Lubricating effect of thin films of styrene-dimethylsiloxane block copolymers

The lubricating effect of styrene-dimethylsiloxane block copolymer films sp in cast on Si wafers was found to increase with increasing size of the poly (dimethylsiloxane) (PDMS) block. Friction properties of symmetrical and clo se-to-symmetrical block copolymers were evaluated in contact with bromobuty l rubber in a plate-on-plate configuration. X-ray photoelectron spectroscop y showed that the copolymer films were all covered by an at least a 10 nm t hick layer of PDMS. The lubricating effect remained constant down to a film thickness comparable to the chain dimensions of the copolymer, R-0, after which it decreased progressively. At thicknesses below R-0, the copolymer d ewetted the wafer and formed discrete domains with sizes comparable to R-0. The copolymer could further be blended with up to 70 wt % polystyrene with out detecting a decrease in the lubricating effect. A mechanism is proposed for the observed frictional behavior where microflakes are abraded from la mellar copolymer surfaces and stick to the contacting rubber surface, where after lubrication may result from sliding of PDMS brushes against each othe r.