Influence of pH on the Growth and the Local Dynamics of Polyelectrolyte Multilayers

Abstract

Polyelectrolyte multilayers (PEMs) have been prepared using the strong polycation poly­(diallyldimethylammonium chloride) and the weak polyanion poly­(ethylene-alt-maleic acid) [P­(E-alt-MA)]. The spin label (SL) 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide was covalently attached to the polyanion [SL-P­(E-alt-MA)] to monitor changes in the signal intensity as a measure of the layer growth and the line shape of the spectra indicative of the polymer chain mobility obtained by continuous wave electron paramagnetic resonance spectroscopy. The aim of this study was to investigate the influence of the preparation conditions. We show the influence of the pH value on the buildup and the mobility of PEMs with 16 and 17 layers, respectively. In the first part, the spin-labeled polyanion was used in the preparation of each polyanion layer to monitor the growth of the multilayers. For all pH values from 3 to 6 a non-linear growth behavior was obtained at the beginning of the PEM buildup where the surface is not yet fully covered, leading to a linear growth after reaching a transition point. This point depends on the pH value. In a second study, the spin-labeled polyanion was selectively placed in one layer to display changes in the rotational mobility of the SL in this layer within the multilayers. As a result, an increase of the rotational mobility of SL-P­(E-alt-MA) with increasing pH value was observed.

EPR Spectroscopy as an Efficient Tool for Investigations of Polyelectrolyte Multilayer Growth and Local Chain Dynamics

“…A reason could be the nonelectrostatic interactions as possibly additional hydrogen bonds at pH 3, leading to more inter- and intramolecular interactions between the polymers, and thus, to lower mobility of SL-P­(E- alt -MA) within the PEMs. At pH 5 and 6, there should be no significant loss of radical concentration visible, as stated in our earlier study . Indeed, the calculated loss of 8% (pH 5) and 6% (pH 6) are within the uncertainty of the plotted values I Int .…”

“…These values have been calculated by the averaged differences (1 d/6 d) of N = 22–30 (last 5 values). This loss of radical concentration at low pH was already described in the literature . At low pH, the radical is not stable.…”

“…Here, the anisotropies of the A and g tensors are averaged out completely. The opposite, the rigid-limit spectrum can be obtained by recording an EPR spectrum of a frozen solution of SL-P­(E- alt -MA) at 123 K. A limiting value of about 77 G was determined in our earlier study . In this powder-pattern-like spectrum, the full anisotropy of the tensors can be displayed.…”

“…For all samples, the spectra show the slow-tumbling component representing the SL-P­(E- alt -MA) in the PEMs. For comparison, a fast regime can be represented by the SL 4-amino-TEMPO in solution with a 2 A zz ′ value of about 40 G . Here, the anisotropies of the A and g tensors are averaged out completely.…”

Polyelectrolyte Multilayers on Soft Colloidal Nanosurfaces: A New Life for the Layer-By-Layer Method

“…However, they neglect the implication of the emergence of heterogeneous structures in the emergence of non-linear growth on polyelectrolyte multilayers. Very recently, Naas et al [129] has shown using electron paramagnetic resonance spectroscopy that the appearance of different types of growth in polyelectrolyte multilayers results from a combination of different aggregation patterns of polyelectrolyte multilayers upon the deposition and modifications on the adsorption dynamics.…”