Effect of temperature on 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 an air–water interface were measured as a function of temperature from 5 °C to 35 °C. The isotherms of the PNIPAM monolayers measured by compression–expansion cycles clearly showed a negative hysteresis, i.e., a decrease in the surface pressure as the number of cycles increased, irrespective of temperature. An increase in temperature caused a decrease in surface pressure in the semidilute region and increase in the surface pressure in the collapse state below the lower critical solution temperature (LCST) of PNIPAM in water, ca. 32 °C. The surface moduli in the semidilute region determined from the first loop of Lissajous orbits at fixed surface pressure and strain, slightly decreased with increasing temperature, indicating that the elastic forces for the entanglement of PNIPAM chains in the monolayers decreased with a decrease in the solvent power conditions of PNIPAM in water.