Using Miscible Polymer Blends To Control Depletion-Attraction Forces between Au Nanorods in Nanocomposite Films

To fully utilize their optical absorption and polarizing abilities, the dispersion of Au nanorods (NRs) in a matrix, such as a polymer film, must be controlled. By functionalizing NRs with a polymer brush chemically similar to the matrix, NR dispersion and aggregation can be controlled by varying the ratio of brush (N) to matrix (P) chain length. For P/N > 2, aggregates containing mainly side-by-side arrangements of NRs are observed. Here, polystyrene (PS) functionalized Au NRs are incorporated into miscible thin film blends of PS and poly(2,6-dimethyl-p-phenylene oxide) (PPO) (P/N approximate to 30) and characterized using a combination of transmission electron microscopy (TEM) and UV-visible spectroscopy (UV-vis). As the volume fraction of PPO (phi(PPO)) increases from 0.00 to 0.50, the NRs remain mainly aggregated; however, at phi(PPO) = 0.75 they begin to disperse and finally completely disperse in a pure PPO matrix. Correspondingly, the longitudinal surface plasmon resonance peak undergoes a red shift, consistent with improved dispersion (i.e., individual NRs). A novel outcome of this work is to utilize UV-vis to detect nanometer-scale changes in Au nanorod dispersion. To understand the role of the PPO matrix chains, which favorably interact with the PS brush, self-consistent field theory (SCFT) calculations were performed to determine the brush and matrix density profiles. The brush profile is initially parabolic for phi(PPO) < 0.25 and has a thickness that is nearly the radius of gyration of the brush. However, for phi(PPO) = 0.50, the brush begins to stretch because of PPO matrix chain penetration. Finally, for phi(PPO) = 0.75 and 1.00, the brush thickness increases by about 50%. These SCFT results help interpret the dispersion of nanorods determined from TEM and UV-vis.