Showing papers on "Differential scanning calorimetry published in 1976"

Dependence of the Fictive Temperature of Glass on Cooling Rate

Abstract: An equation derived by Ritland relating the cooling rate and fictive temperature for glasses without memory is extended to those with memory, i.e. those which exhibit a spectrum of relaxation times. Provided that the spectrum of relaxation times is temperature-independent, the limiting fictive temperature, T′f, obtained when a glass is cooled through the transition region, is shown to be related to the cooling rate, q, by d In |q|/d(1/T'f)=-Δh★/R where R is the ideal gas constant and Δh★ the activation enthalpy for the relaxation times controlling the structural relaxation. Values of T′f vs q obtained from enthalpy measurements by differential scanning calorimetry are presented for B2O3, 0.4Ca(NO3)2—0.6KNO3, and borosilicate crown glasses; Δh★ is equal, within experimental error, to the activation enthalpy for shear viscosity. Values of T′f from volume and enthalpy measurements obtained at the same cooling rate for the borosilicate crown glass are equal.

Investigation of phase transitions of lipids and lipid mixtures by sensitivity differential scanning calorimetry.

Abstract: High sensitivity differential scanning calorimetry is applied to the study of the thermotropic behavior of mixtures of synthetic phospholipids in multilamellar aqueous suspensions. The systems dimyristoylphosphatidylcholine dipalmitoylphosphatidylcholine, and dimyristoylphosphatidylethanolamine-distearoylphosphatidylcholine, although definitely nonideal, exhibit essentially complete miscibility in both gel and liquid crystalline states, while the system dilauroylphosphatidylcholine-distearoylphosphatidylcholine is monotectic with lateral phase separation in the gel state. Comparison of the observed transition curves with theoretical curves calculated from the calorimetrically determined phase diagrams supports a literal interpretation of the phase diagrams.

Differential scanning calorimetry of crystallized PVA hydrogels

Abstract: Crosslinked poly(vinyl alcohol) hydrogels produced via electron beam irradiation of aqueous PVA solutions were crystallized via a two-stage dehydration-annealing process to produce swollen hydrogels of varying degrees of crystallinity. Evaluation of the degree of crystallinity of these hydrogels was done by differential scanning calorimetry (DSC) analysis. The swollen hydrogels had degrees of crystallinities varying between 30% and 50% depending on the temperature–time history of the specimens.

A calorimetric study of the thermotropic behavior of aqueous dispersions of natural and synthetic sphingomyelins.

Abstract: A recently developed differential scanning calorimeter has been used to characterize the thermotropic behavior of aqueous dispersions of liposomes containing sphingomyelin. Liposomes derived from sheep brain sphingomyelin exhibit a broad gel-liquid crystalline phase transition in the temperature range of 20-45 degrees C. The transition is characterized by maxima in the heat capacity function at 31.2 and 37.1 degrees C and a total enthalpy change of 7.2 +/-0.4 kcal/mol. Beef brain sphingomyelin liposomes behave similarly but exhibit heat capacity maxima at 30, 32, and 38 degrees C and a total enthalpy change of 6.9 kcal/mol. The thermotropic behavior of four pure synthetic sphingomyelins is reminiscent of multilamellar lecithin liposomes in that a single, sharp, main transition is observed. Results obtained for liposomes containing mixtures of different sphingomyelins are complex. A colyophilized mixture of N-palmitoylsphingosinephosphorylcholine, N-stearoylsphingosinephosphorylcholine, and N-lignocerylsphingosinephosphorylcholine in a 1 : 1 : 1 mol ratio exhibits a single transition with a Tm below that observed for the individual components. On the other hand a 1 : 1 mixture of N-stearoylsphingosinephosphorylcholine and 1-palmitoyl-2-oleylphosphatidylcholine exhibits three maxima in the heat capacity function. It is clear from these results that the thermotropic behavior of sphingomyelin-containing liposomes is a complex function of the exact composition. Furthermore, it appears that the behavior of the liposomes derived from natural sphingomyelins cannot be explained in terms of phase separation of the individual components.

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

Abstract: We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T G . The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T X . Values of T g and T X have been tabulated for all alloys investigated and the compositional dependence of T g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge 20 Se 40 Te 40 in which a GeSe 2 -rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe 2 -rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.

