Thermoresponsive Segments Retard the Formation of Equilibrium Micellar Interpolyelectrolyte Complexes by Detouring to Various Intermediate Structures

The kinetics of interpolyelectrolyte complexation involving architecturally complex (star-like) polymeric components is addressed. Specifically, the spontaneous coupling of branched cationic star-shaped miktoarm polymers, i.e., quaternized poly(ethylene oxide)(114)-(poly(2-(dimethylamino)ethyl methacrylate)(17))(4) (PEO114-(qPDMAE-MA(17))(4)), and temperature-sensitive linear anionic diblock copolymers poly(vinyl sulfonate)(31)-b-poly(N-isopropyl-acrylamide)(27) (PVS31-b-PNIPAM(27)) and further rearrangements of the formed complexes were investigated by means of stopped-flow small-angle X-ray scattering (SAXS). Colloidally stable micelles were obtained upon mixing both polymers at a 1:1 charge molar ratio in saline solutions. The description of the time-resolved SAXS data with appropriate form factor models yielded dimensions for each micellar domain and detailed the picture of the time-dependent size changes and restructuring processes. A fast interpolyelectrolyte coupling and structural equilibration were observed when mixing occurs below the lower critical solution temperature (LCST) of PNIPAM, resulting, in small spherical-like assemblies with hydrated PNIPAM coronal blocks. Above the LCST, the collapsed PNIPAM decelerates equilibration, though temperature as such is expected to boost the kinetics of complex formation: after a fast initial interpolyelectrolyte coupling, different nonequilibrium structures of spherical and worm-like shape are observed on different time scales. This study illustrates how a thermoresponsive component can modulate the influence of temperature on kinetics, particularly for rearrangement processes toward equilibrium structures during interpolyelectrolyte complexation.