Showing papers on "Crystallization published in 1975"

Melting Point Depression and Kinetic Effects of Cooling on Crystallization in Poly(vinylidene fluoride)-Poly(methyl methacrylate) Mixtures

Abstract: Thermal analysis of solution cast mixtures of poly(viny1idene fluoride) (PVF2) and poly(methy1 methacrylate) (PMMA) has been carried out with various programmed temperature profiles. Results from experi- ments conducted in a quasi-equilibrium state show that PVF:! crystallizes in the mixture upon cooling. Further- more, depressions were observed in the melting and the crystallization temperatures. The melting point depression phenomenon is found to be explicable in terms of thermodynamic mixing of a crystalline polymer with an amor- phous polymer. To this end, an analytical expression appropriate to the crystalline-amorphous polymer pair is de- rived from Scott's equation for thermodynamic mixing of two polymers. From this expression the interaction pa- rameter for the polymer pair is found to be -0.295 at 16OoC, indicating that the system is compatible in the molten state. The depression of crystallization temperature which depends strongly on both the composition and the cool- ing rate is attributed to the ability of the PVF2 segments to migrate and to the change of composition in the melt during crystallization. For mixtures with a PVF2 content by weight of less than 0.5, it is possible to suppress the crystal transformation at room temperature with a moderate cooling rate. Finally, it is pointed out that because of the strong kinetic effects of cooling on the thermodynamic state of the mixture, caution should be exercised in the calorimetric study of compatibility of the polymer pair.

Criteria for bombardment-induced structural changes in non-metallic solids

Abstract: Structural transformations in non-metallic solids induced by energetic heavy ions at intermediate (∼1013−1016 ions/cm2) and high (∼ 1017 ions/cm2) doses normally take the form of amorphization, crystallization, or stoichiometry changes. Using the results of 72 substances both a temperature-ratio and a bond-type criterion are shown to be successful in predicting the occurrence of amorphization or crystallization. The former, based on a physical model involving thermal spikes, states that amorphization should occur whenever the ratio (crystallization temperature)/(melting point) exceeds 0.30. The latter, of more or less empirical origin, states that amorphization should occur whenever the ionicity is ≤0.47. Stoichiometry changes are basically different from amorphization and crystallization and this is reflected in the criterion. It is shown that whether or not a substance (such as AgBr or TiO2) loses a component (such as Br or O) during bombardment correlates with the heat of atomization showing a...

Mg/Ca Ratio and Salinity; Two Controls over Crystallization of Dolomite

Abstract: Carbonate precipitation is governed by both thermodynamic and kinetic considerations. The concentration and ionic composition of natural solutions influence not only the mineralogy of the precipitate, but also the crystal size and habit of both metastable and stable phases. Under marine conditions sparry calcite does not precipitate because of poisoning of sideward crystal growth by Mg, which forces Mg-calcite and aragonite to assume a micritic, steep-rhombic, or fibrous habit. Blocky calcite can form only in low Mg/Ca waters, either from primary meteoric solutions, or where Mg has been removed from marine water by dolomite or clays. In hypersaline environments precipitation is commonly rapid and, together with the high concentrations of foreign ions, it is difficult for dolomite to form because of the precise Ca-Mg ordering required. Instead, aragonite or Mg-calcite crystallizes. Dolomite can form only if the Mg/Ca ratio exceeds 5-10:1, and even then it is aphanitic and poorly ordered. At progressively reduced salinities dolomite is able to nucleate at still lower Mg/Ca ratios, approaching 1:1 in meteoric waters. The absence of foreign ions and slow crystallization, commonly under phreatic conditions, cause both dolomite and calcite to form exquisitely limpid, euhedral rhombs. Thus, dolomite probably forms most readily by a reduction in salinity, particularly in a schizohaline environment (alternating between hypersaline and near-fresh conditions, as in a floodable sabkha or a phreatic mixing zone). Flushing marine saline waters with fresh water lowers salinity but maintains a high Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced.

