Phase transition of aqueous solutions of poly(N,N-diethylacrylamide-co-acrylic acid) by differential scanning calorimetric and spectrophotometric methods

The phase transition of aqueous solutions of poly(N,N-di-ethylacrylamide-co-acrylic acid) (DEAAm-AA) is studied by differential scanning calorimetry (DSC) and UV-vis spectro photometry. The copolymer aqueous solutions are shown to have well-defined lower critical solution temperatures (LCSTs). The LCST values obtained from the maximum of the first derivatives of the DSC and optical transition curves agree well. DSC can be used to measure the phase-transition temperature of more dilute polymer solutions. On increasing the AA composition in the copolymers. the LCST values of the copolymer increase. then decrease at higher AA composition. For the aqueous solution of the copolymers, the transition curve obtained by the spectrophotometric method is highly wavelength dependent. The LCST values are found to be concentration-dependent. The changes in the heat of the phase transition of the copolymer solutions measured from DSC are lower than that of the homopolymer PDEAAm solution. This is consistent with the suggestion that the polymer chains of the copolymers collapsed only partially at temperatures above the LCST. The added salt (sodium chloride) decreases the transition temperature of the polymer solution.