Morphology-sensitive infrared absorption bands of polymers derived from surface polaritons

Infrared (IR) spectroscopy is mostly used for analysis of normal modes, which provides rich chemical information of molecular structure at each chemical group. In practice, however, we sometimes encounter an unusually broad peak that cannot be assigned to a normal mode or overlapped multiple normal modes. This unusual peak can readily be understood by taking the concept of polariton into account even for a polymeric material. In general, however, ‘polariton’ is used for discussing inorganic crystals, in which the real part of the relative permittivity falls in negative. Here, we show some polymeric materials exhibit apparent negative relative permittivity in an IR region, which yields the unusual peaks. Through the measurements and theoretical simulations of spectra of polymer thin films, polariton is found to be necessary to comprehensively understand IR spectra of a polymer not only for a thin film, but also for powder dispersed in a KBr pellet, since the polariton peaks are found to be deeply correlated with the surface morphology.