Showing papers on "Differential scanning calorimetry published in 1972"

Melting behavior of isotactic polystyrene

Abstract: The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature Tc. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with Tc, yielding an extrapolated value for the equilibrium melting temperature Tc° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of Tc° found by extrapolation. For normal values of Tc and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of Tc, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.

Thermally induced phase separation of polystyrene‐poly(vinyl methyl ether) mixtures

Abstract: Past differential scanning calorimetry and dielectric relaxation measurements have established that polystyrene (PS)-poly(vinyl methyl ether) (PVME) mixtures exhibit a degree of compatibility when cast from toluene, whereas they are incompatible when cast from chloroform or trichloroethylene. The present study reports that toluene-cast mixtures can be phase-separated by thermal treatment at temperatures exceeding 125°C. This is true for samples containing 20–80 wt-% PS. The temperature of phase separation varies with heating rate; isothermal heating times needed to cause phase separation increase rapidly as the temperature approaches 125°C. Reversibility of the phase separation process depends upon such factors as cooling rate, annealing time, treatment temperature, and thermal history. By annealing and/or slow cooling, all thermally phase-separated mixtures have been brought back to their original state of compatibility. That is, there is no evidence for true irreversiblity of phase separation in thermally treated samples. Quench-cooled samples remain phase-separated indefinitely at room temperature, but this is attributed to rapid cooling below the glass transition of the PS. Chloroform-cast and trichloroethylene-cast mixtures have not been brought to a compatible state by thermal treatment, even after lengthy annealing and slow cooling steps.

Compositional dependence of the thermal, structural, and electrical properties of AsTeI glasses

Abstract: Thermal, structural, and electrical properties of semiconducting AsTeI (and AsTe) glasses have been examined as a function of concentration. Analytical techniques have been developed for quantitative chemical analysis of all three components. Differential scanning calorimetry data indicate a broad endothermic reaction, Tmin ∼ 145 °C, above the glass transition (Tg = 120 ± 8 °C) for iodine compositions of 0 to ∼ 20 at%. Above this reaction temperature the thermal data are composition dependent. For glasses with I ⪆ 35 at %, Tg is much lower (65–70 °C) and the endothermic reaction is much sharper with a minimum at ∼ 133 °C. The wide variation in thermal properties with composition suggests that electrical effects associated with high temperatures (e.g. switching and memory phenomena) may also be composition dependent, as well as being dependent upon kinetics. Structural studies show that phase segregation above Tg is dependent upon kinetics as well as upon temperature. Thermal, structural, and electrical data give evidence that As2Te3 exists as a unit in the non-crystalline state. This structural unit, stable at high temperatures, is present in the molten material and is thought to be present as the supercooled liquid is quenched. The short-range order extends to long-range order upon devitrification and the first crystalline phase detected is monoclinic As2Te3. Apparently metastable under the conditions of formation, this phase converts to a previously unreported fcc phase upon further heat treatment. Similar crystalline structures are known to be associated with thermally- and electrically-induced memory phenomena in AsTeGe and AsTeI glasses. Density measurements at room temperature show that (1) there are no phase miscibility gaps in the glass substructure or different crystalline phases segregating as the I content is varied, and (2) there is a large change in molar volume with increasing I concentration, whereas the molar weight does not change significantly. Electrical conductivity, σ, data in the region around room temperature, show that the σ0 values, determined from σ = σ0e−E/kt, range from 102 to 103 (Ω-cm)−1. A possible dependence of σ0 on I content may be due to changes in molar volume. The more homogeneous glasses appear to show breakpoints in the σ versus 1/T data; the corresponding changes in E are only 0.02–0.06 eV, with E = 0.04 eV being most frequently observed. For As50Te50−xIx glasses, σ at a given temperature increases as the iodine concentration is increased to about 5 at % and then decreases with further increase in I content. Correspondingly there is a minimum in the E versus I concentration data at I ∼ 5 at %. Results suggest that dependence of the σ breakpoints on glass homogeneity and the variation of E with I concentration may be due to trapping effects.

Identification of Elastomers by Thermal Analysis

Abstract: Degradation of polymers by thermal and thermooxidative reactions is associated with either evolution or absorption of heat which can be followed by differential scanning calorimetry. In the present investigation, DSC thermographs of elastomers between 0° C and 500° C in nitrogen (thermal degradation) and oxygen (thermooxidative degradation) were used for identification of the elastomers from their degradation patterns. Based on the nature of the thermographs of the raw polymers in nitrogen, the elastomers studied may be divided into the following three main groups: (a) those which show an exotherm, (b) those which show an endotherm, and (c) those which show multiple peaks of endo- or exotherms. Identification is aided in some cases by the thermographs in oxygen as well as by the physical transitions. The nature (exo- or endo-) and shape of the thermal patterns and the temperature domain at which the reactions take place depend on the molecular structure, configuration, chemical composition, and n...

