Coexistence Curve and Theta Temperature of Polymer Grafted Nanoparticle (PGN) Solutions

PGNs are emerging as an enabling technology, not only for nanocomposite formulation, but also for separations, energy storage, devices and coatings. Their tunability arises from the modularity of the hard-soft architecture, and understanding its impact on phase behavior in small molecules is crucial to fabrication and processing. Unfortunately, minimal data or theoretical insight is available; or how PGNs relate to the role of macromolecular architecture on solution phase behavior. Herein, we discuss the upper critical solution coexistence curve and theta temperature (Θ) of polystyrene (PS)-grafted Au-nanoparticles in cyclohexane. As molecular weight of the PS graft increases (20→50kDa), the coexistence curve shifts to higher temperatures (e.g. T_c~12→20^oC @ 10nM (Φ~3e-5)), as determined by spectroscopy, dynamic light scattering, and x-ray scattering. Θ, estimated from extrapolating the low concentration region of the co-existence curve, also increased with graft size (Θ~25–27^oC). Overall, these values are less than prior reports of PS stars (Θ~30^oC) and linear chains (Θ~33^oC ). Understanding the macroscopic and mesoscopic solution behavior of these soft colloids in context to structured macromolecules and curved brushes is essential to their future development.