The transport and diffusion properties observed for solid n-alkanes and the premelting phenomena in polymethylene systems imply that molecular chains possess conformational flexibility coupled to longitudinal mobility. Most of the authors have proposed models based on the creation and mobility of conformational kinks whose existence is not proven by spectroscopy. The existence of 'twistons' has been postulated. In this work vibrational i.r. and/or Raman spectroscopy is used as a probe for twistonic excitations. The conditions for the spectroscopic observation of twistons are discussed. The temperature dependence of the spectra of n-alkanes from the orthorhombic to 'alpha' (or 'pseudohexagonal') and to melt phases is studied and the spectra are interpreted in the attempt to find evidence of twistons. As a test case of the theoretical predictions the spectra of n-alkanes in urea clathrates are discussed. Indications are found that twistons start being produced while still in the orthorhombic phase when the lattice starts expanding. The 'alpha' phase (or premelting state) may be considered as a 'soft' phase consisting of a collection of twistonic chains out of register because of the propagation of twistons which also generate the spectroscopically observed surface disordering and diffusion.
TWISTONS AND PHASE-TRANSITIONS IN POLYMETHYLENE SYSTEMS - A SPECTROSCOPIC STUDY
DEL ZOPPO, MIRELLA ELVIRA ANGELA;ZERBI, GIUSEPPE
1992-01-01
Abstract
The transport and diffusion properties observed for solid n-alkanes and the premelting phenomena in polymethylene systems imply that molecular chains possess conformational flexibility coupled to longitudinal mobility. Most of the authors have proposed models based on the creation and mobility of conformational kinks whose existence is not proven by spectroscopy. The existence of 'twistons' has been postulated. In this work vibrational i.r. and/or Raman spectroscopy is used as a probe for twistonic excitations. The conditions for the spectroscopic observation of twistons are discussed. The temperature dependence of the spectra of n-alkanes from the orthorhombic to 'alpha' (or 'pseudohexagonal') and to melt phases is studied and the spectra are interpreted in the attempt to find evidence of twistons. As a test case of the theoretical predictions the spectra of n-alkanes in urea clathrates are discussed. Indications are found that twistons start being produced while still in the orthorhombic phase when the lattice starts expanding. The 'alpha' phase (or premelting state) may be considered as a 'soft' phase consisting of a collection of twistonic chains out of register because of the propagation of twistons which also generate the spectroscopically observed surface disordering and diffusion.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
