JOURNAL OF MATERIALS SCIENCE 29 (1994) 3451-3457 FeCl3-doped polyvinylidene fluoride Part I Interpolaron hopping and optical properties A. TAWANSI, H. I. ABDEL-KADER, M. EL-ZALABANY*, E. M. ABDEL-RAZEK Department of Physics, Faculty of Science, Mansoura University, 35516, Egypt *Department of Electrical Engineering, Faculty of Engineering, Mansoura University, 35516, Egypt Infrared (350-4000 cm -1) and optical (1.1 5 x 1 04 - 2.95 x 1 04 cm -1) spectra, differential thermal analysis (DTA) and d.c. electrical resistivity of FeCI3- doped polyvinylidene fluoride (PVDF) films, over the doping mass fraction range 0 ~< w ~< 0.40, have been measured. The i.r. spectra provided evidence of: (a) the presence of both ~ and y phases in the undoped, and a y phase in the doped PVDF films; (b) a head-to-head content of 20%; and (c) a different doping mode beyond a 0.25 doping level. The optical spectra resulted in two induced energy bands, and a probable interband electronic transition, due to doping. Dipole relaxation and premelting endothermic peaks were identified by DTA. Electrical conduction is thought to proceed by interpolaron hopping among the polaron and bipolaron states induced by doping. The hopping distance, Ro, is calculated according to the Kuivalainen model. A numerical equation is adopted to formulate the dependence of Ro on doping level and temperature. It is found that Ro < CC separation length. This implies that, in doped PVDF, charge carrier hopping is not an intrachain process. 1. Introduction Since the discovery of polyvinylidene fluoride as a polymer electret [1, 2] much effort has been devoted to the better understanding of the nature of permanent polarization [3, 4], and how it gives rise to piezoelec- tricity [5] and pyroelectricity [6]. PVDF crystallizes into several crystal forms, namely ~, 13, Y [7], the planar zigzag, 21 helix of the [3 phase and the 31 helicai structure [8]. PVDF and related semicrystalline poly- mers are receiving increasing attention, as these poly- mers find use in wide applications. The present work is part of a systematic study of the effect of transition metal halide doping on the physical and structural properties of PVDF and their possible applications. The present work is devoted to an invest- igation of the effect of FeC13 doping on i.r. and optical spectra, and on the electrical conduction of PVDF films. The effect of FeC13 doping on magnetic suscept- ibility and on the microwave response of PVDF is investigated by Tawansi et al. [9]. 2. Experimental procedure Samples were made from PVDF resin provided by Solvey (Belgium) and referenced as SOLEF 1008. Dimethylformamide was used as a solvent for the resin and FeC13 dopant. Films were prepared by casting the desired solution onto glass, such that the films were approximately 0.2-0.5 mm thick after the solvent was removed at 343 K. An infrared spectrometer (Perkin Elmer 1430) was used for measuring the i.r. spectra in the wave number range, of 350-4000 cm- 1. A Perkin Elmer (1800) spectrometer was used for measuring optical transmittance in the wave number range of 1-.15 x 104 - 2.95 x 10 4 cm- i. A thermoanalyser (GDTD 16, setaram), with a measuring temperature range of 10-200 ~ a heating rate of 2 ~ min- 1 and sensitivity 2.5 gV, was used for differential thermal analysis. Electrical resistivity was measured by a standard technique [10] using an autorange multi- meter (Philips 175) of accuracy_+ 0.2%. The films were in the form of circular discs, of 1.6 + 0.001 cm diameter. Contacts were of highly conductive silver paste, with an area of 1 cm 2. A guard ring was used. The sample was short-circuited for about two days, at a constant temperature of 300 K, before the d.c. volt- age was applied. Resistivity was measured in the steady-state to avoid errors due to relaxation phe- nomena. 3. Results and discussion 3.1. Infrared analysis The infrared spectra (4000-350 cm - 1) of the undoped and 5, 15, 25, 30 and 40 wt %) FeC13 doped PVDF samples are shown in Fig. 1, and the frequency values of the observed absorption bands (compared with the bands characterizing the ~ and Y phases [7]) are listed in Table I. Both of the main bands of the ~z and y phases are noticed in the spectrum of the undoped sampR. On the other hand, the spectra of the doped samples revealed the presence of a single y phase, and the disappearance of the ~ phase. The absorption bands, which are clearly influenced by doping, will be discussed as follows. The band at 1750 cm-1, which is assigned to C = C 0022-2461 9 1994 Chapman & Hall 3451