Co-nonsolvency effects in the thermoprecipitation of oligomeric polyacrylamides from hydro-organic solutions

Co-solutes and co-solvents influence the thermoprecipitation of stimulus-responsive polymers from aqueous solution. Taking the behavior of oligomeric poly-(N-isopropylacrylamide) prepared by chain transfer polymerization as reference, the influence of organic solvents (concentration <2 M) on the thermoprecipitation of polyacrylamides with critical solution temperatures (CST) in pure water between 30 and 75 °C is investigated using turbidity and differential scanning calorimetry. Depending on the system, both increase and decrease but also the disappearance of the CST is observed. The strength of the observed effect is related not only to the size but also the structure of the hydrophobic domain of the solvent molecule. Contrary to the effects observed upon the addition of simple salts as additives, the chemistry of the investigated polyacrylamide is of direct consequence for the effect of a given solvent. Certain parallels can hence be drawn to the behavior previously observed for additives such as anionic surfactants and alkylamines.