Showing papers on "Crystallization published in 1976"

Poly(vinyl alcohol) hydrogels: reinforcement of radiation-crosslinked networks by crystallization.

Abstract: Aqueous poly(vinyl alcohol) solutions were crosslinked via electron-beam irradiation to form transparent hydrogels of varying crosslinking densities. Typical crosslinked hydrogels with anti M/sub c/ between 3500 and 8000 were weak, easily shattered, nonextensible materials with very low tensile moduli (up to 70 psi) and tensile strengths at break (less than 10 psi). Reinforcement by induction of partial crystallization was accomplished by a two-stage drying process, consisting of a slow dehydration stage at room temperature and an annealing stage at elevated temperatures, which was mainly responsible for the introduction of the crystallites. The swollen hydrogels after the annealing process had crystallinities widely varying between 30 and 65 percent and polymer volume fractions between 30 and 60 percent, depending on the temperature--time history of the specimen. These materials showed greatly improved mechanical properties (modulus, ultimate tensile strength, tear strength), as compared to the uncrystallized hydrogels.

An experimental study of the kinetics of polymer crystallization during shear flow

Abstract: Some principles of rheology are applied to the study of the shear-induced crystallization of molten polymers. A new technique is described for measuring crystallization kinetics during isothermal flow at constant shear rate in a parallel plate rheometer. The crystallization rate is characterized by the time elapsed from the start of shearing until the rise in melt viscosity due to crystallization. The measured-viscosity and induction time for crystallization are shown to be independent of sample geometry. Kinetic data are presented for crystallization of three linear polyethylenes at shear rates of 0.03 – 30 sec−1. It is shown that shear flow has a strong accelerating effect on crystallization when the deformation rate exceeds a critical value. Comparison of results for the different polyethylenes reveals that higher molecular weight materials crystallize faster at a given shear rate and temperature. Finally, shear-induced crystallization of propylene polymers is shown to be unaffected by the presence of either a carbon black additive or a heterogeneous nucleating agent. It is concluded that the hydrodynamic origin of the shear-induced crystallization is elastic chain extension due to entanglement couplings between molecules. Furthermore, it is suggested that transient orientation effects during the startup of shear flow may have a dominant influence on the observed phenomena.

Crystallization history of lunar picritic basalt sample 12002 - Phase-equilibria and cooling-rate studies

Abstract: Experimental crystallization of a lunar picrite composition (sample 12002) at controlled linear cooling rates produces systematic changes in the temperature at which crystalline phases appear, in the texture, and in crystal morphology as a function of cooling rate. Phases crystallize in the order olivine, chromium spinel, pyroxene, plagioclase, and ilmenite during equilibrium crystallization, but ilmenite and plagioclase reverse their order of appearance and silica crystallizes in the groundmass during controlled cooling experiments. The partition of iron and magnesium between olivine and liquid is independent of cooling rate, temperature, and pressure. Comparison of the olivine nucleation densities in the lunar sample and in the experiments indicates that the sample began cooling at about 1 deg C/hr. Pyroxene size, chemistry, and growth instability spacings, as well as groundmass coarseness, all suggest that the cooling rate subsequently decreased by as much as a factor of 10 or more. The porphyritic texture of this sample, then, is produced at a decreasing, rather than a discontinuously increasing, cooling rate.

The drawing behavior of linear polyethylene. I. Rate of drawing as a function of polymer molecular weight and initial thermal treatment

Abstract: The drawing behavior of a series of linear polyethylene homopolymers with weight-average molecular weight (Mw) ranging from 67,800 to ∼3,500,000 and variable distribution (Mw/Mn = 5.1−20.9) has been studied. Sheets were prepared by two distinct routes: either by quenching the molten polymer into cold water or by slow cooling below the crystallization temperature (∼120°C) followed by quenching into cold water. When the samples (2 cm long) were drawn in air at 75°C using a crosshead speed of 10 cm/min it was found that for low Mw polymers the initial thermal treatment has a dramatic effect on the rate at which the local deformation proceeds in the necked region. At high Mw such effects are negligible. An important result was that comparatively high draw ratios (λ > 17) and correspondingly high Young's moduli could be obtained for a polymer with Mw as high as 312,000. It is shown how some of the structural features of the initial materials (mainly studied by optical microscopy, small-angle x-ray scattering and low-frequency laser Raman spectroscopy) can be interpreted in terms of the molecular weight and molecular weight distribution of the polymers. Although crystallization and morphology can be important at low Mw, it suggested that the concept of a molecular network which embraces both crystalline and noncrystalline material is more helpful in understanding the drawing behavior over the whole range of molecular weights.

