TIME EVOLUTION OF POLYETHYLENEIMINE (PEI) CONFORMATION IN THIN FILMS PROBED BY SUM-FREQUENCY GENERATION (SFG) SPECTROSCOPY

Polyethyleneimine (PEI) is a cationic polyelectrolyte widely used in biology, CO2 capture, and organic electronics. In the latter, it is known that even ultrathin PEI layers are powerful surface modifiers that can shift the electrode work function and hence improve charge injection in electronic devices. This effect may depend on the conformation and the dipolar order of the adsorbed PEI ultrathin film. However, a detailed experimental evaluation of the conformational order and evolution in these layers has not been reported yet. In our work, we use sum-frequency generation (SFG) spectroscopy – a nonlinear optical technique intrinsically specific to interfaces and sensitive to the orientation of polar groups – to probe the conformational organization of PEI in ultrathin layers. For this, we study films fabricated via dip-coating from aqueous solution (at pH 5) on glass substrates. In particular, we consider the organization of PEI in both linear (l-PEI) and branched (b-PEI) structures. The obtained SFG spectra in the regions of N-H and C-H stretches indicate that the conformational ordering at the interface changes over days. We obtained spectra with more intense signals fourteen days after fabrication, indicating a higher degree of ordering. These preliminary results are consistent with a dynamic mechanism for the interaction of PEI with glass, probably affected by the varying degree of hydration of the adsorbed film.