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Nonlinear and kinetic processes at demixing transitions of polymer solutions

Martine Philipp Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München

Due to the comparably low entropy of mixing of macromolecules, demixing transitions frequently occur in soft matter. Such self-organization processes, as well as their kinetics, result from a subtle balance between macromolecular conformational changes, molecular interactions and transport processes. For many polymer solutions, hydrogels, polymer brushes, etc., phase separation can happen upon slightest changes of temperature, pH value, ionic strength or irradiation by light. As such stimuli-responsive polymers are responsive to various environmental changes they are predestined for manifold applications, e.g. in engineering (sensors, actuators) or medicine (smart drug delivery systems). Even though numerous theoretical and experimental studies have been carried out, a molecular-mechanistic understanding of this type of self-organization is still lacking.
For thermo-responsive polymers in aqueous environment, demixing transitions of the lower critical solution temperature type often occur upon heating. Using aqueous solutions of the model thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM), we focus on the leading role of nonlinear and kinetic processes, which co-determine the molecular ordering during phase separation. It will be demonstrated that immense strain-softening governs the demixing process, instead of, as expected, the linear elasticity. During repeated switching of the demixing transition, different kinetic processes can be responsible for an unsteady evolution of both adjacent phases. In order to better assess the relationship between the macroscopic properties and the relevant molecular mechanisms, we focus on the molecular order parameters of the demixing transition. Important variations in H-bond and hydrophobic interactions are probed between the PNIPAM chains and water molecules within the phase-separating polymer solutions. The diffusion behaviour of the molecules is also highly affected by the demixing transition, as shown by our quasi-elastic neutron scattering studies. Novel insights into a more mechanistic understanding of this kind of self-organization are gaisned by relating the molecular properties to the macroscopic behaviour of the demixing solutions.