Surface forces between adsorbed quaternarized poly(2-vinylpyridine) layers: Molecular rearrangements and bridging interactions

The surface forces between quaternarized poly(2-vinylpyridine) (QP2VP) laye rs adsorbed onto mica have been measured over a range of solution pH condit ions. It was found that QP2VP overcompensates the mica's negative charge, r esulting in a positively charged interface. The surface potential at the mi ca/polyelectrolyte interface is a balance of the charge on the adsorbed pol yelectrolyte and the charge on the underlying mica surface. It was also fou nd that, below and near the pK(a) of the polyelectrolyte, segment-segment r epulsion results in the polyelectrolyte adopting a conformation with signif icant extension into solution. Interpenetration of the extended chains lead s to a bridging attraction. At high pH a decrease iri the polyelectrolyte c harge density decreases segment-segment repulsion, resulting in a compact a dsorbed conformation where no bridging attractive jump is observed. An adhe sion is measured between adjacent layers under all pH conditions, regardles s of the presence of an attractive jump in. Here the important distinction is made between an attractive force caused by the overlap of anchored polye lectrolyte chains extending into solution and chain entanglement that occur s due to molecular rearrangements when polyelectrolyte layers are under com pression.