On the Growth Rate of Spherulites and Axialites from the Melt in Polyethylene Fractions: Regime I and Regime II Crystallization.

Abstract: Th e growth rate C of the cr ys tallin e bodies a ppearing in each of a se t of 35 characte rized po ly. e th ylene fractions ra ngin g from 3600 to 807 ,000 in molec ular we ight has been meas ured as a fun ction of the unde rcooling tJ.T. In isothe rmal c rys tallization, only ax ial ites we re found from M\",= 3600 to 18 ,000. (For th ese runs, tJ.T < 17.5 °C.) From M\".= 18,000 to M\". \"\" 115,000 coa rse·grained non ·banded s phe ru · lit es we re found for tJ.T > 17.5 °C , and axialites for tJ.T < 17.5 °C : a rathe r s ha rp break occurred in th e log .o C ve rsus T data at tJ.T \"\" 17.5 0c. The morphologica l changes we re more gradual. Above Mil' \"\" 115,000, only nea rly s tructureless \" irregular\" spherulit es we re found a t all unde rcoolings corresponding to isothermal growth. Typica l ringed sphe rulites were o btained only on quenching. Wide·ang le x·ray data showed that the usual orthorhombic subce ll predominated in all the morphologies encounte red. Low· angle x·ray data showed tha t the s pecime ns exhibit ed lamellar crystallization irrespective of the particular gross morphology involved. The growth rat e data on each frac tion we re analyzed us ing

Near‐surface nucleation and crystallization of an ion‐implanted lithia‐alumina‐silica glass

Abstract: Surface crystallization of a Au+‐ion‐implanted lithia‐alumina‐silica glass has been realized. Annealing of a glass sample implanted with 285‐keV Au+ ions at 550 °C results in the growth of colloidal Au particles of 18–35‐A radius. The Au particles constitute sites for the precipitation of lithium metasilicate crystals at 550 °C. Further annealing at 750 °C allows the growth of quartz and β‐spodumene crystals. The crystallized surface obtained after the high‐temperature anneal was characterized by a Knoop microhardness number of 626, which is of the order of that obtained for commerical volume‐crystallized glass‐ceramics of similar composition.

