TEMPERATURE-DEPENDENCE OF ABSORPTION-SPECTRA AND OPTICAL-PARAMETERS FOR POLYESTER FILMS OF DIBENZYLIDENE CYCLOHEXANONE

Effect of temperature on the optical absorption spectra and optical pa rameters is investigated in new synthesized polyester films of poly al oyloxy(2-methoxy-p-phenylene)methylidyne(2-oxo-1 exanediylidyne)methyl idyne(3-methoxy-p-phenylene)] (I) and poly[oxysebacoyloxy-(2-methoxy-p -phenyle methylidene nediylidene)-methy-lidene-(3-methoxy-p-phenylene) ] (II). Absorption measurements are carried out in the temperature ran ge 80-370 K and in the photon energy range 1.9-4 eV. The optical absor ption spectra of polymers I and II reveal excitonic absorption bands a t 80 and 120 K respectively. These bands are associated with n --> pi optical transitions along polymer backbone chains. Analysis of optica l absorption data indicates that the Tauc equation for indirect optica l transitions successfully explains the optical absorption process in polymeric film I, whereas Urbach's law is more suitable for describing the optical absorption in polymeric him II. Temperature dependence of the optical bandgap, E-g(opt), the Tauc slope root beta, and the widt h of Urbach's tail states (E-e) exhibits anomalous parabolic behaviour which can be explained in terms of various current theories. Moreover , DSC, x-ray diffraction and morphological studies by TEM are used in the explanation of the experimental results. The values of optical par ameters, as well as the temperature coefficient of the optical bandgap for these polymers, are calculated and reported.