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 > pK (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.