Polymer Compatibility by Gas-Liquid Chromatography

Abstract: Reliable methods for identifying soluble polymer pairs have long been limited to the solid state even though the possibility exists that a pair of polymers which is immiscible in the solid is miscible in the melt. Inverse chromatographic procedure has been used to study polymer-polymer miscibility in the molten state. Results for poly( 6-capro1actone)-poly(viny1 chloride) blends indicate that complementary dissimilarity is the rule of polymer compatibility when specific interacting forces are involved. Such forces in PCL-PVC blends are of the same order of magnitude as those between PVC and its usual plasticizers. When these results are combined with earlier treatments of Flory equation of state, it turns out that variation of mixture parameters leads to bimodality of the phase diagram as well as asymmetry of the critical concentration. In the study of polymer compatibility, reliable methods for the identification of soluble polymer pairs are applicable in the solid ~ t a t e ; ~ , ~ ~ those applicable in the melt are generally limited by experimental difficulties. Microscopic methods are applicable only where there are substantial differences in refractive indices and light scattering techniques, including the recently developed pulse-induced critical ~ c a t t e r i n g , ~ are applicable for polymer-solvent systems only. Yet, it is conceivable that two polymers would be miscible in the melt but not in the solid on account of morphological differences, thermodynamic or kinetic changes accompanying crystallization, or vitrification or mere temperature effects on solubility. It is of interest to identify these systems; furthermore, it may be possible to induce miscibility down to the solid state via chemical or physical means. Gas-liquid chromatography (GLC) has received general recognition as an effective simple technique for rapid measurement of polymer-solvent interaction and solvent activity coefficient in molten homopolymers.4-1z I t has also been used in determining such properties10 as the glass transition phenomena and the glassy state, crystallinity, adsorption isotherms, heats of adsorption, surface area, interfacial phenomena, diffusion coefficients, and complex equilibria in solution as well as curing processes in nonvolatile thermoset systems. For these type applications, Guillet has suggested”J2 the name “inverse gas chromatography” based on the fact that conventional usage of GLC determines the property of an unknown sample in the moving phase with a known stationary phase whereas inverse chromatography determines the properties of an unknown stationary phase with the aid of a known vaporizable solute in the moving phase. He considers the latter as a “molecular probe’) experiment where the vaporizable molecules are designated “probe” molecules. In this paper, we describe an inverse chromatographic procedure whereby studies of two homopolymers and their blends, analyzed in terms of Scott’s ternary solution treatment13 of the Flory-Huggins theory,14 yield the polymer-polymer interaction parameter. Relative measures of various contributions such as polar, complexing and noncomplexing interactions are obtained based on the most significant strength of selected probe molecules. Four types of polymer interactions investigated are: (i) proton accepting strength, probed with chloroform and ethanol; (ii) proton donor strength with methyl ethyl ketone and pyridine; (iii) polar strength with acetonitrile and fluorobenzene; (iv) nonpolar strength with hexane and carbon tetrachloride. The division and choice of solutes are based on the relative magnitude of dipole moments, polarizabilities, and hydrogen bonding capabilities. It is recognized, though, that no such clear-cut division exists and tha t association complexes stabilized by electronic and/or electrostatic interactions are possible15 with all the probe molecules chosen. What is proposed is a scale of interaction by which the relative strength of different polymers could be measured. Plausible indices of interaction are the FloryHuggins interaction parameter,14 xil, its counterpart based on conditions of an hypothetical liquid at O’K, xLj*, or the exchange energy parameter of Flory equation of state,1621 XL, . The usual sign convention is assumed; i.e., a large positive value indicates unfavorable interaction, a low value indicates favorable interaction, while a negative value indicates a rather strong specific interaction. A nonpolar probe would generally yield positive quantities representative of noncomplexing contributions whereas specific interacting probes could yield negative values. The total specific interactions of such probes with a stationary solvent comprise54 at least four different kinds: 7~ electrons, dipolar, n electron, proton donor-acceptor interactions. These are, therefore, the basic interpretative concepts that will be applied in the discussion. The above procedure has been applied to poly(viny1 chloride) (PVC), poly(ecapro1actone) (PCL), and their blends. The results indicate that complementary dissimilarity is the rule of polymer miscibility when specific interacting forces are involved. Such forces in PCL-PVC blends are of the same order of magnitude as those between PVC and its usual plasticizers. When these results are combined with McMaster’s applicationzz of Flory equation of state16 z1 to polymer-polymer phase relations, asymmetry of the critical concentration as well as bimodal lower critical solution temperature (lcst) behavior are predicted. Experimental Section Apparatus. A Micro-Tek 2500R gas chrqmatograph equipped with thermal-conductivity detector was used for this study. Temperatures of the injection block, column outlet block, and detector cell were monitored by pyrometer but the column temperature was doubly checked with a Digitec Model 551-4 plug-in platinum resistance thermometer. The average error in column temperature is f0.5’. Flow rate of the helium carrier gas, controlled by a MicroTek regulator valve, is measured by a soap-bubble flowmeter. Column pressure was measured differentially against the atmospheric outlet pressure with a U-tube manometer filled with mercury. The elution profiles were recorded by a Honeywell Electronik 19 chart recorder. Column Preparation. All stationary phases were coated onto Fluoropak-80, 60-80 mesh, by dissolution in appropriate solvent and slow evaporation in a Breeze-Away Packing Dryer (Chemical Research Services, Inc.) which is essentially a fluidized bed. The coated support was packed by a gentle tapping procedure into a 5 ft X 0.25 in. 0.d. stainless steel silanized tubing, the ends of which were loosely plugged with glass wool. The tubing was then coiled to fit the oven chamber. After each experiment, the weight of polymer used was determined by 1-week extraction with refluxing solvent in a Soxhlet extractor equipped with ceramic thimble. Vol. 8, No. 3, May-June 1975 Polymer Compatibility by Gas-Liquid Chromatography 317 Table I Polymer Coating Analysis Table I1 Characteristic Parameters for Solvents and Polymers

