Morphology dependent anomalous frequency shifts of infrared absorption bands of polymer crystals: Interpretation in terms of transition dipole–dipole coupling theory

Abstract

For such crystalline linear polymers as trigonal poly(oxymethylene) (t-POM), orthorhombic poly(oxymethylene) (o-POM), and poly(ethylene oxide) (PEO), it has been found that the infrared bands polarized parallel to the chain axis exhibit anomalously large high-frequency shifts as the crystal morphology of the sample goes from the extended-chain crystal (ECC) to the folded-chain crystal (FCC). The common features of the remarkable spectral change were investigated systematically and summarized as follows: (1) the anomalous bandshifts were observed only for the infrared-active parallel bands, (2) the magnitude of the shift was proportional to the oscillator strength of the band, (3) the overtones corresponding to the parallel fundamental bands did not show such a remarkable shift, and (4) the appearance of the high-frequency bands in FCC was ascribed not to the folded molecular structure, but to the lamellar-type crystal morphology. The morphology-dependent bandshifts were interpreted quantitatively in terms of the transition dipole–dipole coupling theory. By integrating the interacting energy over cylindrical crystal having various radius/height ratios, it was demonstrated that the energy due to the dipole interaction vanished for the idealized ECC and became maximum for the infinitely thin lamellar crystal. The magnitudes of the bandshifts between the two extreme cases were calculated on the basis of the absolute values of the oscillator strengths of the bands measured on highly crystalline samples of t-POM, o-POM and PEO and their crystal structures. The results agreed well with the observed band gaps between the ECC and FCC samples. The change in the absorption profiles of the parallel bands with change in crystal morphology was simulated. The result reproduced well the observed spectral change.