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

Abstract: We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T G . The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T X . Values of T g and T X have been tabulated for all alloys investigated and the compositional dependence of T g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge 20 Se 40 Te 40 in which a GeSe 2 -rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe 2 -rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.

Structure and properties of polyethyleneterephthalate crystallized by annealing in the highly oriented state

Abstract: The structure of polyethyleneterephthalate bristles drawn about five times in the amorphous state and subsequently crystallized at temperatures between 100 and 260‡ C has been studied by means of small-angle X-ray scattering. In addition density, heat of fusion and wide-angle scattering behaviour were measured. For comparison, similar experiments were carried out with undrawn samples. The results showed that the degree of crystallinity of PET cannot be calculated from density data on the basis of a simple two-phase model, since the effective densitiesρc* andρa* of the crystalline and amorphous regions depend strongly on crystallization and drawing conditions. With rising crystallization temperature the size of the mosaic blocks building up the crystalline layers and their longitudinal mutual order increase whereas the volume fraction of the crystalline region is only rather slightly effected by the annealing temperature. The difference between the effective densityρc* and the “X-ray density”ρc of the crystalline layers is supposed to be caused by lattice vacancies in the boundaries of the mosaic blocks.

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

Abstract: A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH2) and chlorination (14–22 Cl/1000 CH2). Both linear and branched polymers were studied. All of the relaxations between the melt and −196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (Tmax at constant frequency or log fmax at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log fmax and Tmax on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing emax with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (∼10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.

Hydrothermal crystallization of silica gel

Abstract: Under hydrothermal conditions (3 kb, 100/sup 0/ to 300/sup 0/C, 25 to 5,200 hr), freshly solidified silica gel crystallized to quartz, predominantly in the form of chalcedonic spherulites. No intermediate crystalline phases in the transition from amorphous silica to quartz were detected. Chalcedony that made up the synthetic spherulites was either length-slow, length-fast, or both, depending on the experimental conditions. These findings contrast with the hypothesis that chalcedony derived from silica gel should be length-fast. In experiments in which blue-green algae had been embedded in the silica gel, spherulite nucleation occurred preferentially on surfaces of particulate organic matter, and the chalcedony generally was length-slow. A summary of spherulitic crystallization mechanisms is presented, and emphasis is placed on impurity and viscosity requirements. Considering these requirements and the experimental results, a viscous colloidal origin is postulated for cherts and other siliceous sedimentary rocks that contain abundant silica microspheres. The seemingly biological attributes of quartz microspheres in Precambrian iron formation cherts can be explained by strictly inorganic processes related to spherulitic crystallization of colloidal precursors.

Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2

Abstract: A series of glasses in the TiO2-SiO2 system was prepared by the flame hydrolysis boule process. Clear glasses containing as much as 16.5 wt% TiO2 were obtained. More TiO2 caused opacity due to phase separation and anatase/rutile crystallization during glass boule formation. Glasses in the 12 to ∼17 wt% TiO2 range were metastable and showed structural rearrangements on heat treatment at temperatures as low as 750CEC (∼200° below the annealing point). These changes were accompanied by large changes in thermal expansion. Thermal treatment can be designed to produce almost any desired expansion between α-200+700=−5 × 10-7/°C and +10 × 10-7/°C. Zero expansion between 0 and 550°C was obtained. Evidence that these changes are due to phase separation and anatase formation is presented. Viscosity data in the glass transition range, refractive index, and density are also presented.

Crystal nucleation in a three‐dimensional Lennard‐Jones system: A molecular dynamics study

Abstract: We have observed crystal nucleation in a Lennard‐Jones fluid by molecular dynamics. In a 108‐particle system cooled slowly below its triple point, crystallization occurs at kT/e=0.50 and 0.57 for densities Nσ3/V =0.90 and 0.91, respectively. For larger systems (256 and 500 particles) crystallization has also been achieved. The supercooling limit decreases with increasing system size. These results are compatible with classical nucleation theory.

A comparative study of “gels” and oxide mixtures as starting materials for the nucleation and crystallization of silicate glasses

