Sponge-like Heterogeneous Gels: Hierarchical Structures in Poly(N-isopropylacrylamide) Chemical Gels As Observed by Combined Scattering and Confocal Microscopy Method

Internal structures of the opaque poly(N-isopropylacrylamide) gel obtained at the preparation temperature (T-P) higher than 24.5 degrees C was found to have hierarchical structures by means of a combined small-angle-scattering (CSAS) method of small-angle light scattering (SALS), ultra-small-angle neutron scattering (USANS), and small-angle neutron scattering (SANS) with an aid of laser scanning confocal microscopy (LSCM). The combined CSAS and LSCM studies revealed that a global structure of the gels had a sponge-like two-phase structure. The polymer-rich phase of the sponge was filled with microgels having highly cross-linked network (tight network) chains that are interconnected by loosely cross-linked network (loose network) chains swollen with water, while the other phase of the sponge was mostly composed of water. These two phases were cocontinuous in three-dimensional space, hence, forming macroscopic gels. The SALS and USANS studies clarified that the sponge had a characteristic spacing of approximate to 12 mu m and the mass fractal structure characterized by its dimension = 2.5, the upper and lower cutoff lengths were approximate to 6 and approximate to 0.5 mu m, respectively. The physical meaning of these values is discussed in the text. The SANS studies further revealed the following characteristic parameters about the local structure of the gels: the microgel of diameter approximate to 60 nm the thermal correlation lengths of the loose and tight network chains approximate to 11 and approximate to 1.2 nm, respectively, though these are averaged values in the range of T-P, 24.5 <= T-P (degrees C) <= 38.0. The swelling behavior of the gels was also studied and discussed in light of the sponge-like gel model. The mechanism of the formation of the hierarchical structures was discussed on the basis of the special reaction field where the two-step cross-linking reaction occurs, as detailed in the text.