SWELLING MECHANISM OF POLY(VINYL ALCOHOL) POLY(ACRYLIC ACID) DENSE GELS MADE BY REPETITIVE FREEZING AND THAWING PROCESS

The osmotic pressure acting on poly(vinyl alcohol)-poly(acrylic acid) (PVA-PAA) dense gels was analyzed. PVA-PAA gels were made by a repetitive freezing and thawing process and then immersed in a mixed solvent of water and acetone. Each pressure term, consisting of osmotic pressure such as pressure due to the network elasticity, the pressure of thermal motion of counter ions, pressure due to mixing entropy, and pressure due to the polymer-solvent interaction was evaluated by measuring the elastic modulus and the concentration of counter ions, and by using the equation of state given by Flory. It resulted in the difference of the pressures due to the mixing entropy and the polymer-solvent interaction being compensated for by the network elasticity. The result of this analysis depends on the validity of the mean field theory for mixing effect of polymers and solvent, appropriate for a semi-dilute system. Actually the PVA-PAA dense gels tested here met this condition in a solvent that had an acetone concentration of below about 60%. Thus, the swelling characteristics of the dense PVA-PAA gels can be understood semi-quantitatively. In the sense of engineering characterization of gels the way of analysis done in this study will be effective for the design improved materials.