Elastic softening in the rotator phase of the perfluoroalkaneC24F50

Abstract

The rotator phase of C24F50 has been characterized in its calorimetric as well as in its elastic properties. To perform the elastic characterization we took advantage of a polytetraAuoroethylene-inducedalignment crystal-growth technique. Special interest is paid to the rotator phase and the premelting behavior therein as reAected by the elastic stifFness coefficients c33 and c44. The anomalous elastic behavior is interpreted in the frame of the Landau theory. The premelting process is related to molecular conformational defects.

Phase-transition behavior ofn-alkanes on nanostructured polytetrafluorethylene films: Brillouin spectroscopic and calorimetric investigations on pentacosane

“…As has been shown for many systems, [6][7][8]10,11 the elastic properties of plastic systems are strongly affected by the morphology and the morphological changes during the different phase transitions. In the following, we will demonstrate the efficiency of high-resolution BS in connection with modulated differential scanning calorimetry ͑MDSC͒ investigations for the understanding of the dynamical properties of the system under study throughout its different phase transitions.…”

“…Usually, even a second crystallographic axis is positioned within the PIA plane. 7,8 The aim of the current paper is to show that the transcrystallization mechanism on the PIA substrate still works for C 25 H 52 and that the created morphology corresponds to that of solution-or melt-grown crystals of that material. Moreover, we want to relate our Brillouin and calorimetric investigations to the phase transition sequence and the related phase-transition mechanisms.…”

“…7,8 This crystal growth method uses the polytetrafluoroethylene ͑PTFE͒ induced alignment ͑PIA͒ technique, to obtain oriented crystal plates of n-alkanes or perfluoroalkanes. The PIA technique, originally developed by Wittmann and Smith, 21 allows one to obtain highly structured crystal mats of very different materials including n-alkanes and perfluoroalkanes.…”

“…This is reflected by acoustic as well as by thermal properties. [6][7][8][9] The competitive influences of the stiff covalent bonded chain molecules and the weak intermolecular interactions between the molecular chains as well as between the crystal lamellas on the elastic properties have been demonstrated for different types of alkanes. [5][6][7]10,11 Consequently, investigations of the static and dynamic elastic properties, combined with investigations of the thermal properties, will give essential information about the interrelations between these properties on one hand, and the molecular structure, the molecular dynamics, and the morphology on the other.…”

“…The PIA technique, originally developed by Wittmann and Smith, 21 allows one to obtain highly structured crystal mats of very different materials including n-alkanes and perfluoroalkanes. 7,8,[22][23][24][25][26] Although the organization mechanism of the nanostructured PIA substrate on the deposited sample material is not well understood, it is clear that the PIA substrate induces a preferential orientation of the sample crystallites with their c axes parallel to the preferential orientation of the PTFE. Usually, even a second crystallographic axis is positioned within the PIA plane.…”