Showing papers on "Differential scanning calorimetry published in 1981"

Studies on Bound Water of Cellulose by Differential Scanning Calorimetry

Abstract: The crystallization and melting of adsorbed water on cellulose samples such as cotton, kapok, linen, jute, various rayons, and wood cellulose have been studied using a differential scanning calorimeter (DSC). Two exothermic peaks of crystallization of adsorbed water on the cellulose samples are observed. One is a sharp peak (Peak I) observed at about 255 K in a DSC curve; the other is a broad peak (Peak II) observed at about 230-250 K. Judging from the amounts of water calculated from the results obtained by the DSC study, there seems to be some nonfreezing water which does not crystallize. Therefore, we have categorized water adsorbed on cellulose samples as one of three different kinds: free water (Peak I), freezing bound water (Peak II), and nonfreezing bound water.The bound water content is dependent on the degree of crystallinity of cellulose samples. The amounts of bound water estimated are from 1.0 to 2.2 moles per one glucose unit of cellulose. However, the amount of water bound to each glucose un...

The Influence of Processing Parameters on Food Protein Functionality I. Differential Scanning Calorimetry as an Indicator of Protein Denaturation

Abstract: Heat denaturation of several proteins from plant sources was studied using differential scanning calorimetry (DSC). Differences in temperatures of denaturation (Td), overall thermal transitions (ΔH), and half band widths (HBW) were noted for these proteins, with oats having the highest Td (112°C) and ΔH (4.45 cal·g −1 ) and the lowest HBW (9.2°C). Vital gluten, on the other hand, did not exhibit a characteristic thermogram so these parameters could not be measured. Using fababean protein as a model system, heat denaturation was shown to be irreversible. The effect of pH on the heat stability of the fababean protein indicated decreases in Td and ΔH on either side of the isoelectric point to a situation where the protein was believed to be completely denatured if the pH were taken below pH 2.5 or above 11.5. The irreversibility of this phenomenon in the alkaline region was demonstrated by DSC of alkaline extracted, isoelectrically precipitated fababean protein isolates.

Effect of surface curvature on stability, thermodynamic behavior, and osmotic activity of dipalmitoylphosphatidylcholine single lamellar vesicles

Abstract: The size and surface curvature dependence of the properties and stability of single lamellar vesicles have been investigated by using a variety of physicochemical techniques. Dipalmitoylphosphatidylcholine single lamellar vesicles of sizes ranging between 200 and 900 A in diameter have been prepared by the French press method and characterized with respect to their size distribution, stability, and thermotropic behavior by negative stain electron microscopy, molecular sieve chromatography, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Vesicles with a diameter smaller than 400 A are unstable below their transition temperature and fuse spontaneously to form larger single lamellar vesicles. Correlation analysis of experimentally obtained size distributions and calorimetric phase transitions profiles allowed estimation of the size dependence of the transition temperature. The phase transition temperature depends on the vesicle size in a sigmoidal fashion. Throughout the entire 200-700 A diamter range, the phase transition parameters are sensitive to size; however, the size dependence is especially pronounced around 400 A in diameter. The anomalous size dependence of the transition temperature for vesicles smaller than 400 A in diameter has been attributed to a decrease in the effective bilayer curvature due to packing rearrangements of the lipid molecules. Changes in the fractional degree of self-quenching of trapped 6-carboxyfluorescein induced by osmotic stress indicate that large single lamellar vesicles are not spherical under isoosmotic conditions. These vesicles are relatively flexible and can sustain almost a 2-fold increase in their internal aqueous volume without any leakage of the internal content.

Mesogenic polymers. III. Thermal properties and synthesis of three homologous series of thermotropic liquid crystalline ``backbone'' polyesters

Abstract: Details of the thermal behavior, including transition temperatures, transition heats and transition entropies, are reported for three series of nematic liquid-crystalline “backbone” polyesters having the general structure All polysters described above were examined by differential scanning calorimetry and were found to have reproducible thermal behavior once the polymer had been annealed by heating to the isotropic phase and then subsequently cooled. Enantiotropic nematic phases were found for all 30 polysters studied. Many polymers showed multiple endotherms on melting to the nematic phase. Plots of solid–nematic and nematic–isotropic transition temperatures versus number of carbon atoms in the diacid segment (x) for each (y) reveal an even–odd alternation reminiscent of trends in homologous series of small-molecule liquid crystals. Enthalpies for the solid nematic and nematic isotropic transitions do not show such a precise alternation.

