Temperature Induced Hydrophobic Adsorption and Desorption of Linear Polymer Chains on Surfactant-Free Latex Nanoparticles

The adsorption of thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) on surfactant-free polystyrene latex was used as a model system to study the effect of chain hydrophobicity on the adsorption of protein on hydrophobic surface. PNIPAM is only soluble in water at lower temperatures with a lower critical solution temperature (LCST) of ∼32 °C. Using PNIPAM, we were able to continuously adjust the chain hydrophobicity and manipulate the adsorption by a simply temperature variation. A combination of static and dynamic laser light scattering was used to study the amount of the adsorption and the thickness of the adsorbed polymer layer. We found that there existed a hysteresis of the adsorption in the heating-and-cooling cycle, indicating an additional adsorption at temperatures higher than the LCST. We also found that the heating rate could greatly affect the adsorption. Our results revealed that the desorption accompanying the swelling of the adsorbed PNIPAM layer was logarithmically proportional to the desorption time, indicating a diffusion-controlled process.