Study of PPV polymer layers on Si substrates by mechanical spectroscopy

Thin layers of the OLED related polyphenylene-vinylene (PPV) deposited by a precursor on micro-fabricated Si cantilevers were studied by applying the vibrating-reed technique during repeated temperature cycling between 100 and 520 K. By means of the Langmuir-Blodgett method for film production, the dependence of damping and elastic modulus on well defined values of film thickness (16 to 69 nm) was determined. Simultaneous measurements of these quantities showed four damping peaks during heating around 130 K (called gamma), 250 K (beta), 350 K (beta'), and 400 K (called C). Three of them (gamma, beta', C) disappeared after heating to the highest temperature (520 K) indicating their presence in the precursor only. The activation parameters of the relaxation peaks (gamma, beta, beta') were estimated and assigned to specific atomic movements in the molecule. Peak C occurs during the conversion process of precursor to polymer. Earlier results are essentially substantiated, indicating only slight differences to those for layers produced previously by spin coating. The observed thickness dependence of damping for the gamma and beta peaks suggests a weaker contribution of molecules in the surface region than of those in the bulk, while the beta' peak is supposed to result from molecules in the interface region between layer and substrate.