MORPHOLOGY-DEPENDENT ANOMALOUS FREQUENCY-SHIFTS OF INFRARED-ABSORPTION BANDS - POLY(TETRAFLUOROETHYLENE) AND ITS LINEAR OLIGOMERS PERFLUOROEICOSANE AND PERFLUOROTETRACOSANE

Morphology-dependent frequency shifts of the infrared parallel bands of poly(tetrafluoroethylene) (PTFE) were investigated using a PTFE sample having a whisker-like crystal morphology and lamellar crystals of linear oligomers of PTFE (perfluoro-n-eicosane (C20F42) and perfluoro-n-tetracosane (C24F50)) The vibrational assignments of the infrared-active zone-center fundamentals mere established through polarization measurements on well-oriented thin films of PTFE and thin layers of C20F42 deposited on a metal surface. Based on the assignments thus decided, it has been demonstrated that the A(2) bands with the transition dipoles parallel to the chain axis are blue-shifted as the crystal morphology of the sample goes from the extended-chain crystal to the lamellar crystal, whereas the E(1) perpendicular bands as well as the infrared-inactive Raman bands (the A(1) and E(2) bands) remain unshifted. The magnitudes of the frequency shifts of the A(2) bands were interpreted quantitatively in terms of the transition dipolar coupling theory. The role of the in-phase oscillation of the parallel transition dipoles in this spectral phenomenon was investigated by the analysis of the frequency shifts of the parallel progression bands of the oligomers in their lamellar type mixed crystals of various compositions.