Showing papers on "Crystallization published in 1973"

Some aspects of nonisothermal crystallization of polymers. II. Consideration of the isokinetic condition

Abstract: In the previous paper a practical method has been applied for an analysis of non-isothermal crystallization in terms of data of isothermal crystallization. The fundamental equation was written on the assumption of the isokinetic conditions in the following form: where X(t) is the degree of phase transformation at time t, and n is the Avrami index determined in the isothermal experiments; K(T) is connected with the crystallization rate constant of the isothermal crystallization, k(T), through the relation K(T) = k(T)1/n. The equation is derived on the basis of the well-known theory of phase transformation. Experiments of nonisothermal crystallization of high-density polyethylene were carried out under various cooling conditions. The change in crystallinity during the process was followed by using the above equation in the course of the primary crystallization. A procedure of the analysis of the whole, including both the primary and secondary processes, is suggested as an eminently practical one on a more general assumption.

Poly(vinyl alcohol) hydrogels. Formation by electron beam irradiation of aqueous solutions and subsequent crystallization

Abstract: Aqueous solutions of poly(vinyl alcohol) were submitted to varying doses of electron beam irradiation. By modification of the classical Flory-Huggins equations appropriate to the initial state of solution of the polymer, the molecular weight between crosslinks, Mc, was calculated as a function of radiation dose, initial polymer concentration, and temperature. Following crosslinking in the solution state, crystallization was induced by dehydrating the network at temperatures above 90°C. Following dehydration, the polymer network was reequilibrated with water and its tensile properties compared with identically prepared hydrogels not subjected to crystallization by dehydration. Greatly enhanced values of ultimate tensile strength and resistance to tear result from the treatment producing crystallization, compared with those of the crosslinked but not previously dehydrated gels.

Melting and crystallization behavior of poly(tetrafluoroethylene). New method for molecular weight measurement of poly(tetrafluoroethylene) using a differential scanning calorimeter

Abstract: Melting and crystallization behavior of virgin polytetrafluoroethylene have been studied using a differential scanning calorimeter. Following quantitative relationship was found between number average molecular weight of polytetrafluoroethylene and the heat of crystallization in the molecular weight range of 5.2 × 105 to 4.5 × 107: Mn = 2.1 × 1010 ΔHc−5.16, where Mn is number average molecular weight and ΔHc is the heat of crystallization in cal/g. The heat of crystallization is independent of cooling rate ranging from 4 to 32°C/min. This relationship provides a simple rapid and reliable method for measuring the molecular weight of polytetrafluoroethylene.

Crystallization of Random Copolymers.

Abstract: A theory of crystallization is formulated for random copolymers which crystallize with the non-crystallizable co-units incorporated into the crystalline lattice as defects. The appropriate melting point equation and other associated thermodynamic properties are derived for this model as a function of crystal thickness and comonomer concentration. The formation of lamellar type morphology is assumed to be a kinetically determined phenomena and nucleation theory is utilized accordingly. The isothermal lamella thickness is predicted to increase in a definitive manner as the noncrystallizable comonomer concentration X increases, while the associated isothermal growth rate is predicted to decrease. The variation of lamella thickness with X when the copolymer is quenched or cooled at a uniform rate is also qualitatively predicted. Under these conditions lamella thickness decreases with increasing X, which is in accord with previous experimental observations on random copolymers of tetrafluoroethylene and hexafluoropropylene as well as other random copolymers. Theory also suggests how the surface free energy parameters σ e and σ can be determined from isothermal crystallization experiments for a series of random copolymers of varying composition.

Heterogeneous nucleation in the crystallization of polyolefins. III. Theory and mechansim

Abstract: The nucleating activity of a large number of nucleating agents for the crystallization of polyolefins is not based on a high surface free energy of the substrate or on epitaxy, or on a minimum particle size just compatible with the thickness of the crystallizing polymer lamellae, or on any persistance of crystalline material in holes of the nucleating particles. The relevant mechanism is thought to be nucleation at steps of limited length in the surface of the nucleating particles. The nucleation is enhanced by a high degree of accommodation of polymer molecules at the surface of the substrate through the presence of ditches in the latter which cause prealignment of polymer chains and thus facilitate crystallization.

