IR laser radiation induced changes in the IR absorption spectra of thermoplastic and thermosetting polymers

This paper demonstrates that powerful laser radiation causes changes in the absorbance spectra of epoxy resin, polyethylene and polysulfone. Thin polymer films were located between IR AgBrCl optical fibres and exposed to the radiation of a CO₂ laser. The output of the laser source was varied in the range 0-8.5 W. The absorbance spectra were recorded using a Fourier transform infrared (FTIR) spectrophotometer. It was revealed that characteristic polymer absorbance peaks decay under exposure to the powerful IR light. The apparent dependence of peak magnitude on IR radiation power has been established. We showed that the phenomenon of the absorbance peak disappearances associated with the polymers-thermoplastic (including an engineering polymer such as polysulfone) and thermosetting-is of a threshold nature. The mathematical theory of the observed effect was derived. We propose that the effect under discussion is caused by the oxygen-free thermal action of IR radiation on the chemical structure of the polymer materials. The revealed effect could be effectively used to lower the losses in adhesive contacts of IR optical elements. The novelty of the proposed method lies in the fact that thermal treatment is localized strictly within the adhesive layer; optical elements to be contacted (fibres, lenses, etc) which are highly IR transparent do not experience the IR radiation, but the polymer adhesive is subjected to a temperature rise.