(715c) Interfacial Friction Control By Surface Free Energy Modification of Elastomers

Polymeric interfaces can be used to manipulate friction in tasks that involve sliding motion in wet environments. These frictional changes can be achieved by controlling the lubricant film thickness and wetting properties of the elastomer poly(dimethyl siloxane) (PDMS) which has robust mechanical properties. To investigate the effect of surface energy on the sliding friction coefficient of wetted PDMS substrates, we add 1-2 wt% poly(ethylene oxide) (PEO)-b-PDMS block copolymer to bulk PDMS (Sylgard 184) and show a decrease in steady state water contact angle from 106.3°± 4.7° for unmodified PDMS to 27.5° ± 1.1° for substrates doped with 2% PEO-b-PDMS. Analysis of the data, using the Owens-Wendt-Rabel-Kaelble (OWRK) model, shows a 109% increase in polar group components of the 2% PEO-b-PDMS samples when compared to that of pristine PDMS, while the dispersive contributions remained statistically similar. Time-of-flight secondary ion mass spectrometry shows the presence of distinct PEO ionic signatures on the surface, indicating that the block copolymer migrates to the interface dynamically. A normal load of 1.5N is applied to the PDMS substrates submerged in different fluids (pure water and 40-80 wt% glycerol-water mixtures) over a sliding speed range of 0.1 - 80 rads s^-1.The results show that the presence of PEO-b-PDMS caused a reduction in the boundary friction coefficient from m = 1.7 in unmodified PDMS to m = 0.72 for 1% PEO-b-PDMS and m = 0.27 for 2% PEO-b-PDMS. However, there is a time dependent migration of the block copolymer to the interface, which affects the nature and size of the interfacial film between the hydrophilic substrates. Prolonged wait times correspond to full surface migration of polar groups and limits lubricant expulsion at the sliding contact, thereby lowering the boundary friction.