Quasi-static and hydrodynamic interaction between solid surfaces in polyisoprene studied by atomic force microscopy

Forces across polymer melts are poorly understood despite their importance for adhesion and the structure of composite materials. Using an atomic force microscope (AFM) this interaction was measured for 1,4-polyisoprene (PI, MW = 1.9-10.2 kDa). Weak repulsive forces which decayed with characteristic decay lengths of 0.4-1 nm were observed on silicon wafers, HOPG, and mica. This indicates that, unlike poly(dimethylsiloxane) (PDMS), PI does not form an immobilized layer. The results confirm theoretical predictions that no long-range force exist across polymer melts in thermodynamic equilibrium. In addition to quasi-static experiments (with microfabricated silicon nitride tips at low approaching velocity), hydrodynamic experiments (with attached glass microspheres as probes at high approaching velocity) were carried out with PI and PDMS (MW = 5.9, 8.0, 18.8 kDa). In some cases slip was observed. Slip was correlated with the quasi-static forces: weak quasi-static forces (observed with PI and short-chain PDMS) were correlated with slip in hydrodynamic experiments, while strong repulsive forces observed with long-chain PDMS are correlated with the absence of slip.