An Effect of Interfacial Aggregation States on Frictional Properties of Hydrogel Thin Films

Frictional Properties, which are closely related to aggregation states at the outermost surface, of hydrogels are pivotal importance for many applications. Recently, we synthesized poly(2-methoxyethyl vinyl ether) (PMOVE) containing photo-cross-linkable 2-(vinyloxy)ethyl methacrylate (VEM) units by living cationic polymerization. Here, interfacial density profiles and frictional properties of cross-linked PMOVE (c-MrV) thin films in water were examined by neutron reflectivity (NR) and lateral force microscopic (LFM) measurements, respectively. The density profile of the c-MrV films in close proximity to the water interface was well described by a parabolic function, which was generally used for polymer brushes in a liquid. The lateral force (F_L) for the hydrogel films swollen in water was a function of normal force (F_N) applied on to a probe tip. For all films, F_L first increased with F_N and then remained constant at an F_N. The critical depth, in which F_L reached a constant, well corresponded to the thickness of the interfacial swollen layer determined by the NR measurement.