Polymorphism in Vanadyl Phthalocyanine

Abstract: Phase transitions in vanadyl phthalocyanine have been studied using differential scanning calorimetry, x-ray scattering and optical absorption spectroscopy. Three phases, I, II and III, were prepared by nonequilibrium vapour quenching, equilibrium crystal growth and melt quenching, respectively. Phase I is metastable and transforms thermally to Phase II. Both Phase II and Phase III are stable up to the melting point. Phase II has a triclinic crystal structure isostructural with SnPc and is prone to systematic disorder in the ac plane. The disorder enhances the optical absorption at 6500 A. Attempts to account for the optical absorption spectra using simple dipole-dipole models have been unsuccessful although dimeric structures are suggested for all three phases.

Modification of membrane lipid: physical properties in relation to fatty acid structure.

Abstract: Differential scanning calorimetry (DSC) and electron spin resonance (ESR) measurements were made to characterize how modifications in the fatty acid composition of Escherichia coli affected the thermotropic phase transition(s) of the membrane lipd. When the fatty acid composition contained between 20 and 60% saturated fatty acids, the DSC curves for isolated phospholipids and cytoplasmic membranes showed a broad (15-25 degree C) gel to liquid-crystalline phase transition, the position of which depended on the particular fatty acid composition. Utilizing multiple lipid mutants, enrichment of the membrane phospholipids with a single long-chain cis-monoenoic fatty acid in excess of that possible in a fatty acid levels less than 20% and gradually replaced the broad peak as the cis-monoenoic fatty acid content increased. These results were obtained with phospholipids, cytoplasmic membranes, and whole cells. With these same phopholipids, plots of 2,2,6,6-tetramethylpiperidinyl-1-oxy partitioning and ESR order parameters vs. 1/T revealed discontinuities at temperatures 40-60 degrees C above the calorimetrica-ly measured gel to liquid-crystalline phase transitions. Moreover, when the membrane phospholipids were enriched with certain combinations of cis-monenoic fatty acids (e.g., cis-delta 9-16:1 plus cis-delta 11-18:1) the DSC curve showed a broad gel to liquid crystalline phase change below 0 degrees C but the ESR studies revealed no discontinuities at temperatures above those of the gel to liquid-crystalline transition. These results demonstrated that enrichment of the membrane lipids with molecules in which both fatty acyl chains are identical cis-monoenoic residues led to a distinct type of liquid-crystalline phase. Furthermore, a general conclusion from this study is that Escherichia coli normally maintains a heterogeneous mixture of lipid molecules and, by so doing, prevents strong lipid-lipid associations that lead to the formation of lipid domains in the membrane.

Heat Capacity and Structural Relaxation of Mixed‐Alkali Glasses

Abstract: Heat capacities of a series of mixed-alkali glasses of composition (in mol%) 24.4(Na2O + K2O)-75.6SiO2 were measured in the transition region by differential scanning calorimetry. The glass heat capacities at 700 K and the equilibrium liquid heat capacities are the same for all glasses on a per-g atom basis and equal, respectively, to 5.6 ± 0.1 and 6.8 ± 0.1 cal/g atom K. The glass transition temperatures exhibited negative deviations from additivity, but the heat capacity curves in the transition region of all the glasses for identical heating rates and thermal histories could be superimposed on the same reduced plot. This behavior indicates that the shapes of the structural relaxation functions are the same for all the glasses. These results support Shelby's conclusion that there is no unique “mixed-alkali effect” on thermodynamic or structural relaxation properties and that the term should be reserved for properties relating to ionic transport.

Influence of water content on the stability of myoglobin to heat treatment

Abstract: The effect of heat treatment on a model protein, sperm whale myoglobin, at various water contents has been investigated at neutral pH. Samples ranging from solid preparations of 3% water content to dilute solutions of 99.6% water content have been examined. Differential scanning calorimetry (DSC) has been used for the heat treatments as well as for analyzing the effect of the heat treatments on the protein samples. At water contents below 30% only a certain fraction of the heat-treated protein sample underwent irreversible transitions. This fraction of the protein sample increased linearly with the amount of water present during the heat treatment. Above 30% water content the entire protein sample underwent irreversible transitions. The heating rate was shown to influence the transition temperature as well as the apparent transition heat. Possible applications in food technology and food processing of the DSC technique used are discussed.

