A unique phase behavior of random copolymer of N-isopropylacrylamide and N,N-diethylacrylamide in water

Thermosensitive phase separation of aqueous solutions of the random copolymers of N-isopropylacrylamide (iPA) and N,N-diethylacrylamide (dEA) (PiPA–dEA) and of iPA and N-isopropylmethacrylamide(iPMA) (PiPA–iPMA) with different compositions has been investigated by using calorimetry, turbidimetry and infrared spectroscopy. Though the phase transition temperature (Tp) of PiPA–iPMA is a linear function of its composition, a deviation from additivity is observed for that of PiPA–dEA, that is, it has a minimum value at iPA/dEA = 1 (mol/mol). IR spectrum at the amide II mode of the copolymer suggests that part of N–H groups of iPA units form a hydrogen bond with CO groups of dEA units at T > Tp as well as with those of the iPA units. Effects of methanol on Tp of these copolymers have also been studied.