Characterization of a third phase transition in multilamellar dipalmitoyllecithin liposomes.

Abstract: The thermotropism of dipalmitoyllecithin in fully hydrated multilamellar dispersions has been reexamined by differential scanning calorimetry, X-ray diffraction, and 31P nuclear magnetic resonance. Apart from the well-known pretransition and main transition, there exists a third transition at about 11 degrees C with a transition enthalpy of approximately 3.7 kcal/mol. Both adjoining phases are lamellar, but they differ in the lateral acyl chain packing of the lecithin molecules and in the dynamics of the polar head groups. The kinetics of this third phase transition are extremely slow in comparison with those of the other two transitions.

Differential Scanning Calorimetry Studies on the Crystallinity of Ageing Wheat Starch Gels

Abstract: The ageing of wheat starch gels stored at 4°C, 21°C and 30°C was examined by differential scanning calorimetry. Kinetics of the crystallisation process were studied using the Avrami model where it was found that the experimental data fitted the model more satisfactorily when the Avrami exponent was less than unity. At higher storage temperature the starch crystals appeared more symmetrically perfect. Addition of wheat flour pentosans to 50% starch gels did not significantly affect the kinetics of ageing. The extent of crystallisation during the ageing process depends on the moisture content of the starch gel. Crystallinity of the starch gel, measured in the temperature range 306–346°K does not occur if the solids content is below 10% or above 80% and a bell shaped curve is obtained. Reduction of the moisture content of the starch fraction of baked goods either directly or indirectly using materials competing for water, might therefore reduce the staling problem.

A calorimetric study of precipitation in aluminum alloy 2219

Abstract: Precipitate microstructures in aluminum alloy 2219 were characterized using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC signatures of individual precipitate phases were established by comparing the DSC and TEM results from samples that had been aged such that only one precipitate phase was present. These signatures were then used to analyze the commercial tempers. It was found that DSC could readily distinguish between the T3, T4, T6, T8 and O tempers but could not distinguish amongst T81, T851 and T87. Small amounts of plastic deformation between solution treatment and aging had a significant effect on the thermograms. Aging experiments at 130 and 190 °C showed that the aging sequence and DSC response of this alloy were similar to those of pure Al-Cu when the increased copper content is taken into account. Further aging experiments at temperatures between room temperature and 130 °C showed pronounced changes of the GP zone dissolution peak as a function of aging conditions. These changes were found to be related to the effect of GP zone size on the metastable phase boundary and on the GP zone dissolution kinetics.

Hexagonal phases in phospholipids with saturated chains: phosphatidylethanolamines and phosphatidic acids.

Abstract: The structure of phospholipids with saturated chains was investigated by differential scanning calorimetry and by X-ray diffraction. It is shown that both phosphatidylethanolamines and phosphatidic acids can exhibit a hexagonal phase at high temperature. The temperature of the transition to the hexagonal phase is dependent on chain length and sodium salt concentration. Increasing the chain length or the sodium salt concentration results in a decrease in the transition temperature. In addition to this transition at high temperature, a calorimetric transition at low temperature, a calorimetric transition at low temperature is detected in some phospholipids.

