Abstract:
The IR spectra of partially crystalline polymers polyacrylonitrile (PAN) and polytetrafluoroethylene (PTFE) in the region of manifestation of the spectrum of their crystal lattices (10 – 150 cm$^{-1}$) at temperatures from 4.2 to 300K are presented and analyzed. The increased sensitivity of the spectral parameters of the bands of these spectra to crystalline effects allows us to trace structural rearrangements at the molecular level. The intensity, half-widths, and position of the maximum of the bands of external translational and rotational modes change especially sharply during phase transitions (PT), indicating a correlation between the PT and the vibrational properties of the external lattice modes. Analysis of these changes shows that during the PT from a low-temperature structure to a more disordered high-temperature structure, conditions arise for the manifestation of relaxation processes (RP) in the system, indicating the relationship between PT and RP, consisting in the fact that the former often precede and are responsible for the manifestation of the latter in the amorphous regions of the polymer.
Keywords:partially crystalline polymers PAN and PTFE, lattice IR spectra, solid-state phase transitions, $\gamma$- and $\beta$-relaxations, the relationship between PT and RP.
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