Phase transitions in the system poly(N-isopropylacrylamide)/water and swelling behaviour of the corresponding networks

The phase behaviour of linear poly(N-isopropylacrylamide) (PNIPA) and chemically cross-linked PNIPA in water has been determined by calorimetric and optical techniques. Experiments fur the linear polymer were conducted for three different molar masses and encompassed the whole accessible concentration range. In addition the binodal of the 'monomer' N-(isopropyl)propionamide (NIPPA) was determined by cloud point measurements. The phase behaviour was analysed in terms of the Flory-Huggins-Staverman theory amended with a strong concentration dependent interaction function. The analysis established that PNIPA/water is exemplar of Type II phase behaviour, indicating that the system does not fit the classic Flory-Huggins scheme, Distinctive of Type II behaviour is the existence of an off-zero liquid-liquid critical composition for a hypothetical polymer of infinite molar mass and the absence of the classic FH Theta -critical state. The Type IT phase behaviour of the linear polymer is also found in the network and is shown to be responsible for the discontinuous volume response of the PNIPA gel. The peculiar swelling behaviour is a direct consequence of the invariant interference of the swelling curve with the liquid-liquid miscibility gap. Finally, the dependence of the enthalpy of melting of water in the polymer solution is not related to the formation of a strong complex between the polymer and the water molecules. Instead, the crystallisation and melting of water art: impeded by the vitrification of the aqueous polymer solution. (C) 2000 Elsevier Science B.V. All rights reserved.