Max-Planck-Institute für Kolloid und Grenzflächenforschung
Rudower Chaussee 5, 12489 Berlin
Ultrathin polymeric films prepared by the layer-by-layer deposition method of oppositely charged polyelectrolytes, as discovered by Decher [1], are of great interest for the fabrication of optical thin film devices. It is a great advantage of this method that different materials, such as conjugated polymers [2], dye molecules [3], and inorganic nano crystallites [4], can be combined to form layered structures with a thickness in the range of a few nanometers. For any optical as well as electrical properties of the films the position of the energy levels of the active species relative to the vacuum level has to be controlled. If dye molecules are used the energy levels are strongly influenced by the polarity of the surrounding polymer matrix as it is the case for solvents [5].
It is the aim of this contribution to investigate the polarity of self- assembled films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS). Therefore, the fluorescence of pyrene (PY) is used as a polarity sensor. The polarity can be measured from the ratio of the first to the third vibronic band of the emission spectrum and compared to values obtained from various solvents (PY-scale). Films of five double layers of PSS/PAH are dipped into Py-solutions of different solvents for a certain time to let the pyrene molecules penetrate into the polymer films. The fluorescence spectra of the films indicate always the same polarity independent of the employed solvent. The dependency of the dielectric constant on humidity will be discussed and the results will be compared to measurements of the dielectric constant by electronic methods.
References
[1] G. Decher, J. D. Hong, Macromol. Chem. 1992, 46, 321
[2] K. Ariga, Y. Lvov, and T. Kunitake, J. Am. Chem. Soc., 1997, 119, 2224
[3] A. C. Fou, O. Onitsuka, M. Ferreira, M.F. Rubner, J. Appl. Phys., 1996, 79, 7501
[4] M. Gao, B. Richter, S. Kirstein, Adv. Mat. 1997, 9, 802
[5] Christian Reichardt, American Chemical Society, 1994, 94, 2319-2358, rew. (Solvatochromic Dyes as Solvent Polarity Indicators)