Chain Dispersity Effects on Brush Properties of Surface-Grafted Polycaprolactone-Modified Silica Nanoparticles: Unique Scaling Behavior in the Concentrated Polymer Brush Regime

Silica nanoparticles (R_h = 70 nm) were functionalized with high dispersity (Đ > 2.3) polycaprolactone at various grafting densities, and the brush properties were investigated using dynamic light scattering. Owing to recent advances in controlled polymerization techniques, low dispersity brushes are easily grafted from nanoparticle surfaces, and these systems have been well studied. However, the effect of high dispersity brushes on nanoparticle surfaces is largely unexplored. Here we discuss the brush properties of high dispersity polycaprolactone-grafted silica nanoparticles. Because of the polymerization conditions used, transesterification events are induced during the polymerization to give brushes with increasing dispersity both as brush length increases and as grafting density is increased (e.g., Đ from 1.32 to 2.39 for σ from 0.21 to 0.61 chains/nm²). All grafting densities showed extended chains in the concentrated polymer brush regime, with brush length, l_b, scaling with degree of polymerization, l_b ∼ N^a, where a = 1.39, 1.47, and 1.84 for the high, mid, and low grafting density sets. This study provides the first experimental insight into the effects of increasing chain dispersity on brush properties of nanoparticle systems. Furthermore, this system offers a facile method to tune dispersity of grafted brushes concurrent with the grafting polymerization. We expect this work to be of significant interest to the ongoing study of fundamental properties of polymer brushes as well as these materials finding use in polymer composite applications and provide enhanced mechanical properties compared to their monodisperse analogues.