Gradient chemical micro patterns: a reference substrate for surface nanometrology

We present fabrication routes for a new type of surface specimen that exhibits a micro pattern with a gradient in chemical contrast between the pattern domains. Design elements in the specimen allow chemical contrast in the micro pattern to be related to well-established surface characterization data, such as contact angle measurements. These gradient specimens represent a reference tool for calibrating image contrast in chemically sensitive scanning probe microscopy techniques and a platform for the high-throughput analysis of polymer thin film behavior.     

Oily Nanocoatings

Tribological performance of molecular thermoplastic elastomeric films grafted onto a silicon surface was enhanced by adding a minute amount of paraffinic oil, which was adsorbed by the rubber matrix. The nanocomposite films show bimodal distribution of the surface elastic modulus, caused by the nanodomain structure. Minute amounts of oil, trapped within the rubber phase after evaporation, modified the nanotribological properties of these layers. We observed a significant decrease of the friction coefficient by 40%, along with a lower elastic modulus of the rubber phase. We suggest that under high shear stresses and sliding velocity, oil molecules can be compressed out of the bulk to the surface, facilitating instant local lubrication of the contact area.     

Towards self-lubricated nanocoatings

The microtribological performance of molecularly thick (<10 nm) thermoplastic elastomeric films grafted to a silicon surface was enhanced by adding a minute amount of paraffinic oil, which was adsorbed from vapor phase and held by the rubber matrix. We studied the kinetics of polymer swelling in oil and the formation of polymer gels. We observed that a vast majority of adsorbed oil evaporated from the ultrathin polymer coating leaving a minute amount of oil trapped within the rubber phase. This resulted in a dramatic enhancement of the microtribological performance of the grafted polymer gel layers. These polymer gel layers exhibited a very steady friction response and a small value of the coefficient of friction as well as greater wear-resistance as compared to the initial polymer coating. The performance of polymer gel coatings was much better than the performance of a classic ‘boundary lubricant’ for silicon surfaces, an alkylsilane self-assembled monolayer. The approach proposed demonstrated a new efficient route towards enhanced tribological performance of ultrathin polymer coatings.     

Tribological Behavior of Grafted Polymer Gel Nanocoatings

A robust molecular lubrication layer on a silicon surface has been fabricated from a grafted polymer gel with thickness below 10 nm. A functionalized rubber-glassy block-copolymer was chemically grafted to a silicon oxide surface and its tribological performance was enhanced by vapor saturation with a minute amount of alkyl-based paraffinic oil. A combination of tribological measurements and Auger electron spectroscopy was used to monitor the polymer layer wearing behavior. We observed that unlike a dry polymer layer and a classic boundary lubricant, an alkylsilane self-assembled monolayer, the polymer gel coating exhibited a steady friction response, a very low value of the coefficient of friction, and possessed much higher wear-resistance.     

Nanotribological behavior of tethered reinforced polymer nanolayer coatings

We fabricated molecularly thick thermoplastic elastomeric films with organized microdomain structure and intriguing nanotribological properties. Molecular films from poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) were obtained by a melt/solution grafting to a functionalized silicon surface modified with epoxy-terminated self-assembling monolayers. We varied the thickness of grafted block–polymer films from 1.35 nm (disordered polymer layer) to 9 nm (well defined nanophase structure) and tested their friction, adhesion, shear and wearing properties on a microscale with scanning probe microscopy. Tethered SEBS monolayers, composed of a rubber matrix reinforced by a two-dimensional net of glassy polystyrene (PS) microdomains, possess a friction coefficient as low as 0.02 and shear strength in the range 0.15–1.5 GPa. Chemically tethered SEBS monomolecular films are much more stable under shear stresses than conventional molecular coatings.