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Controlling the Assembly of Nanoparticles in Polymer Blends

While many novel methods have been devised for directing the assembly of nanoparticles in block copolymers, the topic has not reached the same level of sophistication for polymer blends. The assembly of particles at the interface between phase-separated domains can serve as a means to compatibilize polymer blends, reducing domain sizes and enhancing interdomain adhesion by impeding coalescence and decreasing interfacial tension. Compatibilization optimizes the performance of blended materials in applications where the properties of both components must be expressed synergistically, such as in plastics requiring both high strength and high toughness and in photovoltaic films. Thus, approaches to robustly control particle location in blends, especially those generating interfacial adsorption, are a much sought-after goal. This dissertation is a discussion of such approaches. Recognizing that Janus particles present a promising route to achieving interfacial adsorption of particles in an immiscible blend, we attempted the synthesis of several types of Janus particles with the goal of producing one that could kinetically stabilize a bicontinuous morphology in a blend during spinodal decomposition. Using ternary blends of polystyrene (PS), poly(methyl methacrylate) (PMMA), and Janus particles (JPs) with symmetric PS and PMMA hemispheres, we demonstrated the stabilization of dispersed and bicontinuous phase-separated morphologies by the interfacial adsorption of Janus particles during demixing upon solvent evaporation. The resulting blend morphology was varied by changing the blend composition and JP loading. Increasing particle loading decreased the size of phase-separated domains, while altering the mixing ratio of the PS/PMMA homopolymers produced morphologies ranging from PMMA droplets in a PS matrix to PS droplets in a PMMA matrix. Notably, bicontinuous morphologies were obtained at intermediate blend compositions, marking the first report of highly continuous domains obtained through demixing in a blend compatibilized by Janus particles. The JPs were found to assemble in a densely packed monolayer at the interface, thus largely preventing coalescence of domains in films annealed above the glass transition temperature. The rate of solvent evaporation from the drop-cast films and the molecular weights of the homopolymers were found to greatly affect blend morphology. In another approach, we used specific interactions to direct the localization of nanoparticles both within each phase and to the interface in a polymer blend. Using hydrogenbond- accepting nanoparticles, gold nanoparticles with poly(styrene-r-2-vinyl pyridine) (P(S-r- 2VP)) ligands, and two copolymers featuring competitive hydrogen-bond donation, poly(styrene-r-hydroxy styrene) (P(S-r-HS)) and poly(methyl methacrylate-r-2-hydroxyethyl methacrylate) (P(MMA-r-HEMA)), we demonstrated that the particles exhibit a distribution of locations strongly favoring the phase in which the total hydrogen-bonding interaction strength is greater. When HEMA/HS interactions were balanced, the particles displayed interfacial adsorption. This apparent balance occurs at a consistent ratio of HEMA/HS across several HEMA compositions. Annealing above the glass transition temperature generally induced adsorption at the interface between the two copolymers. Favorable hydrogen bonding interactions between phases increase the compatibility of the copolymers and can induce miscibility; the lower prevalence of hydrogen bonding at elevated temperatures is thus associated with increased interfacial tension, providing a greater driving force for the interfacial adsorption of 5 particles. This work marks one of the few reports regarding stimuli-responsive relocation of nanoparticles in a polymer blend, and could have fundamental application in gaining better understanding of the effect of particle location on the rheology and structural development of blends.
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