Smart surfaces using functional polymer brushes prepared via organocatalyzed living radical polymerization

• Main Author: Sim, Xuan Ming
• Other Authors: Atsushi Goto
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Chemistry
• Online Access: https://hdl.handle.net/10356/173437
• Institution: Nanyang Technological University

This thesis aims to synthesize functional polymer brushes prepared via organocatalyzed living (reversible-deactivation) radical polymerization to create smart surfaces i.e., multi-stimuli responsive reversibly crosslinking/decrosslinking polymer brushes, rewritable polymer brushes on surfaces, and aggregation-induced emission (AIE)-bearing polymer brushes to recognize sizes of external molecules. Chapter 1 describes the introduction and literature reviews of living radical polymerization and the general approaches for the preparation of well-defined chain-end functionalized polymers via living/controlled polymerization techniques. The concepts of polymer brushes on surfaces, click reaction, Diels–Alder reaction, and aggregation-induced emission, along with the motivation and aim of each chapter are presented. Chapter 2 presents the synthesis of a reversible crosslinking/decrosslinking polymer brush via the Diels–Alder (DA) and retro-Diels–Alder (rDA) reactions. A bismaleimide crosslinker was used to crosslink poly(furfuryl methacrylate) (PFMA) brushes at 70 °C via the DA reaction to form a crosslinked PFMA brush. Subsequently, the crosslinked PFMA brushes underwent decrosslinking via the rDA reaction at 110 °C, creating a thermal-responsive crosslinked PFMA brush. The surface wettability of the brushes is reversibly tailored by the reversible crosslinking/decrosslinking processes. Disulfide-containing bismaleimide provides another crosslinking/decrosslinking pathway of the crosslinked PFMA brush by reversible oxidation/reduction. Binary and striped patterned brushes were obtained from the disulfide-containing crosslinked PFMA brushes using a photo-induced reduction. Reversibility, multi-stimuli responsiveness and metal-free polymerization approach are appealing aspects in the present work that may find application in adsorption/desorption interfaces, rewritable interfaces, and sensing interfaces. Chapter 3 presents the facile yet quantitative post-polymerization modification of polymer-iodide (Polymer-I) using HSCH2CH2SH to obtain thiol-terminated polymers (Polymer-SH) at room temperature. Polymer-SH is also modified to obtain pyridyl disulfide-terminated polymers (Polymer-SS-Py). The hetero-disulfide exchange between Polymer-SH and Polymer-SS-Py generated a polymer possessing a disulfide bond (Polymer-SS-Polymer). SH-functionalization polymer chains were grafted to a Py-SS-functionalized solid substrate to obtain a polymer disulfide brush layer on the solid surface via the hetero-disulfide exchange (writing). A reducing agent is used to cleave the disulfide bond and the brush polymers are detached from the surface (erasing). Other SH-functionalized polymer chains can then be attached (rewriting) to the solid substrate, demonstrating a rewritable smart surface. Such rewritable smart surfaces may be exploited for rewritable microarray technologies and sensing interfaces. Chapter 4 presents the syntheses of an AIE-bearing fluorescent polymer brush, i.e., poly(4-(1,2,2-triphenylvinyl)phenyl methacrylate) polymer brush. The polymer brush was fabricated in patterned manners with different graft density domains and served as a conceptually new molecular size analyzer of external molecules in the oligomeric molecular regions. The sizes of external molecules are recognized via changes in the photoemission of the polymer brushes. Different graft density domains selectively capture external molecules of different sizes. Small external molecules can enter the highly-density polymer brush domain on the surface, concomitantly inducing the AIE side groups in the polymer brush chains to aggregate to give a more intense AIE. On the other hand, large external molecules cannot enter (are excluded from) the highly-density polymer brush domain on the surface, giving a relatively less intense AIE. The graft density of the brushes determines the size exclusion (molecular weight) threshold of the external molecules. Using AIE polymer brushes with different graft density domains, the present work distinguished external molecules with molecular weights of 300 and 1000 by the change in the AIE emission of the polymer brushes. The present technique can tune the graft density in a wide range and may be used to analyze a wide range of molecular sizes. Chapter 5 presents the conclusion of the thesis, offering the novelty, significance, and key achievements highlighted in the preceding chapters, as well as providing insights on potential practical applications that can be exploited in the future.