To better understand surface forces across polymer melts, we measured the force between two chemically well-defined solid surfaces in a melt of polymer chains with a functional end group. As for surfaces, we used self-assembed monolayers (SAMs) of alkyl thiols with different end groups (methyl, amino, and hydroxyl) on gold. The polymer was a hydroxyl-terminated polyisoprene. To measure the force, an atomic force microscope was used. Between methyl-terminated SAMs, a weak and short-range repulsion was detected. Between hydroxyl or amino-terminated SAMs, a strong and long-range repulsion was observed up to distances of 16 nm. This indicates that the hydroxyl group of the polymer binds to the hydroxyl or amino groups of the SAMs. It forms a brush-like structure, which leads to steric repulsion. On amino-terminated SAMs, force-versus-distance curves on approach and retraction were monotonically repulsive and reversible. With hydroxyl-terminated SAMs, a jump was observed on approach when the load exceeded a certain threshold. On retraction, an adhesion had to be overcome. We interpret the jump as a rupture of the polymer layer. It indicates that the kinetics of bond and brush formation is faster on OH-SAMs than on NH2-SAMs.

The biomimetic adhesion polymer poly[(dopamine acrylamide)-co-(butylamine acrylamide)] shows strong adhesion even in aqueous environments. With single-molecule atomic force microscopy experiments, we show that the adhesion force does not depend on the density of the functional dopamine groups.

Using atomic force microscopy, the force between solid surfaces (oxidized silicon) was studied in a melt of end-functionalized polymers (hydroxyl-terminated polyisoprene). The surfaces repelled each other in both approach and retraction. Comparison to results obtained with methyl-terminated polyisoprene shows that the hydroxyl groups bind to the silicon oxide surfaces, and a polymer brush is formed. Thickness and stability of the brush increase with the molecular weight, possibly due to entanglement. With increasing humidity the repulsive force decreases and changes to attraction in the retracting part. This implies that water adsorbs to the interface and destabilizes the bond between the hydroxyl group and the oxidized silicon.

Forces between solid surfaces across polymer melts are poorly understood despite their fundamental importance and their relevance for making composite materials. Such force measurements reveal information on the structure of polymers at surfaces and of confined polymers. Experiments with the atomic force microscope and polyisoprene ( PI) confirmed theoretical predictions that no long-range force should be present in thermodynamic equilibrium. In poly( dimethyl siloxane) ( PDMS) repulsive forces are observed at high molar mass. We attribute this to the formation of an immobilized layer caused by a slow release of adsorbed segments enhanced by entanglement. In low molar mass PDMS attractive forces were observed which we can not yet explain. Attaching a hydroxyl end group to PI or PDMS chains lead to repulsive forces caused by the formation of a brush-like structure.