Showing papers on "Differential scanning calorimetry published in 1985"

The synthesis of amorphous NiTi alloy powders by mechanical alloying

Abstract: Crystalline powders of Ni and Ti were mechanically alloyed by high-energy ball milling in an inert atmosphere at T < 240 K. The alloyed NixTi1−x powders were characterized by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. For 0.28 < x < 0.72, the powder is amorphous. This composition range is in good agreement with that deduced from a free energy diagram for the crystalline terminal solutions and the amorphous phase. Outside this regime, the powder is a two-phase mixture of an amorphous phase and the crystalline terminal solution of the major element. After ball milling, the solubility limit of Ti in FCC Ni is approximately 28%, which is significantly larger than that in annealed Ni. Atomic pair distribution functions, G(r), were calculated from the X-ray patterns for x = 0.32, 0.4, 0.6, and 0.7. For x = 0.4, the G(r) of the mechanically alloyed powder is almost identical with that of rapidly quenched amorphous Ni40Ti60. For Ni33Ti67, the crystallization temperature, the crystallization enthalpy, and the apparent activation energy for crystallization are 712 K, 0.74 kcal/g at., and 65.8 kcal/g at., respectively. These values are within 10% of those measured by Buschow for rapidly quenched Ni32Ti68. The amorphization by mechanical alloying is attributed to mechanical mixing and to a solid state interdiffusion reaction taking at or near clean boundaries between polycrystalline Ni and Ti.

Plasticization of glassy polymers by CO2

Abstract: A technique is described which uses differential scanning calorimetry to estimate the glass transition of polymers containing a dissolved gas. The technique is simple and appears to give reliable results. The effects of CO2 sorption at pressures up to 25 atm were examined in detail for poly(methyl methacrylate) and its blends with poly(vinylidene fluoride). Less extensive results for polystyrene, polycarbonate, poly(vinyl chloride), and poly(ethylene terephthalate) are also given. Reductions in Tg of up to 50°C are observed. A theoretical relation by Chow predicts results in reasonable agreement with the experimental data. These findings are relevant to various applications such as membrane separation processes for gases.

Calorimetric studies of crystallization and relaxation of amorphous Si and Ge prepared by ion implantation

Abstract: Amorphous Si and Ge layers, produced by noble gas (Ar or Xe) implantation of single crystal substrates, have been crystallized in a differential scanning calorimeter (DSC). The MeV implantation energies resulted in amorphous layers of micron thickness whose areal densities were determined using the Rutherford backscattering and channeling of 1‐MeV protons. These techniques allow determination of the amorphous‐crystal interface velocity (which is proportional to the rate of heat evolution ΔHac) and the total enthalpy of crystallization ΔHac. Amorphous Ge was found to relax continuously to an amorphous state of lower free energy, with a total enthalpy of relaxation of 6.0 kJ/mol before the onset of rapid crystallization. The interface velocity for crystallization on (100) substrates, was found to have an Arrhenius form with an activation energy of 2.17 eV. The value of ΔHac was found to be 11.6±0.7 kJ/mol, the same as for samples prepared by deposition. For Si, ΔHac was determined to be 11.9±0.7 kJ/mol wit...

The application of differential scanning calorimetry (DSC) to the study of epoxy resin curing reactions

Abstract: This review is on the use of differential scanning calorimetry as a method of monitoring and investigating the kinetics of epoxy resin curing reactions. Some instrumental and experimental aspects are discussed, including methods of analysing the kinetic data. A brief survey is made of epoxy resin curing reactions and results of DSC studies are reviewed. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents.

Diacylglycerols, lysolecithin, or hydrocarbons markedly alter the bilayer to hexagonal phase transition temperature of phosphatidylethanolamines.

