Showing papers on "Differential scanning calorimetry published in 1973"

Kinetics and thermal characterization of thermoset cure

Abstract: The differential scanning calorimeter (Perkin-Elmer DSC-1) is used to characterize the cure of a general-purpose polyester during isothermal and scanning experiments. The technique is based on a new proposed model for the kinetics of isothermal cure. The model yields results which are in good agreement with experimental isothermal rate of reaction and integral heat of reaction data. It also gives some information about the residual reactivity of the sample after an isothermal cure experiment. With the aid of the proposed kinetic model, it is possible to obtain integral heats of reaction and rates of heat generation at different temperatures during a scanning experiment. The difference between the rate of heat input to the sample and the heat of reaction at any instant during scanning may be used to calculate the specific heat of the sample at the same instant. Specific heat data show two maxima during each scanning experiment. These maxima may be associated with transitions occurring during cure in the melt and rubbery states.

Annealing effects in poly(vinylidene fluoride) as revealed by specific volume measurements, differential scanning calorimetry, and electron microscopy

Abstract: Specimens of poly(vinylidene fluoride), crystal form II, annealed at different temperatures between 130 and 180°C were characterized by specific volume measurements, differential scanning calorimetry (DSC), and electron microscopy. The degree of crystallinity calculated from the specific volume changed only by 15% i.e., from 50% to 65%. On the other hand, the melting behavior changed with annealing conditions. When a specimen was annealed above 170°C, two endothermic peaks appeared on either side of the annealing temperature. Results from DSC measurements made at different heating rates and electron microscopy showed that the two endotherms were caused by a bimodal distribution of lamellar thicknesses. The equilibrium melting point was found to be 210°C from the linear relation of the melting point and the annealing temperature. The equilibrium enthalpy and entropy of fusion were found to be 1.6 keal/mole and 3.3 eu/mole of repeat units by measurement on polymer–diluent mixtures. The surface free energy was found to be 5.1 kcal/mole of lamellar sequences from the plot of melting point versus reciprocal lamellar thickness obtained by electron microscopy. From a plot of enthalpy of fusion versus reciprocal lamellar thickness the surface enthalpy was found to be 20 keal/mole of lamellar sequences. These data lead to the estimate that a chain fold consists of about 30 repeat units.

Differential scanning calorimetry of the epoxy cure reaction

Abstract: From a review of the literature it is shown that kinetics from programmed heating rate experiments are often in serious disagreement with published isothermal data. In every case, including the present, the dynamic kinetic parameters measured on epoxies are significantly larger than the isothermal. Use of a dynamic rate equation provides good agreement between the two experiments. It is also shown that the Kissinger method is correct when applied to DSC of epoxies.

Liquid crystalline phases in a doubly homologous series of benzylideneanilines‐textures and scanning calorimetry

Abstract: We have investigated the phase behavior of 35 N ‐(p ‐alkoxybenzylidene)‐p ‐n ‐alkylaniline compounds, CnH2n + 1O–φ–CHN–φ–CmH2m + 1′ with n values ranging from 1 to 7 and m from 4 to 8 by means of thermal microscopy and differential scanning calorimetry. We report transition energies and entropies in addition to transition temperatures. From textures we determined the existence of nematic as well as smectic A, C, and B phases in many of the compounds. The microscopy and calorimetry evidence are consistent with a possible second order nature for the smectic C to smectic A transition. An interesting and useful smectic textural transition phenomenon is reported.

Melting and crystallization behavior of poly(tetrafluoroethylene). New method for molecular weight measurement of poly(tetrafluoroethylene) using a differential scanning calorimeter

Abstract: Melting and crystallization behavior of virgin polytetrafluoroethylene have been studied using a differential scanning calorimeter. Following quantitative relationship was found between number average molecular weight of polytetrafluoroethylene and the heat of crystallization in the molecular weight range of 5.2 × 105 to 4.5 × 107: Mn = 2.1 × 1010 ΔHc−5.16, where Mn is number average molecular weight and ΔHc is the heat of crystallization in cal/g. The heat of crystallization is independent of cooling rate ranging from 4 to 32°C/min. This relationship provides a simple rapid and reliable method for measuring the molecular weight of polytetrafluoroethylene.

Anomalous heat capacities of supercooled water and heavy water.

Abstract: Emulsification makes it possible to supercool water to the homogeneous nucleation temperature. Accordingly, the heat capacities of water and deuterium oxide have been determined from the respective equilibrium melting points to —38° and to —34°C, respectively. Two methods, drift calorimetry and differential scanning calorimetry, have been used. Both methods reveal a striking rise in the constant-pressure heat capacity below —20°C. This indication of an apparently cooperative behavior should serve to test current theories of water, most notably perhaps, the pair potential model of Ben-Naim and Stillinger. Some implications of possible meteorological significance are mentioned.

