In situ Buildup
In the multilayering method of preparing PEC films it is known that, for a given pair of polyelectrolytes, the use of higher salt concentrations, or more hydrophobic salts at the same concentration, yields thicker films. A strong function of thickness on polyelectrolyte hydrophobicity is also observed. The mechanism of buildup is a combination of thermodynamic and kinetic factors. Charge overcompensation of the “top layer” of polyelectrolyte by the adsorbing polymer is driven by thermodynamics, but the range, lcp, that this overcompensation penetrates into the multilayer, which determines the amount absorbed on each cycle, is controlled by the rate of diffusion of the polymer into the film. Doping, a thermodynamic parameter, provides sites (defects) that accelerate the diffusion in a strongly nonlinear fashion which increases lcp. Some polyelectrolyte combinations are so hydrophilic that they are easily doped and lcp is always greater than the film thickness. Such multilayers grow exponentially.
The dependence of film buildup rate on salt and polyelectrolyte type, monitored by FTIR-ATR, is illustrated in this figure. In the upper panel of the buildup of a PDADMA/PSS multilayer is depicted. The polyelectrolytes were assembled in 1.0 M solutions of NaCl, LiCl or KCl. An oscillating pattern in the (strong) sulfonate signal is observed, similar to other previously reported ATR-FTIR work. Whenever an odd (PDADMA) layer is added, the sulfonate band intensity decreases, whereas one would expect the sulfonate absorption band to remain unchanged, since PDADMA contains no SO3-. In fact, when a PDADMA layer is added, the PEMU swells, taking more SO3- groups further from the ATR crystal, where they are sampled more weakly by the evanescent IR wave. Oscillations in water content in PEMUs during buildup have been attributed to a surface charge effect. It is more accurately described as an oscillation in hydration of the outer layer: when the outer layer (of thickness lcp) comprises excess PDADMA compensated by chloride ions, it is more hydrated than PSS balanced with Na+ ions.
With a sufficient number of layers, the PEMU is thick enough to entirely contain the evanescent wave, hence no additional signal is observed, though the multilayer is still growing linearly. The fact that the oscillations also cease is consistent with the claim that they are from PEMU surface effects. For PDADMA/PSS grown in 1.0 M NaCl, we calculate an evanescent wave penetration depth of about 600 nm at the SO3- wavelength. The dry thickness of the (PDADMA/PSS) in 1M NaCl (i.e. “30 layers”) was 980 nm, which would yield a wet thickness of about 2 µm.
In the alkali metal series Li+, Na+, K+, lithium ion is the most hydrated while potassium is the least. K+ is therefore the most effective at swelling the PEMU (see below), providing thicker films. The amount of oscillation seen depends on the thickness of each “layer”: since KCl gives thicker layers there is a greater change when PDADMA is adsorbed. PDADMA is compensated by Cl- in all cases. It may be possible to use “hydration matched” salts where Pol+A- and Pol-M+ are equally hydrated, which should reduce or eliminate the oscillations.
Panel B shows that a more hydrophobic polyelectrolyte couple, P4VMP/PSS, grows much more slowly than PDADMA/PSS in 1.0 M NaCl, which dopes the former PEMU to a much lesser degree. Each “layer” is much thinner, as polyelectrolyte cannot diffuse as far into the multilayer. Since the layers are thinner, the oscillations are also less prominent, as shown in the inset.