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.) 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 congruent to 1.3-1.5) consisting of congruent to 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 similar to 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.