Origin of Higher Friction for Elastomers Sliding on Glassy Polymers

We have designed a novel approach to couple infrared−visible sum frequency generation (SFG) spectroscopy with adhesion and friction measurements to determine the molecular structure of the surface molecules at the contact and sliding interfaces. The frictional force of crosslinked poly(dimethyl siloxane) (PDMS) lens sliding on poly(styrene) (PS) is a factor of 4 higher than that of PDMS sliding on poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC). This higher frictional force of PDMS/PS interface cannot be explained by its adhesion energy or hysteresis. The SFG results indicate that the hydrocarbon PVNODC surface consists of well-ordered crystalline side chains that do not restructure upon mechanical contact and during sliding of the PDMS lens. On the contrary, the SFG results show that the PS phenyl groups are more tilted upon mechanical contact and during sliding compared to that at the PS surface before contact. The change in phenyl orientation can be due to local chain interpenetration that occurs at temperatures much below the T g of PS at the PDMS/PS interface and is the reason behind higher friction forces for PDMS elastomer sliding on glassy PS compared to that on crystalline PVNODC.