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′-ethyl hexyloxy)-p-phenylene vinylene) (MEH-PPV) to photolysis, heat treatment, plasma and X-ray exposure was monitored by UV-VIS spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and angle resolved X-ray photoelectron spectroscopy (XPS). Samples were either neat films of MEH-PPV or solutions of MEH-PPV in xylene. A significant broadening of the absorption spectrum with considerable blue shift and a rapid loss in photoluminescence intensity 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 observed 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 functional groups on the polymer film surface was monitored by angle resolved XPS. The polymer film also lacks long-term stability when exposed to a moderate temperature of 160°C in ambient atmosphere, and suffers a considerable loss in effective conjugation. Finally, the loss in the effective conjugation length of the polymer film during X-ray exposure under ultra-high-vacuum environment is associated with the crosslinking of the polymer chains.