Phase Transitions in Mesomorphic Benzylideneanilines

Abstract: We have synthesized(1) 35 compounds of structural formula where n and m take on values from 1 to 7 and 4 to 8 respectively. (2) We have studied the phase behavior and identified many of the mesomorphic phases of these compounds by means of differential scanning calorimetry(3) (DSC) and thermal microscopy(4) (TM)

Heats of thermally induced helix-coil transitions of DNA in aqueous solution.

Abstract: Thermal denaturation of calf thymus DNA at both alkaline and neutral pH values was studied by differential scanning calorimetry. It was shown that the dependence of the enthalpy of transition on pH and salt concentration could be accounted for on the basis of a heat capacity change of +40 cal deg−1(base pair)−1. In the pH range between 10.3 and 11.3, a release of 0.6 proton per base pair was calculated from the pH dependence of the melting temperature. The heat effect associated with the release of this proton was calculated to be 5 kcal mole−1.

Heat capacity of molten polymers

Abstract: Heat capacities of molten polyethylene, polypropylene, poly-1-butene, polystyrene, and poly(methyl methacrylate) were measured over a wide range of temperature by using a differential scanning calorimeter. The upper limit of temperature was established for each polymer at about 10°K below the beginning of thermal decomposition. For poly-1-butene and poly(methyl methacrylate) the solid-state heat capacity was also measured starting from room temperature. Several samples of each polymer were used so that average values of heat capacities could be established (reported in 10°K intervals). The data revealed for all polymers a nearly linear increase of heat capacity with increasing temperature over the whole temperature range investigated.

Phase Studies on Binary Systems of Cholesteryl Esters A. Two Aliphatic Ester Pairs

Abstract: Binary blends for two pairs of esters of cholesterol, the myristate plus acetate and the myristate plus the nonanoate have been characterized to determine the equilibrium phase conditions for the isotropic liquid, the cholesteric and smectic mesophases, and the stable crystals. The phase diagrams were determined using a Differential Scanning Calorimeter (DSC-1B), which also provided the heats of transition. For the more complicated acetate system, morphology was also studied with a polarizing microscope puls hot stage and structure determinations were made using X-ray diffraction. Results indicate that both ester pairs form a single eutectic mixture. The crystal melting points of the pure components in both pairs follow the van't Hoff equation for melting point depression due to impurity over most of the composition range. A homogeneous cholesteric mesophase was formed in both pairs. The temperatures and heats of the smectic-cholesteric transition for the acetate blend decrease as the concentrati...

Phase Studies on Binary Systems of Cholesteryl Esters B. Three C18 Ester Pairs

Abstract: Three binary systems for the cholesteryl esters of C18 aliphatic acids, the stearate plus oleate, the stearate plus linoleate, and the oleate plus linoleate were studied in order to determine the phase transitions and the conditions for the cholesteric and smectic mesophases. Phase diagrams were determined using Differential Scanning Calorimeter (DSC-1B) and a polarizing microscope with hot stage. Both individual unsaturated esters exhibit a polymorphism in the solid state. The metastable modifications are formed from highly supercooled melts. Prolonged standing produces recrystallization in the stable form. The formation and stability of metastable modifications are enhanced in investigated binary systems and result in complex behavior with solid-solid transitions and regions of metastability. Both stearate systems are of a eutectic type with a limited solid solubility. A continuous series of a solid solutions over the entire composition interval was found in the oleate-linoleate system. The mes...

Studies of polymer blends. I. Compatibility of poly(vinyl chloride) and poly(ethylene‐co‐vinyl acetate‐co‐sulfur dioxide)

Abstract: Various methods of determining polymeric molecular compatibility were applied to blends of poly(vinyl chloride) (PVC) and poly(ethylene-co-vinyl acetate-co-sulfur dioxide) (E/VA/SO2). In one series, where the E/VA/SO2 had a mole composition of 72.7/18.5/8.8, true compatible blends were demonstrated by phase-contrast microscopy, torsion pendulum studies, and differential scanning calorimetry experiments for blends containing up to 40% E/VA/SO2. These blends exhibited a single Tg whose compositional variation was found to follow the Fox expression. Experimental densities were slightly greater than predicted on assuming volume additivity. This observation implies better packing and a negative heat of mixing and thus is in harmony with a negative free energy of mixing and the observed molecular compatibility.

