Kinetic processes at the demixing transition of PNIPAM solutions

Kinetic processes, which are joined with mass transport, are studied in the vicinity of the sharp LCST-type
demixing transition of semi-dilute aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions. These
processes are slow as compared to the highly cooperative collapse of individual polymer chains. Purely
elastic properties, that are particularly sensitive to this phase transition, are addressed depending on the
temperature, space and time by Brillouin spectroscopy. Above the demixing temperature Tc, we
discriminate between kinetics related to the phase separation into PNIPAM-rich and PNIPAM-poor
domains and kinetics connected to the impact of gravitation on the on-going phase separation. Using
shallow temperature jumps of 0.3 C, the growth of compact PNIPAM-rich agglomerates with identical
gel-like mechanical consistency is provoked independently of temperature and position within the
sample above Tc. Astonishingly, the transition temperature does not vary while heating or cooling the
solutions across the phase transition, although the elastic properties depend strongly on space and time
during the equilibration of PNIPAM concentration gradients following the re-entrance into the lowtemperature
phase.