Entropy-Driven Phase Separation in Polymer-Grafted Nanoparticles Blends

Polymer-grafted nanoparticles, or PGNPs, bring an interesting twist to conventional polymer blend systems. In these materials, both entropic and enthalpic effects play important roles. On one hand, nanoparticles tend to disperse to maximize the system’s configurational entropy; on the other, the tethered polymer chains still try to preserve their own conformational freedom and favorable enthalpic interactions. In this work, we investigate the miscibility and structural organization of a binary PGNPs blend composed of PMMA-g-SiO₂ and PSAN-g-SiO₂ and compared its behavior to that of corresponding homopolymer blends. The reference homopolymer blend PMMA/PSAN blend (14%AN) with matched molecular weights remains fully miscible under all thermal conditions and compositions due to a favorable mixing free energy (single phase, ΔG_mix > 0). However, when the same polymers are grafted to nanoparticles, the blend PMMA-g-SiO2/PSAN-g-SiO2 exhibits clear phase separation (two-phase, ΔG_mix < 0), particularly near the critical composition 50/50. This behavior is attributed to the loss of configurational degree of freedom due to grafting, which suppresses the entropic driven forces for mixing and allows even weak enthalpic interaction to dominate. A detailed qualitative and quantitative analysis of AFM images will be presented to support the observed morphological evolution. These findings reveal how the grafting can fundamentally alter phase behavior in polymer blends, offering new design pathways for tunable nanostructures materials.