(161d) Polymer Dispersity Affects Conformation of Brushes Grafted on Nanoparticles

The efficacy of polymer-grafted nanoparticles in applications such as drug delivery systems, reinforced composites, and water purification depends, in part, on the conformation of polymer brushes. Though parameters controlling conformation such as grafting density and length of polymer brushes are well-studied, brush dispersity (i.e. breadth in the molecular weight distribution) remains underexplored. Theories and simulations suggest that dispersity markedly alters the conformation of polymer brushes grafted from nanoparticles, with long polymers in a disperse brush extending outward and short polymers collapsing toward the surface.¹ Experimental measurements of the conformation of disperse brushes, however, remain scarce. We investigate the effect of dispersity on conformation of polymer brushes grafted from nanoparticles. We synthesized poly(tert-butyl acrylate)-grafted nanoparticles at nearly constant grafting density with varying brush dispersities by atom transfer radical polymerization in the presence of a chain termination agent. As the dispersity was increased from 1.1 to 1.7 at constant number-average degree of polymerization (N_n), polymer brushes adopted a more extended conformation, as assessed via hydrodynamic radius using dynamic light scattering. We show that the lengths of brushes of varying dispersities, adopting semi-dilute polymer brush conformation, can be collapsed onto a master curve as a function of weight-average degree of polymerization (N_w) for high molecular weight brushes (N_w > 200). At lower N_w, however, high-dispersity brushes are up to twice as thick as low-dispersity brushes, and brushes similarly adopt concentrated polymer brush conformation. By additionally accounting for the varying grafting densities, we show that brush length measurements from the literature can be collapsed onto this bifurcated curve. We hypothesize that the drastic differences in brush length at low N_w are related to conformational differences among low and high dispersity brushes. The more extended polymer brushes attained via increase in dispersity can be leveraged to improve nanoparticle dispersion in complex media.

Reference:

1. Dodd, P. M.; Jayaraman, A. Monte carlo simulations of polydisperse polymers grafted on spherical surfaces. J. Polym. Sci. B Polym. Phys. 2012, 50 (10), 694-705 DOI: 10.1002/polb.23057.