Showing papers on "Differential scanning calorimetry published in 1987"

Biochemical Applications of Differential Scanning Calorimetry

Abstract: Differential scanning calorimetry (DSC) and the closely related differential thermal analysis (DTA) have been widely employed during the last several decades in the thermodynamic study of processes that are initiated by either an increase or a decrease in temperature. This review focuses on biochemical applications of DSC. Macromolecular and polymolecular structures stabilized by the cooperation of numerous weak forces are important to most biochemical processes. Since such highly cooperative structures undergo conformational or phase transitions upon being heated, significant information concerning these structures can be obtained by DSC. Small molecules cannot be studied by DSC unless they form aggregates showing intermolecular cooperation, as in crystals. This is illustrated in Table 1. Since the enthalpies of chemical processes rarely are as large as 20 cal g-l, it is evident that molecules having molecular weights, or molecular aggregates having aggregate weights, in the thousands of daltons are required to give transitions sufficiently sharp for useful DSC observation. In a scanning calorimeter, one measures the specific heat of a system as a function of the temperature. For a solution, the apparent specific heat of the solute, c2, is given by the expression

Relaxation behaviour of the amorphous components of wood

Abstract: The viscoelastic properties of mod were investigated using dynamic mechanical thermal analysis and differential scanning calorimetry. Under a limited set of conditions, two separate glass transitions (T g) could be identified with both techniques. These two transitions were identified as arising from the amorphous lignin and hernicellulose matrix in the wood cell wall. Moisture dramatically affected the temperature at which the two dispersions occurred and, consequently, the ability to resolve their independent responses. The relationship betweenT g and moisture for both components could be modelled with the Kwei equation, which accounts for the presence of secondary interactions. Annealing and specific interactions of a series of organic diluents were wed in an attempt to enhance the resolution of the two components values ofT g. Time-temperature superposition was shown to be applicable to wood plasticized with ethyl formamide, following Williams-Landel-Ferry behaviour over the temperature rangeT g toT g + 85° C. These observations allow certain conclusions to be drawn concerning the applicability of existing models of the wood cell wall's supermolecular morphology. Most notably, models of thein situ morphology of the three wood components can be limited to those which consider the amorphous matrix of lignin and hemicellulose to be immiscible.

A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains.

Abstract: The thermotropic phase behavior of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chains of 10-22 carbons was studied by differential scanning calorimetry. When fully hydrated and thoroughly equilibrated by prolonged incubation at appropriate low temperatures, all of the compounds studied form an apparently stable subgel phase (the Lc phase). The formation of the stable Lc phase is a complex process which apparently proceeds via a number of metastable intermediates after being nucleated by incubation at appropriate low temperatures. The process of Lc phase formation is subject to considerable hysteresis, and our observations indicate that the kinetic limitations become more severe as the length of the acyl chain increases. The kinetics of Lc phase formation also depend upon whether the acyl chains contain an odd or an even number of carbon atoms. The Lc phase is unstable at higher temperatures and upon heating converts to the so-called liquid-crystalline state (the L alpha phase). The conversion from the stable Lc to the L alpha phase can be a direct, albeit a multistage process, as observed with very short chain phosphatidylcholines, or one or more stable gel states may exist between the Lc and L alpha states. For the longer chain compounds, conversions from one stable gel phase to another become separated on the temperature scale, so that discrete subtransition, pretransition, and gel/liquid-crystalline phase transition events are observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Double-melting behavior of poly(ether ether ketone)

Abstract: : The melting behavior of semicrystalline poly(ether-ether-ketone) (PEEK) has been studied using differential scanning calorimetry (DSC). When cold-crystallized from the amorphous glassy state at high supercooling or when melt-crystallized at low supercooling, PEEK usually shows two melting peaks. This double melting behavior can be explained by the sum of four contributions: melting of most original crystals, their recrystallization, remelting of recrystallized PEEK and finally the melting of residual crystalline regions.

Effect of Moisture on the Thermal Behavior of Strawberries Studied using Differential Scanning Calorimetry

Abstract: Differential scanning calorimetry was used to determine the thermal transitions in fresh and freeze-dried strawberries and the moisture dependence of these transitions. The freeze-dried strawberry samples had a glass transition at 30-6O”C, and the melting endotherm of the dried products was similar to that for freeze-dried sugars. The glass transition temperature of humidified samples was a linear function of the water activity; it decreased with increasing moisture content. Ice was found to melt at a moisture content of 21.4% or above. The melting of ice in strawberries was similar to that in sugar solutions and fruit juices.

