Studies of phase transition of aqueous solution of poly(N,N-diethylacrylamide-co-acrylic acid) by differential scanning calorimetry and spectrophotometry

The phase transitions of aqueous solutions of poly(N,N-diethylacrylamide-co-acrylic acid) (DEAAm–AA) were studied by micro-differential scanning calorimetry (DSC) and UV–visible spectrophotometry. The lower critical solution temperatures (LCST) obtained from the maxima of the first derivatives of the DSC and optical transition curves were in good agreement. The LCST values of the copolymers varied with the AA content. The optical transitions of the copolymers were wavelength dependent and were much broader than that of the homopolymer PDEAAm. It is reasoned that the AA copolymers formed smaller aggregates compared to the homopolymer PDEAAm. The stability of the smaller particles is attributed to the electrostatic repulsion among the partially ionized carboxylic groups in the copolymers. The LCST values were slightly concentration dependent in the relatively narrow concentration range studied. The enthalpy change ΔH for the phase transition in solution was found to be lower for the copolymer than that for the homopolymer PDEAAm.