The morphology and thermal response of high‐temperature–crystallized poly(vinylidene fluoride)

Abstract: In this study, optical microscopy, differential scanning calorimetry, and infrared spectroscopy are used to study the high‐temperature isothermal crystallization of high‐molecular‐weight poly(vinylidene fluoride) It is shown that there exists a temperature domain in which both the α and β phases of PVF2 can be grown concurrently and in competition with each other to form two distinct populations of spherulites which are characterized by different diameters, band periods, and melting points In addition, a time‐ and temperature‐dependent crystal‐crystal transition from the α phase to the γ form can be induced in this high‐temperature crystallization region to produce spherulites which melt 15–20 ° above the melting point of the original α phase This transformation exhibits nucleation and propagation characteristics which in some regions can compete with the normal growth of the α phase to produce unique ’’wagon‐wheel’’ spherulitic structures

Devitrification characteristics of GexSe1−x glasses

Abstract: Amorphous Ge x Se 1− x compounds have been prepared and carefully characterized in the range 0 ⩽ x ⩽ 0.43 (density and micro-hardness measurements and DTA experiments). By heating samples from the amorphous phase, the crystallization process has been investigated for 0.15 ⩽ x ⩽ 0.30. An immiscibility gap in the vitreous region is determined by optical microscopy, from phase-separation observations; moreover, a new metastable crystalline phase appearing as an intermediate step between phase separation and stable GeSe 2 is described.

Transformation behavior of PdAuSi metallic glasses

Abstract: The transformation behavior of roller-quenched amorphous Pd 0.82− x Au x Si 0.18 and Pd 0.835− x Au x Si 0.165 alloys, where x ⩽ 0.10, after rapid heating to temperatures near to or above T g , is reported. The calorimetrically determined glass ( T g ) and kinetic crystallization ( T c ) temperatures both increased with x up to x ≈ 0.04. With increasing x , at x ⩾ 0.04, T c decreased rapidly while T g varied little. Binary Pd 0.82 Si 0.18 alloys crystallized to an fcc phase without apparent composition segregation. The tendency to phase separate at T near T g , as manifested by small- (SAXS) and large-angle X-ray scattering and calorimetry, increased with increasing Au substitution. Pd 0.8 Au 0.035 Si 0.165 alloy apparently phase separated by a nucleation and growth mechanism, with a growth rate limited by the melt viscosity, to form an fcc phase dispersed in an amorphous phase which later crystallized. Pd 0.74 Au 0.08 Si 0.18 alloy phase separated initially to two melts, each of which later crystallized in turn. The initial separation behavior was generally consistent with the predictions of the spinodal theory but with some deviation from Cahn's linear relation.

Structure development during polymer processing: Studies of the melt spinning of polyethylene and polypropylene fibers

Abstract: Experimental studies of structure development in melt spinning of polyethylene and polypropylene fibers are described. Emphasis is given to the influence of applied stresses on the rates of crystallization and on the development of crystalline morphology. The relationship of fiber morphology to mechanical properties, especially “hard elastic fibers” is considered. The relevance of such studies to other polymer processing operations such as film extrusion is discussed.

Effect of carbonate and biological macromolecules on formation and properties of hydroxyapatite.