Polyethylene networks crosslinked in solution: Preparation, elastic behavior, and oriented crystallization. I. Crosslinking in solution

Abstract: A study has been made of the crosslinking of linear polyethylene in solution. Networks containing a low number of trapped entanglements and elastically ineffective chain ends were prepared by crosslinking high molecular weight linear polyethylene in 1,2,4-trichlorobenzene solutions with dicumyl peroxide at 120°C. No syneresis was observed during crosslinking except at high peroxide concentrations. The networks were characterized by swelling measurements, infrared spectroscopy, and differential scanning calorimetry. The crosslinking efficiency, calculated from swelling, was found to be proportional to the square of the polymer volume fraction. The proportionality constant was 0.8, indicating close to unit efficiency for undiluted polymer. Chemical modification of the polyethylene chains by attachment of peroxide and solvent fragments was of the order of one foreign unit per elastically active network chain, depending on peroxide and polymer concentration. Sol–gel analysis indicated that no chain scission occurred. These results are shown to be consistent with a “cage” mechanism for crosslinking. The possible topological consequence of this mechanism, preferential crosslinking of entanglements, is discussed. The concentration of trapped entanglements was also found to be proportional to the square of the polymer volume fraction. The proportionality constant corresponds to a molecular weight between entanglements of 4000 for the undiluted polymer, which is close to the value of 4200 found for networks prepared from the undiluted polymer. Since the results obtained are based mainly on the use of the swelling equation, different aspects of the applicability of this equation for the evaluation of the crosslinking process are discussed. As regards the reference dimensions, which should be known for a quantitative application of the elastic theory, the results strongly support the use of the dimensions of the network chains after completion of crosslinking.

A study of the freezing of supercooled water dispersed within emulsions by differential scanning calorimetry

Abstract: Supercooling of microsize droplets of water dispersed within emulsions has been studied by means of a differential scanning calorimeter Perkin-Elmer DSC 2. In an emulsion cooled steadily at 2.5K min-1, the breakdown of supercooling in individual droplets is distributed in temperature over the range -37.5 degrees C to -40.5 degrees C, the most probable temperature of freezing being T*=-(39.0+or-0.5) degrees C. When an emulsion is maintained at TC, 0 degrees C>TC>T*, the freezing of individual droplets is distributed in time, the time taken to achieve total freezing of the emulsion becoming shorter as TC becomes closer to T*. If an emulsion which has been partially frozen by being maintained at TC and then melted is cooled steadily at 2.5K min-1, then the distribution of freezing temperatures shows two most probable values of T*, -(34.0+or-0.5) degrees C and -(39.0+or-0.5) degrees C. This phenomenon is briefly discussed.

Melting and degradation behaviour of needle‐like poly(oxymethylene) crystals

Abstract: DSC/TG (differential scanning calorimetry/thermal gravimetry) simultaneous measurements have been carried out for needle-like and other poly(oxymethylene) crystals and the occurrence of degradation, overlapping the melting, has been confirmed for needle-like and fibrous extended-chain crystals. A theory to analyze the DSC curves with reference to the corresponding TG has been formulated and the contributions from the melt and the degradation have been resolved by means of computer curve-fitting. For needle-like crystals, the heat of fusion, 2260 cal/mol (9450 J/mol) and the melting point, 183–184°C (as well as the heat of degradation, 13,17 kcal/mol [55,05kJ/mol]) have been obtained and discussed with reference to the data from other samples. Systematic experiments for the effect of running conditions on the shape of DSC curves have been made and described in the Appendix.

