Sprecher
Beschreibung
Pressure is a key variable in the phase behavior of thermoresponsive polymers [1]. In the prototype system poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution, the cloud point temperature, Tcp, depends on pressure, and the coexistence line in the temperature-pressure frame is described by an ellipse with a maximum at ca. 60 MPa and 35 °C. Small-angle neutron scattering (SANS) has shown that, above Tcp, mesoglobules are formed, with their size, water content, and inner structure depending strongly on pressure [2]. Rapid pressure changes (“pressure jumps”) across the coexistence line combined with kinetic SANS (50 ms – 1500 s) allow to resolve the pathway of mesoglobule formation in dependence on the target pressure [3,4]. At low pressure, kinetic factors are important, such as the formation of a polymer-rich, rigid shell, which hampers further growth by coalescence. In contrast, at high pressure, the growth proceeds by diffusion-limited coalescence, due to the weaker dehydration of PNIPAM than at low pressure. Pressure jumps from the two-phase to the one-phase region reveal that the disintegration of the mesoglobules evolves either via chain release from their surface or via swelling, depending on the osmotic pressure of the water [5]. Finally, kinetic processes in solutions of self-assembled micelles, formed by diblock copolymers from a PNIPAM block and a hydrophobic poly(methyl methacrylate) block, are elucidated in dependence on target pressure.