Temperature-sensitive properties of poly(N-isopropylacrylamide) mesoglobules formed in dilute aqueous solutions heated above their demixing point

The kinetics of merging and/or chain exchange between mesoglobules formed in dilute aqueous solutions of fluorescently labeled poly(N-isopropylacrylamides) (PNIPAM) heated above their demixing temperature (T-dem) were monitored at specific temperatures between 30 and 50 degrees C via nonradiative energy transfer (NRET) using polymers carrying similar to 0.25 mol% of either naphthyl (Np, energy donor) or pyrene (Py, energy acceptor). Dynamic and static light scattering measurements (DSL and SLS) indicated that PNIPAM-Py and PNIPAM-Np solutions form stable mesoglobules when heated above 30 C and that the size and size distribution of the mesoglobules depend on solution concentration and, more importantly, on the sample thermal history. Fluorescence depolarization measurements performed on mesoglobular solutions of PNIPAM-Np gave the temperature dependence of the probe anisotropy, an indication of the microviscosity sensed by the probe. The results show that samples heated within the similar to 31 degrees C < T < 36 degrees C range consist of fluidlike particles able to merge and grow in size. At higher temperatures the mesoglobules act as rigid spheres unable to merge upon collision. These observations are interpreted in terms of the various mechanisms invoked to account for the stability of the PNIPAM mesoglobular phase.