Probing Molecular and Surface Interactions of Comb-Type Polymer Polystyrene-Graft-Polyethylene Oxide (PS-g-PEO) with an SFA
Using a surface forces apparatus (SFA), we have directly measured the molecular and surface interactions of well-defined comb-type polymer polystyrene-graft-poly(ethylene oxide) (PS-g-PEO). The interaction forces of PEO side chains end-functionalized with–OH were investigated in both symmetric (polymer vs polymer) and asymmetric (polymer vs mica) configurations. Long-range repulsive forces were measured between the swollen polymer brushes in aqueous solutions, which were shown to have a steric origin and could be well described using the Alexander–de Gennes model. Contact angle measurement showed the water contact angle on PS-g-PEO surface decreased by over 20° in about 1 min after the water droplet contacted with the polymer surface, which indicates that the PEO side chains are able to extend into water due to the strong van der Waals forces and hydrogen bonding between hydrophilic PEO side chains and water. The extended PEO side chains can act as a swollen brush, leading to the strong steric forces measured by SFA. Atomic force microscope (AFM) was also employed to provide complementary information regarding the surface morphology before and after the polymer was exposed to water. Solution ionic strength showed negligible impact on the molecular interactions of the comb-type polymer. The surface energy of PS-g-PEO film was determined to be γ ≈ 38.0 ± 1.0 mJ/m2 by both adhesion mechanics test and three-probe-liquid contact angle measurement. Our results provide important insights into the fundamental understanding of molecular interaction mechanisms of comb-type copolymers and the development of novel functional polymers/coatings with strong antifouling capabilities for engineering and biomedical applications.