The friction behavior of polymer films on solid surfaces has received great interest, due to their importance in practical applications, e.g. lubrication, joint implants, M/NEMS (micro/nano electromechanical devices), etc. The frictional performance of such near-surface polymers is significantly different from that of bulk polymers, and depends upon the interaction strength with the shearing surfaces, which leads to enhanced or reduced interfacial polymer mobility. The glass transition temperature (Tg) of such polymer films is also dictated by the polymer-substrate interaction strength, and this in turn influences the friction force.
We utilized the spin-coating method, and by controlling spin speed and initial polymer solution concentration, prepared a series (10-800 nm thickness) of densely grafted and well-defined polyethylene oxide (PEO), polystyrene (PS) and polyethylene (PE) films on planar graphite, mica, silicon substrates. The friction behavior in these model polymers was then examined by using atomic force microscopy (AFM). It was found that even though the polymer is glassy, the region very close to the surface wall can be liquid-like, so that the near-surface mobility is enhanced and a lubrication layer forms. The stronger or weaker polymer-substrate interaction strength, accompanied by the upward or downward shift of Tg, will result in the reduced or enhanced mobility of polymer films. The friction coefficient µ obtained for PEO, PS and PE films supported by various substrates for a range of thicknesses is further utilized to characterize the mobility. The different molecular motions of polymers result from the shift in Tg which is dictated by polymer-substrate interaction strength, and further influence the friction force. It is noteworthy that the downward shift of the friction coefficient occurs as the polymer thickness is reduced for very small values of polymer-substrate interaction strength but, for greater interaction strength, this trend is reversed. By coating the AFM tip with polymer molecules, the ratio of polymer-substrate adhesion force (FAps) to polymer-polymer adhesion force (FApp) were used to estimate the competitive strength of polymer-substrate attraction. The values of FAps / FApp exhibit the same behavior as above: greater values of experimental FAps / FApp correspond to greater values of polymer-substrate interaction strength, and in this situation, the substrate restricts the near-surface mobility, and Tg rises to further increase the friction force. By contrast, at smaller values of FAps / FApp, a decrease in Tg will be experienced for polymers that have no specific attractive interactions with the substrate on which they have been placed, resulting in a smaller friction force.