of nanoscale metallic particles being embedded in a matrix of non conducting material should exhibit higher sensitivity to mechanical stress and strain compared to homogeneous films. The production of heterogeneous films may follow different routes. This paper describes the attempt to embed Ag nanoclusters emitted from a gas aggregation cluster source into a growing matrix of alumina originating from sputter sources. The characteristics of the cluster source are first resumed, with their mean masses ranging from approx. 1000 to 100,000 atoms per cluster. The expelled and soft landed clusters are extensively examined by transmission electron microscopy verifying their crystalline form. Yet the use of a radio frequency driven sputter source for the embed material destroys and annihilates the Ag clusters even at very low sputter power. If a reactive direct current sputter process is performed within an oxidising sputter gas instead, the Ag clusters are oxidised to different oxides, but they survive as crystalline entities as verified by X-ray diffraction investigations. A simple subsequent heat treatment reduces the Ag oxides to metallic Ag clusters.

Shear induced crystallization of poly( m-xylylene adipamide) with and without nucleating additives

The shear induced crystallization of the poly( m-xylylene adipamide) (MXD6) which is a semi-aromatic polyamide, was studied for a virgin (PA1) and a nucleated (PA2) grades using a shearing hot stage coupled with a microscope. Half crystallization times were measured according to the crystallization conditions (crystallization temperature, shear rate and shearing time). The effect of shear on the crystallization kinetics was shown by a strong decrease of the crystallization times for both materials. PA2 sensitivity to shear was much lower than that of PA1. This was attributed to the presence of nucleating agents which increased the primary nucleation density in the unsheared quiescent melt, leading to a higher necessary shear rate to overcome the quiescent nucleation. Kinetic models were proposed to predict the crystallization process as a function of the crystallization conditions. They were based on both Avrami and Hoffman–Lauritzen theories and modified to take into account the effect of shear. In the model the nucleation rate of the crystalline entities was related to the shear rate by a power function. Besides, crystalline morphology and orientation were studied by wide and small angle X-ray scattering to confirm the orientation effect of the shear in the crystalline part of the material.

Fatigue resistance of continuous glass fiber/polypropylene composites: consolidation dependence

The effect of consolidation on the fatigue resistance of continuous glass fiber/polypropylene (CGF/PP) composites was studied. Five sets of conditions, chosen to approach plant-like processing conditions, were used to obtain samples with varying consolidation quality. These conditions resulted in different void content, degree of crystallinity, size of crystalline entities (spherulites) and level of fiber dispersion. Although they led to different short-term (flexural, tensile or short-beam shear) mechanical results, a single linear correlation could be established between the interlaminar shear strength (ISS) and the void content for all consolidation conditions except one. The fatigue resistance of the CGF/PP composites showed that these consolidation conditions also resulted in different fatigue damage evolution and fatigue resistance curves (or S–N curves). From the S–N curves, two useful correlations could be established: the first is a linear correlation between the Basquin's fatigue strength and the short-term flexural strength; the second is a linear correlation between the Basquin's fatigue sensitivity and ISS. The results emphasize the predominance of the fiber–matrix interface in determining the general mechanical properties of continuous fiber composites and suggest that short-term flexural and short-beam shear tests can be useful empirical indicators of the long-term performance of CGF/PP composites.

Structural Characterization and Relaxation Processes of the Inner Crystalline Core in Foams Based on Polyethylene/Polypropylene Blends

Injected foams obtained from blends with different compositions of isotactic polypropylene, iPP, and two different low density polyethylenes, LDPE, were analyzed by X-Ray diffraction, differential scanning calorimetry, and dynamic mechanical thermal analysis. Most of the results on the blends can be simulated as a weighed addition of the two pure components, indicating practical immiscibility. Some features about the crystallization of the iPP component pointed out inhibition and, consequently, a delay in the development of monoclinic crystalline entities.

