Kinetics of the volume phase transition in poly(n‐isopropylacrylamide) gels prepared under high pressure

Abstract

New poly(N‐isopropylacrylamide) gels were prepared under high pressure (ca. 200 MPa) during gelation. The preparation‐pressure dependence of the deswelling speed of the gels was measured with a conventional T‐jump method. The deswelling time of a gel rod 2.2 mm in diameter prepared at 193 MPa was about 200 s, 1000 times faster than that of a homogeneous poly(N‐isopropylacrylamide) gel. Moreover, the collective diffusion coefficient, the thermal fluctuation, and the ensemble‐average intensity of the swollen gel networks were obtained with dynamic light scattering measurements. Both the enthalpy and entropy of the gels were estimated from equilibrium swelling curves with the Flory–Huggins interaction parameter evaluated with mean field theory based on the Flory‐type of the Gibbs free‐energy formula. It was found that the networks of the gels had an inhomogeneous structure newly introduced by the preparation pressure. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2315–2325, 2001

Photochromic Cross-Links in Thermoresponsive Hydrogels of Poly(N-isopropylacrylamide): Enthalpic and Entropic Consequences on Swelling Behavior

“…23,29 Kitada and co-workers have shown that the characteristic time for these processes can be altered by manipulating the pressure during gelation. 30 Incorporation of hydrophobic comonomers with NIPAM can also lead to gels with a sigmoidal shrinking behavior. 31 Visual observations indicate that all of these effects stem from either heterogeneities in the gel or formation of a dense skin layer that restricts outward permeation of water.…”

“…The exact nature of the departure has been reported to depend on the chemical and physical structure of the gel. For example, shrinking of PNIPAM gels cross-linked with MBAA near T c has been known to occur in multiple stages with different rates of water loss. , Kitada and co-workers have shown that the characteristic time for these processes can be altered by manipulating the pressure during gelation . Incorporation of hydrophobic comonomers with NIPAM can also lead to gels with a sigmoidal shrinking behavior .…”