Confined polymer melts studied by atomic force microscopy

Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar solid surfaces have been measured across a long-chain poly(dimethyl siloxane) (PDMS, M_W = 18,000 g/mol), a short-chain PDMS (M_W = 4200 g/mol), a poly(ethylmethyl siloxane) (PEMS, M_W = 16,800 g/mol), and a diblock copolymer consisting of one PDMS and one PEMS block (PDMS-b-PEMS, M_W = 15,100 g/mol). The interaction changed significantly during the first 10 h after immersing the solids in the polymer melt. This demonstrates that the time scale of structural changes at a solid surface is much slower than in the bulk. On mica and silicon oxide both polymers formed an immobilized “pinned” layer beyond which a monotonically decaying repulsive force was observed. Attractive forces were observed with short-chain PDMS on silicon oxide and PEMS on mica and silicon oxide. On the basal plane of graphite PEMS caused a stable, exponentially decaying oscillatory force.