Lubricating effect of thin films of styrene-dimethylsiloxane block copolymers
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The lubricating effect of styrene-dimethylsiloxane block copolymer films spin cast on Si wafers was found to increase with increasing size of the poly(dimethylsiloxane) (PDMS) block. Friction properties of symmetrical and close-to-symmetrical block copolymers were evaluated in contact with bromobutyl rubber in a plate-on-plate configuration. X-ray photoelectron spectroscopy showed that the copolymer films were all covered by an at least a 10 nm thick 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 dewetted the wafer and formed discrete domains with sizes comparable to R-0. The copolymer could further be blended with up to 70 wt % polystyrene without detecting a decrease in the lubricating effect. A mechanism is proposed for the observed frictional behavior where microflakes are abraded from lamellar copolymer surfaces and stick to the contacting rubber surface, whereafter lubrication may result from sliding of PDMS brushes against each other.