A versatile osmometer for polymer gels and solutions with applications to cellulosic gels

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Abstract
An osmometer is described which is capable of measuring swelling pressures of polymer and polyelectrolyte gels up to 200 atm., as well as measuring osmotic pressures of solutions down to approximately 0.1 mm. H2O pressure. For the latter, use can be made of the deswelling of a solvent swollen gel that occurs when it is placed in contact with the polymer solution. In this case no semipermeable membrane is required. Useful thermodynamic information regarding the network structure of gels can be obtained, which in the case of polyelectrolytes cannot be obtained through vapor phase measurements. Measurements on a gel obtained by crosslinking cellulose acetate in solution allow the determination of the elastic free energy and the degree of swelling q0 at which the coiling chains are relaxed, if Huggins' parameter in the expression for the free energy of mixing is known. If the degree of swelling q is not too far removed from q0, it is shown that the “rubber elasticity” theory of networks holds, and yields the number of crosslinks in the gel. Pressure measurements of a cellulose gel in water and in alkali solutions are presented. The maximum in the swelling pressure at 10% alkali is qualitatively explained with Neale's theory, which considers the cellulose hydroxyl groups to be weakly acidic.