The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses

Abstract: The role of oxygen and other impurities on the crystallization characteristics of Ni‐Zr glasses near the composition NiZr2, as well as for FeZr2, CoZr2, and NiHf2, has been investigated. For NiZr2 glasses with 1 at. % oxygen, the first crystallization product is the metastable E93 structure with a =1.227 nm instead of the equilibrium C16 structure. A similar effect is found for samples containing ≳3 at. % B. For FeZr2, CoZr2, and NiHf2 the first crystallization product is also E93 structure, even with very small levels of oxygen (≤0.2 at. %). The formation of the E93 structure is always accompanied by an increase in the electrical resistivity, an increase which transmission electron microscopy shows is intrinsic to the phase and unrelated to crystallite size. For Ni36.5Zr63.5 and Ni42Zr58 the crystallization is also accompanied by an increase in electrical resistance and the evolution of a crystal structure similar to the E93 structure in the size of the unit cell and packing fraction but with a different...

Curing studies of a polyimide precursor

Abstract: Complexes between diphenylcarbamido-dicarboxybenzene and 1-methyl-2-pyrrolidinone, 1/4 and 1/2 M, are obtained by reacting pyromellitic dianhydride with aniline in the solvent 1-methyl-2-pyrrolidinone Decomplexation and imidization of this diamic acid are studied by thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectrometry, polarizing microscopy, and gas chromatography/mass spectrometry Side reactions competing with the imide formation are discussed The presence of solvent is found to markedly influence imidization

Crystallization and Morphology of Poly(β-hydroxybutyrate) and Its Copolymer

Abstract: The morphology and structure of the bacterial plastics of poly(β-hydroxy-butyrate-β-hydroxyvalerate) P(HB-HV) were studied as well as the reference homopolymer, poly(β-hydroxybutyrate) (PHB). Because of bacterial origins they are exceptionally pure. Crystallization behavior of them is expected to be almost ideal, so that we can easily infer how the minor component (HV) acts within the major component (HB). The solution-grown single crystals of PHB and P(HB-HV) showed linear increase in melting points and hyperbolic increase in long spacings as the crystallization temperature increased. The HV component in the polymer chain was inclined to be excluded outwards as much as possible from the crystal of HB component during the crystallization. A small part of the HV component included in the HB component crystal acts as a defect, by which small expansion of the a-parameter of the unit cell is mainly caused with increasing HV content. The severe effect of the HV component was observed on crystallization characteristics and morphological changes.

Differential scanning calorimetry study of solid‐state amorphization in multilayer thin‐film Ni/Zr

Abstract: Differential scanning calorimetry (DSC) has been used to study solid‐state amorphization and subsequent crystallization in sputtered multilayer Ni/Zr thin films Initial results provide quantitative information about the thermodynamics and kinetics of these processes An analysis of DSC data enables the activation energy and pre‐exponential factor for interdiffusion of Ni and Zr in a‐NiZr to be found

Thermal degradation of polyacrylamide and poly(acrylamide‐co‐acrylate)

Abstract: The thermal degradation behavior of polyacrylamide and poly(acrylamide-co-acrylate) was studied by differential scanning calorimetry, thermogravimetric analysis, gas chromatography/mass spectrometry, and carbon-13 solid state nuclear magnetic resonance. The degradation products over the temperature range of 115–450°C were characterized. Mechanisms are proposed for the degradation processes involved.

Experimental and Numerical Study of the Thermal Degradation of PMMA

Abstract: Thermal degradation of PMMA subjected to a radiant heat flux was studied both experimentally and numerically. Very accurate measurements of mass loss rate and temperature profiles were performed for three constant heat fluxes: 1.5,2.3 and 3.0 W·cm-'. The results show that the mass loss rate cannot be directly related to the surface temperature and a contribution of the sub-surface region of the sample to the rate of gasification must be taken into account in a thermal degradation odel. A heat source term was introduced in the conduction equation and a kinetic equation was used to locally calculate the mass loss rate in each slab of the solid. The density and the thermal conductivity of the material were assumed temperature independent while for the specific heat the values measured by differential scanning calorimetry in the range 20-400·C were retained. The predictions of the model are in very close agreement with the experimental results for the evolution with time of both the temperature profi...

