Thermoresponsive polymeric gels are a class of smart materials with the ability to absorb or release large amounts of solvent when changes in their environment occur. Scaling theories of gel swelling (de Gennes' c* theorem and Obukhov's model) predict that the swelling of a gel correlates with that of a linear chain with equal degree of polymerization to that of a network strand. Data on linear and cross-linked poly(N-isopropylacrylamide) (PNIPAM) in aqueous solutions of different molar masses under θ temperature and good solvent conditions are examined. The Kuhn length of PNIPAM is evaluated to be LK = 4 ± 1 nm, in strong disagreement with previous estimates by single molecule force spectroscopy. The excluded volume strength (B) varies with the reduced temperature (τ) as B ? 4.7τ nm. While the power law exponent for the equilibrium swelling as a function of degree of cross-linking is close to the scaling predictions, the degree of swelling observed in cross-linked networks is two to three orders of magnitude larger than that expected from scaling models. The large differences between theoretical predictions and experimental results probably arise from the highly inhomogeneous nature of PNIPAM gels.
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