Hydration of Odd−Even Terminated Polyelectrolyte Multilayers under Mechanical Confinement

Using a combination of neutron reflectivity and a surface force type apparatus, the hydration of polyelectrolyte multilayers, PEMs, is investigated under mechanical confinement. The samples consist of poly(styrenesulfonic acid), PSS, and poly(allylamine hydrochloride), PAH, which were formed by spin-coating with 11, 23, and 47 bilayers. The terminating layer polyelectrolyte of the PEMs was also varied. Selectively deuterating some layers and using a model that separately considers the last layer from the other strata representing the material structure of the PEM allow a low-resolution hydration distribution within the PEM to be determined. When swollen with H2O and unconfined, PSS-terminated PEMs are approximately 26% v/v water, and this decreases to 15% with an applied pressure of 5 bar. By comparison, PAH-terminated PEMs are 20% water when unconfined and dehydrate to 10% water at 5 bar. For all samples, the final adsorbed layer is significantly more hydrated at approximately 40% v/v. The water is generally uniformly distributed within the bulk of a PEM; however, there is a significant increase in hydration in the bilayers adjacent to the final layer in PAH-terminated PEMs. Also considered is an experiment where the inherent charge of the final adsorbed layer of a PEM is neutralized by compressing it with an oppositely charged PEM. This determines the hydration for an uncharged PEM and demonstrates that the influence of the potential of the final layer on a PEMs’ internal hydration is a perturbation. A detailed scheme is then presented whereby the amount and distribution of internal charge are directly responsible for the hydration of a PEM.