Using a combination of neutron reflectivity and a surface force type apparatus, the hydration of polyelectrolyte multilayers, PEMs, is investigated whilst under a confining pressure. PEMs were fabricated from poly(styrenesulfonic acid), PSS and poly(allylamine hydrochloride), PAH, by spin coating on a silicon substrate. The polyelectrolyte used as the terminating layer of the PEM was varied. By selectively deuterating some layers, a low resolution distribution of the water volume fraction within the PEM can be determined. When swollen with water and unconfined, PSS-terminated samples are approximately 26% v/v water and this decreases to 15% when a confining pressure of 5 bar is applied. By comparison, PAH terminated samples are 20% water when unconfined and dehydrate to 10% water at 5 bar. The water is generally uniformly distributed within the bulk of the sample, though bi-layers nearest to the final adsorbed layer of a PAH-terminated sample are found to have a significant increase in their hydration. Also considered is an experiment where the inherent charge of the final adsorbed layer of a PEM is neutralised by compressing it with an oppositely terminated PEM, adsorbed onto the flexible membrane of the apparatus. This allows the level of hydration expected for an uncharged PEM to be assessed and demonstrates that the influence of the excess charge in the final adsorbed layer only acts as a perturbation to the internal hydration of the sample. A detailed scheme will be discussed whereby the amount of internal charge is directly responsible for the hydration of a PEM.

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Last edited: Friday September 10, 2010

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