The Synthesis of Crystalline Rare Earth Carbonates

Abstract: The factors which influence the crystallization of rare earth carbonates were studied. Crystalline rare earth carbonates were precipitated at various temperatures from aqueous solutions by using sodium carbonate, sodium bicarbonate, trichloroacetic acid, and urea as precipitants. The crystal parameters, compositions, and factors concerning the formations of various carbonates, such as lanthanite-type Ln2(CO3)3·8H2O(Ln=La, Ce), tengeritetype Ln2(CO3)3·nH2O(Ln=Nd, Sm, Gd, Dy, Ho, Er, and Y, n=2–3), monoxocarbonate-type Ln2O(CO3)2·nH2O(Ln=La, Ce, Nd, and Sm, n=1–2) and a hydrated double carbonate of rare earth and sodium (rare earth= La, Ce, Nd, Sm, Gd, Dy, and Y) were determined by chemical analysis and X-ray powder diffractometry.

Melting and Crystallization Process of Polyethylene under High Pressure

Abstract: Melting and crystallization temperatures of extended- and folded-chain crystals under high pressure are precisely obtained by a microdifferential thermal analysis. In the cooling process from the fused state the crystallizations of extended and folded chains occur successively by two steps. The growth rate of extended-chain crystals is fairly large at the crystallization temperature of the extended chain, and is accelerated by raising pressure. The extended-chain crystal grows directly at its crystallization temperature under high pressure and it does not need long duration to form a complete extended-chain crystal. When the sample is quenched to the crystallization temperature of the folded-chain crystal, the folded-chain lamellae grow for the first time, and gradually thicken into an extended-chain crystal for the reason that the extended chain has a lower free energy than the folded chain under high pressure. These facts are clarified by the use of morphological study.

Crystallization and structure of a MgO-Al2O3-SiO2-TiO2 glass-ceramic

Abstract: The structure and changes occurring during the volume crystallization of a MgO-Al2O3-SiO2-TiO2 glass induced by heat-treatment at temperatures in the range 740 to 1300° C were investigated by means of DTA, scanning electron microscopy, X-ray diffraction, electron microprobe analysis, density and other measurements. Crystallization proceeds by liquid phase separation and coalescence of small particles forming larger microcrystalline regions dispersed in the glassy phase, these regions fill the bulk of the glass during the heat-treatment. Such a mechanism of crystallization leads to the formation of a fine-grained homogeneous structure whose chemical composition is identical to that of the glassy matrix (as is shown by the electron microprobe). The scanning electron microscope revealed two crystalline phases in the initial stage (850 to 890° C) of crystallization while X-ray diffraction identified only a μ-cordierite type structure, which at temperatures above 1100° C is transformed into the α-form. The X-ray diffraction patterns showed further that a cordierite 2MgO.2Al2O3.nSiO2 solid solution may be formed dependent on the heattreatment temperature. Stereoscan micrographs of various stages of crystallization show the development of the glass-ceramic. In addition, the influence of the technique employed to prepare the electron microscope specimens, on their surface topography is also shown.

Phase transformations at bombardment of Al and Fe polycrystalline films with B+, C+, N+, P+, and As+ ions

Abstract: The phase composition of aluminium films after bombardment with boron, carbon, nitrogen, phosphorus, and arsenic ions is investigated by the electron-diffraction method. It is shown that in all the cases except of bombardment with B+ ions the known compounds of aluminium with the indicated species are formed. The phase formed as a result of B+ bombardment could not be identified with the known aluminiumborides. It is shown that the texture of the formed phases is determined by the texture of the initial aluminium films indicating the epitaxial (or endotaxial) character of growth of these phases. In some cases AIP and AlAs phases are formed in an amorphous state. This is explained by insufficient substrate temperature for the crystallization or by amorphization under ion beam action. It is further shown that at bombardment of α-iron films with C+ ions ϵ-carbide is formed which after annealing at T ≧ 300 °C transforms into cementite. [Russian Text Ignored].

