Selected X-ray diffraction and differential scanning calorimetry investigations of the binary system K-palmitate/glycerol

Three mole fractions of the binary system K-palmitate/glycerol (KC₁₆/Gl) x_KC16 = 0.30, 0.37 and 0.50 have been investigated as a function of temperature by small- and wide-angle X-ray diffraction investigations and differential scanning calorimetry (DSC) measurements. The existence of a gel-like region, named G₁ in the preliminary binary phase diagram [4], could not be confirmed. Consequently, a corrected version of the phase diagram of the KC₁₆/Gl system is established. According to this corrected phase diagram at low K-palmitate concentrations, x_KC16 < 0.25, the transitions crystalline phase (C) ⇆ hexagonal phase, chains fluid (H_α) ⇆ isotropic, micellar phase (S) occur with rising temperature. At x_KC16 > 0.25 the transitions C ⇆ gel phase (G) ⇆ lamellar phase, chains fluid (L_α) were observed. X-ray diffraction and DSC measurements provided concordant results. Only differences in the phase transition temperatures from DSC curves obtained at rising and falling temperatures were observed. The phase transitions C ⇆ G, G ⇆ L_α and G ⇆ H_α correlate with a sharp shift in the d value of the first small-angle reflection. The occurrence of the G phase is accompanied by a distinct split of the first small-angle reflections. Simultaneously, the wide-angle reflections change and the peak intensity is reduced. Received: 14 April 1999 Accepted: 28 June 1999     

A low-temperature X-ray diffraction study of the Cu2ZnSnSe4 thin films

Low-temperature XRD measurements were performed to confirm the phase composition and structural parameters of the electrochemically deposited Cu₂ZnSnSe₄ thin films on flexible metal substrates.     

On the spline approximation of low-temperature calorimetry data

An algorithm for the spline approximation of heat capacity taking into account the Debye or Tarasov asymptotes in extrapolation from the first experimental temperature to 0 K was developed. The possibility of applying other boundary conditions that do not contradict the modern concepts of the behavior of heat capacity as a function of temperature is considered. A procedure for selecting the smoothing parameter based on a priori specifying the number of inflection points in the smoothing dependence is suggested. Possibilities for estimating the accuracy of approximation with the use of the orthogonal functions are discussed.     

Simultaneous small-angle X-ray scattering and wide-angle X-ray diffraction

The simultaneous SAXS/WAXD technique is shown to provide an unambiguous method for following structural changes taking place during the programmed heating of a range of multiphase polymeric materials. Results are given for polyethylene, block copolyurethanes and block copolyesters containing liquid crystalline hard segments. UK Thermal methods Group Award Lecture     

The role of differential scanning calorimetry and X-ray diffraction in identifying crystal continuity in highly oriented samples of die-drawn polypropylene

X-ray diffraction and differential scanning calorimetry studies of die-drawn polypropylene have indicated the presence of a crystalline structure which does not identify with any of the usual crystallographic forms and has a melting point slightly below that of the monoclinic a form. The melting of this structure is stress sensitive and shows the superheating effect expected from the melting of an extended chain structure, whose chain ends are physically constrained. It is postulated that this structure bridges the gaps between adjacent monoclinic crystal blocks and provides the framework necessary to maintain mechanical stability at temperatures well above that of the glass transition. Moreover, together with taut tie molecules, this structure is considered to be the main source of the high material stiffness.     

The use of a continuous-flow helium cryostat in low-temperature calorimetry

For experimental investigation of the temperature-dependences of specific heat and thermal conductivity in the range 4–300 K a continuous-flow helium cryostat has been developed. Its adaptation for low-temperature calorimetry and its use for measurement of the temperature-dependences of the specific heats of bulk samples of metals and insulators are described in this note. The phase transition from the normal to the superconductive state has been measured on NbTi and its critical temperature determined. Two methods of determination of the temperature-dependences of the specific heats of metals and insulators have been developed. The inaccuracy of specific heat determination did not exceed 2 % with metal materials and 5 % with insulating materials.

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Zusammenfassung Ein Heliumkryostat mit kontinuierlichem Strom wurde für Versuchszwecke zur Untersuchung der Temperaturabhängigkeit der spezifischen Wärme und der Wärmeleitfähigkeit im Bereich von 4 bis 300 K entwickelt. Sein Einsatz als Tieftemperaturkalorimeter sowie seine Anwendung zur Messung der Temperaturabhängigkeit der spezifischen Wärme einer Anzahl von Metall- und Isoliermaterialproben werden beschrieben. Der Phasenübergang vom normalen in den Supraleitungszustand wurde an NbTi gemessen und seine kritische Temperatur bestimmt. Zwei Methoden wurden zur Bestimmung der Temperaturabhängigkeit der spezifischen Wärme von Metallen und Isolatoren entwickelt. Die Ungenauigkeit der Bestimmungen der spezifischen Wärme war unterhalb von 2 % bei metallischen und unterhalb von 5 % bei isolierenden Materialien.

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Résumé Un cryostat à circulation continue d'hélium, destiné à la détermination expérimentale de la chaleur spécifique et de la conductivité thermique en fonction de la température entre 4 et 300 K, a été mis au point. Son utilisation comme calorimètre à basses températures est décrite ainsi que son application à la mesure de la chaleur spécifique en fonction de la température d'échantillons massifs de métaux et isolants. La transition de phase de l'état normal à l'état supraconducteur a été étudiée sur un échantillon de NbTi et la température critique a été déterminée. Deux méthodes ont été mises au point pour déterminer la variation de la chaleur spécifique en fonction de la température des métaux et isolants. L'imprécision de la détermination de la chaleur spécifique est inférieure à 2 % pour les métaux et 5 % pour les isolants.

