Solid surface structure affects liquid order at the polystyrene–self-assembled-monolayer interface

We present a combined x-ray and neutron reflectivity study characterizing the interface between polystyrene (PS) and silanized surfaces. Motivated by the large difference in slip velocity of PS on top of dodecyl-trichlorosilane (DTS) and octadecyl-trichlorosilane (OTS) found in previous studies, these two systems were chosen for the present investigation. The results reveal the molecular conformation of PS on silanized silicon. Differences in the molecular tilt of OTS and DTS are replicated by the adjacent phenyl rings of the PS. We discuss our findings in terms of a potential link between the microscopic interfacial structure and dynamic properties of polymeric liquids at interfaces.

Figure 1

X-ray reflectivity curves normalized to the Fresnel reflectivity on a log${}_{10}$ scale of the bare silicon wafers covered with (a) DTS (lower purple bars) and (b) OTS (lower green bars) as well as the reflectivities for the silanized samples covered with PS (shifted by one, upper red and blue bars, respectively). The solid lines represent fits. (c) and (d) Corresponding scattering length density (SLD) profiles in the same color code as the reflectivity data. To exclude interfacial roughness as the origin of the depletion layer, the dotted black line in (d) was obtained by adding the SLDs of the OTS and PS profiles in air. The resulting simulated reflectivity (dotted line) in (b) demonstrates that the experimental reflectivity data cannot be explained by just a roughness mismatch between OTS and PS. In order to highlight the difference in the depletion layer thickness for OTS and DTS, the blue dash-dotted lines in (a) and (c) denote a simulation of the DTS data with a thicker interfacial layer.

Figure 2

Neutron reflectivities on a log${}_{10}$ scale of the OTS (blue bars, shifted by one) and DTS (lower red bars) substrates covered with deuterated PS. The solid lines are fits corresponding to the SLD profile from the inset in the same color code as the data points ($d$PS on OTS on the left).

Figure 3

Sketch of the molecular conformation at the (a) PS-OTS and (b) PS-DTS interfaces as explained in the text.