Rapid Crystallization of SiO2-Al2O3 Glasses

Abstract: The crystallization of Al2O3-rich glasses in the system SiO2-Al2O3 which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature (Tx) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al2O3 region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al2O3 region mullite and γ-Al2O3 cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective Tx's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al2O3 content. In light of the Tx data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO2-Al2O3 system is questioned.

Factors affecting the precipitation of calcium phosphate in vitro

Abstract: The precipitation of calcium phosphatein vitro was studied in order to define the processes of homogeneous and heterogeneous nucleation and to study the effect of certain inhibitors of crystallization on these processes. It was shown that the solubility-determining surface phase of the calcium phosphate formed by homogeneous nucleation had a Ca/P molar ratio close to that of octacalcium phosphate (OCP) and that the most constant formation product in solution at the onset of spontaneous precipitation was that of OCP. Known inhibitors of crystallization had no effect on the formation product of homogeneous nucleation. Below the formation product of spontaneous precipitation nucleation was essentially heterogeneous and was markedly influenced by adding nucleating material to the system, by increasing the time of incubation, and by adding inhibitors of crystallization containing the grouping P-X-P (where X=O, C, or N). The level of supersaturation at the point of maximum inhibition was close to the formation product of spontaneous precipitation of calcium phospate. The implications of these observations are discussed in relation to the precipitation of calcium salts in urine.

Properties and structure of AsTe glasses. (I). Glass-forming ability and related properties

Abstract: AsTe glasses have been investigated for determining the molecular mechanisms of glass formation and of crystallization. Stable and homogeneous bulk glasses are prepared throughout the range of concentration from 20 to 65 at % As, by using an improved quenching method. GFA (glass-forming ability) is determined with respect to the rate of cooling. Density, microhardness, T g and ΔC p at T g are measured in the whole range of glass formation. Finally a quantitative analysis of the kinetics of crystallization is carried out. It appears that most properties which involve molecular motions exhibit anomalies for As 40 Te 60 composition (As 2 Te 3 is the unique definite compound of the system). So GFA shows a sharp depression for this critical composition; similarly enthalpy and kinetics of crystallization are drastically modified at As 40 Te 60 . For Te-rich glasses, crystallization occurs at low temperature by homogeneous nucleation with a small energy of activation. For As-rich glasses, crystallization occurs at much higher temperatures. Above 200°C it is controlled by growth with an high activation energy which indicates deep rearrangements of the relative positions of the atoms by chemical diffusion. A coherent analysis of all the data is developed which shows that important differences of local order exist between amorphous and crystalline states and also between the two well-distinct types of glasses we are dealing with. The aim of paper II is to determine these differences from diffraction data.

Memory switching in a type i amorphous chalcogenide

Abstract: Amorphous chalcogenides may be divided among three classes based upon the changes in local order and bond type on transition from the amorphous to the crystalline phases. These types include: (I) Stoichiometric compounds which do not undergo significant changes in local order on crystallization (e.g., As2Se3). (II) Stoichiometric compounds which undergo significant changes in local order on crystallization (e.g., GeTe). (III) Non-stoichiometric alloys which phase separate on crystallization.

Laser‐beam writing on amorphous chalcogenide films: crystallization kinetics and analysis of amorphizing energy

Abstract: By using laser pulses, the kinetics of crystallization of Te‐rich films were studied as a function of light intensity and pulse duration. The initial devitrification was found to be temperature activated (ΔE≃1–1.6 eV), and no evidence for photocrystallization was found. Amorphizing energies for 100‐nsec pulses ranged from 0.13 to 0.55 nJ/μ2, depending on the degree of crystallization. Part of this increase is due to heat‐of‐fusion effects, but most of it is due to increased thermal conductivity with increased crystallization.

