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 calor imetry (DSC) and UV-visible spectrophotometry. The lower critical solution temperatures (LCST) obtained from the maxima of the first derivatives of th e 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 small er aggregates compared to the homopolymer PDEAAm. The stability of the smal ler particles is attributed to the electrostatic repulsion among the partia lly ionized carboxylic groups in the copolymers. The LCST values were sligh tly concentration dependent in the relatively narrow concentration range st udied. The enthalpy change DeltaH for the phase transition in solution was found to be lower for the copolymer than that for the homopolymer PDEAAm. ( C) 2001 Elsevier Science Ltd. All rights reserved.