Rubidium and cesium meta-phosphates were prepared in crystalline and glassy forms and their heat capacities measured. The results were analyzed along with those on analogous sodium and potassium salts in terms of the difference in the normal mode frequencies of the glassy and crystalline modifications. Non-linear least squares fitting using the data between 5 and 40 K gave a result that 0.3 – 0.5 vibrational modes per chemical unit come down from above 100 cm−1 in the crystals to 20 – 40 cm−1 in the glasses. These rather large numbers of the displaced modes may actually represent the number of loosely bound ions in the disordered glassy structure.
The orientational disorder and reorientational motion at room temperature in crystalline 5CNB results in the presence of the glass transition in contrast to PBCB. Different behaviors presented by PCNB, PBCB and 5CNB implies that the molecular dynamics is govered by the details of molecular and crystal structures and intermolecular interactions. The molecular structures mentioned here are certainly simple in compounds on the earth. The third law of thermodynamics, though it has a clear statistical-physical meaning, may be seemingly violated in most real compounds.
Phase diagrams of some compounds showing optically isotropic mesophases are compared as a function of the number of paraffinic carbon atoms per molecular core. In spite of a wide range of the transition temperatures, the number of paraffinic carbon atoms required for the appearance of the isotropic mesophases is limited within a narrow range, irrespective of molecular structure and/or intermolecular interaction. Combining this finding with the previous estimates for entropy contributions of molecular core and chains, a possible framework toward the molecular-statistical modeling is proposed.
A cubic mesogen ANBC(22) shows two cubic mesophases with the space groups Im3m and Ia3d. Heat capacity of ANBC(22) was measured with an adiabatic calorimeter in the 14 – 485 K range, and with a DSC in the 193 – 483 K range. Phase transitions were observed between the adjacent phases in the phase sequence, Cr1 – Cr2 – SmC – Cub I – Cub II – isotropic liquid. The enthalpy (170 J mol−1) and entropy (0.37 J K−1 mol−1) gained at the Cub I – Cub II phase transition at 453.8 K were extremely small, compared with the entropy (about 5 J K−1 mol−1) gained at SmC – Cub I phase transition. This implies that the two cubic phases bear close resemblance from a thermodynamic viewpoint.
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