Direct Force and Friction Measurements Reflecting Structural Changes in Confined Diblock Copolymer Liquids

We have measured the normal and shear forces for a symmetric polystyrene-polyisoprene (PS-PI) diblock copolymer liquid in toluene, a good solvent for both blocks, between two mica surfaces. The aim was to investigate the structure of the macromolecular liquid when confined between surfaces separated by molecular-sized distances. The surface forces apparatus was used for these studies, and the microstructure was manipulated by varying the fraction of diblock in solution. Measurements of the equilibrium normal force profiles (intermolecular force versus separation distance between the confining surfaces) were made. All of these normal force profiles exhibited monotonic repulsive behavior. We found that in the disordered regime, as the fraction of diblock was increased, the intermolecular forces became much longer-ranged. As the fraction of diblock was further increased to concentrations near the bulk order-disorder transition (ODT), the ranges of the force profiles unexpectedly collapsed to shorter distances. In a series of dynamic surface forces experiments, the frictional force required to slide steadily one surface past the other with the macromolecular liquid intervening was measured as a function of shear rate. The shearing results reflected the same pattern of behavior exhibited in the normal force experiments. We determined that the frictional force of the most concentrated liquid was distinctly lower than that of the disordered samples at all shear rates and attribute both the equilibrium and dynamic results to be reflective to the inherent structure of the liquid. We believe that the reduction in the range of the normal force profiles and the reduction in frictional forces at the highest concentrations reflect pretransitional ordering in the confined sample. Our results are discussed in terms of surface-induced ordering and composition fluctuations, which are not accounted for in the mean-field description of polymer phase behavior.