Modern experimental methods for the observation of the far infrared spectra of polymers

The introduction of very sensitive detectors and the use of phase modulation has enabled interferometric spectroscopy to be extended downwards in frequency until the spectral coverage overlaps that provided by microwave dielectric methods. The radiometric precision available has been steadily improved by taking advantage of both instrumental and computational advances and the accuracy which can now be attained is comparable to that of dielectric or mid‐infrared methods. Both the real and the imaginary components of the complex dielectric constant (or the complex refractive index) can now be obtained for moderately transparent materials over the millimetre and submillimetre bands for a wide range of specimen temperature, and reflectance methods have been developed for the study of heavily absorbing specimens. The far infrared band is particularly appropriate for the study of high polymers. With these instrumental innovations considerable advances may be expected in the understanding of molecular motion in these materials.