Near surface concentration profile of sheared semi-dilute polymer solutions

TL;DR

The study addresses how semi-dilute polystyrene solutions organize near a smooth solid interface under rest and shear. Using a custom neutron reflectivity setup complemented by SANS and XRR, it directly quantifies a depletion layer with thickness $d$ that decreases from about 109 Å at $\phi=3\%$ to 65 Å at $\phi=6\%$, and detects a small adsorbed fraction near the surface. Under shear with $Wi$ up to about 0.03, the depletion layer remains essentially unchanged, indicating limited interfacial restructuring at these flow rates. The results imply that solvent–surface affinity controls depletion and point to tuning strategies via solvent choice or higher shear rates to uncover potential shear-rate dependent interfacial behavior, with implications for interfacial rheology and slip in polymeric flows.

Abstract

Controlling the structure of polymer solutions near a solid surface is crucial for many industrial processes, as it significantly impacts solution flow and influences slip at the interface. To date, only a few techniques have been developed to experimentally investigate this type of interface at the nanometric scale of solid/liquid interaction. In this study, we probe the interface between a smooth sapphire surface and a semi-diluted polystyrene solution, using neutron reflectivity. A special setup for flow measurements under shear has been designed and optimized. Our results show that, at rest, polymer chains are globally depleted from the solid surface. Contrary to common assumptions, some polystyrene chains do adsorb onto the wall. Under flow conditions, we experimentally demonstrate that the depletion layer remains stable, a finding that has been hypothesized but only vaguely confirmed in the literature.

Figures (5)

• Figure 1: (a): 3D view of the shear cell used for neutron reflectivity. The fluid is injected through the PTFE block (white) block and flows on the substrate represented in dark gray. The fluid gap is 1 mm thick. (b): COMSOL flow simulation of a newtonian fluid ($\eta=11$ Pa.s)
• Figure 2: Effect of bulk volume fraction $\phi_{b}$. (a): reflectivity curves. Solid lines correspond to the Fresnel reflectivity if the solution was homogeneous until the solid surface. Dashed lines correspond to fits from which the SLD profiles (b) and thus the volume fraction profiles (c) are extracted.
• Figure 3: Top: Depletion layer thickness $d$ issued from the fit of the reflectivity data and blob size of the bulk solution measured using SANS as a function of the polymer volume fraction. The dashed line correspond to the scaling law of the blob size in good solvent conditions ($\xi\approx\phi^{-3/4}$). Bottom: Cartoon of the interface. The size of the depletion layer $d$ is typically the diameter of the blob $2 \xi$
• Figure 4: (a): Neutron reflectivity curves of DEP on clean sapphire (blue diamonds, fit is the blue dashed line) and on sapphire which has been incubated with dPS/DEP solution prior to the experiment (orange squares). (b): X-ray reflectivity curve of the sapphire surface that has been incubated with PS/DEP solution ($\phi = 6 \%$, $M_{n} = 708$ kg/mol), and then rinsed and dried (orange points). The black line is a fit with a rough interfacial layer. Green points correspond to the reflectivity of the clean substrate. Inset: Electronic density as a function of the distance from the interface. Corresponding cartoons are plotted on the right.
• Figure 5: Effect of the flow on the reflectivity profiles. (a): reflectivity curves and corresponding SLD profiles in the inset. (b): Size of the depletion layer extracted from the fits as a function of the Weissenberg number Wi. These measurements have been done with $1.56$ Mg/mol dPS/DEP at a volume fraction $\phi = 6\: \%$.