Abstract: Nucleation and crystallization behaviour of glasses in SiO2-La2O3, SiO2-La2O3-Al2O3 and SiO2-La2O3-ZrO2 systems were investigated using glasses prepared by the fusion of “gels” and the mixtures of oxides in solar/image furnace. Two methods for the preparation of multicomponent homogeneous non-crystalline products in the form of gels were developed. The phase separation, devitrification and micro-hardness of the above glasses were investigated in relation to the starting materials and the composition. The results show that the glasses made from gels are more homogeneous than those made from oxide mixtures. The phase separation characteristics of glasses made from gels are markedly different from those of glasses made from a mixture of oxides. The addition of Al2O3 to the binary SiO2-La2O3 glasses improves the homogeneity but reduces the micro-hardness and the devitrification tendency, whereas the addition of ZrO2 causes a considerable increase in micro-hardness and enhances the devitrification. The rates of nucleation and crystallization of glasses of different compositions made from gels are much higher than those made from the mixture of oxides. The formation of the high temperature crystal form, (Β-La2Si2O7 is more evident with the crystallization of gel-glasses. When the rate of nucleation is low, (in the case of glasses from the mixture of oxides), the curve representing the relation between the micro-hardness and the time of heat-treatment shows a distinct minimum, whereas this minimum is not obtained with the gel-glasses. With most of the gel-glasses, the micro-hardness rises very sharply with the length of heat-treatment. The curve showing the relation between the micro-hardness and the volume fraction of the dispersed crystalline phase also gives a distinct minimum which can be explained on the basis of the fracture mechanism consisting of the processes of crack nucleation and of crack propagation around the dispersed crystalline particles.

The study of transformation kinetics of the amorphous PdSi alloys

Abstract: The kinetics of transformation behavior of roller-quenched amorphous Pd 83 Si 17 and Pd 80 Si 20 alloys after linear and isothermal heating is reported. The transformation was examined with electron microscopy, electron diffraction and electrical resistivity measurement. The crystallization of amorphous alloys with 17 at% Si begins with metastable ordered fcc solid solution. The ordered fcc solution is transformed to a ordered metastable phase with, probably, orthorhombic structure. The crystallization of amorphous alloys with 20 at% Si begins by formation of spherulites with lamellar structure. Using electron diffraction we found that spherulites consist of two phases - orthorhombic Pd 3 Si silicide and Pd-rich silicide. From resistivity measurements, activation energies of 28.5 kcal/mol for Pd 83 Si 17 and 80 kcal/mol for Pd 80 Si 20 , respectively, were calculated.

Reusable heat pack containing supercooled solution and means for activating same

Abstract: A heat pack is made by enclosing supercoolable aqueous sodium acetate solution together with a metallic activator strip in a sealed, flexible container. The activator strip is a flexible metal strip having one or more fissures or slits extending therethrough. To prepare the heat pack for activation, its contents are first heated to a temperature above the melting point of sodium acetate to completely melt it. Thereafter, the sodium acetate solution is supercooled. Activation or crystallization of the sodium acetate (with evolution of heat) is produced by bending the activator strip.

Gelation‐crystallization in isotactic polystyrene solutions and its implications to crystal morphology, to the origin and structure of gels, and to the chemical homogeneity of polyolefins

Abstract: As part of a wider study on the crystallization of isotactic polystyrene solutions it was observed that at sufficient concentrations (> 3–5%) gelation sets in below a certain (very high) supercooling in competition with the usual single crystal formation which in itself produces turbid suspensions. It was established that gelation is a form of crystallization (mode A) which must be of fringed micellar type to provide the connectedness as opposed to the chain folded lamellae (mode B) which gives rise to discrete particles. The gel crystals (A) display sharp melt endotherms and produce distinct x-ray diffraction patterns both of which, however, differ decisively from those provided by crystals B, a distinction which can be preserved even after removal of the solvent. The melting points of A are significantly lower than those of B and the x-ray diffraction patterns of A are incompatible with the recognized structure of polystyrene (31 helix) possessed by B; they point to a broadly planar zig-zag arrangement of the chain. This strongly suggests that we have blocks of chemically distinct sequences which could be syndiotactic or head-to-head tail-to-tail (presently with substantial support for the latter) which is responsible for the gel forming crystallization. However, so far the C13 nuclear magnetic resonance (NMR) results do not provide the evidence for these distinct species but explanations for our observations on any other basis seem to lead to unsuperable difficulties from other points of view. Consequently, the paper is left open ended with the possibilities discussed. Amongst these the existence of a very few but long, chemically distinct sequences seems most attractive. The wider implications of the facts as they stand for crystal morphology (fringed micelles versus lamellae), for the origin and structure of gels in general, for the crystallization of block copolymers and for issues relating to chemical homogeneity (tacticity, head-to-head tail-to-tail) are discussed and preliminary effects are quoted which indicate that these issues may also be relevant to the usual atactic polymers.

