Effect of Polymer Side-Chain Architecture upon Nanoparticle Incorporation

The mixing of additives with polymers through chemical or physical attraction is of broad importance for nanocomposites, drug delivery, and template-based synthesis. Modern synthetic chemistries enable diverse polymer architectures, from linear to bottlebrush; however, the architecture effect upon additive mixing remains unclear. Here, nanoparticles were mixed with block polymer micelles having three different corona architectures, ranging from linear poly(ethylene oxide) (PEO, O0) to bottlebrush poly(oligo ethylene oxide methacrylate) (OEGMAs, OM) with different side-chain lengths, all of which had similar overall molecular masses. The vitrified poly(cyclohexyl methacrylate) (PCHMA, C) micelle cores prevented micelle equilibration by maintaining a constant aggregation number (DLS, SAXS, and TEM) during additive mixing experiments. SAXS and SEM morphology measurements showed that O0C exhibited mixing at all loading ratios examined (M:T < 5.0), as evidenced by uniform short-range ordering. In contrast, O4.5C and O9C exhibited a loss of short-range ordering, indicating demixing at 71 wt % nanoparticles (M:T ∼ 2.5), as quantified by nearest-neighbor statistics and excess entropy calculations. The additive demixing from bottlebrush chains was attributed to their shorter spatial extent and greater stiffness from an entropic elasticity perspective that limits the volume of corona interactions needed to maintain mixing and ordering. Thus, under constant molecular mass, the shorter backbone and stiffer bottlebrush chains limit additive mixing as compared to linear chains. These results highlight how polymer architecture significantly alters polymer mixtures.