Microstructure design and formation of organic/inorganic thin film nanocomposites
There is significant interest in understanding and exploiting the extraordinary property enhancements of polymers, enabled by adding small concentrations of nanoparticles to polymer hosts to create polymer nanocomposites (PNCs). Thin film PNCs hold potential for novel technological applications in areas such as optoelectronics or photovoltaic devices. One of the key challenges that limits the potential of PNC-based technologies is the control of nanofiller dispersion throughout the matrix. This requires a fundamental understanding of the energetic interactions that affect dispersion. Thin film PNCs pose a greater challenge than bulk PNCs, largely because interfacial interactions become increasingly important as the material is confined. It is equally important to find effective processing schemes that promote nanofiller dispersion in a manner that can be readily scalable for industrial operations. Accordingly, the last few years have seen an upsurge in processing schemes involving supercritical solvents, due in part to their tunable solvent strength. To this end, our research is aimed at gaining control of nanoparticle dispersion within thin film hosts using supercritical CO₂ (scCO₂) as a processing aid. This research examined a series of related problems. For the first project, we investigated the effects of scCO₂ sorption on the structural stability and kinetics of destabilization of homopolymer films. We showed that the films are metastable under these conditions, and the barrier to nucleation is larger than that encountered in air/vaccum. We also examined the issue of nanofiller dispersion within homopolymer thin films. In a model athermal mixture, polystyrene-coated gold nanoparticles in polystyrene hosts, interfacial segregation was generally observed, and was shown to be a function of the wetting characteristics of the brush-matrix interface and the ratio of the size of the particles to the unperturbed dimensions of the host chains. In a separate system, we show how scCO₂ can serve to prevent coarsening, which is ubiquitous in air/vacuum environments at elevated temperatures, for these nanofillers. Finally, we made nanocomposite micellar structures from block copolymers, with a fluorinated block. Gold nanoparticles were sequestered within the discontinuous domain. We then showed how scCO₂ could be used to invert the structure, placing the nanoparticles in the continuous phase.