Swelling and stress-relaxation of poly(N-isopropylacrylamide) gels in the collapsed stateThe structure of environment-sensitive gels consisting of poly͑N-isopropylacrylamide-co-acrylic acid͒ ͑NIPA/AAc͒ hydrogels was investigated by small-angle neutron scattering ͑SANS͒. Effects of the variation of ionic circumstance were observed both on the volume phase transition and on the structure factors: ͑1͒ The volume phase transition temperature, T c , increases with increasing pH from 4 to 9 and then decreases with further increasing pH. ͑2͒ the volume gap between the shrunken and swollen gels at T c is highest around pH 7 to 9. ͑3͒ Though the structure factor, I(q), has a maximum above the T c of the corresponding NIPA homopolymer gels, the peak intensity decreases with increasing pH, where q is the magnitude of the scattering vector. When NaCl is added, a similar scattering maximum appears for TϾT c . However, strong ionic screening leads to a lower swelling power and a decrease in T c , resulting in precipitation at a lower temperature than for the gel at a moderate pH. The roles of pH and salt are discussed on the basis of Donnan membrane equilibrium.

ABSTRACT:Inhomogeneities, that is, nonrelaxing frozen concentration fluctuations, are inevitably present in polymer gels because they are introduced during the crosslinking of the constituent polymer chains in a solvent. Therefore, inhomogeneities increase as the number of crosslinks increases in a gel. The ionization of polymer gels is one of the methods used to suppress inhomogeneities. However, because crosslinking also means a freezing-in of the conformation and topology of polymer chains in a solvent according to the chemistry of crosslinking, inhomogeneity control is quite sophisticated. In this article, we discuss the relationship between the inhomogeneities and the molecular/environmental parameters of polymer gels, such as the polymer concentration, the degree of crosslinking, the degree of ionization, and the interaction parameter, by considering the memory effect of gels.

Analysis of the Rabin-Panyukov theory of scattering from polyelectrolyte gels shows that the theory predicts an anomalous cross-link density ͑CD͒ dependence of amplitude of spatial inhomogeneity in charged polymer networks. By increasing the interaction parameter or by decreasing the degree of ionization, one can pass from a regime in which the calculated structure factor is a monotonically increasing function of CD, to one in which an inflection in the CD dependence is predicted. These theoretical predictions were experimentally verified by light scattering experiments on a series of weakly charged polymer gels of N-isopropylacrylamide-acrylic acid copolymer. The ensemble average scattered intensity, a measure of the spatial inhomogeneity, increased with CD for Ͻ c and decreased with CD for Ͼ c , where and c are the interaction parameter and the critical value of , respectively.

Thermoreversible gels of poly(vinyl alcohol) / Congo red in aqueous solutions have been studied by dynamic light scattering. Below T gel ͑ഠ 43 ± C͒, a speckle pattern appeared, while it disappeared for T . T gel . This indicates that nonergodicity observed in chemically cross-linked gels also exists in thermoreversible physical gels. Analogous to the b and a relaxations of glasses, the intensity correlation function showed a power-law behavior for T # T gel and a stretched exponential behavior for T $ T gel , respectively, while the thermal fluctuations characteristic of polymer gels were present independent of T gel . [S0031-9007(99)

The complexation of poly (vinyl alcohol) (PVA) with congo red (CR) ions in aqueous solutions was investigated as functions of PVA (Cpva) and congo red concentrations (Ccr) and temperature. The PVA-CR complexation resulted in a gelation around pH 7 when both Cpva and Ccr were high enough. However, in the case of intermediate PVA concentrations in monomer unit, i.e., 0.6 mol/L, a sol-gel-solgel-type transition was observed with increasing CR concentration. The origin of this "re-entrant"-type phase transition was studied by estimation of the enthalpy of cross-link formation and by capillary viscometry. The gelation was classified into two steps: (a) weak cross-link and (b) strong cross-link formations. These mechanisms were characterized by the value of the enthalpy of cross-link, AH. The former is in the order of -5 kcal/mol and the latter -20 kcal/mol. This viscosity behaviors in a dilute polymer concentration regime were also classified into several regions: When both CCr and Cpva were low, the complex solution showed a typical viscosity behavior for a polyelectrolyte solution. With increasing Ccr and Cpva, there appeared a region where a time-dependent viscosity thickening took place. With further increase in Cpva, the viscosity recovered a behavior similar to that for a PVA solution without CR. The connection between these rheological properties and sol-gel transition is discussed by taking into account the ionic screening effect and complexation equilibria.