Abstract: The bilayer to hexagonal phase transition temperatures of dielaidoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylethanolamine are 65.6 and 71.4 degrees C, respectively. Using high-sensitivity differential scanning calorimetry, I have shown that these transition temperatures are extremely sensitive to the presence of small amounts of other lipid components. For example, at a mole fraction of only 0.01, dilinolenin lowers the bilayer to hexagonal phase transition temperature of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine by 8.5 degrees C. Other diacylglycerols have similar effects on this transition temperature, although the degree of unsaturation of the acyl chains has some effect, with distearin being less potent. In comparison, the 20-carbon alkane eicosane lowers this transition temperature by 5 degrees C, while palmitoyl-lysolecithin raises it by 2.5 degrees C. Similar effects of these additives on the bilayer to to hexagonal phase transition temperature are observed with dielaidoylphosphatidylethanolamine. At these concentrations of additive, there is no effect on the gel-state to liquid-crystalline-state transition temperature. The observed shifts in the temperature of the bilayer to the hexagonal phase transition can be qualitatively interpreted in terms of the effects of these additives on the hydrophilic surface area and on the hydrophobic volume. Substances expanding the hydrophobic domain promote hexagonal phase formation and lower the bilayer to hexagonal phase transition temperature. The sensitivity of the bilayer to hexagonal phase transition temperature to the presence of additives is at least as great as that which has been observed for any other lipid phase transition.

Order–disorder transitions in Langmuir–Blodgett monolayers. I. Studies of two‐dimensional melting by infrared spectroscopy

Abstract: A two step melting process in Langmuir–Blodgett (LB) monolayer assemblies of cadmium arachidate (CdA) has been studied by infrared spectroscopy in conjunction with thermal measurements. These techniques have allowed, for the first time, the investigation of detailed structural changes which occur during heating, melting, and annealing cycles. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed. This was followed by a catastrophic breakup of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry (DSC).

Physicochemical characterization of indomethacin polymorphs and the transformation kinetics in ethanol.

Abstract: Methods for the preparation of polymorphs of indomethacin (IMC) were studied in order to obtain the pure polymorphs. The physicochemical properties of IMC polymorphs were measured by using X-ray diffraction analysis. infrared (IR) spectroscopy, differential thermal analysis (DTA) and differential scanning calorimetry (DSC), and two polymorphs (α and γ forms) and one benzene solvate (β form) were identified. The pure α form was obtained when distilled water at room temperature was poured into IMC ethanol solution at about 80°C, and the precipitated crystals were filtered off and dried. The pure β and γ forms were obtained by recrystallization from benzene and ethyl ether, respectively, at room temperature. The melting points of the α and γ forms were 148 and 154°C, respectively, and their heats of fusion were 7.49±0.27 and 8.64±0.13kcal/mol, respectively, as determined by DSC. A mixture of α and γ forms was obtained by the method previously reported for α form preparation ("recrystallization method"), since the pure α form was transformed to the γ form in ethanol at room temperature. The transformation of α form to γ form in ethanol was analyzed by the kinetic method using 9 kinds of kinetic models. It was concluded that the transformation followed kinetics corresponding to two-dimensional growth of nuclei (Avrami equation), and the activation energy was calculated to be 14.2 kcal/mol from the Arrhenius plot. The solubilities of the α and γ forms in distilled water were 0.87 and 0.69mg/100ml, respectively.

Sol → gel → glass: II. Physical and structural evolution during constant heating rate experiments

Abstract: Porous, multicomponent gels were converted to dense glasses at temperatures less than 700°C and at heating rates ranging from 0.5 to 40°C/min. The results of shrinkage, weight loss and differential scanning calorimetry experiments were used to elucidate mechanisms responsible for gel densification. We propose that: (1) capillary contraction, (2) condensation polymerization, (3) structural relaxation, and (4) viscous sintering are the principal gel densification mechanisms. Condensation-polymerization and structural relaxation result in skeletal densification, the magnitude of which closely accounts for all the observed shrinkage between 150 and 525°C. Viscous sintering is the predominant shrinkage mechanism above 525°C. Due to the complex interdependency of the densification mechanisms, the kinetics of gel densification depend strongly on thermal history and, therefore, general constant heating rate analyses are inappropriate for deriving meaningful kinetic information regarding the gel → glass conversion.

