Calorimetric data, phase transitions

Poly (diethyl siloxane) was suggested by Beatty et al. 1651 based on DSC, dielectric, NMR, and X-ray measurements to possess liquid crystalline type order between about 270 and 300 K. The macromolecule shows two large lower temperature first order transitions, one at about 200 K, the other at about 270 K166 ll,7). The transition of the possible mesophase to the isotropic liquid at 300 K is quite small and irre-producible, so that variable, partial crystallinity was proposed 165) [measured heat of transition about 150 J/mole1S8)], Very little can be said about this state which may even consist of residual crystals. It is of interest, however, to further analyze the high temperature crystal phase between 200 and 270 K. It is produced from the, most likely, fully ordered crystal with an estimated heat and entropy of transition of 5.62kJ/mol and 28J/(Kmol), respectively [calculated from calorimetric data 1S6)... 

Abstract. Gas interstitial fullerenes was produced by precipitation of C6o from the solution in 1,2 dichlorobenzene saturated by O2, N2, or Ar. The structure and chemical composition of the fullerenes was characterized by X-ray powder diffraction analysis, FTIR spectroscopy, thermal desorption mass spectrometry, differential scanning calorimetric and chemical analysis. The images of fullerene microcrystals were analyzed by SEM equipped with energy dispersive X-ray spectroscopy (EDS) attachment. Thermal desorption mass spectroscopy and EDS analysis confirmed the presence of Ar, N and O in C60 specimens. From the diffraction data it has been shown that fullerite with face centered cubic lattice was formed as a result of precipitation. The lattice parameter a was found to enhance for precipitated fullerene microcrystals (a = 14.19 -14.25 A) in comparison with that for pure C60 (a = 14.15 A) due to the occupation of octahedral interstices by nitrogen, oxygen or argon molecules. The phase transition temperature and enthalpy of transition for the precipitated fullerene microcrystals decreased in comparison with pure Cgo- Low temperature wet procedure described in the paper opens a new possibility to incorporate chemically active molecules like oxygen to the fullerene microcrystals. 

Their study was made prior to discovery of the five crystalline phases, and their drop-calorimetric data reveal only the transition near 748 K. Reinterpretation of the data above 748 K is complicated by the proximity of the furnace temperatures to the transition temperatures and by the tendency of high-temperature forms to revert to both o- and 0-forms under different conditions of cooling. BJorge and and Jenssen (4) and Rolin et al. (5) also reported crystalline enthalpy data, but these are relatively Imprecise and Include similar uncertainties in the final state after the drop. These difficulties preclude the derivation of accurate heat capacities for the high-temperature forms. 

The Effect of Polymer Heterogeneity on the Enthalpy. The kinetics of the isotropic-smectic phase transition were studied for two of the polymers HPX-C9 and HPX-C11, and in Figure 8 a summary of the calorimetric data for the former is presented. The behaviour of the HPX-C11 polymer was similar. Two processes are in fact revealed by these data (a) at high temperatures (7 471.9 K) the two processes have approximately the same induction time and rate and are therefore not resolved, (b) at intermediate temperatures (468.9heat evolved in the slower process is 30-50% of that evolved in the more rapid process. There is a tendency for an increase in Ah0 for the slow process with increasing temperature for HPX-C9, Ah0 was equal to 13 kJ/kg at 468.9 K and 20kJ/kg at 470.9 K. The same trend was observed for HPX-C11. (c) at low temperatures (TS467.9 K), the slower process was much retarded and not observed within the experimental window. 

Calorimetric data on the various phase transitions are given in Table 7.18. 

The ac heat capacity and vapor pressure isotherm experiments of CO on exfoliated graphite foam [112] allowed the detailed investigation of the narrowing of the commensurate solid-fluid coexistence region as the tricritical point is approached see Fig. 51 for this part of the phase diagram and Fig. 52 for the calorimetric data corresponding to the paths I to VII marked in the phase diagram. The vapor pressure isotherms indicate that the transition... 

