Frictional properties play a key role in the performance of hydrogels in applications such as soft contact lenses, cell sheets, artificial articular cartilage, etc. Here we characterize the swollen state and frictional properties of thin hydrogel films composed of poly(2-methoxyethyl vinyl ether) on a nanometer scale using neutron reflectivity (NR) in conjunction with lateral force microscopy, leading to the discussion about the extent to which surface chains impact the frictional properties of gels. NR measurement revealed that the density profile for the hydrogel films in the interfacial region with water along the direction normal to the interface was well described by a parabolic function, which was generally used for swollen polymer brushes in a liquid. Lateral, or frictional, force (FL) first increased with increasing normal force (FN) and then reached a region where the dependence of FL on FN was extremely subtle. That is, there exist two regimes, I and II, for the relationship between FN and FL. The thickness of the interfacial layer composed of dangling chains, which should behave like brush chains, as determined by NR measurement, was in good accordance with the depth at which the regime transited from region I to II. The frictional properties of the thin hydrogel films could be better understood by considering the two contributions from the interfacial layer and the internal bulk region.
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