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

Large monocrystalline domains of pentacosane (C25H52) 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 GPa, c(33)=35 GPa, c(44)=2 GPa. In the R-V orthorhombic rotator phase the elastic constants are c(22)=3.35 GPa, c(33)=3.67 GPa, c(44)< 0.05 GPa. The evidenced thermal and acoustic anomalies are discussed in the frame of the accompanying structural changes.