On the elastic nature of the demixing transition of aqueous PNIPAM solutions

Mechanical instabilities accompanying the demixing transition of semi-dilute aqueous poly(Nisopropylacrylamide)
(PNIPAM) solutions are probed for the first time with Brillouin spectroscopy,
densitometry and refractometry. The particular role of the elastic moduli and the mass density at this
coil-to-globule transition followed by molecular aggregation is investigated. Even though the demixing
transition of PNIPAM solutions is denoted as a volume phase transition, it turns out that this transition
is governed by the elastic properties, instead of the volume properties. This is consistent with earlier
findings made for the demixing transition in chemically cross-linked PNIPAM hydrogels. Above the
demixing temperature, Brillouin spectroscopy discriminates compact PNIPAM-rich agglomerates with
sizes larger than 200 nm. Interestingly, these agglomerates possess a sharp distribution of elastic
moduli, which can be attributed without any doubt to a material with gel-like mechanical consistency.
Thus the phase-separated PNIPAM-rich agglomerates are not in the glassy state.