Unsteady‐state heat transfer in a crystallizing polymer

Abstract: The processing of polymers typically includes steps involving heat transfer, such as quenching and crystallization. The mechanical properties of a polymer are thus affected by processing conditions, such as the amount of time spent by each portion of the polymer at each temperature in the processing sequence. This paper presents the results of a mathematical modeling study of the unsteady-state temperatures inside a crystallizing polymer during quenching. This model differs from usual methods in including temperature dependence of induction times and kinetic processes, and in not treating the polymer as a simple, small molecule system. The model permits the prediction of the final morphological distribution within the polymer. Use of the model will aid in tailoring processing conditions to produce a product with specified morphology. The method is applicable to a wide variety of problems in polymer processing, particularly for those polymers which exhibit crystallization.

Crystallization of amorphous selenium films. I. Morphology and kinetics

Abstract: We report a study of the morphology and kinetics of crystallization of films of a‐Se vapor deposited on mica substrates. Morphological examination was facilitated by the complete dissolution of the amorphous component of the films by CS2, while the crystalline component was left essentially intact. Crystallization begins (in the temperature range 30–100°C) with the formation and radial growth of crystalline aggregates, with a cylindrite (two‐dimensional analog of spherulite) morphology, at the mica‐film interface. Filamentary crystals stem from the less regular regions of these cylindrites and grow upwards through the film. Upon reaching the vicinity of the free surface, some of the filamentary tips develop into cylindrites which grow in and parallel with the free‐surface plane. The interface cylindrites were about 500 A thick and their presence apparently is responsible for the phenomenon of ``visual'' darkening often seen in a‐Se films. They are composed of small crystal domains each oriented so that the Se chain axes are parallel with the interface and perpendicular to the disk radius. In the regular morphology, favored by higher‐temperature crystallization, the domains are stacked into lamellae which extend radially from the inner regions to the outer edge of the cylindrites. These lamellae are interconnected by less regularly oriented domains and are much thicker and less regular in cross section than the crystals formed by linear polymers. The radial growth rates of the regular interface cylindrites in films of the highest purity were constant in time and quite reproducible at a given temperature. They are among the highest rates reported for a‐Se and about two orders of magnitude higher than the corresponding rates for free‐surface cylindrites. Their temperature dependence is described by u = 6 × 1015 exp[(− 32.7 kcal/mole)/ RT] cm/sec.

The crystallization of cordierite glass

Abstract: The results of a study of the crystallization characteristics of a glass near the cordierite composition are discussed. The devitrification of the base glass was studied by the method of DTA and the development of this technique is described. The effects of additions of vanadium pentoxide and tungstic oxide on the behaviour of the base glass were also determined and are discussed.

Terephthalic acid recovery by continuous flash crystallization

Abstract: Inordinately slow crystallization at rate controlled evaporative cooling of terephthalic acid from liquid phase aqueous solutions also containing from 500 to 6,000 ppm p-toluic acid based on dissolved terephthalic acid at temperatures in the range of 400° to 550°F. can be overcome by more rapid continuous flash evaporation of solvent in two or more stirred crystallization zones to obtain a terephthalic acid product having 150 ppm or less p-toluic acid. Such product is readily separated from mother liquor by continuous centrifuge.

Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

Abstract: The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at −25°C, the long period continuously increases from about 140 A at −25°C to about 250 A at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron...

Making surface crystallized glass bodies and resulting product

Abstract: A process for the production of a high-strength translucent or opaque glass body which is resistant to temperature change and which has a partially crystalline surface layer and the product produced thereby. The glass comprises a beta -spodumene and/or h-quartz solid solutions and a residual glass phase, and which differs in its oxidic composition from the basic glass by a higher Li2O content and correspondingly lower content of the other alkali oxides, the total mol content of the alkali oxides being however substantially the same. In the process, a heat treatment for the controlled surface crystallization is carried out on the glass, during which an ion exchange occurs within the glass between Li ions in the interior of the glass and Na-and/or K-ions on that side of the growing crystal front which is facing the interior of the glass.