Showing papers on "Differential scanning calorimetry published in 1982"

Crystallization characteristics of Cu‐Zr metallic glasses from Cu70Zr30 to Cu25Zr75

Abstract: The crystallization characteristics of glassy Cu‐Zr has been studied by differential scanning calorimetry, x‐ray diffraction, and measurement of electrical resistance and magnetic susceptibility. Our results allow us to remove many discrepancies between previously published data; they also suggest that the equiatomic Cu‐Zr does not exist as a single phase in the crystalline state. We observe parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which we interpret through changes in the d density of states.

Crystallization kinetics in Fe−B−Si metallic glasses

Abstract: Crystallization kinetics of Fe‐B‐Si metallic glasses, as characterized by the effective activation energy Ec and the Avrami exponent n have been studied using Differential Scanning Calorimetry. The data show that in the hypoeutectic (Fe‐rich) region, the glasses crystallize into α‐(Fe,Si) and Fe3B in that order. The growth of crystallites is diffusion controlled, and Ec falls off rapidly away from the eutectic trough. In the hypereutectic region, both crystalline phases are obtained simultaneously. Thermal stability is greatly enhanced by addition of Si(∼3 at. %) and the growth process is no longer purely diffusion controlled. The behavior of Ec and n versus composition is discussed. Effects on Ec and n of additions of small amounts of Ni and Mo are also reported. Such additions have direct relevance to the design of new glasses for applications at high frequencies.

Liquid Crystal Polymers. V. Thermotropic Polyesters with Either Dyad or Triad Aromatic Ester Mesogenic Units and Flexible Polymethylene Spacers in the Main Chain

Abstract: The thermal properties of two new series of thermotropic, liquid crystalline polyesters were studied by differential scanning calorimetry and on a hot-stage of a polarizing microscope. The first series contained an aromatic ester triad with a central terephthaloyl and two terminal oxybenzoyl units connected by a flexible polymethylene spacer containing from two to ten methylene units. The second series contained a head-to-head tail-to-tail aromatic ester dyad with one terephthaloyl and one oxybenzoyl unit and either a dimethylene or hexamethylene spacer. The melting temperatures and the transition temperatures for conversion from the mesophase to the isotropic phase (the clearing temperature) of the polyesters in the first series initially decreased in a zig-zag manner for polymers with up to nine methylene units but increased for the decamethylene polymer. The temperature ranges over which the mesophase formation occurred were generally broader for polymers containing an odd number of methylene units than for those with an even number of units. Most, if not all, of the polymers in both series formed nematic states on melting. The enthalpy and entropy changes for the clearing transition both increased with the length of the polymethylene spacer. Polymers of the second series possessed lower transition temperatures and narrower temperature ranges for mesophase formation, as would be expected for the shorter mesogenic unit.

Structure and thermotropic behavior of phosphatidylserine bilayer membranes.

Abstract: The structure and thermotropic properties of a homologous series of diacylphosphatidylserines (PS) in the anhydrous and hydrated states have been examined with low-angle X-ray diffraction and differential scanning calorimetry. In the anhydrous state at low temperatures both acidic PS and its NH4+ salts exhibit lamellar bilayer crystal forms that transform to liquid-crystalline hexagonal (type II) structures at higher temperatures. The crystal leads to liquid-crystal transition temperature increases with increasing chain length, the transition temperature of an NH4+-PS being higher than that of its corresponding acidic form. In contrast, the transition enthalpies of the acidic PS are higher than those of the NH4+ salt forms. Hydrated cyclic PS and NH4+-PS exhibit reversible lamellar gel leads to liquid-crystal transitions. In this case the acidic form undergoes this chain length dependent transition at a higher temperature, but with a lower enthalpy change, than the NH4+-PS. Both below and above the hydrocarbon chain melting transition, hydrated lamellar bilayer structures are present. The temperature-composition phase diagram of the NH4+-dimyristoyl-PS/H2O system has been studied in detail. The chain melting transition decreases with increasing hydration, reaching a limiting value of 39 degrees C. X-ray diffraction shows that both the bilayer gel structure and the bilayer liquid-crystal form take up water continuously (i.e., no hydration limit), a characteristic of lipid bilayers with a net charge. Electron-density profiles a NH4+-dimyristoyl-PS at different hydration levels permit detailed analysis of the structural parameters of the PS bilayer.

Calorimetric determination of base‐stacking enthalpies in double‐helical DNA molecules

Abstract: Differential scanning calorimetry was used to directly determine the transition enthalpies accompanying the duplex-to-single-strand transition of poly[d(AT)], poly(dA)·poly(dT), poly[d(AC)]·poly[d(TG)], and d(GCGCGC). The calorimetric data allow us to define the following average base-stacking enthalpies: Interaction ΔH (kcal/stack) AC/TG, TG/AC 5.6 AT/TA, TA/AT 7.1 AA/TT 8.6 GC/CG, CG/GC 11.9 Comparison with published data on the corresponding RNA interactions reveals remarkably good agreement. By assuming transition enthalpies to result from the pairwise disruption of nearest-neighbor stacking interactions, we used the enthalpy data listed above to predict the transition enthalpies for three oligomeric DNA duplexes. Excellent agreement was found between the predicted and the calorimetrically determined values.

