AC calorimetry at the first order phase transition point

A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C₂₀H₄₂). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.

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Zusammenfassung Es wird ein Modell für die AC-Kalorimetrie (ACC) von Phasenumwandlungen erster Ordnung vorgeschlagen. Das Modell wird mit den Ergebnissen von ACC in Nähe des Schmelzpunktes des n-Paraffins C20H42 verglichen. Die beobachtete Frequenzabhängigkeit von ACC stimmt mit dem Modell überein. Im Phasenumwandlungspunkt kann eine harmonische Komponente der Temperaturmodulation mit einer Frequenz festgestellt werden, die doppelt so hoch wie die Heizfrequenz ist. Es wird gezeigt, daß die harmonische Komponente anhand des vorgeschlagenen Modelles erklärt werden kann.

     

Extra meridional reflections of drawn polyoxymethylene

Extra meridional reflections (00l: l = 3, 5, 13, 15, 21) were observed in drawn polyoxymethylene (POM). The 003, 0015, and 0021 x-ray diffraction intensities increase on annealing, while the 005 and 0013 intensities decrease. It is concluded that crystalline regions are of two kinds: in the first, which generates the 003, 0015, and 0021 reflections, the molecular conformation is slightly distorted from the uniform helix by intermolecular steric hindrance between the CH₂ groups; and in the second, which generates 005 and 0013 reflections, the molecular conformation is distorted in another way. The second form is converted into the first by annealing. The displacements in the c direction of the CH₂ groups and the oxygen atoms from the uniform helix in the first form are about 1.0% and 0.75% of the interval between a CH₂ group and an oxygen atom along helix axis of the POM crystal; thus the correct conformation in the stable state is known. The nature of distortion in the second region is not clarified but several characteristics are offered.     

Entropy production in TMDSC

The non-equilibrium process due to irreversible heat exchanges occurring during a temperature modulated differential scanning calorimetry (TMDSC) experiment is investigated in detail. This enables us to define an experimental frequency dependent complex heat capacity from this calorimetric method. The physical meaning of this dynamic heat capacity is discussed. A relationship is clearly established between the imaginary part of this complex quantity and the net entropy created during the experimental time-scale.     

Application of the light heating dynamic DSC to polymeric materials

Light heating dynamic DSC was used to study the melting transition of polyethylene. The results show that melting and crystallization are different phenomena from each other in terms of the complex heat capacity. Frequency dependence of the complex heat capacity was examined from 0.01 Hz to 0.2 Hz. It is found that at the lowest frequency the phase of the complex heat capacity exceeds /2 radians. Thermodynamic considerations were made for the large phase of the complex heat capacity.     

Spontaneous formation of wurzite-CdS/zinc blende-CdTe heterodimers through a partial anion exchange reaction

Ion exchange of ionic semiconductor nanoparticles (NPs) is a facile method for the synthesis of type-II semiconductor heterostructured NPs with staggered alignment of band edges for photoelectric applications. Through consideration of the crystallographic orientation and strain at the heterointerface, well-designed heterostructures can be constructed through ion exchange reactions. Here we report the selective synthesis of anisotropically phase-segregated cadmium sulfide (CdS)/ cadmium telluride (CdTe) heterodimers via a novel anion exchange reaction of CdS NPs with an organic telluride precursor. The wurtzite-CdS/zinc blende-CdTe heterodimers in this study resulted from spontaneous phase segregation induced by the differences in the crystal structures of the two phases, accompanying a centrosymmetry breaking of the spherical CdS NPs. The CdS/CdTe heterodimers exhibited photoinduced spatial charge separation because of their staggered band-edge alignment.     

A new model for the molecular conformation in polyoxymethylene crystals

Nonintegral indices are found for the extra meridional reflections in the x-ray diffraction of drawn polyoxymethylene. The index of one of the extra reflections is estimated at 00l: l = 2.830 ± 0.003. A new model for the molecular conformation which accounts for the nonintegral indices is proposed. The new model is a helix with defects at constant intervals along the helix axis. The defect is an unwinding of the helix by 20° around the helix axis. This defect is believed to be characteristic of crystals of helical polymers. The interval between the defects is estimated as 18 monomers. The crystal structure with these defects is consistent with precise measurements of the layer line positions.     

Frequency Dependence of the Dynamic Heat Capacity at the Melting Temperature of Polyethylene Crystals

The dynamic heat capacity of polyethylene was measured in the heating process over two decades of the modulating frequency using the light heating modulated temperature DSC. The dynamic heat capacity exhibited clear frequency dependence from 95°C to the end of the melting of the crystals. Frequency dependence of this work was compared with that of the quasi-isothermal measurement. The relaxation time estimated in this work was much shorter than that of the quasi-isothermal measurement. It was found that notable heat exchange between the sample and reference sides occurred between 120 and 135°C. Frequency dependence of the heat exchange was studied.This revised version was published online in November 2005 with corrections to the Cover Date.     

Quasi-isothermal measurement of frequency dependent heat capacity of semicrystalline polyethylene at the melting temperature using light heating modulated temperature DSC

Frequency dependence of the heat capacity of semicrystalline polyethylene was measured using light heating modulated temperature DSC (LMDSC). The quasi-isothermal measurement was carried out in the melting temperature range of the crystal. Two decades of the frequency, from 0.01 to 1 Hz, was covered by the LMDSC instrument constructed in the author's laboratory. It was found that in the melting temperature range polyethylene exhibited Debye relaxation with the relaxation time of 14 s and had excess heat capacity independent of the kinetics. The excess heat capacity and the relaxation strength could be attributed to the disordered crystals generated during rapid cooling from the molten state and/or its surrounding amorphous region instead of the stable crystals reorganized during the quasi-isothermal measurement.     

Solid-state phase transitions of polytetrafluoroethylene crystals studied with an ultra-high-pressure ac calorimeter

The solid-state phase transitions of polytetrafluoroethylene crystals have been studied with an ultra-high-pressure ac calorimeter developed in the authors’ laboratory. The temperature dependence of heat capacity has been measured at various pressures from 0.1 to 580 MPa. It was found that excess heat capacity had long tails around the phase transition temperatures. A phase diagram was drawn and compared with reported one. In the high-temperature phase called phase I, the boundary of different phases was confirmed in this work.