Gel to liquid-crystalline transition temperatures of water dispersions of two pairs of positional isomers of unsaturated mixed-acid phosphatidylcholines

Abstract: The gel to liquid-crystalline phase transition temperatures of dispersions of mixed-acid sn-1,2-lecithins which contain one unsaturated and one saturated fatty acid have been studied by differential scanning calorimetry. The temperature for 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (containing no reversed isomer) was -9.3 degrees C while that for 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (containing 8% of the reversed isomer) was -2.6 degrees C. The temperature for 2-oleoyl-1-stearoyl-sn-glycero-3-phosphocholine (containing 6% of the reversed isomer) was 6.3 degrees C while that for 1-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (containing 18% of the reversed isomer) was 8.6 degrees C. The differences in transition temperatures for the isomers of a pair containing the same two acids were consistent with those observed for positional isomers of saturated mixed-acid lecithins in that the isomer of the pair which had the longer fatty acid in the sn-1 position had the lower temperature. The phase transition temperatures of pairs of isomers containing palmitate and oleate at the sn-1 and -2 positions were different by at least 6.7 degrees C, while those containing stearate and oleate were different by at least 2.3 degrees C. Differences in the chain lengths of the fatty acids at the two positions of the glycerol appear to predominate over differences in the depths of the double bonds in the bilayer in determining the transition temperatures.

Evidence of a phase transition in the rotator phase of the odd‐numbered paraffins C23H48 and C25H52

Abstract: We present here a study of the ’’rotator phase’’ displayed by the odd‐numbered paraffins CnH2n+2 with n = 23 and n = 25. From x‐ray and D.S.C.(differential scanning calorimetry) experiments we show that the so‐called rotator phase of these compounds exhibits a phase transition from a pseudohexagonal symmetry modification to a hexagonal symmetry modification when increasing the temperature. The corresponding phase transition seems to be a weak, first order transition.

Effect of ionizing radiation on the chemical composition, crystalline content and structure, and flow properties of polytetrafluoroethylene

Abstract: This article describes a study of ionizing radiation-induced changes in the chemical composition, crystalline content and structure, and flow properties in polytetrafluoroethylene (PTFE). Irradiations conducted in the presence of oxygen cause acid fluoride end groups to be formed, which on exposure to water vapor hydrolyze to form carboxylic acid end groups. Analyses by infrared (IR) spectroscopy indicate that when irradiated in a vacuum PTFE exhibits defect absorption bands which have been attributed to branch and crosslink formation. The crystalline content of PTFE which increases after exposure to radiation was monitored by IR spectroscopy, density, x-ray diffraction on unoriented samples, and differential scanning calorimetry (DSC) as the measurement probes. The melt viscosity of PTFE exposed to various radiation doses in air decreases dramatically after irradiation. Between 2.5 and 5 Mrd an increase in viscosity is attributed to the formation of branches and crosslinks. The effects of preirradiation crystallinity and postirradiation heat treatment were studied. A model is presented to explain the mechanism of the observed radiation effects.

Influence of water on thermal transitions in natural polymers and synthetic polyamides

Abstract: The glass transition temperature Tg and to some extent the melting temperature Tm of collagen, cellulose, polyamide 6 and 66, and of the statistical copolyamide 6/66 were investigated with the differential scanning calorimeter after equilibration of the specimens at various humidities at 295K. The results showed that smaller amounts of absorbed water cause a pronounced shift of Tg to lower temperatures due to the breaking of hydrogen bonds and the reduction of the cohesive energy between the polymer chains. Additional water molecules form clusters with previously absorbed water and Tg decreases are less pronounced. The plasticizing effect of water on natural polymers exceeds by far the effect on synthetic polyamides, as does its influence on Tm in the case of collagen.

Thermal stability of amorphous Zr‐Cu alloys

Abstract: By means of differential scanning calorimetry we have studied the glass transition and crystallization in various amorphous Zr1−xCux alloys in the concentration range 0.25 ? × ?0.90. Included in this investigation are several amorphous alloys of Cu and either Hf, La, Gd, Dy, Er, or Y. A linear relationship is observed when the crystallization temperatures are plotted versus the energy neeeded to create a hole the size of a Cu atom in the various alloys. Experimental values for the activation energies of the crystallization processes were dervived by studying these processes with different heating rates. These activation energies are in general larger than the hole‐formation enthalpies.

