Phase-transition behavior of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>n</mml:mi></mml:math>-alkanes on nanostructured polytetrafluorethylene films: Brillouin spectroscopic and calorimetric investigations on pentacosane

Abstract

Large monocrystalline domains of pentacosane $({\mathrm{C}}_{25}{\mathrm{H}}_{52})$ have been prepared on nanostructured polytetrafluoroethylene films. The acoustic and thermal properties have been investigated around the various phase transitions of the crystalline state of pentacosane using high-performance Brillouin spectroscopy and modulated differential scanning calorimetry. In the crystalline orthorhombic phase the values of the elastic constants are ${c}_{22}=7\mathrm{GPa},$ ${c}_{33}=35\mathrm{GPa},$ ${c}_{44}=2\mathrm{GPa}.$ In the ${R}_{V}$ orthorhombic rotator phase the elastic constants are ${c}_{22}=3.35\mathrm{GPa},$ ${c}_{33}=3.67\mathrm{GPa},$ ${c}_{44}&lt;0.05\mathrm{GPa}.$ The evidenced thermal and acoustic anomalies are discussed in the frame of the accompanying structural changes.