Showing papers on "Differential scanning calorimetry published in 1984"

Curing reaction of epoxy resins with diamines

Abstract: The curing reaction of a commercial bisphenol A diglycidyl ether (BADGE) with ethylenediamine (EDA) was studied by differential scanning calorimetry. Different kinetic expressions were found with isothermal (low temperature range) and dynamic (high temperature range) runs. Two competitive mechanisms are shown to be present: an autocatalytic one (activation energy E = 14 kcal/mol) and a noncatalytic path characterized by a second-order reaction with E = 24.5 kcal/mol. At low temperatures both mechanisms took place simultaneously, showing a significant decrease in the reaction rate after the gel point. At high temperatures only the noncatalytic reaction was present, without showing a noticeable rate decrease in the rubber region. Also, a third-order dependence of the glass transition temperature on reaction extent is shown.

Synthesis and characterization of sulfonated poly(acrylene ether sulfones)

Abstract: Ionic groups incorporated into a polymer have a decided effect on its physical properties. A number of ionomers and polyelectrolytes have been widely applied. In particular, sulfonated bisphenol-A polysulfone (SPSF) has been used as a composite or single-component membrane for the desalination of water. In this article, the synthesis and physical characteristics of sulfonated polysulfone are addressed. A detailed synthesis route is provided and methods that yield determinable levels of sulfonation are described. These ion-containing polymers retain an excessive amount of residual salts, which, of course, are impurities to the system. Therefore, before any analyses were made the polymers were subjected to a thorough soxhlet extraction process with boiling water, which appeared to be quite effective. The degree of sulfonation was assessed by several methods such as 1H NMR and FT-IR. A new 1H NMR method was derived because the method cited in the literature proved to be too inconsistent for our work. The new 1H NMR method used a quaternary ammonium counterion [N(CH3)4]. These methyl protons are easily measured and may be ratioed against the isopropylidene protons in the polymer backbone that act as an internal standard. Characterization of the physical properties of SPSF consisted of water uptake, differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and solubility studies. Its physical appearance and mechanical behavior were improved by the solution procedure. Also addressed were the effects of different counterions (Na+ & Mg++) with SPSFs of low levels of sulfonation. The variation in physical properties between the divalent and monovalent counterions is dramatic, especially when observed by TMA in the rubber plateau above the apparent glass temperature.

The thermodynamic properties of polytetrafluoroethylene

Abstract: Polytetrafluoroethylenes of different crystallinity were analyzed between 220 and 700 K by differential scanning calorimetry. A new computer coupling of the standard DSC is described. The measured heat capacity data were combined with all literature data into a recommended set of thermodynamic properties for the crystalline polymer and a preliminary set for the amorphous polymer (heat capacity, enthalpy, entropy, and Gibbs energy; range 0–700 K). The crystal heat capacities have been linked to the vibrational spectrum with a θ3 of 54 K, and θ1 of 250 K, and a full set of group vibrations. Cv to Cp conversion was possible with a Nernst–Lindemann constant of A = 1.6 × 10−3 mol K/J. The glass transition was identified as a broad transition between 160 and 240 K with a ΔCp of 9.4 J/K mol. The room-temperature transitions at 292 and 303 K have a combined heat of transition of 850 J/mol and an entropy of transition of 2.90 J/K mol. The equilibrium melting temperature is 605 K with transition enthalpy and entropy of 4.10 kj/mol and 6.78 J/K mol, respectively. The high-temperature crystal from is shown to be a condis crystal (conformationally disordered), and for the samples discussed, the crystallinity model holds.

Thermodynamic properties of undercooled liquid metals

Abstract: In the form of a droplet emulsion, a number of liquid metals and alloys have been undercooled by substantial amounts (0.3–0.4 T m ) before crystallization. For example, pure Hg, In, Sn, and Bi can be undercooled by 90°C, 110°C, 187°C and 227°C respectively. Differential scanning calorimetry has been applied to determine the heat capacity difference, ∪Cp, between the undercooled liquid and crystalline solid. With decreasing temperature an increasing value of ∪Cp is observed which implies a reduction in configurational entropy of the liquid. In addition the heat capacity measurements allow for the determination of the free energy and the onset of the hypercooled regime which are useful in the evaluation of the crystallization kinetics.

Crystallization in fluoride glasses: I. Devitrification on reheating

Abstract: The devitrification of glass from the BaF 2 LaF 3 ZrF 3 AlF 3 quaternary system is studied as a function of sample size and heating rate by differential scanning calorimetry. Results are analyzed in terms of a theory of non-isothermal crystallization kinetics similar to that of Matusita and Sakka but modified to allow for diffusion controlled growth of the crystallites.

