Temperature-Induced Transition from Odd-Even to Even-Odd Effect in Polyelectrolyte Multilayers Due to Interpolyelectrolyte Interactions

Within a liquid cell the linear growth of polyelectrolyte multilayers from poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAN) is observed with multiple angle null ellipsometry. The salt content is varied between 1 and 4 mol/L NaCl and the temperature between 20 and 55 degrees C. In the linear growth regime, the outermost layer is investigated. At low temperature, a top PSS layer is twice as thick as a top PAH layer (odd-even effect), consistent with the respective monomer volumes and the same water content for both kinds of top polyelectrolyte layers as confirmed by refractive index measurements. On heating, the thickness of a polycation/polyanion bilayer increases. For temperatures exceeding a crossover temperature, a top PAH layer is thicker than a top PSS layer (even-odd effect). Simultaneously, the index of refraction of the respective top layers indicates a compact PSS and a swollen PAR layer. It is suggested that, at elevated temperature and high salt conditions, secondary forces gain importance in comparison to electrostatic forces: therefore, a transition from an odd-even to an even-odd effect occurs, as well as the decreased film stability on drying as described before (Cornelsen, M., et al. Macromolecules 2010, 43, 4300). The ellipsometric data indicate that PAH/PSS layer pairs exceeding 8.6 nm thickness in solution are unstable in air.