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

Abstract

Large monocrystalline domains of pentacosane (C 25 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 ϭ7 GPa, c 33 ϭ35 GPa, c 44 ϭ2 GPa. In the R V orthorhombic rotator phase the elastic constants are c 22 ϭ3.35 GPa, c 33 ϭ3.67 GPa, c 44 Ͻ0.05 GPa. The evidenced thermal and acoustic anomalies are discussed in the frame of the accompanying structural changes. ͓S0163-1829͑97͒00737-6͔

Oriented growth of n-alkanes crystals on nanostructured poly(tetrafluoroethylene) substrates

“…Despite the larger lattice mismatch involved ͓e.g., ⌬ϷϪ15% for the RII phase, ͑100͒ contact plane͔, a possible reconstruction of the rotator crystal at the interface, to account for the 0.566 nm interchain distance of PTFE, is therefore suggested by the very low C 22 elastic constant measured by Brillouin scattering experiments. 17 Finally, it is worth noting that the stabilization of the ͑200͒ contact plane on PIA substrates is not very strong: We sometimes observe-as for pure glass substrates-on slow cooling a ''dewetting'' effect also on PIA substrates, i.e., the growth of unoriented ''lying flat'' ͑00l͒ crystalline layers. This observation clearly shows that the interfacial interactions are not strong enough to completely hinder the nucleation of conventional ͑00l͒ crystals.…”

“…This procedure finally yields crystalline layers with a thickness close to 10 m. Subsequently, the samples were either rapidly cooled at room temperature ͑i.e., quenched from the melt͒, isothermally crystallized in the rotator phase and/or slowly recrystallized in an unidirectional temperature gradient ͑Bridgman technique͒. [17][18][19] In the latter case, the crystallization was performed at a rate of ⌬T/⌬t ϭ0.005 K/min, the temperature gradient being directed perpendicular to the PTFE sliding direction. After the crystallization procedure, the sample plates, a few micrometers thick, were floated off on water to remove the glass slides.…”

“…This method yields thin macroscopic, nearly defectless crystal plates particularly suitable to characterize their elastic properties using Brillouin spectroscopy. [17][18][19] In the present study, the orientation of alkane layers, achieved by melt crystallization between two nanostructured PIA substrates, is determined from optical microscopy and grazing incidence x-ray diffraction. The results will serve as a basis for a discussion about the role of the interfacial effects on the structure of organic layers.…”

First One-Dimensional Stress-Strain Experiments inside an Aperiodic Inclusion Compound: Evidence of Depinning Effects

“…Most of the data are coming from fits of thermodynamical properties of the liquid or critical state, which give values between 0.5 and 5 N=m [10,11]. In the solid state a value of 2 N=m can be deduced from Brillouin scattering on C 25 H 52 [12]. By knowing the intermolecular force constant, it is easy to derive the characteristic values of the acoustic wave dispersion of the alkane sublattice.…”

Simple Landau model of the RIV-RIII-RV rotator phases of alkanes

“…A large number of experimental studies [3][4][5][6][7][8][9] are devoted to describe the structure and the phase transitions between different rotator phases. According to literature, 6,7 in the crystalline form, n-heptacosane ͑C 27 H 56 ͒ exhibits the following four phases: Low temperature orthorhombic crystalline phase ͑X͒, a R V phase, a R III phase, and a R IV phase.…”