P-NIPAM in water-acetone mixtures: experiments and simulations

Area of Science:

• Polymer Science
• Physical Chemistry
• Materials Science

Background:

• Poly(N-isopropylacrylamide) (p-NIPAM) is a well-known thermoresponsive polymer exhibiting a lower critical solution temperature (LCST) in aqueous solutions.
• The LCST behavior of p-NIPAM is sensitive to the addition of co-solvents, but the exact mechanism and resulting phase behavior require further elucidation.

Purpose of the Study:

• To investigate the effect of acetone concentration on the LCST of p-NIPAM microgels.
• To elucidate the molecular mechanisms underlying the observed thermoresponsive behavior in the presence of acetone using computational simulations.

Main Methods:

• Dynamic Light Scattering (DLS) was employed to measure the hydrodynamic radius (RH) of p-NIPAM microgels across a range of temperatures and acetone concentrations (0.00–0.25 molar fraction).
• All-atom molecular dynamic (MD) simulations were performed on a single 30-mer p-NIPAM chain under similar solvent and temperature conditions to complement experimental findings.
• Density profiles were analyzed from simulations to understand the preferential interactions between the polymer, water, and acetone.

Main Results:

• DLS measurements and MD simulations revealed that increasing acetone content leads to an augmentation in the size of the shrunken polymer state.
• A minimum in the globule-to-coil transition temperature, corresponding to the minimum LCST, was observed with increasing acetone concentration.
• Density profiles indicated preferential interaction of p-NIPAM with acetone over water, with water being expelled from the polymer interior at higher temperatures.

Conclusions:

• The observed minimum in LCST with increasing acetone concentration is attributed to the competition between water and acetone for the polymer surface.
• Acetone's preferential interaction with p-NIPAM influences the polymer's hydration and, consequently, its thermoresponsive behavior.
• The findings highlight the role of co-solvent composition in tuning the phase transition temperature of p-NIPAM and provide insights into the underlying molecular interactions.