ves /3/ have been also undertaken
THE ELLIPSOMETRIC STUDY OF PCX-YPYRROLE FILMS
T. Silk, A. Hallile, J.Tamm Tartu University, Tartu
Polypyrrole CPPD is one of the most studied conducting polymers thanks its good stability to temperature and to acids y\y. PP films can be easily generated electrochemically on various substrates. There have been several el1ipsometriс studies of conducting polymer films: polyaniline /2/, pol yvi nyl f er rocene /3/, polypyrrole /4/, polythionine /5/, polythiophene /6/. In most cases the complex refractive index С n-iM has been determined from the el 1 ipsometriс angles Л and Ф. According to the majority of investigations the refractive index shows only a slight dependence on the thickness d of polymeric films, i.e. the films seem to be homogeneous. But it is known that such films have complicated structure: the рю1утег1с chains may be either fibrillar or globular; the dopant ions distribution is not uniform; according to SEM pictures the film is porous etc.
This contradiction probably springs from the fact that the authors have been determined the parameters only at some limited thickness range or at relatively large d values С above lOO nnO. It is very important by the computing of n and M values to know the exact film thickness, since it can be easily shown that in case of conducting films СМЮ^ we can find greatly different quantities for the parameters depending on the assumptive d. In most cases the tS-values are calculated from coulometric data, assuming, a full current efficiency which may not be correct. According to /7/, >*^iere the d of freestanding PP films were determined from cross sections by SEM the slope of the charge—thickness dependence changes with the synthesis potential. The actual d can fluctuate ca. 1.5 times at the same amount of charge.
Another difficulty in interpreting the ellipsometric data is caused by the influence of surface roughness. Taking into account the actual roughness is a very serious problem in el 1 i psometr y. Therefore it is net^essary to try to reali^^u the meai;urements on аз smooth surfaces as possible. For the ellipsometric studies of conducting polymer films it means
191
thai the d. cannot exceed a certain value /8/.
In present investigation the PP films parameters n and h at (i=20, AO, 70, lOO, 150 , 200, 400 and 700 run. were determined. The PP in oxidized state was formed electrochemi-cally on a mirror-finished surface of Pt-electrode from acetonitrile solution, containing 0.1 M of pyrrole and 0.15 M NaClO^ as a supporting electrolyte under galvanostatiс conditions С i=l тАУсп? D. The reduced state of PP was formed in the same solution applying the potential -0.6 У in respect to the saturated aqueous AgCl reference electrode throughout 1 and 5 minutes. We expect to obtain the thickness of 1 with the passage of 0.4 Сусп? according to /О/. A L119 el 1 i psometer equippted with a L118RA automatic rotating analyzer interfaced to a HP-85 desk-top computer was employed. All measurements were performed in air at the wavelength of 632.8 run. and the angles of incidence were 60**
and 70°.
Experimentally obtained trajectories on the Д,Ф-р1апв cannot be described in the framework of homogeneous model, i.e. the n, and h must be changed with d, especially in the vicinity of the electrode. The measurements on reduced PP show that in the same region С up to 100-150 nm from the electrode D a alteration of prop>erties takes place. We should note that the scattering of Д and Ф values was greater for reduced PP than for oxidized state.
In order to model the PP films behaviour we used a multilayer model with a layer thickness of- 5 nm with several regions. The functional depiendence of n and M upon d in each region was proposed in the form n=n.-*-C n.. —ex-pI-ctd **3 , where n and n, are the initial and final values, restsectively;d -V f * relative thickness in given region and a,b are formal parameters determining the shape and position of the transition from n. to In this way we were able to simulate the experimental data with a satisfactory accuracy using 3-6 different regions. We have detected that in all ca ses there is a deep minimum in n,Л-values at range of 40-100 nm. from the electrode and the optical constants achieve a more or less fixed values at 200 iva. The reduction affects mainly the h, which decreases 4-5 times in the vicinity of' the electrode, the гг-minimum becomes deepter and the maximum
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at ~ 100-150 ntn appears. It is known that the quantity of h is connected with the conductivity of the films. Consequently the diminishing of h may be associated with the decrease in the dopant concentration. This is in accordance with the inf luence of reduction time on such a behaviour. On the other hand, the removal of dopant ions affects strongly the diameter of polymer fibres which may be decrease up to 3 times/8/. It also influences the values of optical parameters.
To summarize the results of simulation of the expc-i-imental datu we can deduce that the transition from the conducting state to nonconducting one begins at the electrode-polymer interface and propagates to the direction of the solution.
The synthesized PP films have complicated structures and thus are to considerable extent inhomogeneous, or at least close to the electrode surface. The ellipsometry is applicable to detect the alterations which take place in the polymeric films of various thicknesses.
References
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2. S. Gottesfeld, A. Redondo, S. W. Feldberg, J. Electrochem. Soc. , 134C1987D272.
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С1985Э353; G.C.Winston, C.M.Carlin, J. Electrochem. Soc. , 135C1988D789.
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6- J.2ferbino, W. J.Plieth, G. Kossmehl , J. Electroanal. Chem. , 260C1989D361 ; W. J. Plieth, J.Zerbino, C. Lahmann, G. iCossmehl, J. Electroanal. Chem. , 274C19895213.
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8. G. Tourillon, in T. A. Skotheim СEd.D,Handbook of Conducting Polymers,V. 1, Marcel Dekker, N. ?. 1986, Ch.9.
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