Thermal analysis of polymers

Abstract: Results of research on the quantitative thermal analysis of polymers by calorimetry are reviewed. Adiabatic and scanning calorimeters are briefly described and data obtained are discussed in terms of current theories. Heat capacities, glass transition phenomena and melting of macromolecules are treated.

Hydrates of Organic Compounds. I. Solid-Liquid Phase Equilibria in the Water+1,4-Dioxane System and Some Properties of 1,4-Dioxane Hydrate

Abstract: The solid-liquid phase diagram for the water +1,4-dioxane system has been determined by the ordinary cooling curve method and with the aid of a differential scanning calorimeter (DSC). The diagram shows the existence of a hydrate which is stable in rather narrow ranges of temperature (−15.8∼−13.5 °C) and of composition (37.0∼46.5 wt% dioxane). From the phase relations and an analysis of the calorimetric data, the composition and the heat of formation (from the liquid states of both components) of the hydrate have been estimated to be C4H8O2·mH2O with m=36∼39 and ΔH1=−51.3 (for m=36)∼−55.6 (for m=39) kcal/mol respectively. This hydrate seems to be the same as that recently reported by Rosso et al., though the compositions and the temperature ranges are very different.

Dissolution Behavior of Solid Drugs. III. Determination of the Transition Temperature between the Hydrate and Anhydrous Forms of Phenobarbital by Measuring Their Dissolution Rates

Abstract: The transition temperature and the heat of transition between the hydrate and the anhdrous forms of phenobarbital were determined to be 36.4° and 1.87kcal/mole, respectively by the initial dissolution rate measurements. These values were in good agreement with values obtained by the conventional solubility equilibrium method and by the differential scanning calorimetry (DSC), as well as with transition temperature estimated by the electrical conductivity and vapour pressure measurements. Furthermore, metastable solubility values calculated from experimentally determined stable solubility values and ratios of dissolution rates for both forms agree closely with values obtained by the conventional solubility method. The assumption that the dissolution processes for both forms of phenobarbital are diffusion controlled was further supported in this study by the equality of dissolution activation energy for both forms, as well as by the magnitude of this activation energy, and by the diffusion layer thickness.

Estimation of butadiene in vulcanized BR and SBR by thermographic analysis

Abstract: A method for the determination of butadiene in vulcanized polybutadiene (BR) and styrene–butadiene rubber (SBR) has been proposed. The method is based on determining the area under the exothermic peak (around 380°C) of the differential scanning calorimetry thermograph in nitrogen. The exotherm area was found to be linearly related to the butadiene content of the polymers and was unaffected by vulcanization or by loading with carbon black. This approach supplements and extends the existing methods of BR determination, which depend upon the estimation of styrene content. The existing evidence indicates that the exothermic reaction is due to cyclization of BR. The energy of activation calculated from the Arrhenius plot is 32–35 kcal/mole; it does not change with vulcanization, loading, or nature of the polymer.

Thermally-induced structural and electrical effects in GeTe-based amorphous alloys

Abstract: The thermal, structural electrical properties of bulk glasses based on GeTe compositions near the binary eutectic, Ge15Te85, are studied Information regarding the non-crystalline state and the transformation from the non-crystalline to the crystalline state is reported The particular alloys studied represent binary (Ge17Te83), ternary (Ge15Te80As5) and quaternary (Ge15Te81Sb2S2) compositions Structural information is obtained using X-ray diffraction techniques and density measurements Thermal data are reported from differential scanning calorimetry (DSC), thermogravimetry (TGA) and mass spectrometry results The electrical conductivity is measured as a function of temperature and, on the ‘as-prepared’ glasses, shows semi-conducting behavior with activation energies, E, of 043–048 eV DSC, TGA and X-ray powder diffraction patterns indicate the samples crystallize as Te and GeTe in a two-step process, and melt at the binary eutectic temperature The binary vaporizes as Te and GeTe in a two-step process GeTeAs and GeTeSbS vaporize by essentially the same mechanism, with As evaporating ( ⋍125−133° C ; crystallization temperature ⋍206−228° C as determined by DSC) DSC, TGA and mass spectrometry results have been correlated to electrical and structural changes Results show that crystalline Te nucleates at the surface and forms a conductive surface layer The conductivity of this surface layer is nearly temperature independent with E ≈ 10−2 eV for all three alloys Crystallization and the associated electrically conductive regions extend into the bulk material with further annealing In these disordered alloys the additives As and Sb + S apparently do not act as electrical dopants in the sense of affecting the conductivity activation energy The additives Sb + S however do retard crystallization of GeTe The secondary crystallization product, GeTe, apparently changes the conduction mechanism to either a metallic or degenerate semiconductor type behavior