Synthesis and Morphology of Well-Defined Mixed Homopolymer Brushes Grafted on 150 - 180 nm Silica Particles

Mixed homopolymer brushes, composed of two distinct homopolymers randomly or alternatively immobilized by one end on a solid substrate, exhibit intriguing phase behavior. This dissertation work focuses on synthesis and morphology study of well-defined mixed brushes grafted on 150 – 180 nm silica particles. The brushes were grown from asymmetric difunctional initiator (Y-initiator)-functionalized silica particles by using two “living”/controlled polymerization techniques.

Chapter 1 is an introduction to mixed brushes. Chapter 2 presents a study on the effect of overall grafting density (s_overall) [Sigma overall] on morphology of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene brushes synthesized by sequential surface-initiated atom transfer radical polymerization of tBA and nitroxide-mediated radical polymerization (NMRP) of styrene from Y-initiator-functionalized particles. By changing the mass ratio of Y-initiator to silica particles in the initiator immobilization step, a series of brush samples with different s_overall values but similar molecular weights (MW) were synthesized. Transmission electron microscopy showed that at s_overall ³ 0.34 chains/nm² [chains/nanometer square], the brushes formed “rippled” structures after cast from chloroform. The normalized ripple wavelength (D) scaled with s_overall^-0.47 [sigma overall to the -0.47 power] in the s_overall range of 1.06 to 0.54 chains/nm².

Chapter 3 describes a study on the effect of MW on morphology of mixed PtBA/PS brushes on silica particles. A series of samples with different average MWs, from 13.8 to 33.1 kDa, but comparable s_overall were made and their morphologies were investigated. For samples cast from chloroform, D scaled with MW^0.70 [MW to the 0.7 power] in the studied MW range. For uniformly collapsed mixed brushes cast from water, D was proportional to MW^0.56 [MW to the 0.56 power].

Chapter 4 presents the synthesis of mixed brushes from Y-initiator-functionalized silica particles by surface-initiated ring-opening polymerization and NMRP. The s_overall of mixed brushes can be tuned by varying the mass ratio of Y-initiator to silica particles in the initiator immobilization step. Chapter 5 describes the synthesis of two polymer brush acid catalysts and their applications for fructose dehydration to 5-hydroxymethylfurfural in water. Both brush catalysts exhibited a higher catalytic activity than the corresponding free polymer catalysts. Chapter 6 presents conclusions and future work.