Influence of bidisperse self-assembled monolayer structure on the slip boundary condition of thin polymer films

TL;DR

This study investigates how bidisperse self-assembled monolayers (SAMs) with varying compositions influence slip boundary conditions for thin polymer films, revealing a continuous and enhanced slip behavior correlated with alkyl chain density.

Contribution

It introduces a method to continuously tune SAM structure using bidisperse mixtures, demonstrating enhanced slip boundary conditions compared to single-component SAMs.

Findings

Mixed SAMs show increased slip length over single-component SAMs.

Slip length correlates with exposed alkyl chain density.

Slip length varies nearly two orders of magnitude.

Abstract

Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [Fetzer et al., Phys. Rev. Lett. 2005; B\"aumchen et al. J. Phys.: Cond. Matt. 2012] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, this change in boundary condition is affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare SAMs containing bidisperse mixed SAMs of alkyl silanes, with the composition as a control parameter. We find that all the mixed SAMs we investigated show an enhanced slip boundary condition as compared to the single-component SAMs. The slip boundary condition is accessed using optical and atomic force microscopy, and we describe these observations in the context of X-ray reflectivity measurements. The slip length, varying over nearly two orders of magnitude, of identical polymer melts on chemically similar SAMs is found to correlate with the density of exposed alkyl chains. Our results demonstrate the importance of a well characterized solid/liquid pair, down to the angstrom level, when discussing friction between a liquid and a solid.