Molecular conformation of polyelectrolytes inside Layer-by-Layer assembled films

Abstract

Among all methods available for the preparation of multifunctional nanostructured composite materials with remarkable functional properties, Layer-by-Layer (LbL) assembly is currently one of the most widely used techniques due to its environmental friendliness, its ease of use and its versatility in combining a plethora of available colloids and macromolecules into finely tuned multicomponent architectures with nanometer scale control. Despite the importance of these systems in emerging technologies, their nanoscopic 3D structure, and thus the ability to predict and understand the device performance, is still largely unknown. In this article, we use neutron scattering to determine the average conformation of individual deuterated polyelectrolyte chains inside LbL assembled films. In particular, we determine that in LbL-films composed of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) multilayers prepared from 2 M sodium chloride solutions the PSS chains exhibit a flattened coil conformation with an asymmetry factor of around seven. Albeit this highly non-equilibrium state of the polymer chain, its density profiles follow Gaussian distributions occupying roughly the same volume as in the bulk complex.

Fig. 1. Specular neutron reflectometry.

a NR (log scale) of a d-PSS/PAH multilayer with 73 layer pairs prepared by dipping from 2 M NaCl solutions (red points, error bars showing the statistical counting error). The black solid line corresponds to a fit to the data. Data were collected on the D17 reflectometer at the ILL. b Part of the SLD profile (red solid line) corresponding to the fit to the NR curve from (a) (Fitting parameters can be found in the Supplementary Table. 2). The broken black line is the SLD profile of a single PSS layer and the blue solid line is a Gaussian fit to the former. The small arrow underneath the maximum of the broken line indicates the radius of gyration of the Gaussian fit.

Fig. 2. Small Angle Neutron Scattering.

Transmission SANS cross-sections for a dipped film (red squares, 75 layer pairs) and a sprayed film (black triangles, 80 layer pairs) on a log-log scale, both prepared in 2 M NaCl solutions. The lines going through the data points are fits as explained in the main text; the other lines are slope indicators. Error bars show the statistical counting error.

Fig. 3. Determination of the single chain radius of gyration.

Guinier plot of the dipped sample scattering cross-section from Fig. 2. The slope is fitted by the solid line indicating a radius of gyration of 180 ± 10 Å.

Fig. 4. Polyanion/polycation interface correlations.

a OSS map of a dipped sample from 2 M NaCl solutions (reflected intensity on logarithmic color scale). The sample consists of 27 layer pairs with every 4th PSS layer deuterated. b DWBA simulation assuming in-plane structural correlation as described in the main text. Both maps are drawn with the same color scale and identical axes.

Fig. 5. Grazing Incidence Neutron Small Angle Scattering.

GISANS pattern (arbitrary counts on linear color scale) of the dipped film from 2 M NaCl solution with 53 layer pairs at a wavelength of 3.2 Å.