Ellipsometry study of the photo-oxidation of poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene]

Autor: Eugene A. Irene, M. Tinani, Rodrigo Fernando Bianchi, Débora Terezia Balogh, Roberto Mendonça Faria
Rok vydání: 2004
Předmět:
Zdroj: Journal of Polymer Science Part B: Polymer Physics. 42:1033-1041
ISSN: 1099-0488
0887-6266
DOI: 10.1002/polb.10734
Popis: Poly(p-phenylenevinylene) (PPV) and its derivatives exhibit strong luminescence, being serious candidates to be used as active layers in organic light-emitting diodes. However, the structural degradation caused by photo-oxidation is an obstacle for commercial applications of such materials. Here, we show that spectroscopy ellipsometry is a useful technique to investigate the photo-oxidation of poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene] (MH-PPV), a PPV derivative, which emits a red color light. Spectroscopy ellipsometry enables determination of the complex dielectric function ∈ * (E) of MH-PPV thin-layer films exposed to air, in the 2.1-4.2 eV energy range, as a function of the light exposure time (t e ). By using the Lorentz model to fit the experimental ∈*(E) curves, it was inferred that the interactions among polymeric chains increase with t e . From ∈*(E), it is also possible to obtain the complex refractive index, N*(E) = n + ik. At higher energies (where k << n), n increases from 1.32 to 1.40 with the photo-oxidation progress. The behavior of n was investigated by using the Lorenz-Lorentz equation, taking into account the contribution for n by the chromophores of MH-PPV. The effect of photo-oxidation, mainly due to the replacement of vinyl C=C by the ketone C=O bonds, is confirmed by Fourier transform infrared measurements, an effect that reduces the average effective polymer conjugation length.
Databáze: OpenAIRE
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