An electron energy loss study of the vibrations of polyimide layers deposited by Langmuir-Blodgett techniques

Single monolayer films of poly-N,N'-bis(phenoxyphenyl)pyromellitimide deposited on gold and graphite by Langmuir-Blodgett (LB) methods have been investigated by high-resolution electron energy loss spectroscopy. Initially, the vibrational spectra show evidence of the long-chain, aliphatic hydrocarbon units used in the deposition process. However, after heating in vacuo these are removed and a spectrum reminiscent of the bulk crystalline polymer is obtained. Of particular interest though is the absence of a strong feature associated with the carbonyl bands. The authors interpret this as a consequence of an adsorption geometry in which the C=O units of the polymer chain are aligned parallel to the underlying substrate, in general agreement with an earlier NEXAFS study of this polymer deposited on graphite. Further investigations of a five-monolayer film deposited on gold also fail to show a carbonyl feature suggesting that the orientation of the polymer is maintained over this thickness