Botryoidal Assembly of Cholesteryl−Pullulan/Poly(N-isopropylacrylamide) Nanogels

Abstract

Hybrid nanogels consisting of cholesteryl-modified pullulan (CHP) and poly(N-isopropylacrylamide) (PNIPAM) were synthesized by graft free-radical copolymerization of N-isopropylacrylamide (NIPAM) onto methacryloyl-substituted CHP nanogels (CHPMA) in water at 50 °C in the presence of a water-soluble free radical initiator. Depending on the initial NIPAM/CHPMA ratio, CHP−PNIPAM (CN) nanogels containing 30.8−84.8 wt % PNIPAM were obtained in the form of self-assembled nanoparticles with a hydrodynamic radius (R_h) of 69.0−116.0 nm in water kept at 20 °C. Hybrid nanogels of sufficiently high NIPAM content, such as the sample CN90, which contains 79.6 wt % NIPAM, exhibited a two-step response to changes in solution (3 mg/mL) temperature: a decrease in R_h from 93 to 57 nm as the temperature increased from 20 to 35 °C, followed by a sharp increase in R_h from 57 nm to 90 nm at 55 °C. Both steps in this temperature response were reversible. The multistep response to temperature of the CN nanogels was attributed to the morphology of the nanogels, which are seen as consisting of grape-like (botryoidal) clusters of associated native nanogels held together via cholesteryl cross-linking points and held together by the grafted PNIPAM chains.