Far infrared and millimetre-wave absorption spectra of some low-loss polymers

Absorption spectra in the range 2–50 cm⁻¹ are reported for polyethylene, polypropylene and 'TPX' (basically poly 4-methyl pentene-1). The former has no resonances in this region and the continuum absorption can be represented by a simple quadratic function of wavenumber which, when extrapolated to radio frequencies, gives a good account of the observed losses. Polypropylene and 'TPX' show discrete absorption bands, but for these too there appears to be an underlying continuum. The millimetre-wave absorption possibly arises from a summation of the high frequency wings of the radio region relaxations and the low frequency wings of the far infrared resonances plus an inherent continuum which may be analogous to the broad absorption shown by non-polar liquids in the far infrared.