The thermal transition behavior of polyorganophosphazenes

Abstract: The thermal transition behavior of poly[bis(trifluoroethoxyphosphazene)] (I) and two samples of poly[bis(p-chlorophenoxyphosphazene)] (II) have been studied as representative alkoxy- and aryloxy-substituted polyorganophosphazenes. Several of the polymers of this class are reported to exhibit two first-order transitions, denoted herein as T (1) for the transition from a crystalline to mesomorphic state and Tm for the true melt. Studies of these two polymers were undertaken to gain a further understanding of this behavior. Optical microscopy on a solution-cast film of I showed that the details of spherulitic morphology persist through T(1) = 90°C and remain undisturbed through the temperature interval up to Tm = 240°C. The study of II by x-ray diffraction reveals that two sharp lines are observed above T(1) = 165°C and that orientation is not randomized upon heating to temperatures as high as 238°C. Considerable improvement in the crystalline diffraction pattern results from the thermal treatment. A detailed examination was also made by differential scanning calorimetry (DSC) of the effects of cycling through T(1), annealing in the temperature interval between T(1) and Tm and for I, the influence of controlled crystallization from the melt. The results indicate that the organization in the mesomorphic state, as influenced by thermal history, has a profound affect on the peak position, area, and sharpness of the endotherm at T(1). For I, the apparent heat of fusion at T(1) is about ten times greater than at Tm, whereas for II, no DSC peak is observed at Tm = 365°C, suggesting that the ratio of the heats of fusion at T(1) and Tm is greater than 50. However, estimated volume changes at the two transitions are nearly equal. These results are compared with those of other polymers which exhibit an intermediate state of order and with molecular liquid crystals.

Morphology development during shearing of poly(ethylene oxide) melts

Abstract: A parallel glass-plate rotary shearing device has been constructed. This device allows direct observation of the crystallization of polymeric melts being subjected to a constant rate of shear under controlled temperature conditions. Polarized light microscopy and small-angle light-scattering techniques were employed to study the kinetics and morphology of poly(ethylene oxide) melts as they crystallized under various combinations of supercooling and shear rate. The techniques of epimicroscopy, differential scanning calorimetry, and wide-angle x-ray diffraction were used to study the already crystallized material. Crystalline aggregates developing from sheared melts showed a sheaf-like lamellar morphology. The long axes of the sheaf-like structures oriented perpendicular to the flow direction when crystallized under shear. This behavior can be explained in terms of hydrodynamic theory.

Molecular motion, phase transitions, and disorder in the pyridinium halides

Abstract: The solid pyridinium chloride, bromide, and iodide salts were studied using 1H nuclear magnetic resonance and differential scanning calorimetry. Phase transitions were observed at 345 K for the chloride, 269 K for the bromide, and 247 K for the iodide. Well below each transition, the pyridinium ions are held rigidly in the crystal lattice, whereas above each transition the ions reorientate rapidly about an axis at right angles to the ring planes. From the temperature dependence of the spin–lattice relaxation times the high temperature phase reorientational activation energies were determined to be 1.55, 2.30, and 4.20 kcal/mol for the chloride, bromide, and iodide, respectively.

Characterization of oriented low‐density polyethylene samples by differential scanning calorimetry

Abstract: The melting behavior of samples of oriented low-density polyethylene is examined as a function of annealing temperature and time, subsequent heat treatment, and irradiation dose. Three types of endotherm are identified which are attributed to primary melting of the lamellae, to melting of the products of reorganization during the scan, and to melting of material crystallized during cooling from the original annealing temperature. Irradiation was found to suppress the refolding which normally occurs when a sample is heated above its annealing temperature, but also produces an increase in melting point. The behavior of similar samples prepared from high-density polyethylene is briefly considered. (auth)

Annealing characteristics of ultraoriented high‐density polyethylene

Abstract: The annealing characteristics of ultraoriented high‐density polyethylene fibers have been studied using differential scanning calorimetry, thermomechanical analysis, and tensile testing. The melting point of unannealed fibers was estimated to be 130 °C using a 10 °C min −1 heating rate. Fiber annealing between 126 and 132 °C split the melting endotherm into two peak melting at 132 and 139 °C. The weight fraction of fiber which had reorganized to the lower melting point increased with annealing time, finally attaining a constant value. However, annealing of laterally constrained fibers below 136 °C did not result in structural changes as detected by calorimetry. Melting‐point variations with heating rates≳0.625 °C min−1 indicated the equilibrium melting point of the unconstrained fiber at zero heating rate as 138 °C, while a similarly extrapolated value using heating rates <0.625 °C was ∼132 °C. The linear expansion coefficients rapidly increased near 132 °C and were attributed to the structural reorganization detected by calorimetry. Fiber annealing markedly decreased the tensile modulus from 50 GPa towards the 1 GPa observed for the unoriented polyethylene. The annealing behavior was consistent with the thermal instability associated with the anisotropy of the surface free energies of the ultraoriented crystalline fibers as well as sources of melting‐point reduction such as defects.