Surface pressure–area isotherms and surface dilational moduli of poly (N-isopropyl acrylamide) monolayers spread at air–water interface

The surface pressure–area isotherms and surface dilational moduli of poly (N-isopropyl acrylamide) (PNIPAM) monolayers spread at the air–water interface were measured during the compression–expansion cycles as a function of surface pressure and frequency at a fixed strain of 10%. The isotherms of the monolayers clearly showed a negative hysteresis, i.e., the decrease in the surface pressure increased with increasing number of cycles. However, the isotherm of an expansion process was in good agreement with that of the following compression process. Therefore, the Lissajous orbits of the monolayers exhibited a negative hysteresis loop with the repeating cycles, and then, the corresponding surface dilational moduli were determined from the first loop of Lissajous orbits. The resulting surface moduli were much larger than those of poly (ethylene oxide) and poly (vinyl acetate) monolayers, while their magnitudes were the same as those of poly (methyl methacrylate) monolayer. Such larger surface moduli may be attributed to the chain entanglements of loops and trains of spread PNIPAM chains in the monolayers.