Synthesis and Application of Polymer Brush-Grafted Nanoparticles as Hydrogel Gelators and Lubricant Additives

This dissertation describes the synthesis of polymer brush-grafted nanoparticles (hairy NPs) and an analysis of their behavior or utility in multiple areas. The hairy NPs were synthesized from silica NPs functionalized with initiating moieties by surface-initiated atom transfer radical polymerization. A brief introduction to hairy NPs, with a focus on the synthesis and behavior of stimuli-responsive polymer brush-grafted particles, is given in Chapter 1 to provide context for this work.

Chapters 2 and 3 present the synthesis of thermosensitive diblock copolymer brush-grafted nanoparticles designed as hairy NP analogues in place of thermosensitive block copolymers micelles for the construction of hybrid, physical hydrogels. Chapter 2 details a series of hairy NPs with a thermosensitive poly(methoxydi(ethylene glycol) methacrylate) (PDEGMMA) inner block and a charge-bearing, poly(DEGMMA-co-2-(methacryloyloxy)ethyltrimethylammonium iodide) (PDEGMMA-co-TMAEMA-I) outer block. These hairy particles underwent a reversible, cooling-induced gelation at moderate concentrations in water, based on the packing of hairy NPs due to the LCST-driven increase in brush volume fraction upon cooling. Another series of thermosensitive hairy NPs was made with brushes composed of P(TMAEMA-I)-b-PDEGMMA, which exhibited a heating-induced reversible gelation at concentrations as low as 3 wt % in water, due to the association of PDEGMMA outer blocks at temperatures above their LCST. The inner hydrophilic polyelectrolyte block served to bridge these domains and NPs to form a three-dimensional gel network.

Chapter 4 details the use of NPs grafted with oil-soluble poly(lauryl mathacrylate) as lubricant additives. These hairy NPs showed superior stability in a poly(alphaolefin) (PAO) base lubricating oil, and the addition of 1.0 wt % hairy NPs to PAO yielded significant reductions in both friction and material wear. These gains were attributed to the formation of a load-bearing tribofilm at the rubbing interface. Chapter 5 explores the brush microphase separation of poly(n-butyl acrylate)-b-polystyrene (PnBA-b-PS) brush-grafted particles with PnBA as inner block. From TEM analysis, there appeared an evolution of phase morphology from a stripe-like nanostructure to a more uniform layered structure with increasing PS molecular weight, in qualitative agreement with simulation studies. Chapter 6 includes a look back on this dissertation work in its entirety and possible future work.