Temperature Dependence of Density of Polymer Gels: 1. A Pycnometry Method Applied to Poly(N-isopropylacrylamide)-Water System


Polymer gels that are in equilibrium with excess solvent(s) are three-component systems consisting of cross-linked polymer networks, solvent(s) in gels, and solvent(s). We have developed a method for the determination of densities of the polymer and solvent components in gels by using a specifically designed pycnometer. The method was applied to the cross-linked poly(N-isopropylacrylamide) and water system from 3.0 to 50.0°C. From the densities of the polymer and solvent components, we have evaluated the total volume of gel per unit mass of the cross-linked polymer (solid) [v(gel)/mp], the ratio of mass of polymer (or solvent) versus mass of gel [mp/m(gel) or ms/m(gel)], and the number of solvent molecules per polymeric residue at various temperatures. The determination revealed how contraction of the total volume of gels is related to temperature dependences of these quantities as follows: (1) The total volume at 3.0°C [v(gel)/mp = 32.61 cm3/(1.0 g of mp)] continuously decreased to a value 10.59 cm3 at 33.5°C. This decrease is essentially brought about by a loss in the mass of water from the gel system to liquid water; (2) The above change corresponds to the changes in mp/m(gel) and ms/m(gel) from their original values 3.055% and 96.94% at 3.0°C to 9.417% and 90.58% at 33.5°C, respectively; and (3) The ratio of mp/m(gel) vs ms/m(gel) asymptotically approached 4.5 vs 5.5 above 34.0°C. The changes in both quantities caused a further shrinkage in total volume but held to less than several percent of the original value (2.017 cm3 at 50°C). The number of water molecules per unit polymeric residue changed from 199 (3°C) to 60.4 (33.5°C) and then to 7.28 (50°C).



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