Yoshitsugu Hirokawa

Volume phase transition in a nonionic gel

Nonionic N‐isopropylacrylamide gel was found to undergo a discontinuous phase transition by changing a solvent composition or temperature. The observation that polymer gel with and without charge can undergo a first order volume phase transition is an evidence for the universality of the phase transition of polymer gels.

Volume‐phase transitions of ionized N‐isopropylacrylamide gels

Equilibrium swelling curves of N‐isopropylacrylamide (NIPA) gel and a series of its ionized counterparts were measured as a function of temperature. Nonionic NIPA gels underwent a sharp yet continuous volume change, whereas incorporation of a small amount of ionizable groups into the gel network drives the transition toward a discontinuous one. The critical ionic concentration and polymer network density at which the get undergoes a critical phase transition were determined. The results were analyzed using a mean field theory. Discrepancy between experimental data and theory is nontrivial, and may require the formulation of a more elaborate theory.

Reentrant phase transition of N‐isopropylacrylamide gels in mixed solvents

Reentrant volume–phase transitions are observed in N‐isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed‐loop instability phase boundary having both upper and lower critical points. The closed‐loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed‐loop phase boundaries, whose physico‐chemical basis are not yet clear.

Influence of casting solvents on microphase-separated structures of poly(2-vinylpyridine)-block-polyisoprene

Abstract Influences of casting solvent on the microphase-separated structures of poly(2-vinylpyridine)-block-polyisoprene films were studied by transmission electron microscopy and small-angle X-ray scattering. The variations of the structures obtained were consequences of vitrification of microdomain structures developed during the solvent casting processes. The various microphase-separated structures were properly understood by considering the Hansens solubility parameters of the solvent and the copolymer, which quantify the polymer–solvent interactions in the system.

Effects of Network Structure on the Phase Transition of Acrylamide-Sodium Acrylate Copolymer Gels

We report preliminary studies on the effect the relative amount of ionizable groups incorporated in the network, main polymer-constituent molecules, and cross-linking agent has on the volume phase transition of acrylamide-sodium acrylate copolymer gels. The experimental results agree with the theoretical prediction that the phase transition is characterized mainly by the number of ionized groups per effective polymer chain.

Volume phase transition in a non‐ionic gel

We report the observation of sharp volume phase transition in the non‐ionic N‐isopropylacrylamide gel. The finding demonstrates that the phase transition of gels may be general and not confined to ionic gels as was previously considered. It will allow us to separate the electrostatic effects and network‐structural effects on the phase transition.