Thermally Controlled Association in Aqueous Solutions of Poly(l-lysine) Grafted with Poly(N-isopropylacrylamide)

The effect of temperature on the association of graft copolymers of poly( l -lysine)- graft -poly( N -isopropylacrylamide), PLL- g -PNIPAM, in water was investigated by light scattering methods and electron microscopy. It was found that the thermosensitive graft copolymers form monodisperse particles above the phase transition temperature of PNIPAM blocks (31 °C). The hydrodynamic radii ( R h ) and molecular weights of supramolecular particles were strongly concentration-dependent. The larger particles ( R h ≈ 130−240 nm) were observed at concentrations above 0.1 mg/mL whereas smaller and more compact particles ( R h ≈ 60−80 nm) were observed at concentrations below 0.1 mg/mL. The molecular weight of the particles increased with increasing content and molecular weight of PNIPAM grafts. The small monodisperse particles observed at low concentrations are probably swollen micelles, and particles observed at high concentrations are their aggregates.