Effect of carbon chain length of monocarboxylic acids on cloud point temperature of poly(2-ethyl-2-oxazoline)

The temperature-induced phase transition of poly(2-ethyl-2-oxazoline) (PEtOx) aqueous solution under mixing with a series of small carboxylic acids has been studied by turbidity measurements and laser light scattering. It has been found that cloud point temperature ( T cp ) of the PEtOx was changed to varying degrees depending upon the pH, ionic strength, molar ratio of acids to 2-ethyl-2-oxazoline unit, and carbon chain length of small carboxylic acids. Significant change in T cp was observed in the case of hexanoic acid. At acidic pH, an increase in the molar ratio of hexanoic acid to the 2-ethyl-2-oxazoline unit gradually decreased the phase transition temperature of the polymer as compared to the T cp of pure PEtOx. At original pH 6 (pH > p K a ), T cp shifts to higher value than that of pure PEtOx for lower molar ratios and decreased later on with increasing the molar ratio. The shift in the T cp is described based on the differences in the driving force of phase transition, including hydrogen bonding between small carboxylic acids and PEtOx polymer and hydrophobic interaction.