Systematic modification of the rheological properties of colloidal suspensions with polyelectrolyte multilayers

Tailoring rheological properties of colloidal suspensions with the adsorption of polyelectrolyte multilayers (PEMs) is based on the idea of controlling macroscopic mechanical properties by modifying the particle surface in a reproducible and well-understood manner. With layer-by-layer self-assembly, monodisperse polystyrene particles are coated with up to ten layers of the oppositely charged strong polyelectrolytes: poly(diallyl dimethyl ammonium chloride) and poly(styrene sulfonate). The conformation of the adsorbed polyelectrolyte is controlled by the ionic strength of the used aqueous polyelectrolyte solution. For 1M NaCl solution, a brushlike adsorption of the polyelectrolyte is expected. The ability of PEMs to serve on a nanoscale level as surface modifiers and influence macroscopic rheological properties like viscoelasticity, yield stress, and shear banding is discussed. The mechanical behavior of these suspensions is qualitatively described by the theory of Derjaguin-Landau-Verwey-Overbeek with short-range repulsion and long-range attraction. A scaling rule is proposed which distinguishes between the precusor and the multilayer regime.

Figure 1

Procedure for static yield-stress estimation: strain $\gamma$ as a function of applied shear stress. The solid line represents the linear stress-strain regime with the slope $1/G_0$. The yield stress ${\sigma }_y$ is indicated by an arrow.

Figure 2

Solidlike properties of the suspensions; evolution of (a) yield stress ${\sigma }_y$ and (b) elastic modulus $G_0$ with the number of polyelectrolyte layers. The lines in (a) and (b) serve to guide the eye [PSS-terminated PEMs (squares), PDADMAC-terminated PEMs (triangles), and bare particles (diamonds)].

Figure 3

Imposed decreasing shear stress vs the strain rate. After performing a stress hystereses loop (a), the strain rates for PSS-terminated suspensions with 6, 8, and 10 layers switch to negative strain rates when the applied stress drops below a critical value (b).

Figure 4

Startup flow curves for all investigated suspensions; (a) PDADMAC- and (b) PSS-terminated PEMs. Inset in (a): An increasing layer number (arrow) lead to a developing stress plateau.

Figure 5

The scaled yields stress ${\sigma }_y/G_0$ as a function of the layer number [PSS-terminated PEMs (squares), PDADMAC-terminated PEMs (triangles), and bare particles (diamond)].

Figure 6

Particle surface characterization. (a) AFM height images of bare particles (left) and of PEM-coated particles with 2 (middle) and 8 (right) layers. The size of the images is 500 $\times$ 500 nm. The scale bar indicates the height. (b) Power spectral density (PSD) vs the spatial frequency $k$.

Figure 7

Calculated interaction potential $U\left(r\right)/k_{B}T$ (solid lines) as a function of the separation distance $r$ with increasing charge heterogeneity using $A=0$ (a), $A=5\times {10}^7$ (b), $A=1\times {10}^8$ (c), and $A=5\times {10}^8$ (d). The dashed lines depict the corresponding attractive contributions $U_{\text{vdW}}+U_{\text{pc}}$.