Formation and growth of amorphous phases by solid‐state reaction in elemental composites prepared by cold working

Abstract: Amorphous alloys of Ni-Zr and Cu-Zr have been synthesized by solid-state reaction of composite metal mixtures produced by mechanical deformation of metal powder mixtures and intercalated foil layers. The materials obtained were investigated by means of x-ray diffraction and differential scanning calorimetry. The results are compared with known data for thin films and rapidly quenched alloys.

Phase transitions in crystalline models of bilayers. 1. Differential scanning calorimetric and x-ray studies of (C12H25NH3)2MCl4 and (NH3C14H29NH3)2MCl4 salts (M = Mn2+, Cd2+, Cu2+)

Abstract: Mise en evidence de deux transitions de phases du premier ordre. La temperature et l'ordre de la transition dependent de la longueur de la chaine organique et de la nature de l'ion metallique. Observation d'hysteresis thermique

Study of the enthalpies of formation and crystallization in the system Zr‐Ni

Abstract: The enthalpies of formation were measured in the intermetallic compounds ZrNi5, Zr2Ni7, Zr7Ni10, ZrNi, and Zr2Ni, and the amorphous Zr1−xNix alloys with x=0.25, 0.35, and 0.65 by means of a solution calorimeter based on liquid aluminium. For the amorphous system Zr1−xNix, the concentration dependence of the crystallization enthalpy was studied by means of differential scanning calorimetry on 12 different compositions. The concentration dependence of the formation enthalpy of the crystalline materials was analyzed in terms of Miedema’s model. The enthalpy effects observed in some of the amorphous alloys point to the occurrence of compositional short‐range ordering.

Synthesis of polyterephthalates derived from dianhydrohexitols

Abstract: By melting condensation of terephthalic acid dichloride and 1,4∶3,6-dianhydrohexitols as well as selectively prepared lower oligomers thereof the formation of polyterephthalates is achieved These polyesters are characterized by 1H-NMR spectra vapour pressure osmometry (VPO), differential scanning calorimetry (DSC), viscosity measurements and combustion analyses Their preparation and properties are discussed

Modulation of membrane fusion by ionotropic and thermotropic phase transitions

Abstract: We have studied the relationship of ionotropic and thermotropic phase transitions to divalent cation induced fusion of large unilamellar phospholipid vesicles. Fusion was monitored by the Tb/dipicolinic acid fluorescence assay for the intermixing of internal aqueous contents of vesicles. The phase behavior of the membranes was followed by differential scanning calorimetry. (1) Sr2+ and Ba2+ shifted the phase transition temperature (Tc) of bovine brain phosphatidylserine vesicles from 6 to 27 and 31.5 degrees C, respectively. These cations induced vesicle fusion at temperatures above or below the Tc of that cation/phospholipid complex, indicating that an isothermal phase change from the liquid-crystalline to the gel phase is not a requirement for membrane fusion. (2) The temperature dependence of the initial rate of fusion of phosphatidylserine/dipalmitoylphosphatidylcholine (1:1) vesicles in the presence of Ca2+ exhibited a pronounced maximum at 17 degrees C, at the lower part of the broad phase transition endotherm whose Tc was about 25 degrees C; fusion was inhibited completely at 30 degrees C when the membrane was in the liquid-crystalline state. These observations suggest that molecular clusters rich in phosphatidylserine, formed when the membrane is in the phase transition region, allow the vesicles to fuse. (3) The fusion of phosphatidylserine/dimyristoylphosphatidylethanolamine (1:1) vesicles, whose Tc was also around 25 degrees C, had a different temperature dependence in that the initial rate increased sharply above the Tc, with a local maximum within the transition region. Phase separation of dimyristoylphosphatidylethanolamine was induced by Ca2+ but not by Mg2+, although both ions induced fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Compatibility of poly(ε-caprolactone) (PCL) and poly(styrene-Co-acrylonitrile) (SAN) blends. II: The influence of the AN content in SAN copolymer upon blend compatibility

Abstract: The compatibility of polymer blends of poly(ϵ-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) containing various acrylonitrile (AN) contents was studied to evaluate the influences of copolymer composition and PCL concentration upon blend compatibility. Blend compatibility was characterized by the occurence of a single glass transition intermediate between the transitions of the respective pure components. The glass transitions were determined by differential scanning calorimetry (DSC) and dynamic mechanical measurement (Rheovibron). It was found that SAN and PCL form compatible blends when the AN content of SAN ranges from 8% to 28% by weight. These blends are compatible in all proportions except for SAN 28 (AN wt % = 28) and PCL blends containing 70 or 85 wt % PCL. Blends of PCL and SAN were found to be incompatible when the AN content in SAN is greater than 30 wt % or less than 6 wt %. Lower critical solution temperature (LCST) behavior, which can be attributed to phase separation, was found to occur when these blends were heated to elevated temperatures. The cloud point, or phase separation, was found to vary with AN content in SAN and the concentration of SAN in the blend.