Crystalline phase in the ultrahigh molecular‐weight polyethylene gel solution and xerogel

AbstractThe transverse and longitudinal sizes of crystallites in thermoreversible polyethylene gels and xerogels were measured using wide‐angle X‐ray scattering and a low‐frequency Raman spectroscopy, and were found to be 10–40 and 4–5 nm, respectively. The experimental data evidence the imperfection of primary crystallites in both dimensions. The gel‐to‐solid transition results in gaining the direct interaction between crystalline entities with increasing the cracking, on the one hand, and forming the clusters of stacked crystalline platelets linked with regular molecular rods, on the other hand. These contact interaction leads, in addition, to the coiling of a great part of regular sequences that emanate from the crystallite cores to the amorphous region. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 373–378, 2003

Effect of the morphology of reactor powders on the structure and mechanical behavior of drawn ultra-high molecular weight polyethylenes

Evolution of the straighten-chain-segment length distributions in three polyethylene reactor powders manufactured using different catalysts has been traced through the melting-drawing procedures carried out under the same temperature-deformation conditions. The powders exhibiting the presence of either folded or chain-extended crystalline entities yielded the oriented samples with quantitatively different interfibrillar structure, and, respectively, with different mechanical behavior concluded from their stress–strain curves and variation of the tangential modulus. The correlation between original and final properties of samples derived from reactor powders evidences a persistence of a kind of memory. The longitudinal disorder in nascent chain-extended crystals transforms virtually to the lateral disorder in the axially oriented interfibrillar material.

Crystallisation process on the stage of the gel-to-solid transformation in thermo-reversible polyethylene gels

The crystalline phase in gels and solid (gel derived) samples of ultra-high molecular weight polyethylene was studied using the low-frequency Raman spectroscopy and the differential scanning calorimetry methods. The Raman data evidence the presence in gels of tiny crystallites whose thickness (3–5 nm) depends on the sort of solvent used. After removal of the solvent by drying or squeezing the original gels, there appear some extra ordered regions (6–12 nm in size) coexisting in solid material with the primary crystallites which remain unchanged. It is concluded that the gel-to-solid transformation is accompanied by a secondary crystallisation process which is weakly (if ever) dependent on the incipient crystalline entities in gels.

“In‐situ” monitoring of the non‐isothermal crystallization of polymers by dielectric spectroscopy

AbstractIn recent years, experimental techniques based on microdielectrometry have attracted increasing interest for continuous monitoring, in a nondestructive way, of the advancement of the reaction of thermoset resins under cure. The extension of this technique for “in‐situ” monitoring of the crystallization of thermoplastics has been carried out. A correlation between the evolution of the dielectric parameters of the material and its viscoelastic properties was done. A dynamic rheometer equipped with a dielectric cell and an instrumented slit die was designed. The crystallization process is depicted by a conductivity drop and by the occurrence of a maximum in the real permittivity. The decrease of the specific conductivity results from a modification of the conductive paths, whereas the increase of the permittivity is due to an interfacial polarization phenomenon (so‐called Maxwell Wagner Sillars effect) between the amorphous phase and the growing crystalline entities. The sudden variation of the dielectric response is envisioned as a means for following the progression of the crystallization front within a mold. The description of the crystallization with Nakamura's model allowed the modeling of the crystallization inside a mold. The evolution of the temperature profile and of the crystallinity rate is modeled by a classical finite difference method. Dielectric measurements performed at controlled cooling rate experiments are in good agreement with the numerical simulation results and confirm the usefulness of dielectric measurements for “In‐situ” monitoring of the crystallization process of a polymer inside a mold.

Localized optical absorption inCs4PbBr6

The fundamental optical absorption of Cs4PbBr6 crystals,which are built up of nearly regular Pb2+(Br-)6octahedra mutually bound by Cs+ ions, exhibits novel features:despite the crystalline entity of Cs4PbBr6, it showsoscillator-like absorption peaks and a wide window just above thefirst peak. The Cs+ ions prevent the Pb2+ 6s and 6p statesfrom taking part in the construction of extended states. These statesform a set of localized states confined to within respectiveoctahedra, similar to the case of isolated Pb2+ ions doped inface-centred cubic alkali halide crystals.

