Piezochromic effect and orientational order in monolayers and LB multilayers of poly(p-phenylenesulfonate)- dioctadecyldimethylammonium bromide complexes

By spreading dioctadecyldimethylammonium bromide (DODA) on a subphase conta ining rigid-rod-like poly(p-phenylenesulfonate)s (PPPS), polyelectrolyte co mplexes are formed in situ at the air-water interface. These complex monola yers are investigated by recording surface pressure/area isotherms, UV/vis spectroscopy, and, after transfer onto a solid substrate, X-ray photoelectr on spectroscopy (XPS) and X-ray reflectivity (XR). XPS and XR measurements indicate that although the polyelectrolytes are aggregated in aqueous solut ion, the complexes are stoichiometric. The isotherms show that the area per amphiphile in the complex monolayer is significantly higher than in the mo nolayer of pure DODA. This is due to the distance of the ionic sites along the rigid-rod-like polymer chain and results in a more tilted arrangement o f the alkyl chains as confirmed by the XR measurements. UV/vis spectroscopy reveals that the A-band of the PPPS is red-shifted in the complex, and a f urther red shift of the a-band upon compression of the complex monolayer is observed. This two-dimensional piezochromic effect is discussed to be a re sult; of the electronic interaction of the pi -systems, which improves with decreasing distance during compression. Upon transfer to solid substrates the rigid-rod-like polyelectrolytes are oriented to be parallel to the dipp ing direction as evidenced by polarized UV/vis spectroscopy. The degree of order in the LB film shows a linear dependence on the axial ratio of the ri gid-rod-like PPPS.