Preparation and properties of castor oil‐based polyurethanes

Abstract: Four series of polyurethane elastomers were synthesized using diphenylmethane diisocyanate (MDI) and castor oil as the polyol. Two types of MDI (crude and distilled) were used. The temperature of reaction and the effect of excess isocyanate above stoichiometric were also varied. The structure of the polyurethanes was analyzed using wide-angle X-ray diffraction, scanning electron microscopy, and density and differential scanning calorimetry (DSC). Thermomechanical and dynamic mechanical (TMA and DMA) as well as mechanical measurements were carried out. The results have shown considerable influence of the type of MDI, excess of isocyanate, and temperature of preparation on the properties of the polyurethanes synthesized.

Thermal stability of polyethylene terephthalate

Abstract: The kinetics of thermal and thermal-oxidative degradation of polyethylene terephthalate (PET) have been investigated as a function of melt temperature, melt residence time, melt environment, and drying environment. Rates of thermal and thermal-oxidative degradation were measured in terms of: weight loss of volatile degradation products, decreasing inherent viscosity, and increasing carboxyl end group concentration. Thermal-oxidative degradation was also investigated by Differential Scanning Calorimetry. Calorimetric results show that thermal-oxidative degradation of PET is an exothermic reaction, with an apparent activation energy of 117 kJ/mol. Melt temperature, melt residence time, melt environment, and drying environment have all been found to affect the degradation of PET. Analysis of inherent-viscosity kinetic data has been carried out, utilizing existing theories based on a random chain scission mechanism. Activation energies of 117 and 159 kJ/mol have been calculated for air-dried and vacuum dried samples respectively.

Differential Scanning Calorimetry

Abstract: The purpose of this paper is to review briefly the technique of differential scanning calorimetry (1), and to indicate how it may be used to determine enthalpies of phase change and of certain chemical reactions.

Thermal Stability of Human Albumin Measured by Differential Scanning Calorimetry

Abstract: . The thermal stability of 5% previously unheated, undefatted human albumin monomer in 145 mM Na+, pH7.0 was investigated by differential scanning calorimetry (DSC) as a function of added caprylate and/or N-acetyl-DL-tryptophanate. Caprylate was substantially more effective than N-acetyl-DL-tryptophanate in protecting the protein against thermal denaturation at a given level or at a saturating level of stabilizer. The tracing of the differential heat capacity versus temperature (thermogram) for this undefatted monomer that contained 1.5 mol endogenous, long-chain fatty acid (LCFA)/mol monomer exhibits two denaturation peaks (endotherms) in the absence of stabilizer. The endotherm with the lower denaturation temperature (Td) comprises 70% of the total heat of denaturation and also corresponds to irreversible denaturation and precipitation of 70% of the albumin. This endotherm is associated with more thermally labile protein species containing low levels of LCFA. The endotherm with the higher Td is associated with more stable protein species containing high levels of LCFA. Thus, the two endotherms are not related to the proposed domain structure of the protein but result from an uneven LCFA distribution that is due to preexisting heterogeneity in the albumin and/or heterogeneity that arises during the DSC experiment. Binding data do not support a preexisting uneven distribution of sufficient magnitude to explain the experimental results. A complementary explanation is that an uneven fatty acid distribution arises during the DSC experiment by migration of LCFA from the more labile species to the more stable as the former unfold; such migration would cause further stabilization of the latter.

Glass formation and crystallization in the GeSe2-Sb2Te3 system

Abstract: The glass formation and crystallization of liquid-quenched (GeSe2)100-y /(Sb2Te3) y alloys was investigated by means of differential scanning calorimetry, X-ray diffraction and optical and scanning electron microscopy. By water quenching glasses are obtained from compositions in the range 5≲y≲30. Qualitative parametrization of glass-forming tendency gives, as best glass formers, alloys with y≅20. Crystallization on heating proceeds in one stage for glasses withy≲20 and in two stages for those with greater Sb2Te3 content. For compositions lying in the GeSe2 primary crystallization region crystals appear preferentially at the surface of the sample, but for the other compositions (24≲y≲30) the crystals emerge in the bulk and often develop in spherulitic or axialitic form.

