An experimental study of adhesion and conformational transitions in polyelectrolytes
We revisit the pH-induced conformational structure of poly(methacrylic) acid (PMAA) chains in dilute solution using a combination of fluorescence techniques: nonradiative energy transfer, fluorescence lifetime, and time-resolved anisotropy. These measurements are supplemented with dynamic light scattering and transmission electron microscopy experiments. The fluorescence lifetime results suggest that the PMAA has a more complex structure than previously considered, with graded changes in density in the collapsed state (low pH). Such structure also suggests that the swelling of these dilute chains is of a progressive nature, with the outermost parts responding to the changing pH before the central regions, rather than a simultaneous swelling. We have used neutron reflectometry to characterize the volume fraction-depth profiles of poly[(diethyl amino)ethyl methacrylate] (PDEAMA) and poly(methacrylic) acid (PMAA) brushes in aqueous solution as a function of pH. The polymers were synthesized by atom transfer radical polymerization in order to create a dense brush layer. In the case of PDEAMA .the brushes are collapsed at high pH, and swell at a pH of between 3 and 4. The depth profiles of swollen brushes have an unusual shape, with depletion in brush concentration close to the substrate, which increases away from the substrate. Such a profile is unexpected for weak polyelectrolytes and we propose that entanglements created during the growth of the brush are an important consideration, creating a long-lived metastable equilibrium. The PMAA brush was exposed to a pH cycle, showing a collapsed and expanded brush at low and high pH values respectively. In addition, we introduce and test here a new technique as a means to measure interactions between polymer networks and brushes when immersed in aqueous solution. The important feature offered by this technique is that measurements are performed in situ. We have performed measurements showing that there is a difference between the behaviour of PMAA gel/blank silicon and PMAA gel/brush systems. These early results show interactions occurring at the interface between gel and brush, which may be due to adhesion.