Curing of compression molded sheet molding compound

Abstract: A recently developed kinetic model has been applied here to describe the polyester-styrene addition copolymerization. By assuming that the termination step is negligible and the reaction rate between inhibitor and initiator free radical is much, faster than any other reactions, the kinetic mechanism can be simplified to be expressed as a single equation. The parameters, rate constant of initiator decomposition and rate constant of propagation, are estimated from the induction time and the time to the peak exotherm of isothermal differential scanning calorimetry (DSC) curves. Temperature profiles inside plate sections of SMC parts during molding are predicted by a mathematical model in which addition polymerization is coupled with heat transfer. The predicted temperature profiles compare well with the experimental results. The model is also used to predict the cure time of different part thicknesses, mold temperature and initiator concentration. Glass fibers playa role as a heat sink as well as heat conductor during curing. Adding glass fibers to SMC not only lowered the maximum exotherm but also reduced the cure time.

Phase Transition of the Bilayer Membrane of Synthetic Dialkyl Amphiphiles as Studied by Differential Scanning Calorimetry

Abstract: The phase transition behavior of dilute aqueous solutions of the bilayer membrane of dialkyl (C12 to C18 in most cases) amphiphiles was investigated by differential scanning calorimetry (DSC). The extent of the sample dispersion affected the DSC behavior for dialkyl amphiphiles with ammonium, phosphate and sulfonate head groups. The well-sonicated aqueous sample of the ammonium amphiphiles showed broader transition peaks at lower temperatures than the corresponding frozen sample did. The discrepancy became less pronounced when the interacting group was introduced at or near the head group. The wel DSC behavior was not observed for amphiphiles with nonionic (polyoxyethylene) and zwitterionic (phosphocholine) head groups. The phase transition temperature (0 – 80°C) was enhanced progressively with increasing alkyl chain lengths (C12 to C18), but the transition enthalpy (2–12 kcal/mol) did not obey simple relations.

A differential scanning calorimetric study of the thermal behaviour of bovine β-lactoglobulin at temperatures up to 160 °C

Abstract: The thermal behaviour of β-lactoglobulin was studied by differential scanning calorimetry (DSC) in the temperature range 40–160 °C. The DSC curves revealed, in addition to the usually observed denaturation peak near 80 °C, a distinct endothermal peak between 130 and 150 °C. When the pH was increased from 6·5, the area under the peak near 80 °C (denaturation heat) decreased significantly, whereas the peak area near 140 °C increased. The temperature of maximum heat absorption in the peaks near both 80 and 140 °C gradually increased as the pH decreased. Addition of sugars and variation of the heating rate both caused a temperature shift of the endothermal heat effect at 140 °C, similar to that at 80 °C. No peak near 140 °C was observed when β-mercapto-ethanol was added to the β-lactoglobulin solution before scanning. The origin and nature of the high temperature denaturation peak is discussed in terms of conformational changes of the protein.

Transition temperatures and heats of crystallization of amorphous Ge, Si, and Ge1−xSix alloys determined by scanning calorimetry

Abstract: The transition temperatures (Tt) and heats of crystallization (ΔH) of a‐Ge, a‐Si, and a‐Ge1−xSix films deposited by rf sputtering have been measured by differential scanning calorimetry (DSC). Both Tt and ΔH (per gram) increase linearly with x in the alloy system. Overlapping exothermic multiple peaks are observed in the DSC traces of a‐Ge and a‐Si samples prepared at high deposition rates, suggesting that multiple amorphous configurations may coexist in these materials.

Thermal Analysis of Lignocellulosic Materials. Part II. Modified Materials

Abstract: Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature while the substance is subjected to a controlled temperature program. The output of thermal analysis represents data predominantly in the form of curves characteristic for a particular chemical substance or a mixture of substances. Changes in latent heat in a chemical substance are detectable by differential thermal analysis (DTA) or by differential scanning calorimetry (DSC). Progressive weight loss is detectable by thermogravimetry (TG) or differential thermogravimetry (DTG). Progressive changes in certain mechanical properties are detectable by thermomechanical analysis (TMA). Many other thermoanalytical techniques are available, but are used appreciably less often.

