Swelling and Shrinking of Poly(N-Isopropylacrylamide) Chains Adsorbed on the Surface of Polystyrene Nanoparticles

Abstract

As a thermally sensitive polymer, poly(N-isopropylacrylamide) (PNIPAM) is hydrophilic at temperatures below its lower critical solution temperature (LCST, ∼132°C), but undergoes a coil-to-globule transition in water at higher temperatures. The adsorption of PNIPAM on polystyrene (PS) nanoparticles resulted in a core-shell nanostructure. In the first heating-and-cooling cycle there existed a hysteresis in terms of the thickness of the adsorbed PNIPAM layer; this could be attributed to additional binding of the adsorbed PNIPAM chains to the surface. Moreover, the surface shifted the coil-to-globule transition to a lower temperature and made the transition less sharp. The temperature dependence of the ratio of the radius of gyration to the hydrodynamic radius (<R_g>/<R_h>) revealed that the PNIPAM shell is highly swollen, and the density of the shell in the collapsed state approached that of the PS core.

Keywords:

• Adsorption of polymer chains on surface
• Laser light scattering
• Poly(N-isopropylacrylamide)
• Polymeric nanoparticles
• Swelling and shrinking of polymer chains

ACKNOWLEDGMENT

The financial support of the National Distinguished Young Investigator Fund (1996, 29625410), the BASF-Sino-German Research and Development Fund (1998), and the Research Grants Council of Hong Kong Special Administrated Region Earmarked Grant 1997/98 (CUHK4181/97P, 2160082) is gratefully acknowledged.