Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide) copolymers

Temperature and pH sensitive polymers were prepared by the copolymerization of N-isopropylacrylamide (NIPAAm) with varying amounts of acrylamide derivatives bearing carboxylic groups attached to spacers with different chain length (C(n)AAn). Aqueous solutions of the copolymers show lower critical solution temperature behaviour (LCST). The LCSTs of the aqueous solutions of these copolymers decrease with increasing comonomer content and spacer chain length. Through the use of suitable amounts of the monomers it was possible to accurately predict the phase transition temperature and values were obtained between 10 and 32 degrees C. The LCSTs also strongly depend on the pH-value of the solution. An increase of the pH-value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. Through altering the pH it is possible to obtain phase transition temperatures between 10 to 50 degrees C. The LCST behaviour was investigated by means of DSC as a standard method which allows the measurement of the phase transition over a wide range of temperatures and pH-values. Comparison of these results with those from other methods (mainly turbidimetric and viscosimeter measurements) shows a two step mechanism for the phase separation. Dependence of the phase transition temperature of aqueous P(NIPAAm co C(2)AAm) copolymer solutions on the pH-value (lines were drawn to make the diagram clearer.).