Entropy-Driven Phase Transition in Polymer-Grafted Nanoparticle Blend Films and Associated Gloss and Dielectric Breakdown Strength

While polymer-grafted nanoparticle (PGNP) mixtures having brush layers with different chemistry undergo phase separation as in ordinary homopolymer blends, knowledge of the underlying phase behavior of such "hybrid" blends currently does not exist theoretically or even empirically. Entropic effects in PGNPs arising from the chain-end tethering constraints of the brush layers reduce the number of configurational states of the polymer segments, compared to their corresponding free chain homopolymer blend, greatly reducing the gain in entropic free energy of mixing (-TΔSmix). However, the enthalpic driving force for phase separation for χ(Τ) > 0, ΔHmix ∼ χ(Τ), is also lowered by reduced unfavorable segmental brush/brush physical contacts, a situation that makes estimation of the overall free energy of mixing, ΔG = ΔHmix - TΔSmix, difficult. The phase stability of miscible polymer blends is normally dominated by a large enthalpic (relative to entropic) contribution to ΔG, so that observations of competitive entropy/enthalpy effects on blend phase behavior are limited to systems having relatively small ΔHmix, i.e., 0 < χ(Τ) Ν < 2. With this general thermodynamic relationship in view, we designed a binary PGNP blend system comprising end-grafted polymer brush layers on silica nanoparticles having nearly balanced entropic/enthalpic interactions. Specifically, we synthesized poly(methyl methacrylate-silica) (PMMA-SiO2) and poly(styrene-acrylonitrile (AN-14%)-silica (PSAN-SiO2) PGNPs and cast blend films that exhibited a reversible and compositionally tunable U-shaped lower critical solution temperature (LCST) phase behavior. In contrast, the corresponding free chain PMMA/PSAN homopolymer blend films under the same conditions were fully miscible. As in a phase-separating homopolymer blend, phase separation in the PGNP blend film leads to spontaneous (spinodal) surface pattern formation, wherein the ratio of the in-plane to out-of-plane phase separation scale, λ/h, is constant. Furthermore, a shift in miscibility based on an asymmetry in the grafted chain length ratio of PGNPs can be compensated by an equivalent asymmetry in the PGNP blend ratio. Practical properties relevant to colloidal paints and coatings, including gloss (sheen) and dielectric breakdown strength (EBD) for capacitive energy storage, are measured. Single-phase PGNPs show high gloss (sheen) and high EBD, while the phase-separated PGNP blend films have low gloss and low EBD, effects that could be exploited for the design of numerous optical and flexible solid-state electrical power devices.