Entropic Phase-Separation in Polymer Grafted Nanoparticle Blends

Polymer grafted nanoparticles (PGNP) systems tend to maximize the system’s configurational entropy by dispersion of the NPs, and the polymer chains’ conformational entropy and enthalpic interactions by mediating the interactions between the NPs. We examine a binary blend of PGNPs with chemically distinct brushes, with respective to entropic and enthalpic and overall miscibility behavior. First, we document that the binary PMMA/PSAN (AN content14%) homopolymer blend (i.e. without attached nanoparticle) films remain in the 1-phase for all blend compositions, wherein for both homopolymers, their Mw is matched to PGNP brush Mw, and studied at same sets of temperature. In contrast, end-grafting of the polymer chains to the NPs apparently significantly reduces the chains’ configurational degrees of freedom, and the PGNP blends phase separate. We hypothesize that the reduced entropic miscibility interactions due to tethering of PGNP brushes, allowing the weakly unfavorable enthalpic interaction to drive 2-phase behavior. This is despite the fact that the enthalpic interactions are themselves reduced in PGNP brush/brush interdigitation contacts compared to untethered systems, due to limited interdigitation within only the semi-dilute polymer brush (SDPB), zone but not in the concentrated polymer brush (CPB) zone. We demonstrate these results in a binary PGNP blend system with chemically distinct grafted-polymers of PMMA-g-SiO2 /PSAN-g-PSAN that undergoes this this kind of phase separation, and that too, only in a narrow window in vicinity of the 50/50 PGNP blend composition, where enthalpic interactions are maximal. In summary, while an identical matched in every respect, chemically distinct PMMA/PSAN homopolymer blend system is miscible at all temperatures and compositions studied, its tethered PGNP/PGNP blend system is not miscible in a limited composition window around 50/50. A delicate balance of reduced entropic degrees of freedom of PGNP brushes and enthalpic interactions is responsible for this phenomena.