Ab Initio Calculation of the IR Spectrum of PTFE: Helical Symmetry and Defects

A detailed analysis of the structure and vibrational properties of PTFE and the assignment of its IR spectrum are carried out by means of density functional theory simulations on infinite, one-dimensional chains. Calculations take into consideration regular polymer chains with different conformations (15(7), 13(6), 10(3), 4(1), and 2(1)) in order to investigate the main features due to the peculiar helical structures in the IR spectra. In addition, also the helix-reversal defect and effects related to conformational disorder are considered, to analyze the contributions in the spectrum due to defects and to the amorphous phase. The present study solves the ambiguities in the interpretation of the 638-626 cm(-1) doublet, assigning the lower frequency component to a normal mode of the helix-reversal defect. This interpretation is consistent with the general belief that the PTFE crystal contains a large concentration of defects already at low temperature and that the two crystalline transitions at room temperatures (19 and 30 degrees C) are accompanied by an order-disorder transition. As an additional result, the 788 cm(-1) band, previously adopted to measure the amount of amorphous material in real samples, is confirmed as a marker of this phase.