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4–300 . ë . NbTi . . 2%, 5%.

     

Determination of ice content in hardened concrete by low-temperature calorimetry

Low-temperature calorimetry has been used to determine the ice content in concrete at different temperatures when exposed to low-temperature environments. However, the analysis of the ice content from the measured data of heat flow is not straightforward. In this study, two important factors influencing the ice content calculation are discussed. The importance of the baseline determination for the calculation of the ice content is realized. Two different methods of generating the baseline are discussed. First, the ‘J-baseline’ is discussed which is a recently proposed extrapolation method based on the accumulated heat curves measured in the freezing and the melting process. Second, the ‘C-baseline’ is discussed in which a calculated baseline is used where the heat capacity of both water and ice and the phase changing behaviour under different testing temperatures are considered. It turns out that both the ‘J-baseline’ method and the ‘C-baseline’ method can be used to calculate the approximate baseline. The heat of fusion of the water confined in small pores is another important parameter to be considered in ice content calculation. This property must be carefully analyzed in order to accurately calculate the ice contents at different temperatures in the freezing and melting process. It should be noted that there is no general agreement on how to obtain the important temperature dependence of the heat of fusion of water confined in small pores. By performing comparison studies, the present study shows the influence of the different values of the heat of fusion commonly adopted on the calculated ice content for the studied concrete samples. The importance and necessity to use an accurate value of the heat of fusion is emphasized. Based on the calculation of the baseline proposed in this work and by carefully selecting the values for the heat of fusion, the ice content in a hardened concrete sample is expected to be estimated with an acceptable accuracy.     

X-ray diffraction relaxation of polyethylene

The change in x-ray diffraction intensity with time following rapid stretching by various amounts is measured for a low-density polyethylene sample at several temperatures. An appreciable decrease in intensity with time at the meridian of the diffraction from the 110 and 200 planes is observed to occur within the first 5 sec following stretching. The change takes considerably longer times at lower temperatures. By use of a calibration curve, the change in crystal orientation functions for the a, b, and c crystal axes were calculated, and an increase in c-axis orientation was shown. From the results of birerringence measurements on the same sample, the crystalline and amorphous contributions to birefringence were calculated. It was shown that stretching is initially accompanied by a greater amount of amorphous than crystalline orientation, and that relaxation involves an increase in crystalline orientation accompanied by a decrease in amorphous orientation.     

Polymorphism in alternating polyketones studied by x-ray diffraction and calorimetry

Differential scanning calorimetry and high temperature x-ray diffraction were used to study the perfectly alternating copolymer of ethene and carbon monoxide (polyketone; POKC2). It was found that oriented POK-C2 fibers show a crystalline phase transition at a temperature between 110–125°C with a 10% change in crystalline density. At this temperature, the crystal structure reported recently (POK-α) is transformed to a crystal structure that was reported in the past for room temperature imperfectly alternating polyketone. The latter structure will be designated as POK-β. The influence of chain defects on the crystal structure was studied by synthesizing terpolymers (POK-C2/C3), in which small amounts of propylene-CO units are incorporated into the polymer backbone. The resulting terpolymers differ from the copolymer by the presence of methyl groups randomly distributed along the polyketone backbone chain. Evidence is presented that indicates that the methyl groups are built into the crystal lattice as defects. With more than 5 mole-% propene the terpolymer fibers crystallize exclusively in the β-modification. Below this level the α/β ratio (at room temperature) increases with decreasing amounts of propene. Both as-synthesized and as-spun POK-C2 were found to consist of both POK-α and POK-β; the α/β ratio depends on the method of preparation. Because the drawn POK-C2 fibers studied here consist exclusively of POK-α, the process of spinning and drawing leads to the transformation of unoriented β-rich material into oriented POK-α. © 1995 John Wiley & Sons, Inc.     

Selected X-ray diffraction and differential scanning calorimetry measurements on nonaqueous liquid crystals and gel phases in the K-myristinate/glycerol binary system

Samples from the three selected concentrations x _KC14 = 0.25,0.37, and 0.50 of the K-myristinate/glycerol (KC₁₄/Gl) binary system have been investigated by means of small- and wide-angle X-ray diffraction measurements as well as differential scanning calorimetry (DSC) measurements as a function of temperature. The results are – The G₁ gel phase, contained in the preliminary phase diagram according to Dörfler and Senst [(1993) Colloid Polym Sci 271: 173], is nonexistent. – The same applies of the isotropic phase in the preliminary phase diagram according to Dörfler and Senst. Initially an isotropic or a cubic phase was assumed based on polarized microscopy texture observations. X-ray diffraction and DSC measurements provided no indication for their existence. – Due to the nonexistence of the G₁ and isotropic phases the preliminary phase diagram of the KC₁₄/Gl binary system had to be corrected. The region of the lamellar phase extends over a wider region. – X-ray diffraction and DSC measurements provided concordant results, which were further confirmed by electron microscopic investigation. Differences in phase-transition points from DSC data obtained for rising and falling temperatures have been observed. – The crystalline-to-gel phase transition correlates with a sharp shift in the d value of the first small-angle reflex. – The gel phase is accompanied by a distinct splitting of the first small-angle reflexes. The wide-angle reflexes show rearrangement and reduced intensity. – Similar to the crystalline-to-gel phase transition, the gel-to-lamellar phase transition is accompanied by a sharp change in the d values.