Towards a kinetic model for the crystallization of magma bodies

Abstract: The kinetics of crystallization of a magma body can be described in terms of the classic Johnson-Mehl-Avrami expression for the rate of crystallization of nucleation and growth-controlled reactions combined with the heat flow equation including a term for the rate of release of latent heat. Although present data for the rates of nucleation and growth in geologic systems are insufficient to warrant quantitative calculations, finite difference solutions of these equations using reasonable values for these rates predict qualitatively many of the observed features in natural systems. These observed features include the variation in mineral composition and crystal morphology with distance from a surface of cooling and fraction crystallized, the form of the fraction crystallized versus time curve, and the relationships between the rates of nucleation and growth and the fraction crystallized. More detailed calculations will require accurately determined values for the appropriate rates of nucleation and growth and a knowledge of the process by which nucleation takes place.

Anodic Oxidation of Copper in Alkaline Solutions I . Nucleation and Growth of Cupric Hydroxide Films

Abstract: The anodic oxidation of copper in LiOH solution has been investigated by galvanostatic, potentiostatic, and voltammetric sweep technique. The structure and composition of the films were determined by x-ray and electron diffraction, and by scanning electron microscopy. Cu(OH)/sub 2/ forms in two layers: a base layer grown by a solid-state mechanism and an upper layer of individual crystals nucleated and grown from solution. The size and number of upper layer crystals are dependent on electrode potential. More anodic potentials produce a large number of randomly deposited crystals, whereas less anodic potentials result in fewer, more highly developed crystals. Increased stirring results in a greater loss of material into solution, and in the extreme, nucleation and growth are completely prevented. For sufficiently low crystallization rates, produced galvanostatically, the thermodynamically stable phase, CuO, is formed. At higher rates the formation of Cu(OH)/sub 2/ dominates. A nucleation and growth mechanism is given and discussed with reference to other metal systems.

Correlation between the thermal stability and activation energy of crystallization in metallic glasses

Abstract: The origin of a large spread in the activation energy of crystallization, ΔE, in metallic glasses is explained based on a structural relaxation model. The calculated ΔE from the available thermal and viscosity data is in fair agreement with the experimental results. It is the structural relaxation which leads to high ΔE (⩾100 kcal/mole) for stable glasses. It is also suggested that the structural relaxation is the rate‐controlling factor during the crystallization of the unstable glasses. Thus ΔE for unstable glasses is low (ΔE⩽20 kcal/mole), and is expected to be lower for amorphous metals prepared by low‐temperature deposition than for the corresponding melt‐quenched metallic glasses.

X-Ray Studies of Polyethylene under High Pressure

Abstract: The phase diagram, growth processes of extended-chain crystal and high pressure phase, and the temperature dependency of lattice parameter in polyethylene have been studied under high pressure up to 6000 kg/cm2 using a new high pressure X-ray diffraction apparatus. The high pressure phase diagram obtained by micro-DTA measurement agrees with that by X-ray measurement. The crystallization process of the high pressure phase seems to be a homogeneous and diffusion controlled two dimensional growth. The growth process of the extended-chain crystal is very rapid, and the direct formation of the extended-chain crystal without passing through the high pressure phase is clarified. The temperature dependency of the lattice parameters of the extended-chain crystal and the high pressure phase at 5000 kg/cm2 were obtained, and the shrinkage of the c-axis in the high pressure phase which shows only a sharp (100) diffraction pattern is discussed with the aid of dilatometric data.

Crystallization from a strained melt or solution

Abstract: The elongation and orientation of randomly coiled macromolecules in a strained melt or solution reduces their entropy and thus increases the crystallization or melting temperature of the ideal lattice. At any given temperature of experiment this enhances nucleation and crystal growth rate. As a rule, linear primary nuclei are formed. They contain more or less extended chains. The existence of row nuclei reduces the local gradient in the liquid to such an extent that further crystallization proceeds by epitaxial overgrowth of folded chain lamellae. Densely packed cylindrites are formed with the ribbon-like lamellae radiating from the central row nucleus. The irregular shish-kebab structure observed in stirred or sonicated solutions seems to be formed by subsequent exial deformation of cylindrites in the flow field. It displaces the lamellae irregularly and thus produces a great many microfibrillar elements parallel to the original row nuclei. The almost completely extended chains in the shish yield a high elastic modulus and tensile strength for exial loading. The shish-kebab morphology in fibers as spun does not affect to a great extent the mechanical properties obtainable by subsequent drawing. The lamellae are transformed into microfibrils in very much the same manner as in spherulitic samples. But the highly regular orientation of lamellae seems to result in a more uniform drawing and hence a stronger fiber. In an extremely high temperature and pressure gradient, the melt extrusion produces hard elastomers where the lamellae of the cylindrites seem to be locally stapled. Upon application of tensile load in the extrusion direction, the intervening sections bend like beams, thus forming thin holes extending in the direction perpendicular to the load. The holes enormously enhance the permeability for gases and liquids. The elastic bending of lamellae yields the high recoverable strain and low tensile modulus.