Structure and functional behaviour of whey proteins

Abstract: Structure and functional properties of the whey proteins are interrelated and are governed by changes in the globular conformation of the molecule. In particular the heat sensitivity of the whey proteins appears to be a characteristic property that affects their structure and physico-chemical properties. After a brief analysis of some of the physico-chemical properties of the major whey proteins, attention is paid to their thermal behaviour as observed by differential scanning calorimetry (DSC) in a temperature range up to 150 deg C. Two distinct heat effects are observed; the first, near 70 deg C, is denoted as the denaturation heat and the second, near 130 deg C, is ascribed to unfolding of residual protein structure. Environmental effects such as pH, lactose and calcium salts are discussed in relation to heat denaturation and protein solubility. It has been shown that heat denaturation of the whey proteins will not necessarily result in an impaired protein solubility when the heating is conducted under the proper conditions. Special attention is paid to the relation between heat stability in milk and the thermal behaviour of beta-lactoglobulin at different pH values and calcium concentrations. Both the heat stability is milk and the thermal behaviour of beta-lactoglobulin change quickly in a very narrow pH range between pH 6.7 and 7.0. The importance of sulphydryl/disulphide groups has been stressed in this relationship.

Properties of polyether‐polyurethane zwitterionomers

Abstract: Polyether polyurethanes based on 4,4′-diphenylmethane diisocyanate (MDI), N-methyl diethanolamine (MDEA), and polytetraniethylene oxide (PTMO) were synthesized with varying levels of hard segment content. The tertiary amine of MDEA was sulfonated with γ-propane sultone thereby converting the polyether polyurethane to a polymeric zwitterion. The effect of the chemical composition and the degree of ammonium sulfonation on the extent of phase segregation, and physical properties were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis and tensile testing. It was found that the MDEA chain extender inhibits microphase separation of the polymer in the solid state. As the degree of ammonium sulfonation increases in the zwitterionomers, an improvement of phase separation and domain structure was observed. In addition the tensile properties change dramatically with increasing sulfonate content going from properties characteristic of soft, weak gum rubbers to those of hard, strong thermoplastic elastomers.

Structural and mechanical properties of polybutadiene‐containing polyurethanes

Abstract: A series of segmented polyurethanes based on hydroxylterminated polybutadienes (HTPBD) and their hydrogenated derivatives (HYPBD) has been synthesized. Thermal, mechanical, and spectroscopic studies were carried out over a wide temperature range to elucidate the structure-property relationships existing in these polymers. Both thermal and dynamic mechanical response showed a soft segment Tg at −74°C for the unsaturated polyurethanes and at −69°C for the hydrogenated samples. In addition, two hard segment transitions are observed by differential scanning calorimetry (DSC) at 40 and 75°C and a softening region by thermal mechanical analysis (TMA) at 190°C. The low Tg, very close to that of the free HTPBD and HYPBD and independent of hard segment content, indicated that these polymers were well phase separated. Results of infrared analysis revealed that at room temperature, 90-95 percent of the urethane N-H groups formed hydrogen bonds. Since hydrogen bonding resides only within the hard segment domain in these butadiene-containing polyurethanes the extent of H-bonding served as additional evidence for nearly complete phase segregation. From dynamic mechanical studies, the plateau modulus above the soft segment Tg and stress-strain behavior depended upon the concentration of hard segments. A slight increase in the modulus, a moderate increase in stress (σb), and decrease in elongation accompanied a higher hard segment content. The thermal and mechanical response of these polyurethanes appears to be consistent with behavior observed for other phase segregated systems. Variations in behavior resulting from hydrogenation of the precursor prepolymer are discussed.

Chiral mesophases of 12-hydroxyoctadecanoic acid in jelly and in the solid state. II. A new type of mesomorphic solid state.

