Piezochromic Effect and Orientational Order in Monolayers and LB Multilayers of Poly(p-phenylenesulfonate)− Dioctadecyldimethylammonium Bromide Complexes

Abstract

By spreading dioctadecyldimethylammonium bromide (DODA) on a subphase containing rigid-rod-like poly(p-phenylenesulfonate)s (PPPS), polyelectrolyte complexes are formed in situ at the air−water interface. These complex monolayers are investigated by recording surface pressure/area isotherms, UV/vis spectroscopy, and, after transfer onto a solid substrate, X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XR). XPS and XR measurements indicate that although the polyelectrolytes are aggregated in aqueous solution, the complexes are stoichiometric. The isotherms show that the area per amphiphile in the complex monolayer is significantly higher than in the monolayer 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 of 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 further red shift of the A-band upon compression of the complex monolayer is observed. This two-dimensional piezochromic effect is discussed to be a result of the electronic interaction of the π-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 dipping 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 rigid-rod-like PPPS.