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Eingeladener Vortrag
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In Order to be able to assess the damage caused to materials by acid deposition the first question to be considered is: Which materials are affected?
A wide ränge of materials are exposed to the extemal environment. These materials have different susceptibility to attack by acid rain and different stock at risk in the Federal Republic of Germany.
Plasma polymer films of vinyltrimethylsilane (VTMS) were deposited by radiofrequency (RF. 13.56 MHz) or Microwave (MW, 2.45 GHz) discharges in two reactors of the same volume and geometry. — The polymer films were characterized by electron spectroscopy for chemical analysis (ESCA) and infrared spectroscopy (FTIR). It turned out, that the RF — and the MW — polymers had almost the same Si/C-composition ratio of 1/5 in the entire range of RF-and MW-power (60-150 W). Although the Si/C film-composition remains almost constant, the binding states of Si-and C-atoms in the respective ESCA-spectra are changing. While the CSi part of the C1s-peak increases with the RF-or MW-power, the C-H/C-C part of the peak decreases, indicating a loss of hydrogen in the polymer films. — Obviously the fragmentation of the VTMS-monomer is quite different for the two types of plasma discharges. While films of the RF-plasma clearly show strong contributions of the methyl and vinyl groups, these groups are completely missing for the MW-plasma polymers. — From these results structure models of the plasma polymers have been developed.
Measurements of polarized excitation and fluorescence spectra were carried out with thin films of PPV and with films of oligomer model compounds in a polystyrene matrix. The degree of fluorrescece polarization r and its spectral dependence are discussed with respect to the transfer of excitation energy and the mutual alignment of the flurophores. In contrast to the behaviour of conventional aromatic polymers, the value of r is non-zero in isotropic PPV and depends on preparation (up to r = 0.25 at λexc = 460 nm and λflu = 555 nm). r can be nearly as high as in the dilute solid solution of the oligomers. This is easily understood if the PPV segments are highly aligned in the distance range of energy transfer.
If the intramolecular distance between the two isoenergetic chromophores of a bichromophoric compound is much smaller than the Förster radius of excitation energy transfer (EET), the probability to fluoresce is equal for both chromophores, because the EET takes place at a shorter time compared with the nanosecond timescale of the fluorescence lifetime. Then, the fluorescence depolarization of diluted solid solutions of such bichromophoric molecules can be measured with stationary excitation and used to evaluate the orientation between the two chromophores; in the most simple case the angle between their main axes is determined. Model calculations for bichromophoric alkane compounds with various conformational freedom reveal the amount of depolarization and thus the limits of the method. The results are compared to experimental data of 1,3-di(N-carbazolyl)-propane and 1,2-trans-di(N-carbazolyl)cyclobutane.