Analysis of the structure-mechanical property relationships of nanoparticle-filled hydrogels

Polymer nanocomposites have received considerable attention because they often exhibit much better properties than neat polymers or polymers filled with micrometer-scale inorganic fillers. Agglomeration of particles in composite polymeric materials, however, is a fundamental issue, and the relationship between the particle distribution and the composite mechanical properties is not fully understood.;The ultimate goal of this dissertation was to study the effects of both the particle distribution and the particle---polymer affinity upon mechanical properties of the composite by carefully decoupling these two effects. Evaluation of individual contributions to the mechanical properties allows us to expedite efficient development of nanocomposites with desired properties. To achieve this goal, a colloidal crystalline array was encapsulated within a polymer matrix to make a model composite that has a well-ordered particle distribution.;First, the effect of particle distribution within the matrix polymer on the mechanical properties of the nanocomposites was studied. We experimentally characterized the particle distribution within the polymer matrix using Bragg diffraction of visible light and compared it with the interaction potential calculated by DLVO theory. The dynamic mechanical analysis of the nanocomposites suggested that the particle distribution plays an important role in the composite mechanical properties.;Next, we investigated how the nanoparticle---polymer affinity relates to the mechanical properties of the nanocomposite by comparing silica and polystyrene nanoparticles. The surface roughness of the particles and the molecular conformation of the interfacial layer between the polymer and the nanoparticles were characterized by synchrotron small-angle x-ray scattering (SAXS) and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. On polystyrene particles, the surface roughness was larger, and the polymer adsorbed strongly. Consequently, the mobility of the adsorbed polymer was reduced compared to that on silica particles. This reduced mobility explains a smaller viscoelastic loss for the polystyrene-filled nanocomposite compared to the silica-filled nanocomposite.;Finally, the hydrogel network structure around the nanoparticles was studied by taking advantage of the responsive nature of the thermosensitive polymer, poly(N-isopropylacrylamide). The analysis using a percolation model suggests that the polymers that play a role in the adsorption are relatively free chains, not constrained segments of polymers bound between crosslink junctions. This analysis opened a way to examine the adsorption behavior of the hydrogel on the particles by simply adding linear polymer chains to the particle suspensions. We prepared polystyrene and silica suspensions with linear polymer chains, and the rheological measurement was conducted. The resultant thickness of the adsorbed layer was larger on polystyrene compared to that on silica particles. This result is consistent with our study using SAXS and QCM-D, indicating the accuracy of the rheological study.