Annealing behaviors of quench-deposited (CsCl) 1− x(PbCl 2) x films studied by optical absorption spectroscopy

All the films of the (CsCl) 1− x (PbCl 2) x system prepared by quench deposition exhibit a similar fundamental absorption spectrum over the full mixing range. On annealing the films, drastic changes occur in the spectra showing three characteristic types of spectral features depending on the mixing ratio. Type 1 ( x≥0.7) suggests the formation of mixed crystals of a PbCl 2 base. Type 2 (0.5≥ x≥0.3) exhibits the spectrum of CsPbCl 3 crystal. Type 3 ( x=0.1, 0.05), the most characteristic, is the spectrum of Cs 4PbCl 6 crystal, which was observed for the first time. The spectrum of the Cs 4PbCl 6 crystal exhibits novel structures. Despite the crystalline entity of the compound, it shows oscillator-like fundamental absorption and a wide (≈1 eV) window just above the first absorption peak, suggesting the peculiarity of the associated energy band structure. To a first approximation, the spectrum is explainable in terms of Pb 2+-ion excitation of the Pb 2+(Cl −) 6 quasi-complexes.

Dielectric studies of PVDF crystallization. Application to in-situ monitoring in injection molding

In recent years, experimental techniques based on microdielectrometry have presented an attractive and increasing interest for continuous monitoring in a nondestructive way of the advancement of thermoset resin reactions under cure. A new application of this technique has been carried out also for in situ monitoring of thermoplastic crystallization. Therefore, a dynamic rheometer equipped with a dielectric cell, as well as an instrumented slit die, were designed. The crystallization process is depicted by a conductivity drop as well as by the occurrence of a maximum in the real permittivity. The decrease of the specific conductivity results from a modification of the conductive paths whereas the increase in permittivity is due to an interfacial polarization phenomenon between the amorphous phase and the growing crystalline entities. As a second step of development, microdielectrometry has been applied for in situ monitoring of the crystallization of thermoplastics during an injection molding process. The electric sensors were located at the walls of the mold cavity so that an analysis of bulk dielectric properties during the filling, post-filling and the cooling phases could be performed. The dielectric method detailed herein provides a new non-invasive technique and could be applied to a closed-loop control of the injection molding process.

What can polymer crystal structure tell about polymer crystallization processes?

Contrary to most or all other materials, crystallization of chiral but racemic polymers such as isotactic polypropylene is accompanied by a conformational rearrangement which leads to helical geometries: the building units of the crystal are helical stems, \(\)-20nm long, which can be either right-handed or left-handed. Helical hand cannot be reversed within the crystal structure: it is therefore a permanent marker and an indicator of molecular processes (in particular segregation/selection of helical hands) which take place during crystal growth, and more precisely during the crucial step of “efficient” helical stem deposition. The issue of proper helical hand selection during polymer crystal growth is mainly illustrated with isotactic polypropylene. Its various crystalline polymorphs (\(\), \(\), \(\) and smectic) display virtually all possible combinations of helical hands, azimuthal settings and even non-parallel orientation of helix axes in space. Furthermore, a specific homoepitaxy which generates a lamellar branching in the \(\) phase “quadrites” and \(\) composite structures makes it possible a) to determine the helical hand and associated azimuthal setting of every stem in the crystalline entities and b) to determine the impact on the crystal structure and morphology of “mistakes” in helical hand of the depositing stem. Analysis of these morphologies demonstrates that the crystallization of isotactic polypropylene (and by implication of other achiral, helical polymers) is a highly sequential and “substrate-determined” process, i.e. that the depositing stem probes the topography of the growth face prior to attachment. These observations appear difficult to reconcile with crystallization schemes in which molecules (helical segments) are prearranged in a kind of pseudo-crystalline bundle (and as such, are not subjected to the high constraints of crystal symmetry) before deposition as a preassembled entity on the substrate.

Anionic polymerization of caprolactam in organic media. Morphological aspects

Crystalline entities formed during anionic polymerization of caprolactam (CL) in nonpolar solvents were examined, mainly by scanning electron microscopy and wide angle X-ray scattering. The morphological development of these entities is governed by the complex interaction between the competing polymerization and physical processes like phase separation and crystallization. The effect of the efficiency of catalytic systems on their interaction, the mechanisms of the phase separation and the crystal growth under topological restrictions set by the phase separation are discussed. Under reaction conditions, for these ones favouring high polymer yields, the final morphology of the polycaproamide (PCA) particles can be controlled by the efficiency of both catalytic species (activator and catalyst). Despite the large range of particle sizes two typical morphologies, namely – connected globules evolving to large blocks and macroporous powders are obtained using more or less efficient catalytic systems, respectively. An adequate selection of the catalytic pairs allows to control the particles’ size and their internal morphology, which is important for certain specific applications of PCA.

