Impact of Dispersed Nanoparticles in Block Copolymer Soft Solids
Block copolymer soft solids are emerging as a means to spatially organize and store nanoparticulate material. Soft solids are formed as a disordered triblock copolymer solution [PEOx-PPOy-PEOx] transitions to a lyotropic liquid crystal. For certain chain architectures, the crystal structure of the soft solid undergoes an order-order transition (OOT) from spherical micelles packed in a cubic crystal structure to close packed cylindrical micelles. The focus of this thesis is to determine the impact of processing conditions and formulation on the structural parameters, flow mechanism, and timescales of soft solids. We study the effect of structural history and applied flow fields on the soft solids while the formulation is varied by incorporating nanoparticles into the polymer matrix. In the cubic phase, we found that the ordering kinetics occur on the timescale of minutes to tens of minutes depending on the structural history and the presence of particles. Furthermore, ordering of the cubic phase through the OOT occurs epitaxially. Without nanoparticles, a short application of high amplitude oscillations will macroscopically align the cubic and cylindrical phases into a persistent near single crystal. Changing the formulation through the addition of nanoparticles makes it more difficult to align the micellar crystals by shear, decreases the persistence of the alignment, and can change the unit cell of the micelle structure. By varying the formulation and phase behavior of the polymer matrix, nanoparticles can access different types of flocculation behavior. Specifically, the particles will aggregate in both the liquid and soft solid, remain dispersed in both phases, or re-disperse upon formation of the soft solid. These findings provide additional design parameters to improve the templating of nanoparticles in the soft solid as well as highlight the feasibility of using the phase behavior of the polymer solution to control the flocculation behavior and aggregate size. Most importantly, the results in this work are crucial for effective processing of these composite systems and for development of future applications.