Direct force measurements on bulk polystyrene using the bimorph surface forces apparatus

Interfacial properties of bulk polymer samples have been characterized by direct measurements of long-range and adhesion forces using a non-interferometric bimorph surface forces apparatus. For this purpose, we developed a method where bulk polystyrene was melted and formed into drop-shaped surfaces with a cylindrical shaft. In the case of very smooth polystyrene samples, the adhesion in air and water was very strong and when brought into contact, cold welding of the polymers sometimes took place. Upon separation, it was found that cohesive failure occurred and the surfaces were locally damaged. In the case of less flat polystyrene surfaces, the adhesion increased with the applied (external) load to a saturation value. We attribute this to local flattening of nanometer-sized protrusions. The long-range interaction in water is dominated by a weak electrostatic double-layer force. At separations below about 10 nm, an attractive force component predominates the interaction. The adhesion in aqueous solutions ...