Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

Elastic nonlinearities are particularly relevant for soft materials because of their inherently small linear
elasticity. Nonlinear elastic properties may even take over the leading role for the transformation at
mechanical instabilities accompanying many phase transitions in soft matter. Because of inherent
experimental difficulties, only little is known about third order (nonlinear) elastic constants within
liquids, gels and polymers. Here we show that a key concept to access third order elasticity in soft
materials is the determination of mode Gr¨uneisen parameters. We report the first direct observation of
third order elastic constants across mechanical instabilities accompanying the liquid–liquid demixing
transition of semi-dilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions. Immense elastic
nonlinearities, leading to a strong strain-softening in the phase-separating PNIPAM solutions, are
observed. Molecular mechanisms, which may be responsible for these immense elastic nonlinearities, are
discussed. The importance of third order elastic constants in comparison to second order (linear) elastic
constants in the demixing PNIPAM solutions evidences the need to focus more on the general role
played by nonlinear elasticity at phase transitions within synthetic and biological liquids and gels.