Abstract: Amorphous calcium phosphate (ACP) was transformed at 25 degrees to hydroxyapatite (HA) in horse and bovine serum; solutions of serum-protein fractions in tris-HC1 buffer (pH 7.4), and pH 7.4 buffers containing from 0.1 to 10 times physiological CO3(2-) concentration. The ACP-to-HA transformation was slower in whole serum and serum fractions than in control buffer solution. The observed adsorption of serum proteins on ACP and HA probably inhibits both the dissolution of the ACP particles and the growth of HA crystals. After 72 h all transformations were complete as determined by X-ray diffraction. The HA crystal dimensions decreased with increasing C03(2-) but the shape, as shown by X-ray linewidths, was relatively constant up to about 4% CO3(2-). At 15% CO3(2-) the crystals were more equiaxial and less needle-like in habit. The radial distribution function (RDF) of HA with 3.7% CO3(2-) is less well resolved than the RDF of HA with ambient CO3(2-) (1.1%). The peaks are less sharp and their amplitude falls more rapidly with increasing atomic separation than for low CO3(2-)-HA. These effects show that CO3(2-) decreases the regularity of the atomic arrangement when incorporated in HA. The rapid decrease, with increasing CO3(2-) content, of the IR splitting of the P-O bending mode of CO3(2-)-HA is attributed to reduced crystal size and possibly to a perturbation of the crystal field due to CO3(2-)-induced lattice distortion. Finally, for bone mineral, it is probable that the poor resolution of the X-ray and IR patterns is due, in large part, to small crystal size and internal disorder caused by CO3(2-).

Flow Induced Polymer Chain Extension and Its Relation to Fibrous Crystallization

Abstract: This work examines the effect that appreciable molecular extension has on the crystallization of long chain molecules. Elementary theoretical considerations presented indicate that to achieve high molecular extensions in solution a longitudinal velocity gradient of strain rate about 103s-1 is required. A method of generating such a velocity gradient, involving flow between opposed jets, is reported and the nature of this flow pattern is examined and quantitatively analysed. The behaviour of polyethylene-xylene solutions in the flow field is presented, notably birefringence observations and measurements indicate that a high degree of molecular alinement can be achieved in specific localized areas of the flow field; also concentration effects are observed which are discussed in terms of entanglement concepts. The effect chain alinement has on crystallization is examined in detail, in particular the ‘shish kebab ’ morphology of the crystals so produced is examined in relation to the hydrodynamic conditions in which they were grown.

DTA and X‐Ray Analysis Study of Nucleation and Crystallization of MgO‐Al2O3‐SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Abstract: Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2O3-SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2. Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2Ti2O4 (Si2O7). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2, but no crystalline TiO2 was detected. In the presence of CeO2, TiO2 was a more effective catalyst than ZrO2. Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.

Effects of annealing and isothermal crystallization upon crystalline forms of poly(vinylidene fluoride)

Abstract: Poly(vinylidene fluoride) exists in three crystalline forms. Optimum conditions for preparing form III were established by infrared spectroscopy, differential scanning calorimetry, and x-ray diffraction measurements. Form III is easily obtained by annealing mats of solution-grown crystals of form II at 175–185°C and is also preferentially formed by isothermal crystallization from the melt between 165 and 175°C. Below 165° crystallization of form II is favored. The melting point of form III is higher than that of form II.

A study of molecular fractionation during the crystallization of polymers

Abstract: Polyethylenes of six different molecular weights and distributions have been crystallized isothermally from the melt at 90 to 129 °C and under elevated pressure (5,000 bar) at 225 to 240 °C. The crystallized polymer and the supernatant melt have been analyzed after quenching to low temperature, by selective dissolution using DSC and viscometry. In addition, data on solution crystallization of the literature were analysed. It was observed that fractionation occurs according to molecular weight. Melt and solution data on the critical molecular weight, chosen so that 90% by weight of the supernatant melt or solution has lower molecular weight and 90% of the crystals have higher molecular weight, agree when shifting the crystallization temperatures by 36 °C. The critical molecular weight-crystallization temperature relationship shows a 1/ΔT2 dependence. The equilibrium segregation on crystallization to hypothetical fully extended chain crystals yields a lower limit of segregation. At low supercooling, molecular lengths many times the equilibrium length and many times the lamellar thickness are reproducibly, selectively and permanently excluded from the crystal. Molecular nucleation is proposed to be the explanation for the observed facts.

