Synthesis and Characterization of Stimuli Responsive Polymer Brushes

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Deodhar, Chaitra Vasant
weak polyelectrolyte brushes; neutron reflectometry; ellipsometry; ion-specific effects; swelling behavior; hydration; Polymer Chemistry
thesis / dissertation description
Polyelectrolytes (PE) are the least understood polymeric systems due to their complex behavior that arises because of the intimate connection between charge and conformation. To address this challenge, my research focuses on understanding the responsive behavior of weak polyelectrolyte brushes to different stimuli such as pH, type of ion and ionic strength. In this work, weak polyelectrolyte brushes made of poly(methacrylic acid) (PMAA) and random copolymer brushes incorporating hydroxyethyl methacrylate (HEMA) as co-monomer were studied. The polymer brushes were synthesized by both “grafting to” and “grafting from” approaches and were characterized mainly by ellipsometry and neutron reflectometry.Several methods for preparing alkyne-decorated substrates suitable for “click” chemistry are described. For each method, factors that control film thickness and content of alkyne groups were investigated. Azide terminated PMAA and poly(vinyl-dimethylazlactone) polymers were tethered to alkyne-bearing poly(glycidylmethacrylate) surface layers via click reactions. It was observed that the alkyne content obtained by these approaches was sufficiently high and was not the limiting factor for the click reaction of azide-capped polymers. A method to self-consistently determine water content of PE films using neutron reflectometry under different hydration conditions is reported. Multiple data sets were modeled with constrained fittings to obtain dry polymer mass density, layer thickness and PE brush profiles into different pH solutions. The influence of monovalent ions from the cationic Hofmeister series on the structure and swelling behavior of weak polyelectrolyte brushes was also investigated. It was found that the scaling behaviors of PMAA brushes with respect to salt concentration are similar in presence of different salts; however, the cross-over salt concentration at which the transition occurs is different for each type of salt and can be explained by its position in Hofmeister series. Finally, some preliminary results concerning calcium carbonate mineralization of PMAA brushes as a function of concentration of organic additive are reported. Calcium carbonate films were obtained by polymer-induced liquid precursor (PILP) strategy, using an ammonia diffusion method. Various characterizations show that as the concentration of additive is increased, the extent of mineralization decreases.