Tuning the Surface Properties of Elastomers Using Hydrocarbon-Based Mechanically Assembled Monolayers
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DD10.3.1
Tuning the surface properties of elastomers using hydrocarbon-based mechanically assembled monolayers KIRILL EFIMENKO AND JAN GENZER. Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905
ABSTRACT We use static contact angle measurements and near-edge absorption fine structure to elucidate the surface structure and molecular orientation of hydrocarbon-based mechanically assembled monolayers (H-MAMs), structures formed by combination of assembly of alkyl moieties onto flexible elastomeric substrates and mechanical manipulation of the substrates. Specifically, we report that the organization of the grafted molecular array (“liquid”-like vs. solid-like) can be tailored by varying the degree of stretching of the elastomeric substrate. We also show that the H-MAM surfaces exhibit excellent stability.
INTRODUCTION Tuning the surface characteristics of materials, including lubrication or wetting, has become of paramount interest for a variety of everyday applications. For example, while in some situations surfaces are required to be completely wettable (i.e., the surfaces of metals before paint deposition), in other applications one needs to prevent the surfaces from wetting. Examples of the latter include non-stick layers, marine anti-fouling coatings, surfaces of car windshields or frying pans, etc. Various experimental techniques, based on either physical of chemical treatment, exist that allow one to modify surface properties of materials. A typical way of adjusting material surfaces is to deposit a self-assembled monolayer (SAM) of silane- or thiol-based molecules on the surfaces of oxides (e.g., silica) or noble metals (e.g., gold), respectively [1]. While simple to perform, this methodology usually produces surfaces, which contain many structural defects [2]. When exposed to polar liquids, such as water, these defect-containing SAMs usually surface reconstruct as the water molecules penetrate through the imperfections in the SAMs. These nondesirable surface reconstruction effects can likely be minimized (or even completely prevented from occurring) by increasing the packing density of the SAMs through increasing the density of the grafting points at the surface. However, tailoring the grafting density of the SAM chains is not an easy task because as mentioned earlier, SAMs are formed through self-assembly processes that are governed by the chemical and structural nature of the SAM molecules as well as the means of their attachment to the substrate. We have recently developed a method that enables us to control the grafting density of endanchored molecules. Specifically, we demonstrated that the combination of the self-assembly with mechanical manipulation of the grafted molecules on surfaces provides a means of fabricating ”mechanically assembled monolayers” (MAMs) [2]. The method for fabricating MAMs made of silane molecules is schematically shown in the upper portion of Figure 1. Previously we have shown that MAMs assembled from semifluorinated (SF) molecules form
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