Stability studies of poly(2-methoxy-5-(2 '-ethyl hexyloxy)-p-(phenylene vinylene) [MEH-PPV]

The stability of the soluble, electroluminescent polymer, poly(2-methoxy-5- (2'-ethyI hexyloxy)-p-phenylene vinylene) (MEH-PPV) to photolysis, heat tre atment, plasma and X-ray exposure was monitored by W-VIS spectroscopy, Four ier transform infrared (FT-IR) spectroscopy and angle resolved X-ray photoe lectron spectroscopy (XPS). Samples were either neat films of MEH-PPV or so lutions of MEH-PPV in xylene. A significant broadening of the absorption sp ectrum with considerable blue shift and a rapid loss in photoluminescence i ntensity were observed when the polymer film and solution were illuminated with UV-visible light, or even with light of wavelength greater than 500 nm , in ambient air. Negligible change in effective conjugation length was obs erved when the polymer film was illuminated under an argon atmosphere with light of wavelength greater than 290 nm. The surfaces of MEH-PPV films were modified using argon, nitrogen and oxygen plasma. The formation of new fun ctional groups on the polymer film surface was monitored by angle resolved XPS. The polymer film also lacks long-term stability when exposed to a mode rate temperature of 160 degrees C in ambient atmosphere, and suffers a cons iderable loss in effective conjugation. Finally, the loss in the effective conjugation length of the polymer film during X-ray exposure under ultra-hi gh-vacuum environment is associated with the crosslinking of the polymer ch ains. (C) 1999 Elsevier Science Ltd. All rights reserved.