Thermochemistry and its applications to chemical and biochemical systems : the thermochemistry of molecules, ionic species, and free radicals in relation to the understanding of chemical and biochemical systems

Abstract: Techniques and Theory of Titration Calorimetry.- Applications of Titration Calorimetry.- Microcalorimetry.- Differential Scanning Calorimetry.- An Oxygen Flow Calorimeter for Determining the Heating Value of Kilogram-Size Samples of Municipal Solid Waste.- Thermogenesis by Heat Conduction Calorimetry.- The Measurement of Vapour Pressure.- The Use of a Simultaneous Torsion and Mass-Loss Effusion Apparatus for Determining Vapour Pressures And Enthalpies Of Sublimation.- Vaporization Studies by Knudsen Cell-Mass Spectrometry and Thermodynamic Stability of High Temperature Gaseous Species.- Transpiration Mass Spectrometry - A New Thermochemical Tool.- Thermodynamic Activity and Vapor Pressure Models for Silicate Systems Including Coal Slags.- Metal-Ligand Heats and Related Thermodynamic Quantities by Titration Calorimetry.- Measurement of Complexing Constants and Enthalpies of Complexing by Batch Calorimetry.- Metal Ligand Bond Strengths and Bonding Theory.- Thermochemistry of ?-diketones and metal-?-diketonates. Metal-oxygen bond enthalpies.- Thermochemistry of Metal-Polyamine Complexes.- Thermochemistry of Main-Group Organometallic Compounds.- Experimental Thermochemistry of Transition-Metal Organometallic Compounds.- Thermochemistry of Molecular Transition Metal Compounds.- Measurement of Enthalpies of Solution of Electrolytes.- Hydrophobic Hydr ation of Alkylammonium Bromides in Aqueous Mixed Solvents.- Solvation Of Alcohols, Amines, Ureas and Amides in Mixed Solvents.- Microcalorimetry as an Analytical Tool In Biology.- Application of Calorimetry to the Life Science.- Superhelicity and Energetics Of Structural Transitions Of Nucleic Acids.- Calorimetric Studies in Binding Reactions In BiochemistrY.- Cellular Thermochemistry: Resting State and Stimulation.- Differential Scanning Calorimetry Applications to Protein Solutions.- Thermochemistry of molecules, molecule-ions, and free radicals. Factors influencing bond energies in these species.- Experimental Methods to Study the Thermochemistry of Gas Phase Ions and Ion-Molecule Reactions.- The Thermochemistry of Gas Phase Ion-Molecule Reactions.- The Application of Low-Temperature Calorimetry to Some Contemporary Problems In Solid State Chemistry.- Thermodynamics of Crystals - 1982.- Calorimetry of Phase, Schottky and Other Transitions.- Thermophysics of Mineral And Rock Systems.- Computerized Adiabatic Thermophysical Calorimetry.- Lectures on Thermochemistry and Kinetics.- Barriers to Internal Rotation in Inorganic Species.- Critical Evaluation of Thermodynamic Data - A Research Activity.- Thermochemistry Today and Its Role in the Immediate Future.- List of Participants.

Differential scanning calorimetry studies of irradiated polyethylene: I. Melting temperatures and fusion endotherms

Abstract: The effect of ionizing radiation on the melting behavior of high-density and low-density polyethylene was examined with data obtained by differential scanning calorimetry. Melting temperatures and fusion endotherms were obtained after absorbed doses of gamma radiation up to 3 MGy. The changes in melting temperatures and heat of fusion for the first and second meltings are related to the radiation chemistry of polyethylene. The changes in the first melting temperature are caused by radiation-induced links that decrease the melt entropy and increase the flod surface free-energy-per-unit area of chain-folded polyethylene crystals. These effects are dependent on the lamellar thickness distribution and on the types of links formed (intramolecular or intermolecular). The changes in melting temperature and heat of fusion obtained in the second melting are related to the inhibition of crystallization caused by the presence of radiation-induced links.

The effect of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water

Abstract: The influence of crystallinity and crosslinking on the depression of the glass transition temperature in nylon 6 by water has been investigated by dynamic mechanical methods. Radiation crosslinking by high-energy electrons was effective in preventing morphological changes during the measurement of the incremental change in heat capacity (ΔCp) at Tg, which was performed by differential scanning calorimetry. The experimentally determined value of ΔCp, when normalized to account for the crystalline phase, was found to deviate from a linear two-phase relation and was reduced further than would be expected based on this model. It is proposed that nylon 6 is best described by a three-phase model which consists of a crystalline domain, a wholly amorphous domain, and an “intercrystalline” region. The importance of this in explaining the relatively large depression of Tg by small quantities of water is illustrated by applying equations derived to account for the compositional dependence of Tg in polymerdiluent mixtures, based on a classical thermodynamic interpretation of the glass transition phenomenon.