From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a
structural instability leads to the collapse and aggregation of the macromolecules at the
temperature-induced demixing transition. The accompanying cooperative dehydration of
the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate
the impact of partial dehydration of PNIPAM on the volume changes related to the phase
separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering
enables us to directly follow the isotropic jump diffusion behavior of the hydration water
and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for
the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM
chains occurs. Dilatation studies reveal that the transition-induced volume changes
depend in a remarkable manner on the PNIPAM concentration of the solutions. The
excess volume per mole of H2O molecules expelled from the solvation layers of PNIPAM
during phase separation probably strongly increases from dilute to concentrated PNIPAM
solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM
solutions.