(Invited) Volume Phase Transition of NIPA Gels and Its Relation to the Inhomogeneity Caused By Extremely Multifunctional Crosslinker

Abstract

NIPA-Bis gels with low fraction of cross-linkers undergo discontinuous volume change at the specified temperature in water (Volume phase transition). Concerning the critical behavior of gels, NIPA hydro-gels with BIS (NIPA-BIS gels) are the most well studied gels. Y. Li et al. (1) and K. D. Arai et al. (2) obtained the critical exponents of NIPA-BIS gel. One of the most extreme gel with multi-functional crosslinker is a hydrogel crosslinked with small crystals of Laponite reported by K. Haraguchi et al. (3); single Laponite crystal consists of about 2000 unit cells on average, and it has discotic form with diameter of about 30 nm and thickness of about 1.0 nm in aqueous solution. When the Laponite crystals are used as cross-linker, each crystal crosslinks a few thousand of chains due to the silanol groups on the surface of the crystal. The NIPA-Laponite gels change the volume drastically by the temperature change and these gels are tough against external force. However, discrete volume change (volume phase transition) was not observed even at the very low fraction of crosslinker. This fact indicates that the number of the crosslinkers are very small compared to the BIS crosslinker, and the structure of the gels become very inhomogeneous. Using the inhomogeneity as the keyword, we will discuss about the hysteresis loop and minor loops observed from NIPA-Lapomite gel. References (1) Y.Li and T.Tanaka, J.Chem.Phys.,90, 5161,(1989). (2) K. D. Arai, A. Saito, K. Ito, Y. Uematsu, T. Ueno, Y. Fujii, and I.Nishio, Phys. Rev. E 87, 022603, (2013). (3) K. Haraguchi, T. Takehisa, and S. Fan, Macromolecules, 35, 10162 (2002). Figure 1