Interdiffusion in Polyelectrolyte Multilayers

Abstract

Using neutron reflectivity, the internal structure of polyelectrolyte multilayers is described on the nanoscale. Each film consists of a protonated and a deuterated block, built from x protonated and y deuterated polycation/polyanion bilayers, respectively. The number of bilayers N = x + y is kept constant; the position of the interface between the blocks is varied systematically. The polyanion is poly(styrenesulfonate) (PSS), and the polycation is poly(allylamine hydrochloride) (PAH) or poly(diallyldimethylammonium chloride) (PDADMAC). Always, the first four to five bilayers are thinner than the average bilayer thickness, but the three terminating bilayers are sometimes thicker. In the core zone, the bilayer thickness is constant. The internal roughness is smallest next to the film/air interface and increases with the number of bilayers away from the film/air interface. This suggests that each deposition step promotes the interdiffusion of the supporting layers. At the selected preparation conditions, the internal roughness increases more for PDADMAC/PSS than for PAH/PSS; the diffusion constants differ by 2 orders of magnitude.

Divalent–Anion Salt Effects in Polyelectrolyte Multilayer Depositions

“…The void formation is exacerbated during the early stages of PEML deposition by the known ability of the substrate roughness and substrate−polyelectrolyte interactions to affect PEML structure and morphology. 21 During the salt rinsing steps, excess PAH solution at the film surface is first replaced by a corresponding polyelectrolyte-free mixed SO 4 2− /Cl − salt solution. The next rinse with 1.00 M NaCl (aq) solution exchanges SO 4 2− dianions in the adsorbed PAH aggregates for Cl − anions, concomitantly reducing the hydration of the aggregates, because SO 4 2− is a kosmotropic anion that binds water well, 74 and forcing partial deprotonation of the PAH amine sites to maintain electroneutrality ( Figure 1A).…”

Depth-profiling X-ray photoelectron spectroscopy (XPS) analysis of interlayer diffusion in polyelectrolyte multilayers

“…These techniques include confocal microscopy (23)(24)(25), FRET (26,27), FTIR (28), neutron reflectivity (29)(30)(31), and X-ray reflectometry (32). Confocal microscopy is limited in spatial sensitivity, as films much thicker than the typical PEM thickness (≤500 nm) are required because of a relatively low z-resolution (23)(24)(25).…”

Anisotropic Diffusion of Polyelectrolyte Chains within Multilayer Films

“…For example, salt-induced diffusion of PEs in the direction parallel to the surface was observed with several PEM systems using FRAP, 7,8 while intermixing of polymer layers was studied in different PEM systems using NR. 10,11,12 Because of the inherently anisotropic structure of PEMs, it is reasonable to suggest that chain mobility might be different in directions parallel and perpendicular to the substrate. However, the experiments testing this suggestion, as well as the effect of the proposed anisotropic chain motions on film layering have not up to now been carried out.…”

“…12 Assuming that diffusion of polymer chains follows a Gaussian distribution, the D ⊥ was estimated from the equation $\Delta {\sigma }_{\mathrm{int}}^2=2D_{\perp }\Delta t$ ,where Δσ int is the interfacial roughness change of the deuterated layers and Δt is the salt annealing time. 11 Neutron reflectivity curves were collected ascast and for 3-, 6-, and 9-day anneals on one pair of samples, as well as as-cast and for two 3-day anneals on another pair. From fits to these data, D ⊥ of dPMAA for the Q100M/PMAA system was (4.0 ± 1.0) × 10 −19 cm 2 /s, which was at least 4-fold larger than the D ⊥ of dPMAA in PDMA/PMAA multilayers of (1.1 ± 0.3) × 10 −19 cm 2 /s.…”