Assembly of oppositely charged macromolecules (proteins,
DNA, polyelectrolytes)
is often used for surface modification and functionalization. Yet,
it remains a challenge to control the position and mobility of the
molecules within the assembly. Using polyelectrolyte multilayers as
model systems, we study the diffusion constant of the polyanion PSS. D
PSS could be varied by 5 orders of magnitude.
Two parameters were found to be important: (i) the conformation of
the polyelectrolytes and (ii) the molecular weight of the polycation
(M
w(PDADMA)); the latter was the dominant
parameter. Independent of conformation, by increasing M
w(PDADMA), D
PSS decreased
by at least 3 orders of magnitude when M
w(PDADMA) increased by a factor of seven. The decrease was stronger
than predicted by any scaling law; it was either exponential or abrupt
after D
PSS was almost constant for low M
w(PDADMA). The polymer conformation was adjusted
with the salt concentration in the preparation solution. Flatter and
less entangled chains led to an increase in D
PSS. These findings on the time dependence of the internal
structure of assemblies are discussed in the context of network theory.
Polyelectrolyte multilayers are usually prepared from polydisperse polyelectrolytes. It is desirable to measure and control multilayer composition when it deviates from the deposition solution. With neutron reflectivity, multilayers prepared from PDADMA and binary mixtures of long deuterated PSSd long (80.8 kDa) and short protonated PSS short (10.6 kDa) were investigated. A small amount of PSS long in the deposition solution led to a disproportionate increase of PSS long in the film, and completely suppressed the exponential growth regime. Adsorption kinetics were studied with in situ ellipsometry: (i) During adsorption of a PSS layer, the PSS long fraction increased with adsorption time; PSS short could desorb, while PSS long adsorbed irreversibly. (ii) During adsorption of a PDADMA layer, a fast thickness increase was followed by a slow thickness decrease. This was attributed to the formation of PDADMA/PSS short complexes, which eventually desorbed. The desorbed thickness depended on the number of layers deposited. The mechanism of layer formation is attributed to the asymmetric growth of PDADMA/PSS multilayers, as supported by the adsorption kinetics of multilayers prepared from one kind of PSS.
Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings, but the dynamics of the constituent polymer chains remain poorly understood. Using neutron reflectivity, the vertical diffusion of polyanion poly(styrene sulfonate) (PSS) (M w (PSS) = 75.6 kDa) within PSS/poly(diallyldimethylammonium) (PDADMA) (M w (PDADMA) = 72.1 kDa) multilayers is probed, while the annealing temperature and salt concentration were varied. Only one fraction of PSS was mobile, and the other PSS molecules were assigned to a second, almost immobile fraction, with the respective diffusion constants D PSS,fast and D PSS,slow differing by about two orders of magnitude. The model with the two different mobile fractions is the simplest one that describes the time dependence of the scattering length density profiles. The relationship between the diffusion constants and the annealing temperature can be described using the Arrhenius equation. From this, the activation energies could be determined. Conditions for Fickian diffusion and for the formation of different mobile fractions are discussed.
For engineering and
biomedical applications, nanometer-thin films
with high electrical conductivity in aqueous solutions are desirable.
Multilayers of polydimethyldiallylammonium chloride (PDADMA) and oxidized
carbon nanotubes (CNTs) were built using the layer-by-layer technique.
CNTs with a low linear charge density were used. The surface coverage
of the CNTs was monitored with optical absorption. The film thickness
and the surface coverage of the CNTs increased linearly with the number
of CNT/PDADMA bilayers deposited. On immersion into aqueous solutions,
the film thickness decreased or remained constant. This finding is
attributed to the hydrophobic character of the CNTs and the backbone
of PDADMA. The films showed ohmic behavior, both in air and in solutions.
The electrical conductivity was 0.95 × 104 S/m in
air and increased to 1.36 × 104 S/m in solution,
provided the thickness of the CNT/PDADMA bilayers was as low as 1.9
nm. We suggest that high electrical conductivity can be achieved by
flat adsorption of the CNTs.
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