Formation and properties of mechanically assembled monolayers (MAMs)

Tuning the surface characteristics of materials, including lubrication or wetting, has become of paramount interest for a variety of everyday’s 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 being wettable. Examples of the latter include non-stick layers, marine anti-fouling coatings, surfaces of car windshields or frying pans, etc.

A typical way of adjusting materials surfaces is to deposit a self-assembled monolayer (SAM) of silane- or thiol-based molecules on the surfaces of silica or gold, respectively. However, when exposed to polar liquids, such as water, these SAMs usually lose their low energy surface properties as the water molecules penetrate through the imperfections in the SAMs causing this surface to reconstruct. These non-desirable 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. SAMs are usually formed through self-assembly processes that are governed by the chemical and structural nature of the SAM molecules and the means of their attachment to the substrate. We have recently developed a method that enables us to control the grafting density of molecules anchored to surfaces. 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). We also showed that MAMs assembled from semifluorinated (SF) molecules form superhydrophobic surfaces with superior long-lasting barrier properties.

The upper panel show a schematic illustrating the technological steps leading to the production of MAMs (“mechanically assembled monolayers”). The lower part denotes the dependence of deinozed DI water contact angle on F8H2-MAM samples on the degree of stretching of the PDMS substrate before the UVO treatment, Dx. The lines are meant to guide the eye. Also shown are photographs of a DI water droplet spreading on the F8H2-SAM and F8H2-MAM (Dx=70%) surfaces.

schematic illustrating the technological steps leading to production of MAMs
Plot of dependence of water contact angle

Experiments aiming at investigating the stability of the MAMs – in particularly the resistance of the F8H2-MAMs to surface reconstruction – revealed that these possess long-lasting superhydrophobic properties that do not deteriorate even after prolonged exposure to DI water. In the figure on the left we plot the dependence of water contact angle of F6H2-MAM (squares) and F8H2-MAM (circles) on the exposure time of the FyHx-MAMs to water. The red symbols denote the contact angles measured on FyH2-MAMs with Dx = 0% (FyH2-SAM). The blue symbols mark the contact angles measured on FyH2-MAMs with Dx=70% taken immediately after the water exposure and substrate drying with nitrogen. The green symbols represent the contact angles from the samples denoted by the blue symbols but measured 6 months later (the samples were stored under ambient laboratory condition in Petri dishes with no temperature or humidity control between the water exposure and the measurement). The lines are meant to guide the eye.

We used carbon K-edge near edge x-ray absorption fine structure (NEXAFS) spectroscopy to study the molecular orientation of the MAMs surfaces. The NEXAFS experiments were carried out on the U7A NIST/Dow Materials Soft X-ray Materials Characterization Facility at the National Synchrotron Light Source at Brookhaven National Laboratory (NSLS BNL). NEXAFS proven invaluable in providing molecular-level information on chain mobility in the SF-MAMs exposed to DI water. The F8H2-MAMs prepared on unstretched PDMS-UVO substrates (Dx=0%) stand almost perpendicular to the sample surface. When exposed to DI water, the chain orientation starts to disappear very rapidly and after about 1 day of DI water exposure, the F8H2-MAMs with Dx=0% disorient completely. In contrast, the NEXAFS experiments revealed that on samples exposed to DI water for up to 7 days is indistinguishable from that measured on the same specimen before the DI water exposure. These results thus provide further evidence that the F8H2 molecules in MAMs with Dx70% are closely packed and mechanically interlocked; this interlocking hinders the chain’s tendency to move and reconstruct on the MAM surface.

Figure showing F8H2 molecules in MAMs closely packed and mechanically interlocked
Graph showing surfaces of Hx-MAMs remain stable

We also formed MAMs from hydrocarbon-based silanes, H(CH2)xSiCl3 (Hx) with x=8 and 16. We showed that the molecular order within the H-MAM can be tailored by varying the length of the grafted molecule, the number of molecules on the surface, and the degree of packing. The data in the figure on the left show that the surfaces of the Hx-MAMs remain very stable even after prolonged exposures to water and their values are similar to those obtained by mechanically assembling the SF moieties. In addition, the stability of the MAMs is much higher than those of SAMs.

There is one very important technological implication resulting from our findings presented here. If one can prepare non-stick substrates with long lasting barrier properties from Hx-MAMs (rather than SF-MAMs), the cost of the coating can be dramatically reduced because the price of hydrocarbon compounds is several times lower than that of fluorocarbons.

This technology has been protected by the US Patent No. 6-423-372 B1.