Heat capacity of semicrystalline, linear poly(oxymethylene) and poly(oxyethylene)†

Abstract: The heat capacities of 38 semicrystalline poly(oxymethylene)s and poly(oxyethylene)s were determined by differential scanning calorimetry from 205 K through the melting transition. By comparison with the well known limiting heat capacities of the supercooled liquids and the crystals of the macromolecules it was found that there are negative and positive deviations from additivity of the heat capacities with crystallinity between the glass transition and the melting transition. The negative deviations are linked with “rigid amorphous” material, and the positive deviations were previously linked to defect formation or early melting. The rigid amorphous fraction in poly(oxymethylene) is constant up to the melting region, in contrast to polypropylene, where it is decreasing with temperature. The proposed mesophase transition in poly(oxymethylene) is shown to be a minor effect. The poly(oxyethylene) heat capacity is governed by positive heat capacity deviations within the rather short temperature range between glass transition and melting.

Crystallization kinetics of polyphenylene sulfide

Abstract: The crystallization kinetics of unfilled and glass-reinforced grades of polyphenylene sulfide has been investigated by using differential scanning calorimetry. The maximum rate of crystallization is observed at about 170°C. From the crystallization data, it is recommended that the molding parameters should be so specified that the polymer spends 10–15 s over the temperature range of 155–190°C during cooling, before demolding, in order to ensure stable morphology of the molded part. The glass fibers have an accelerating influence on crystallization resulting in a 15–25% reduction in crystallization time. The kinetic data have been interpreted by using Avrami analysis followed by a discussion of the possible crystallization mechanisms.

New eutectic alloys and their heats of transformation

Abstract: Eutectic compositions and congruently melting intermetallic compounds in binary and multicomponent systems among common elements such as Al, Ca, Cu, Mg, P, Si, and Zn may be useful for high temperature heat storage. In this work, heats of fusion of new multicomponent eutectics and intermetallic phases are reported, some of which are competitive with molten salts in heat storage density at high temperatures. The method used to determine unknown eutectic compositions combined results of differential thermal analysis, metallography, and microprobe analysis. The method allows determination of eutectic compositions in no more than three steps. The heats of fusion of the alloys were measured using commercial calorimeters, a differential thermal analyzer, and a differential scanning calorimeter.

Formation And Characterization of Amorphous Erbium-Based Alloys Prepared By Near-Isothermal Cold-Rolling of Elemental Composites

Abstract: We report the formation of bulk single‐phase amorphous Cu‐Er and Ni‐Er alloys by extensive cold‐rolling of elemental foils. The reaction is driven by the negative enthalpies of mixing of the constitutent elements and occurs near ambient temperature. The crystallization behavior of the alloys obtained was studied by means of differential scanning calorimetry and found to agree closely with that of the corresponding sputtered and liquid‐quenched alloys. Radial distribution functions were measured for sputtered and rolled Cu72Er28 and were found to be in good agreement.

Crystallization and transformation mechanisms of α, β- and γ-polymorphs of ultra-pure oleic acid

Abstract: Crystallization and transformation mechanisms of ultrapure (99.999%) oleic acid were examined by Differential Scanning Calorimetry (DSC) and X-ray diffraction. X-ray diffraction spectra revealed three different polymorphs newly named α, β and γ, which differ from each other most significantly in the short spacing spectra. α and β were found to be equivalent to the previous data which Lutton named low and high melting polymorphs, whereas γ was newly identified in the present study. DSC studies have clarified the thermodynamic stability of the three polymorphs in a range of temperature from -20 to 16.2 C. β is always most stable, whereas α and γ are metastable, undergoing a reversible first-order solid-state transformation at -2.2 C (on heating). DSC also showed that the crystallization behaviors are strongly dependent on the polymorphs; α crystallizes at a much higher rate than β; despite the fact that they have close melting points (α, 13.3 C; β, 16.2 C). It was demonstrated for the first time that the above peculiar polymorphic behaviors of oleic acid are quite different from those of stearic acid, a saturated fatty acid with the same carbon chain length.

