Polymer Dispersity Affects Conformation of Brushes Grafted on Nanoparticles

Polymer-grafted nanoparticles (PGNPs) exhibit prominent performance in applications, such as tough thin films,1-3 optics and electrics,4-7 ultrafiltration membranes,8 biomolecular and drug delivery vehicles,9-12 colorimetric sensors,13-15 and lubrication,16-18 partly due to uniform dispersion and/or morphology of polymer brushes with varying chain conformation. Polymer brush properties, such as grafting density \sigma and degree of polymerization N, can drastically affect conformation of neutral and pH-responsive PGNPs and further influence their efficacy in applications. Surprisingly, how dispersity Đ (related to the breadth of the polymer molecular weight distribution) affects the brush conformation is still underexplored. First, we examined the brush length l_b of neutral poly(tert-butyl acrylate) (PtBA) brushes in dilute solutions. l_b bifurcated as a function of weight-average degree of polymerization N_w: l_b increased with Đ at low N_w in the concentrated polymer brush (CPB) regime but was less dependent on Đ at high N_w in the semidilute polymer brush (SDPB) regime. We proposed that the variation of l_b with Đ in the CPB regime was due to the presence of a stretched conformation of the stem near the particle surface in high-Đ brushes. SANS data obtained from low- and high-Đ PtBA brushes in the CPB and SDPB regimes were analyzed using a core-chain model and demonstrated the differences in l_b obtained from SANS and DLS increased with Đ in the SDPB regime due to the long chain effect. The monomer density \rho profiles at the periphery decayed in a linear and power-law for brushes in the CPB at low and high Đ, respectively. In the SDPB regime, \rho decayed in a power-law at the periphery regardless of Đ. Next, we explored the conformation of pH-responsive poly(acrylic acid) (PAA) brushes, in which the extent of pH-response of l_b was enhanced with increased N_w or Đ (when N_w is low). We hypothesized the low-N_w and low-Đ brush adopted a highly extended conformation at all pH values, whereas the other brushes with higher N_w and/or Đ transitioned from a collapsed to extended conformations with increasing pH. Together, these results suggest Đ is a tunable parameter to control chain conformation of PGNPs depending on brush molecular weight.