Kinetics of aggregation in thermoresponsive polymer solutions upon a pressure jump

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The research project planned addresses the relation between the hydration of thermoresponsive polymers in aqueous solution and the kinetics of their aggregation upon a pressure jump from the one-phase to the two-phase state. In dependence on temperature and pressure, non-ionic thermoresponsive polymers exhibit an elliptical coexistence line. It is well known that, at high pressure, the hydrophobic hydration of polymers at the phase transition decreases less than at atmospheric pressure. Moreover, the hydrogen bonds of the polar groups play an important role for the phase transition. Pressure jumps across the phase transition are an alternative to temperature jumps and offer the possibility to characterize the kinetics of aggregation and to relate it to the hydration state. In contrast to temperature jumps, also the early stages are accessible. Moreover, the dissolution of aggregates can be investigated in the reverse pressure jumps. At this, time-resolved measurements of the light transmission as well as time-resolved small-angle neutron and X-ray scattering are used. In addition, the interactions between polymer and water shall be investigated using Raman spectroscopy in dependence on temperature and pressure. Three systems are in the focus: (A) PNIPAM, (B) PNIPMAM, and (C) PS-b-PNIPAM diblock copolymers, all in aqueous solution. System A is a reference system, which we have investigated in numerous static investigations. System B features two-step behavior at the temperature-induced phase transition at atmospheric pressure, which is due to the enhanced hydrophobic interactions compared to PNIPAM. System C forms micelles with a thermoresponsive shell, and the results from system A can be applied to this self-assembled system. From the characterization of the aggregate growth, the growth mechanisms, the energy barriers and the collision times of the aggregates shall be determined and shall be related to the hydration. This way, a relation between the molecular interactions and the kinetics at mesoscopic length scales will be derived, which are applied to a more complex system.