Crystallization of Polyolefins from Rheological MeasurementsRelation between the Transformed Fraction and the Dynamic Moduli

Experimental results from the literature and from this work show the reliability of the dynamic mechanical spectroscopy as a complementary tool to follow the crystallization of polymers from the melt. However the problem of the interrelation between the transformed fraction and the mechanical data is not simple and remains a topic open to discussion. To get a better understanding of these relations, the method was applied to the study of two polyolefins which show very different morphologies during their crystallization from the melt. Their morphological study has shown that, though they both crystallize in a well-defined spherulitic structure, because of the differences of size of the crystalline entities, one can be considered as a suspension of spherical particles in a liquid matrix whereas the other behaves as a colloid of small particles. The study of the rheological behavior of the suspension-like material shows the existence of two critical values of the volume fraction. In agreement with the perco...

Crystallization kinetics of polypropylene: II. Effect of the addition of short glass fibres

The effect of short glass fibres on the polypropylene (PP) cyrstallization kinetics and thermodynamics has been investigated. Although at high fibre percentages in the composite the spherulitic limits are not clearly defined, the presence of crystalline entities is evident. A marked decrease of the half time of PP crystallization, τ 1 2 , as well as a sensible increase of the overall crystallization rate, K n, has been observed in the presence of fibres. However, at any crystallization temperature, a maximum of τ 1 2 is reached at 20% glass fibre content in the composite followed by a continuous decrease as fibre percentage increases. A slight increase in the Avrami exponent as temperature increases is obtained although it is relatively independent of composite composition. In all cases an instantaneous nucleation takes place, a nucleant effect of fibres is evident, and no transcrystallinity has been observed with these short glass fibres. Fibres give rise to a marked increase in the crystallinity of the samples. The chain folding energy for PP crystallization ( σ e) decreases with fibre content, and a minimum is observed at 20% fibre content.

Enhancement of tensile properties of isotactic polypropylene spunbonded fabrics by a surface-modified silica/silicone copolymer additive

Previous work at Kimberly–Clark Corp. (R. S. Nohr and J.G. MacDonald, Kimberly–Clark Corp., unpublished results, 1990) demonstrated that the addition of an ultrafine-particle (submicron-size), surface-modified silica predispersed in an alkyl silicone to isotactic polypropylene (iPP) results in dramatic improvements in the tensile properties of fibers and spunbonded fabrics. For both fibers and spunbonded fabrics prepared with a high concentration of the low melt flow index (MFI) resin the incorporation of the additive resulted in markedly improved tensile properties. In this article it is shown that under quiescent conditions the surface-modified silica/silicone copolymer additive acts as an effective nucleating agent for iPP. This results in a reduction in the size of the crystalline entities, hence a more homogeneously distributed crystalline phase and load-bearing “tie” molecules. Indeed, scanning transmittance electron microscopy (STEM) studies show that the crystalline entities are substantially smaller at the surface and mid-radius of fibers prepared with additive containing iPP. Furthermore, the bond points of the resulting spunbonded fabrics have a dramatically increased nucleation density and smaller crystallite dimensions. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1759–1772, 1997

Reinforcement effect and molecular motions in semicrystalline PEEK films: Mechanical and physical modelings. I

The influence of the crystalline phase on the viscoelastic behavior of poly (aryl ether ether ketone) (PEEK) films is assessed by dynamic mechanical spectrometry. Prediction of the viscoelastic behavior near T g of semicrystalline films is performed through mechanical and physical modelings. Changes in the a relaxation induced by the crystalline phase are related to both the mechanical coupling between phases and the decrease in the molecular mobility of chains, which is improved for samples showing a broad crystallite size distribution. Crystalline phase also induces some modifications in the characteristics of the β spectrum. The reinforcement effect brought by the crystalline phase in such a temperature range is predicted through a mechanical model. Then, changes in tan δ level in the β 1 region induced by the crystalline phase result from the mechanical coupling between phases. The magnitude of such changes only depends on the crystallinity ratio and it is not controlled by the crystallite size distribution. The crystalline phase also induces changes in the pattern of the β 2 transition, which could be attributed to modifications in the conformations of the chains near the crystalline entities and/or the magnitude of interactions between chains. Such modifications seem to be sensitive to the thermal history of PEEK samples.

