As a new class of high-strength hydrogels, we designed a tetra-PEG gel by combining two symmetrical tetrahedron-like macromonomers of the same size. Because the nanostructural unit of the gel network was defined by the length of the tetrahedral PEG arm, the gel had a homogeneous structure and resultant high mechanical strength comparable to that of native articular cartilage. Furthermore, since the gel was formed by mixing two biocompatible macromonomer solutions, the gelation reaction itself and the resultant gel were also biocompatible. The breaking strength had local maxima at the overlap concentration of the macromonomers (C*) and at 2C*. Dynamic light scattering measurement indicated the near absence of inhomogeneities in the network at C*. Thus, we successfully designed and fabricated a high-strength hydrogel by controlling the homogeneity of network structure for the first time, which will lead to multiplied effects, i.e., contributing to the understanding of ideal networks, providing a universal strategy for designing high-strength gels, and opening up the biomedical application of hydrogels.

SUMMARY Polymer gels have two types of concentration fluctuations; liquidlike and solidlike fluctuations. The former are time-dependent (thermal) fluctuations and the latter are time-independent spatial inhomogeneity. The static structure factor, obtained by small-angle X-ray (SAXS) or neutron scattering (SANS), consists of both contributions. Several methods which allow to decompose the static structure factor into the individual contributions are reviewed. In practice, however, the decomposition is not as easy as that predicted by the theory due to the lack of information about dynamic fluctuations. The dynamic light scattering technique (DLS), on the other hand, provides the complimentary information of the dynamic fluctuations. It will be shown that DLS becomes a powerful tool to distinguish the two contributions if it is coupled with ensemble averaging. In this article, recent developments of the structure investigation of polymer gels, particularly those of environmental sensitive polymer gels, are reviewed from both experimental and theoretical points of view. The advantage to use the environmental sensitive gels is that the structure of a gel can be examined as a function of its environmental parameters, e. g., temperature, concentration, and pH. Furthermore, the characteristic features of these environmental sensitive gels, such as a volume phase transition and phase separation, are extensively discussed in terms of the thermal fluctuations and the spatial inhomogeneity.

Effect of chemical structure on the volume-phase transition in neutral and weakly charged poly( Nalkyl(meth)acrylamide) hydrogels studied by ultrasmall-angle x-ray scattering Prediction of re-entrant swelling behavior of poly(N-isopropyl acrylamide) gel in a mixture of ethanol-water using lattice fluid hydrogen bond theory Small-angle neutron scattering study on weakly charged poly(N-isopropyl acrylamide-co-acrylic acid) copolymer solutionsThe small angle neutron scattering experiments were conducted on N-isopropyl acrylamide (NIPA) gels in D 2 0 and on the corresponding NIPA solutions. The NIPA gels underwent a sharp, but a continuous volume phase transition at 34.6 ·C from a swollen state to a shrunken state with increasing temperature. In the case of the gels, an excess scattering due to the presence of crosslinks was observed at low q region (q < 0.02 A-I), where q is the magnitude of the scattering vector. The scattered intensity function for the gel was well described with a combination of Gauss and Lorentz-type functions, i.e.,as proposed by Geissler etal. /G(O) and h(O) are the intensities at q=O for the contributions of Gaussian and Lorentzian functions, respectively. The Gaussian part results from solidlike inhomogeneity, having a characteristic size of E, which is due to the introduction of crosslinks into the system. The Lorentzian part is originated from the liquid nature of the local concentration fluctuations of the gel characterized with a thermal blob of dimension S. E decreases systematically with polymer volume fraction, and Ytf> are --1. These exponents were interpreted by taking account of the effects of crosslinking on the FIory's interaction parameter.Small-angle neutron scattering (SANS) is a relevant technique for investigations of such a spatial inhomogeneity in the length scale from a few tens of angstroms to a thousand of angsroms.6-12 Geissler and co-workers studied the crosslinking density dependence of the gel inhomo...

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