(363g) Electrostatic Adsorption of Polyelectrolyte Brush-Grafted Nanoparticles At the Solid/Liquid Interface

Nanoparticulate polymer brushes consist of densely packed, end-attached polymer chains extending from a nanoscale core.  Examples include multi-arm star polymers with cross-linked cores or inorganic nanoparticles with chains grafted at high density from their surfaces.  Similar to block copolymer micelles, they have a core/corona structure, but unlike micelles they are permanent, non-dissociating objects.  When block copolymer micelles adsorb, they can disassemble so that all chains found at the surface are in direct contact with it.  Adsorbed nanoparticulate brushes cannot do this, and a large fraction of their chains may have no direct contact with the surface. Adsorption of nanoparticulate brushes may therefore be advantageous for surface conditioning technologies that benefit from dense layers of highly extended polymer chains, such as steric stabilization or boundary lubrication. 

Adsorption thermodynamics differ in significant ways for nanoparticulate brushes relative to linear polymer chains, particularly with regard to the strong inter-segment repulsions that exist within the brush and how they influence the configurational part of the adsorption entropy.  Here we experimentally compare adsorption of nanoparticulate brushes and chemically similar linear polymer chains to the solid/liquid interface.  This presentation focuses mainly on an annealed nanoparticulate polyelectrolyte brush, consisting of the weak polybase poly(2-(dimethylamino)ethyl methacrylate) grafted at high density from silica nanoparticles by surface-initiated atom transfer radical polymerization (PDMAEMA-Si).  These are adsorbed from aqueous suspensions to negatively charged silica surfaces and compared to linear PDMAEMA adsorption.  For both systems, adsorption balances electrostatic attraction to the surface with lateral electrostatic repulsions among neighboring species or intramolecular/intra-brush electrostatic repulsions.  These are readily tuned by pH and ionic strength changes. The main points of discussion will concern the effects of altered electrostatic conditions on the extent of adsorption. In general, the extent of adsorption is significantly larger at any pH for the PDMAEMA-Si nanoparticulate brush than for linear PDMAEMA, due to the high density packing of polymer segments in the former. Consistent with a prior investigation of linear PDMAEMA adsorption to silica, a critical pH threshold is observed, above which little or no adsorption occurs. This happens because the weakly charged chains have insufficient attraction to the surface at elevated pH to offset the unfavorable adsorption entropy.  In contrast, no such pH threshold is observed for PDMAEMA-Si nanoparticulate brushes, since the adsorption entropy is less unfavorable for this system, and even the weak residual attraction to the surface is sufficient to allow adsorption. This is consistent with prior models of star polymer adsorption. The overall structure of the adsorbed layers is interpreted in terms of pH effects on brush charge and swelling, aided by electrophoretic mobility and dynamic light scattering measurements on bulk suspensions, as well as streaming potential measurements and atomic force microscopy images of PDMAEMA-Si covered surfaces. When possible, this electrostatic-dominated system will be compared and contrasted with behaviors displayed by nonionic poly(ethylene oxide) star polymers and linear polymers.