PNIPAM is a well-known thermoresponsive polymer, with a lower critical solution temperature (LCST) of approximately 32 °C in water. In ethanol, PNIPAM exhibits an upper critical solution temperature (UCST) that is below the freezing point of pure ethanol, indicating excellent solubility at all temperatures. In mixtures of water and ethanol, PNIPAM exhibits a much more complex behavior. As water is added to ethanol, the solution becomes an increasingly poor solvent for the polymer as the volume fraction of water increases. A similar behavior is observed as ethanol is added to water, with the LCST decreasing from 32 °C to near 0 °C for approximately 20% volume fractions of ethanol. In the middle region of the phase diagram, between 60% and 80% water (by volume), a region of compositions emerges where the polymer is not soluble at all, despite favorable interactions with both solvents separately. Here, the phase behavior of PNIPAM, and two analog polymers poly(N-cyclopropylacrylamide) and poly(N,n-propylacrylamide), is explored in mixtures of water, ethanol, and propanol using a combination of turbidimetry, small-angle neutron scattering (SANS), and dissipative particle dynamics (DPD) simulations. The results indicate a complex interplay between polymer architecture, terminal group chemistry, as well as isometry on the phase behavior of the polymers; furthermore, these findings represent an intriguing way to tuning the thermoresponsiveness of PNIPAM-based materials in the future.

Cononsolvency of poly(N-isopropylacrylamide) (PNIPAM) and its analogs in water/alcohol mixtures

255th National Meeting (2018)

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