The effect of addition of high-density polyethylene on the crystallization and mechanical properties of polypropylene and glass-fiber-reinforced polypropylene

Abstract: High-density polyethylene up to about 30% by weight was melt-mixed with polypropylene and short-glass-fiber-reinforced polypropylene. The presence of high-density polyethylene and glass fibers in the polypropylene matrix affects its crystallization characteristics, which were studied with the help of differential scanning calorimetry. The blend and composite samples have a large number of polypropylene domains apparently due to an abundance of surface nuclei; as a result, the tensile strength, tensile modulus, and toughness are enhanced. The temperature dependence of shear modulus and logarithmic decrement indicate that high-density polyethylene can have plasticizing effect below the glass transition temperature of polypropylene. The scanning electron micrographs of fractured ends show the presence of dispersed domains in the composite samples.

A scanning calorimetric study of small molecule-lipid bilayer mixtures

Abstract: The influence of small molecules on the main phase transition of dimyristoyl phosphatidylcholine was observed by high-sensitivity differential scanning calorimetry. The variation with temperature of the excess heat capacity is markedly affected by solid solution formation, which appears to be of frequent occurrence in solutions in lipid bilayers. The experimental results are in some cases in reasonable agreement with ideal solution theory.

The Effect of Polyhydric Alcohols on the Thermal Denaturation of Lysozyme as Measured by Differential Scanning Calorimetry

Abstract: The thermal denaturation of lysozyme in aqueous solutions of polyhydric alcohols at pH 2 was investigated by differential scanning calorimetry (DSC). The polyhydric alcohols employed were inositol, sorbitol, xylitol, erythritol, glycerol, and ethylene glycol. These alcohols tended to enhance the thermal stability of lysozyme; both the temperature and the enthalpy of denaturation increased almost linearly with increasing alcohol concentration. The increase in denaturation temperature at a given alcohol concentration was a linearly increasing function of the number of hydroxyl groups per alcohol molecule. The denaturation enthalpy at identical temperature increased only slightly by the addition of polyhydric alcohol. The hypothesis may be supported that the dominant mechanism by which polyhydric alcohols stabilize proteins to thermal denaturation is through their effect on the water structure, which determines the strength of the hydrophobic interactions between the nonpolar groups of the protein.

Phase separation in segmented polyurethanes

Abstract: Phase separation in a segmented polyurethane has been studied by differential scanning calorimetry. The glass transition temperature of the soft phase decreases logarithmically with time to a limiting value at each annealing temperature. The magnitude of the decrease is larger at low annealing temperatures, but the normalized rate of logarithmic decrease is smaller. At high annealing temperatures, long sequences of hard segments are excluded from the soft phase in which short segments are still soluble. At low temperatures, even short hard segments separate from the soft phase. The exclusion of the hard segments from the soft phase is a relatively fast process, but the development of order in hard domains takes longer time to reach steady state.

Phase transitions in mesophase macromolecules. III. The transitions in poly(ethylene terephthalate‐co‐p‐oxybenzoate)

Abstract: The glass and melting transitions of poly(ethylene terephthalate-co-p-oxybenzoate)s have been studied by differential scanning calorimetry. Despite the higher glass transition expected for polyoxybenzoate, there is almost no change in the glass transition temperature up to 63 mol % oxybenzoate (353 ± 4 K). At high oxybenzoate concentration there seems to be a discontinuous jump to a glass transition of 450 K. This high glass transition has been observed in two-phase materials down to 30 mol % oxybenzoate. The melting transition shows signs of isodimorphism with a minimum in melting temperature at about 60–70 mol % oxybenzoate, 60 K below the melting temperature of poly(ethylene terephthalate). The thermal properties are little affected by the change of the noncrystalline parts of the molecules to a mesophase structure. The transition to the isotropic phase could not be analyzed because of prior decomposition.

A calorimetric study of aluminium alloy AA-7075

Abstract: The differential scanning calorimetry (DSC) technique has been used to examine the precipitation process in the commercial Al-Zn-Mg-Cu alloy AA-7075. In general the results are in good agreement with previous DSC data on a similar alloy and can be related to the precipitation process observed by electron microscopy. However, an additonal precipitation process has been detected at low temperatures, which is usually masked by coarsening and dissolution of Guinier-Preston (G-P) zones formed during room-temperature ageing. This precipitate, which is referred to as the “precursor” to η′ after Lyman et al., does not redissolve on going to higher temperatures and is an important step in the ageing process in these alloys.

Differential Scanning Calorimetry of Forest Fuels

Abstract: Previous investigations for developing fire spread models in wildland forest fuels have not adequately assessed the heat-sink characteristics of the different fuels. This research uses differential scanning calorimetry to determine the total heat required to change the temperature of a sample of forest fuel from 25/sup 0/ to 500/sup 0/C. The incremental heat input is recorded for 10/sup 0/C increments. Thermogravimetric analysis data are given for equivalent samples. Significant differences are noted between major fuels in the total heat required at 500/sup 0/C. Generally, the heat required for pyrolysis of these fuels is as important as that for evaporating moisture from the fuel.

The thermodynamic properties of fluor'topaz

Abstract: The standard thermodynamic properties of fluor-topaz, AlzSiOnFz, have been calculated from low- and high-temperature heat-capacity measurements and from high-temperature, oxide-melt calorimetry. Fluor-topaz (from Topaz Mountain, Thomas Range, Utah) containing 0.04 wt. percent water was used in all the experiments. Adiabatic calorimetry performed from 10.6 to 379.2 K gives Si9E - Si of 105.4 * 0.2 J/mol'K. Combined heat capacities determined by adiabatic calorimetry (200-380 K) and differential scanning calorimetry (340-800 K) were fit to the following polynomial (equation valid 200-1,000 K): c; (ymol.K) = 471.41- 0.08165r + l.2695xlO6T-2 - 5485.510'5 eO.1%).