Journal of Adhesion Science and Technology, Vol.13, No.1, 79-96, 1999
Direct force measurements on bulk polystyrene using the bimorph surface forces apparatus
Interfacial properties of bull; polymer samples have been characterized by direct measurements of lune-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 decreased dramatically in the presence of a non-ionic surfactant, n-octyl beta-D-glucopyranoside. The surfactant adsorbs onto hydrophobic surfaces, such as the polystyrene surface used here, and close to the critical micellar concentration it forms an oriented monolayer where the polar sugar units are directed towards the solution. The range of the short-range repulsion between polystyrene surfaces coated with a monolayer of n-octyl beta-D-glucopyranoside is about 2-3 nm, slightly larger than observed previously for such surfactant layers adsorbed onto very flat hydrophobized mica surfaces. Further, a considerably stronger attractive force was observed upon separation of the surfactant-coated surfaces in the case of polystyrene compared with when hydrophobized mica was used. The influence of the substrate surface on the interaction can be rationalized by their differences in surface roughness.