Characterization of lubricants for polyvinyl chloride

Abstract: The proper use of lubricants and other additives in the compounding of PVC is hampered by lack of knowledge of their interactions with the polymer. Differential scanning calorimetry has been applied to measure the effect of ingredients on the glass transition temperature of PVC. An interaction parameter, based on changes in the glass transition temperature, has been proposed which quantitatively defines the degree of compatibility and which relates to the more empirical definition of lubricants as internal or external. This study of the compatibility of compounding ingredients as a function of molecular weight and chemical structure provides fundamental information of value in formulating, compounding and processing of PVC.

Precision differential calorimetry and the heat of fusion of polyethylene

Abstract: A procedure is described for the analysis by computer of the output from a differential scanning calorimeter. Enthalpy calibration with a variety of materials confirms that alumina is excellent for this purpose. Both the crystallinity, xT, and the heat of fusion of polyethylene, ΔHT, vary with temperature and it is important that both are measured. The use of a room-temperature crystallinity with a high-temperature heat of fusion can be very misleading. Interfacial effects are shown to be negligible in most cases and the heat of fusion of perfectly crystalline polyethylene is given by ΔHT/xT. It is 309 Jg−1 at the melting point.

Thermal transitions in gelatin: optical rotation and enthalpy changes.

Abstract: Enthalpy changes [as determined by differential scanning calorimetry (DSC)] and optical rotation changes over the helix coil transition were measured for various gelatin solutions and films. From these studies it has been concluded that: (1) a linear correlation between ΔH and Δ[α] exists for gelatin solutions, independent of the temperature at which gelation occurred; (2) the amount of triple helical structure regained when a melted gelatin solution is quenched can be calculated from DSC data, but the values obtained will be dependent on assumptions about the number and strength of hydrogen bonds; (3) the anomalously high levorotation values found for cold-dried films of gelatin do not reflect the presence of an extraordinarily large amount of triple helical structure; rather, the large rotations appear to be the result of orientation of helices in the plane of the film.

Determination of cure of diallyl phthalate using differential scanning calorimetry

Abstract: Kinetic parameters (reaction rate constant, activation energy, and frequency factor) were determined by differential scanning calorimetry using a linearly rising temperature program for the polymerization of diallyl phthalate monomer catalyzed with dicumyl peroxide. Cure times for specific degrees of cure at constant temperature were calculated and found to agree with the degree of cure measured by infrared spectroscopy. The relationship between non-isothermal and isothermal cure cycles is discussed.

DSC study of the kinetics of the thermal dehydration of BaCl2 · 2H2O and BaCl2 · H2O

Abstract: The kinetics of the thermal dehydration of various kinds of BaCl2 · 2H2O and of BaCl2 · H2O are investigated using a differential scanning calorimeter. The loss of H2O proceeds in two steps: BaCl2 · 2H2O→BaCl2 · H2O→BaCl2 and is therefore revealed by two endothermic peaks.

Thermal Analysis of Calcium Sulfate Dihydrate and Supposed α and β Forms of Calcium Sulfate Hemihydrate from 25 to 500 ° C.

Abstract: Thermal studies were carried out on CaSO4 · 2H2O and the supposed α and β forms of CaSO4 · 1/2H2O in the region of 25 to 500 ° C. using differential thermal analysis, thermogravimetry, and differential scanning calorimetry methods. Two large endothermic effects and a smaller exothermic effect were found in the differential thermogram of CaSO4 · 2H2O while a single endothermic as well as an exothermic effect were found in the thermograms of the supposed α and β forms of CaSO4 · 1/2H2O. However, the exotherm of α-CaSO4 · 1/2H2O had lower peak and first-break temperatures than that of the β-form. The peak and first-break temperatures of the endothermie effects of CaSO4 · 2H2O) and α and B-CaSO4 · 1/2H2O and the exothermic effect of α-CaSO)4 · 1/2H2O were pressure dependent, shifting to lower temperatures as the gaseous pressure within the DTA cell was decreased. The noted differences in the thennogrms possibly are due to kinetic factors because of the differences in crystallinity of the supposed α- and β-CaSO4 · 1/2H2O. No differences in the supposed a and β forms of CaSO4 · 1/2H2O were detected by thermogravimetry and differential scanning calorimetry studies. The formulation of a two-step dissociative process in the complete dehydration of CaSO4 · 2H2O was supported by the results of the thermogravimetry study.