The freezing transition from the reentrant fluid to the commensurate solid monolayer seems to be of first order at low temperatures and changes to a continuous transition at a tricritical point near 85 K. This interpretation of the heat capacity and compressibility data [112], however, needs further experimental confirmation, and the corresponding tricritical point shown in the phase diagram Fig. 51 as an open triangle is only tentative. There are also conflicting theoretical predictions concerning the order of the RF - CD transition [77, 78, 151, 260], and the present resolution of the calorimetric data [112] does not allow us to draw a firm conclusion in this respect. 

Figure 12. Temperature-dependence of the fraction of liquid crystalline lipid (() and the heterogeneity parameter ("Aj, proportional to the mean square deviation of the order parameter) for the H NMR spectra of Acholeplasma laidlawii membranes shown in Figure 11 (C-14 0-m-d3 90%). The calorimetric phase transition temperature is indicated by Tc. (Unpublished data of H. C. Jarrell, R. Deslauriers, and...

The dipole interaction model of the phase transitions in the hydro-quinone clathrate compounds was conceived several years ago. It turned out to be tenable in the light of the experimental data that became available since. The clathrate compounds of hydrogen chloride and hydrogen sulphide have been studied mostly by spectroscopic methods so far to show that the small molecules rotate freely or quasi-freely in the cavity. Low temperature calorimetric data are becoming available. The entropic aspect of the rotational degree of freedom including the effect of the nuclear spins can now be studied. 

Unlike the first two scenarios, where the parameters were set by fitting tmly the experimental My i(T) data, for the mixed scenario the fit was made simultaneously for both the My i(T) and My T) datasets. In this way, the relative values of the parameters were determined with relatively high precision (the typical maximum error for any parameter among a, B, C, or G for the displayed fit is about 15%). Actually, since these parameters are not independent, only the relative values (ratios) of these parameters can be determined from the H-NMR data, e.g. G/Gc-Their absolute values are accessed from an additional calorimetric measurement, e.g. a measurement of the latent heat released at the phase transition. 

A confirmation of the above finding also follows from the calorimetric measurements. Eigure 25 compares the Cp(T) responses of the highly doped Xlc = 0.28 LCE and the extracted control sample Xlc = 0.006. While the latter is clearly supercritical, as zero latent heat is released at its phase transition, a mismatch between the ac and relaxation runs is detected for Xlc = 0.28 LCE. The measured latent heat 0.24 0.05 J g is in good agreement with the value 0.30 0.05 J g calculated by (14) from the parameters obtained in the fit of the Mi y T) and M2 v(T) experimental data. 

Differential scanning calorimetry is widely used to obtain phase information. Many phase diagrams are inferred from calorimetric data jointly with some other techniques. Calorimetry has been used extensively for physical studies of polar lipids and provides invaluable information about the numerical values of the thermodynamic variables [9]. A sample cannot be viewed during calorimetric studies, but in combination with observations in hot stage microscopy (with common and/or polarized light) the nature of phase transitions and their temperature can be determined. 

In the first case, the course of the molar heat of the pure substance is measured down to the very low temperature region, generally to 12 K. The value of 5 may then be calculated from the third law of thermodynamics, Debye s relation which extrapolates the interval from the lowest calorimetric data to 0 K, and enthalpic changes of phase transitions. The procedure is very well described by Scott S Vvedenskii and others . 

More evidence for a critical point comes from adiabatic and nonadiabatic scanning calorimetry measurements [138], [139], a comparison of which yields the latent heat effects associated with a first-order phase transition. Figure 7.19 shows data from both techniques for two chiralities of a CE2 sample. The sharp peaks, which are due to the nonabiatic technique only, represent first-order helical-BPI and BPI-BPIII transitions the BPIII-isotropic peak only shows first-order behavior for X = 0.40. Repeated runs for various chiralities establishes that, for this system, Xc x 0.45. A complete experimental analysis of the rotatory power and calorimetric experiments has been performed by Kutnjak et al. [139], who conclude that the data is consistent with mean field behavior. Since that time, however, new theory has appeared which allows different conclusions. 

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