AN ATOMIC-FORCE MICROSCOPE STUDY OF GRAFTED POLYMERS ON MICA

Atomic force microscopy is used to measure force profiles and friction forces for the block copolymer PEO/PS physisorbed on mica in xylene, 2-propanol, n-dodecane, and air. The force profiles show the distinctive repulsive forces associated with brushlike configurations in good solvents and shorter range attractive forces in poor solvents. The brushlike profiles show that in addition to being compressed between the surfaces, the polymer chains can also bend out of the tip-surface contact region. The friction data show that the tip is best regarded as a single asperity contact and on solid polymer surfaces there is a transition to plowing type friction as the applied force is increased which can be associated with the yielding of the polymeric material. No friction signal could be measured within the polymer brush in a good solvent. Topographic images of the adsorbed polymer in poor solvents are also shown. At submonolayer coverages the polymer agglomerates and during imaging the agglomerates were either broken up or moved if the tip scanning speed was too slow. The polymer could be more eaily imaged by adding 2-propanol which further collapsed the polymer chains and thus strengthed the agglomerate structures. In good solvents the tip tends to displace the molecules along the surface and it is concluded that further studies on these systems will be best undertaken with polymers chemisorbed onto the surface.