Phase Stability in Heterogeneous Compositions of Butadiene-Acrylonitrile Copolymer

Abstract: Multiple glass transitions in emulsion polymerized butadiene—acrylonitrile copolymer at leass than 36% acrylonitrile by weight were studied using the Differential Scanning Calorimeter. The appearance of two rather than one transition was shown to be related to the drift in composition with degree of monomer conversion, when all monomer was introduced initially. Material responsible for either glass transition in 29% ACN copolymer was concentrated by phase separation in solution. Phase separation in the butadiene—acrylonitrile copolymer system when the width of the distribution of compositions is above a critical value is shown to be predicted by Scott's thermodynamic treatment of multi-component copolymer mixtures. The partitioning of species of different compositions between phases was calculated for a model distribution of polymeric compositions.

Melting behavior of polyethylene crystallized in a pressure capillary viscometer

Abstract: The Melting of various polyethylene structures is compared by using data obtained on the Perkin-Elmer differential scanning calorimeter (DSC). Transparent, high-density samples crystallized under both orientation and pressure in the Instron capillary rheometer are compared with samples crystallized from dilute solution by stirring and with samples crystallized under high pressure. The latter two structures are assumed to contain extended-chain crystallites. By comparison, the melting points and the superheatability of the Instron samples are consistent with the presence of an extended-chain crystal component. The melting of irradiated samples crystallized in the rheometer is also observed to be consistent with this conclusion. In addition, DSC data are compared with the melting points defined with a polarized light microscope equipped with a hot stage.

Thermal Behaviour and Monomerization Kinetics of 9-CN Anthracene and 9-CN, 10-Acetoxy Anthracene Dimers in the Solid State

Abstract: The thermal stability of 9-CN-anthracene dimer (9-CNAD) and 9-CN, 10-Acetoxy anthracene dimer (9-CN, 10-Ac AD) was studied by differential scanning calorimetry and by direct microscopic examination. Both compounds monomerize below the melting point of the corresponding monomer. From thermograms, first order kinetics are found for both. 9-CN, 10-Ac AD monomerizes, even at room temperature, to an amorphous phase which then crystallizes. Thermal data and activation energies are reported.

D. S. C. — Studies on transitions in polyolefins between −100 °C and+300 °C

Abstract: Transitions in polyolefins in the temperature range between −100 °C and +300 °C were studied on both isotropically crystallised materials and materials crystallised under shear, with the aid of differential scanning calorimetry.

Standard enthalpy of formation of crystalline gold(III) oxide

Abstract: The heat capacity and the enthalpy of the quantitative thermal decomposition of crystalline Au2O3 have been measured by differential scanning calorimetry. A value of –13.0±2.4 kJ mol–1 is calculated for ΔfH⊖(Au2O3,s).

Effect of molecular weight on conformation of poly(β-benzyl L-aspartate) in films

Abstract: In order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β-benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 103. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left-handed α-helical form. All samples studied which have molecular weights above 2.3 × 104 are similar in crystallinity and the left-handed α-helices in them crystallize to ω-helices during slow evaporation. In the low molecular weight region, however, the left-handed α-helices reverse to right-handed α-helices during slow evaporation, and the right-handed α-helices, in turn, reverse and crystallize to highly ordered ω-helices upon heat treatment, although there is some simultaneous conversion to the β-form. The transition temperatures of the quick-dried films for conversion from the left-handed α-helix to the ω-helix and from the ω-helix to the β-form increase linearly with increasing molecular weight up to about 2 × 104, but no large molecular weight dependence is observed beyond that region.

Influence of thermal history on the melting behavior of isotactic polystyrene

Abstract: Multiple melting transitions of thermally crystallized isotactic polystyrene were studied. This behavior was attributed to a very rapid reorganization during the melting of the crystalline structure originally formed. From differential scanning calorimetry and optical microscopy it was shown that the rate of reorganization decreases with increasing reorganization temperature TR. This reorganization will only occur if its rate is larger or equal to the scanning rate. A TR value can be defined for each scanning rate and the resulting melting temperature TRM increases when the scanning rate decreases. This increase of TRM is also examined by annealing at different temperatures. A linear relationship between TRM and TR is observed and gives an extrapolated equilibrium melting point of 240°C; moreover the slope is smaller than for isothermal crystallization (TM versus TC). These phenomena are confirmed by optical microscopic observations.

Glass Formation in the Pseudobinary Metallic Selenide Systems MmSen‐Ge1.5As0.5Se3

Abstract: The glass-forming regions were investigated for pseudobinary systems of the composition XMmSen-(1–X)Ge1.5As0.5Se3 (MmSen=PbSe, CdSe, BaSe, ZnSe, or La2Se3). Systems containing PbSe, CdSe, and BaSe contain substantial regions of glass formation, and differential scanning calorimetry, electron microscopy, and water corrosion tests were used to characterize these glasses. The glasses were generally phase-separated, and the available data suggest that for PbSe melts the region of liquid-liquid immiscibility spans almost the entire pseudobinary composition range. A few studies of glass formation were conducted for CdSe-MmSen-Ge1.5As0.5Se3 pseudoternary and MmSen-GeSe2 binary melts.