Annealing effects on the crystallinity of polyetheretherketone (PEEK) and its carbon fiber composite

Abstract: The degree of crystallinity of polyetheretherketone (PEEK) has been measured using both the density gradient technique (DGT) and differential scanning calorimetry (DSC). The difference in results between the methods was shown to depend on crystallization taking place during the heating scan in the DSC. By freezing the sample at different stages of the DSC thermogram and measuring its crystallinity in the density gradient column, the existence of induced crystallization for PEEK was established. Though this induced crystallization is not visible in the DSC thermogram, it must be taken into account when comparing the degree of crystallinity measured by the two methods. The induced crystallization was in turn interpreted as a result of an increase in crystal perfection that is also commonly observed during the initial stages of the annealing process. Accordingly, the effect of annealing on the crystallinity was also investigated. DSC scans on annealed samples exhibited a small endothermic peak at approximately 10°C above the annealing temperature. This peak was observed in both neat PEEK and its carbon fiber-reinforced composite. Annealed PEEK shows, therefore, two melting transitions, a low one which depends on the annealing temperature and a high one which is independent of annealing temperature conditions. Collectively, the results of this study demonstrate that processing conditions and morphological features must be considered in characterizing semicrystalline-based matrix polymers for high performance composites.

Polystyrene-polyethylene melt blends obtained through reactive mixing process

Abstract: Reactive polystyrene (OPS) and reactive polyethylene (CPE) with oxazoline and carboxylic acid functionality, respectively, were melt blended in a Rheomix mixer under a variety of conditions. The properties of these blends were examined and correlated with the compositions and mixing conditions such as shear rate, time, and temperature. An increase in torque was observed, which is believed related to chemical reaction between OPS and CPE. The difference between the maximum and minimum torque (Tmax-Tmin), increases from 48 to a maximum of 510 m-g for 10 and 40% CPE reacted blends, respectively, But on further increase in the CPE amount in the blend the torque increase drops reaching a final minimum value of 133 m-g for a blend with 90% CPE. Differential Scanning Calorimetry (DSC) studies reveal a single first order transition, due to CPE, for each of these polymer blends. Furthermore, evidence of the glass transition temperature for OPS diminishes with increasing CPE content and mixing time. Scanning Electron Micrographs (SEM) show a fine dispersion in these reactive blends, with particle size much smaller than a micron. Blends with 50% or more CPE have no distinguishable features as such. Mechanical properties such as elongation at break of reacted blends are improved over the nonreactive polyethylene (PE) and polystyrene (PS) blends. An intermolecular reaction between the OPS and CPE results in a graft polymer, which imparts improvement in the overall properties of these reacted blends. The maximum grafting reaction corresponds to 40% CPE blend, which is being evaluated as a potential compatibilizer.

Study of Thermal Properties of Oat Globulin by Differential Scanning Calorimetry

Abstract: The thermal behavior of oat globulin was studied by differential scanning calorimetry (DSC). The effects of pH, salts, and of various structure perturbants upon thermal characteristics were determined. Raising or lowering pH from near neutrality reduced denaturation temperature. (Td), enthalpy (ΔH) and cooperativity indicated by increase in width at half height (ΔT1/2). The effect of salts on thermal stability was related to their position in the lyotrophic series and suggests involvement of hydrophobic interaction in the thermal stability of oat globulin. Increasing concentrations of urea progressively lowered Td and ΔH and increased ΔT1/2; sodium dodecyl sulfate (SDS) lowered ΔH without affecting Td; ethylene glycol (EC) lowered Td without changing ΔH. Dithiothreitol did not affect DSC characteristics suggesting that disulfide bonds do not contribute to the thermal response of oat globulin.

Isothermal DSC method for evaluation of the kinetics of crystallization in the Ge-Sb-S glassy system

Abstract: The isothermal DSC method was used to study the crystallization kinetics of Sb2S3 in glassy (GeS2)03(Sb2S3)07 The effective activation energy of crystallization (Ea/n) was evaluated from the DSC curves using the Johnson-Mehl-Avrami transition equation