Differential scanning calorimetry of phenol-formaldehyde resols

Abstract: Differential scanning calorimetry was used on a range of synthesized phenol–formaldehyde (PF) resols to discover relationships between formulation parameters or physical properties of resols, and their thermal behavior during cure. The thermograms showed either one or two exothermic reactions. The lower exothermic peak temperature varied between 98 and 129°C with changes in the free formaldehyde content. This exotherm is caused by the addition of free formaldehyde to phenolic rings. The upper exothermic peak temperature varied from 139 to 151°C, with the higher temperatures occurring when the formaldehyde-to-phenol molar ratio was low or the total amount of sodium hydroxide relative to phenol was high. These two factors led to resins which contain a somewhat higher level of unreacted ortho or para aromatic ring positions and no free formaldehyde. Consequently, condensation is probably not solely by the faster self-condensation through hydroxymethyl groups, but also includes the slower condensation of hydroxymethyl groups with unreacted ring positions. Gel times show trends with changes of formulation parameters somewhat similar to trends of the upper exothermic peak temperatures.

Structural evolution during the gel to glass conversion

Abstract: We have used differential scanning calorimetry (DSC) to measure quantitatively enthalpic changes which accompany gel densification and have related these changes to the evolving gel structure using Raman spectroscopy, gas adsorption, and thermal analysis. We show that the network structure, which results principally from skeletal dehydration (via condensation) during gel densification, is considerably different from the melt-glass structure. A dramatic reduction in viscosity and the formation of metastable MOM bonds as a product of condensation reactions are examples of these differences. Despite the complex manner in which the gel evolves toward a glass, once the gel has been densified and heated above T g , its structure and properties, e.g. viscosity and distribution of relaxation times, become indistinguishable from those of the conventionally melted glass.

Blends of nylon 6 with an ethylene-based multifunctional polymer. I. Rheology–structure relationships

Abstract: An experimental study was conducted to investigate the rheological behavior of a heterogeneous polymer blend system consisting of nylon 6 and an ethylene-based multifunctional polymer (CXA 3101, DuPont Co.). For comparison purposes, we also investigated the rheological behavior of two additional blend systems, namely blends of nylon 6 with a chemically modified polyolefin (Plexar 3, Chemplex Co.) and blends of nylon 6 with ethylene–vinyl acetate copolymer (EVA). We have investigated the thermal and thermomechanical behavior of the blend systems, using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Also, we have attempted to identify the chemical structure of the functional groups present in the CXA 3101 and Plexar 3 resins, using infrared (IR) spectroscopy. This information has enabled us to interpret the observed rheological behavior. Furthermore, we have used both optical and scanning electron microscopies to investigate the state of dispersion of the constituent components in each of the blend systems. We have concluded that, during melt blending, chemical reactions have taken place between carboxyl or anhydride groups present in the CXA 3101 resin and the amino end groups of the nylon 6, forming a graft copolymer which then acted as an “interfacial agent.”

Simultaneous modeling of phase and calorimetric behavior in an amphiphilic peptide/phospholipid model membrane.

Abstract: Differential scanning calorimetry (DSC) experiments have been performed on the amphiphilic peptide/1,2-bis(perdeuteriopalmitoyl)-sn-glycero-3-phosphocholine system for which partial phase diagrams have been measured by deuterium nuclear magnetic resonance. The solute concentration dependence of the transition enthalpy in such systems is often interpreted in terms of an annulus of lipid withdrawn, by the solvent, from participation in the transition while the bulk lipid melts with its fully enthalpy. This idea is equivalent to postulating ideal mixing between the lipid and the peptide/lipid complex, and there is little justification for such an assumption. Adaptation of regular solution theory to this system demonstrates that the peptide concentration dependence of the transition enthalpies can be incorporated into a thermodynamic model which reproduces the observed phase behavior fairly well without postulating that a complexing annulus of lipid around the peptide be withdrawn from participating in the chain-melting transition. The model parameters determined by simultaneous fitting of the phase behavior and transition enthalpies are used to simulate the DSC scan shapes. The asymmetry of the calorimetric scans for chi 2 less than or equal to 0.02 is reproduced by the model, but a broad component observed for higher concentration is not. In light of the results presented here, previous analyses of the calorimetric behavior of two-component systems in terms of symmetric transitions which do not account for the possible extent of a region of two-phase equilibrium must be questioned.