The dynamic behavior of poly(N-isopropylacrylamide) at the air/water interface

The dynamic surface tension of poly(N-isopropylacrylamide) (PNIPAM) homopolymers has been studied as functions of total polymer concentration, molecular weight, temperature, and interfacial area oscillation frequency. PNIPAM lowers the surface tension of water both below and above the lower critical solution temperature (LCST). In the latter case, colloidal polymer particles in the bulk phase diffuse to the interface to unwrap and spread to lower surface tension. The time scale for surface tension lowering decreases with increasing polymer concentration and decreasing molecular weight in the range of 13 100–547 000. The kinetics of surface tension lowering were not very sensitive to temperature. The kinetic curves were fitted to the Hua-Rosen equation yielding a meso-equilibrium (steady-state) surface tension of about 42 mJ m−2 at 25°C. The meso-equilibrium surface tension showed a slight (∼0.3 mJ m−2°C) decrease with increasing temperature with the most dramatic changes occurring around the LCST. Surface tension was measured as a function of time under conditions where the pendent drop surface area varied sinusoidally. At long times, the surface tension showed sinusoidal oscillations in phase with the area change and the amplitude of the oscillations increased with the extent of surface dilation.