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°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 ≈12 μm and the mass fractal structure characterized by its dimension = 2.5, the upper and lower cutoff lengths were ≈6 and ≈0.5 μm, 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 ≈60 nm; the thermal correlation lengths of the loose and tight network chains ≈11 and ≈1.2 nm, respectively, though these are averaged values in the range of T_P, 24.5 ≤ T_P (°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.