Effect of Temperature on the Adsorption of Poly(N-isopropylacrylamide) at the Air−Solution Interface

The adsorption of poly(N-isopropylacrylamide) (PNIPAM) at the air-solution interface has been studied as a function of temperature using surface tension measurements and specular neutron reflectivity. At temperatures below 25 °C, water is a good solvent for PNIPAM and surface tension measurements show that a saturated layer of polymer is formed at the air-solution interface at very low bulk polymer concentrations. Characterization by neutron reflectivity shows that the adsorbed layer consists of a thin monomer-rich zone followed by a more dilute zone which extends toward the bulk solution up to a thickness proportional to the radius of gyration of the polymer. At temperatures above 25 °C, the polymer adsorption increases gradually until around 28 °C, beyond which it increases very steeply. This is attributed to a decrease in solvent quality. In the adsorbed layer, the monomer-rich zone also increases steeply beyond 28 °C while the central more dilute zone increases only gradually over the entire range of temperature studied. At temperatures above the bulk solution cloud point around 33 °C, an inhomogeneous layer is formed which exhibits very slow kinetics to reach equilibrium. We attribute this to a possible glassy nature of the adsorbed layer or a possible existence of a surface gel.