Studies on physical properties and structure of silk. Glass transition and crystallization of silk fibroin

Abstract: Amorphous silk fibroin with random coil conformation shows endothermic and exothermic peaks and endothermic shift on the DSC (differential scanning calorimetry) curve. The endothermic shift observed at 175°C was due to the glass transition. The exothermic peak at 212°C is recognized to be the crystallization, which later was confirmed by x-ray diffraction pattern. The endothermic peak at 280°C is shown to be the degradation.

The crystallization and interfacial bond strength of nylon 6 at carbon and glass fibre surfaces

Abstract: The nucleation and crystallization of nylon at the interface in glass-fibre and carbon-fibre reinforced nylon 6 composites has been investigated by electron microscope studies of sectioned and etched bulk specimens and solution cast and melt crystallized thin films. The fracture energies of the composites were obtained from tensile strength tests and the interfacial bond strengths were calculated from fibre pullout measurements. The fibres are shown to nucleate a columnar structure at the interface with marked differences between the structures nucleated by glass fibres and by carbon fibres and also between that nucleated by type I and type II carbon fibres. The structure around glass fibres was non-uniform and influenced to some extent by the presence of the size coating on the fibre surface. In the carbon-fibre composites the columnar structure was due primarily to physical matching of the graphite crystallites. Surface treatment of the carbon fibres to improve chemical bonding is shown to have a significant effect on bond strength which cannot be explained in terms of the columnar structure at the fibre surface. The treated fibres gave rise to only small amounts of fibre pull-out and low fracture energies whereas the untreated fibres showed extensive pull-out which was reflected in high fracture energies.

Effect of phospholipids on the transformation of amorphous calcium phosphate to hydroxyapatitein vitro

Abstract: The conversion of amorphous calcium phosphate (ACP) to crystalline hydroxyapatite (HA) was studiedin vitro in the presence or absence of phosphatidyl serine (PS) and other phospholipids. ACP transformation and HA crystal growth were monitored by electron microscopy, selected-area electron diffraction and X-ray diffraction, and by measuring supernatant calcium and phosphate. PS, and other acidic phospholipids, had a significant stabilizing effect on ACP. With preformed ACP at ratios of only 1 lipid molecule per 30–50 Ca atoms, PS markedly delayed HA crystal formation. When PS was present during ACP precipitation, inhibition of conversion to HA was less pronounced, but crystal habit and aggregation were greatly altered resulting in stacks of thin, membrane-like sheets approximately 38–42 A thick. PS appeared to be most effective in blocking ACP to HA conversion when oriented primarily on the surface; it most affected subsequent crystal formation when distributed throughout the amorphous precursor. Phospholipids possessing anionic, and hence Ca-binding properties, were effective in stabilizing ACP; neutral zwitterion lipids, which have amphipathic properties but do not bind Ca, were not. In view of the presence of anionic lipids in matrix vesicles and their association with early mineral deposits, the current findings add further evidence that lipids may play a role in the control of normal mineralizationin vivo.

Crystallization and Crystal Data of Monellin

Abstract: Crystals of monellin, a sweet protein from Dioscoreophyllum cumminsii, were grown by vapor diffusion of 20% ethanol into buffered protein solution. The crystals are orthorhombic, belonging to space group P21212, with a = 54.4 A, b = 113.0 A, c = 40.8 A, and V = 250,300 A3. The asymmetric unit contains two complete molecules of monellin. The diffraction pattern of this crystal form extends to at least 2.5 A, indicating that x-ray structural analysis is possible to near-atomic resolution.

The crystallization kinetics of glassy Pd0.775Cu0.06Si0.165

Abstract: The crystallization kinetics of a Pd 0.775 Cu 0.06 Si 0.165 metallic glass have been determined by calorimetry over the temperature interval 665–680 K. We observe a change in the time dependence from t 4 at 665 K to t 3 at 680 K. In this temperature interval the activation enthalpy for crystallization was observed to be (95 ± 5) kcal/mol, in good agreement with the activation enthalpy for viscous flow as determined previously by Chen.