Calcium Phosphate/Calcium Oxalate Crystal Association in Urinary Stones: Implications for Heterogeneous Nucleation of Calcium Oxalate

Most stones contain more than one type of crystals, and some combinations, such as calcium phosphate/calcium oxalate, are more common than others. Epitaxy between the crystals has been suggested to play a role in growth of such stones. The specific aim of this study is to investigate the involvement of calcium phosphate in crystallization of calcium oxalate. Twenty calcium oxalate stones or stone fragments were examined using various microscopic techniques, including scanning, transmission and back-scattered electron microscopy. Similarly, calcium oxalate stones induced on a plastic foreign body implanted inside urinary bladders of laboratory rats were also investigated. Examination of the interface between calcium phosphate and calcium oxalate crystals was emphasized. Close association between crystals of calcium phosphate and calcium oxalate were found in both the human and rat stones. All crystals examined were associated with an organic matrix on the surface and contained copious amounts of organic material within the crystalline entities. Interface between the crystals also appeared to be occupied by organic matrix. Results of this and other studies from our laboratory indicate that epitaxy between various crystals, even though theoretically possible, appears unlikely in vivo. The appearance of specific crystalline combinations in stones is probably a result of the urinary environment being conducive for crystallization of those components. Heterogeneous nucleation of calcium oxalate is most probably induced by biological elements, including membranous cellular degradation products.

Mechanical relaxations and diffusive changes in linear low density polyethylene (LLDPE) films subject to induced stretching

SYNOPSIS An analysis of the diffusion of oxygen and carbon dioxide through linear low density polyethylene (LLDPE) subjected to longitudinal and transversal induced stretching in the ratio 2 : 1, is presented in this paper. The relaxation behavior of two coextruded LLDPE films prepared from copolymers of ethylene-1-octene is repod as well. The spectra, expressed in terms of loss tan 6, present a y relaxation shifted 5°C to 10°C in the LLDPEl with respect to the LLDPE2 when the stretching was longitudinal. This relaxation is lower in intensity than the one exhibited for conventional low-density polyethylene of the same crystallinity. Increasing the temperature order, a fi relaxation process appears as an ostensible shoulder of the first of two relaxation processes, called d and a, detected in the a region. The @ relaxation, which is believed to be produced by motion taking place in the amorphous and interfacial regions, appears as two overlapping peaks centered at -36°C and -30°C for a longitudinal stretching and at -34OC and -28°C for a transversal stretching at 1 Hz. In relation to this fact, we observed a slight increase in the T, of the LLDPE2 with respect to the LLDPE1, which is greater in the transversally stretched polymers than in the longitudinal ones. The values of the activation energy corresponding to the second peak of the fi relaxation were obtained for the films subjected to stretching in longitudinal and transversal directions to the processing orientation. The study of the diffusional characteristics of oxygen and carbon dioxide through the films shows the temperature is related to the region for which the a processes are given. An anomalous behavior of the diffusion coefficient with the temperature is observed, suggesting general movements around the amorphous segments and crystalline entities. The increase with the temperature of both parameters (diffusion and permeability) can be attributed to a change in the gas solubility. This increase is greater for COP than for 02, which we interpret as a plastificant effect of the CO,. Finally, the activation energies from diffusion coefficient and permeability are analyzed in terms of Arrhenius. The results show that the temperature dependence of the diffusive parameters may not be a simply activated process as a consequence of the fact that the diffusional characteristics of the films depend on their morphology which, in turn, is changing with temperature. Little changes are observed when the films are subjected to any kind of stretching. In this sense, we think that the orientation by tensile drawing will decrease the conformational entropy involved in melting processes and, as a consequence, will reduce both the permeability and the apparent diffusion coefficients. 0 1996 John Wiley & Sons, Inc.