Abstract: A new type of chiral solid mesophase was obtained by evaporating the solvents from solutions or jellies of chiral 12-hydroxyoctadecanoic acid (12HOA). The mesomorphic solids exhibited some optical properties characteristic of a helicoidal mesophase: (1) spherulitic or Schlieren texture under a polarizing microscope, (2) reflective circular dichroic band, and (3) induced circular dichroic spectra with a single sign due to achiral molecules added to the solid. From the circular dichroic and X-ray studies, it was concluded that the helicoidal mesophase structure exists locally as small domains with a chiral smectic structure within the mesomorphic solids. Microscopic observation revealed that, upon heating, the transition of the mesomorphic solid to the crystalline solid occurred. This process was studied by differential scanning calorimetry. Formation of a variety of helicoidal systems from chiral molecules such as 12HOA and polypeptides was regarded as a phenomenon characteristic of chiral assemblies of am...

Calorimetric and fluorescence depolarization studies on the lipid phase transition of bacteriorhodopsin--dimyristoylphosphatidylcholine vesicles.

Abstract: The thermotropic lipid phase transition of dimyristoylphosphatidylcholine vesicles reconstituted with bacteriorhodopsin was investigated as a function of the lipid to protein ratio by means of differential scanning calorimetry and fluorescence depolarization of the embedded probe 1,6-diphenyl-1,3,5-hexatriene. Two attractive features of this system are that the lipid phase transition induces lipid-protein segregation and that the state of aggregation of the protein is known. Above the lipid phase transition and above molar lipid to protein ratios of about 100, bacteriorhodopsin is monomeric. Well below the phase transition, bacteriorhodopsin is aggregated in a hexagonal protein lattice. With increasing amounts of incorporated bacteriorhodopsin, the calorimetric transition broadens, and a second component develops at a temperature which is lower than that of the unperturbed transition. The latter transition was assigned to the disaggregation of the bacteriorhodpsin lattice which occurs 6-7 degrees C below the phase transition of the protein-free lipids according to previous measurements. The van't Hoff enthalpy of the phase transition, as determined from the fluorescence depolarization of diphenylhexatriene, is in surprisingly good agreement with that obtained from differential scanning calorimetry over a wide range of lipid to protein ratios. The differential scanning calorimetry data can be simulated on the basis of a model which takes the protein segregation and crystallization specifically into account. The essential feature of this model is that calorimetrically detectable lipid melting occurs in the temperature region of the protein crystallization, since for the disaggregation of the close packed bacteriorhodopsin lattice additional lipids of an intermediate chain conformation are required.

Differential scanning calorimetry of chloroplast membranes: identification of an endothermic transition associated with the water-splitting complex of photosystem II.

Abstract: The structure of spinach thylakoid membranes has been investigated by sensitive differential scanning calorimetry. Six endotherms are observed between 20 and 85 degrees C, corresponding to order--disorder transitions of different structural domains within the thylakoid membrane. In a medium of relatively high ionic strength, endothermic transitions occur at 42, 54, 65, 72, 79, and 84 degrees C, with the 65 degrees C transition being particularly prominent. At a lower ionic strength, transitions are centered at 44, 61, 66, 70, 78, and 83 degrees C. The 42--44 degrees C endothermic transition (the A transition) can be correlated with the modification of three electron-transport components or properties associated with photosystem II: (i) release of manganese from the membrane, (ii) the loss of O2 evolution with water as a donor, and (iii) a decrease in the redox potential of the hydroquinone-reducible cytochrome b-559. Both the A transition and the ability to evolve O2 are irreversibly lost after heating to 49 degrees C and also after exposure to trypsin, suggesting the involvement of protein in this transition. The interpretation of these observations is that one effect of the A transition involves the thermal disruption of a protein component on the donor side of photosystem II.

Physical aging studies of semicrystalline poly(ethylene terephthalate)

Abstract: : Physical aging of semicrystalline poly(ethylene terephthalate) has been investigated as a function of crystalline content. Stress-strain, stress relaxation, and differential scanning calorimetry experiments were used to monitor the physical aging process. It was found that both the overall extent and the rate of physical aging in this material decrease with increasing crystallinity. Several possible reasons for this behavior are advanced and discussed. It was also found that the drawing behavior of amorphous PET changes significantly as physical aging progresses. Specifically, for samples aged and then elongated, the extent of localized deformation (necking) and associated strain-induced crystallization was greater for samples aged for longer periods of time. (Author)