Temperature-Dependent Properties of Telechelic Hydrophobically Modified Poly(N-isopropylacrylamides) in Water: Evidence from Light Scattering and Fluorescence Spectroscopy for the Formation of Stable Mesoglobules at Elevated Temperatures

The self-assembling properties of hydrophobically modified (HM) telechelic poly( N -isopropylacrylamides) (PNIPAM) were studied in aqueous solutions of concentration ranging from 0.1 to 11 g L -1 by fluorescence spectroscopy, using N -phenyl-1-naphthylamine as a probe, and by static (SLS) and dynamic (DLS) light scattering over a temperature domain encompassing their cloud point ( T cp ) and coil-to-globule transition temperature ( T M ). The telechelic HM−PNIPAM samples bear n -octadecyl termini, and their molar mass ( M n ) ranges from 12 000 to 49 000 g mol -1 with a polydispersity index lower than 1.20. In cold aqueous solution, the HM−PNIPAM samples associate in the form of flower micelles (10.8 < R H < 17.5 nm, R G / R H ≅ 1.3−1.5) consisting of ≅16−27 polymer chains, depending on their molecular weight. In solutions heated under equilibrium conditions above T M, individual flower micelles with collapsed loops associate to form stable mesoglobules ( R G / R H ∼ 0.80) comprising a few hundreds chains with a more rigid and more polar interior than the hydrophobic core of hydrated flower micelles. The size of the mesoglobules increases with increasing polymer concentration (19 < R H < 115 nm), but in all cases the mesoglobule size distributions are narrower than those of the corresponding polymer micelles in cold solutions.