Formation and Growth of Mesoglobules in Aqueous Poly( N -isopropylacrylamide) Solutions Revealed with Kinetic Small-Angle Neutron Scattering and Fast Pressure Jumps.

The phase transition from swollen chains to polymer mesoglobules of an aqueous solution of poly( N -isopropylacrylamide) is investigated with kinetic small-angle neutron scattering with 50 ms time resolution in conjunction with millisecond pressure jumps across the coexistence line. The time-resolved study evidenced three distinct regimes: fractal clusters form during the first second and transform into compact mesoglobules. During the following ∼20 s, these grow by diffusion-limited coalescence. The final step consists of a slow growth characterized by an energy barrier of several k B T . The method opens opportunities for kinetic structural studies of multicomponent systems over wide length and time scales and gives a structural picture spanning from the chain collapse to mesoscopic phase separation.