The collapse and aggregation of thermoresponsive poly(2-oxazoline) gradient copolymers: a time-resolved SANS study

We have investigated the collapse transition of aqueous solutions of gradient copolymers from poly(iso-propyl-2-oxazoline)s (PiPrOx), which contain few hydrophobic moieties (n-nonyl-2-oxazoline (NOx) monomers). We extend our previous investigations (Salzinger et al., Colloid Polym Sci 290:385-400, 2012), where, for the gradient copolymers, an intermediate regime right above the cloud point was identified where small aggregates are predominant. Large aggregates are present in significant numbers only at higher temperatures. To investigate the stability of the intermediate regime, we performed time-resolved small-angle neutron scattering (SANS) experiments during temperature jumps starting below the cloud point and ending in the intermediate regime or in the high-temperature regime. We found that the intermediate regime is stable during the time investigated (similar to 1 h). Moreover, the collapse of the small aggregates and the surface structure of the large aggregates are related to the number of hydrophobic moieties and the quench depth. The present results elucidate the structural evolution of these polymers and relate them to their final state as well as to their macroscopic behavior.