Asymmetric Growth in Polyelectrolyte Multilayers
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Abstract
Radioactive
counterions were used to track the ratio of positive
to negative polymer repeat units within a polyelectrolyte multilayer
made from poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene
sulfonate), PSS. For this widely employed pair of “linearly”
assembled polyelectrolytes it was found that the accepted model of
charge overcompensation for each layer is incorrect. In fact, overcompensation
at the surface occurs only on the addition of the polycation, whereas
PSS merely compensates the PDADMAC. After the assembly of about a
dozen layers, excess positive sites begin to accrue in the multilayer.
Treating the surface as a reaction–diffusion region for pairing
of polymer charges, a model profile was constructed. It is shown that
different reaction–diffusion ranges of positive and negative
polyelectrolyte charge lead to a blanket of glassy, stoichiometric
complex growing on top of a layer of rubbery, PDADMAC-rich complex.
Though overcompensation and growth was highly asymmetric with respect
to the layer number, entirely conventional “linear”
assembly of the multilayer was observed. The impact of asymmetric
growth on various properties of multilayers is discussed