Date of Award
Doctor of Philosophy
Materials Science and Engineering
Materials Science; Engineering
Martin M. Thuo
The discovery of macromolecules has catalyzed immense interdisciplinary efforts between chemistry and materials science in the past century. A myriad of polymeric materials have been synthesized and are often studied for their potential use(s). The current developments of synthetic routes for these materials have, however, been largely governed by key assumptions and strategies adopted at the infancy of the field. Herein, we challenge this conventional dogma across three classes of polymeric materials and provide an alternative ‘soft’ approach based on felicitous choice of synthons or application of new design paradigms.
First, chain growth polymerization, for example of olefins, is initiated using a labile molecular fragment as an initiator. The key step in these systems, however, lies in the homolytic disproportionation of the π-bonds with no significant perturbation on the associated π-bond. This process, therefore, can be simplified as an addition (radical) or abstraction (cationic) of electron density from the olefinic π-system. We, therefore, inferred that introduction of a free electron presents the simplest way to initiate olefin polymerization. The use of subatomic particles (electrons) in a solvated state negates the need for a dissociating initiator molecule and a presents an opportunity to exploit quantum mechanics aspect of the electron during initiation. Potential to tunnel across solvent molecules under slight perturbation offers new possibilities in olefin polymerization. Among them include rapid uniform initiation, hence, low polydispersity index (PDI), pathway to 3-electron polymerization (radical and anion) leading to bimechanistic polymerization, and essentially end-group free chains.
On the other end of the spectrum, coordination polymers are conventionally synthesized by inducing chemical potential gradients by thermal and pressure changes (solvothermal). A major challenge in this approach is inability to maintain steady-state concentration hence rate of polymerization decreases with reaction progression. Alternatively, chemical potential gradients can be established by using a slow-release reagent reservoir and exploit solubility (ΔGm) to induce an in situ self-assembly driven precipitation of the polymerized material. Controlling the coordination sphere geometry ensures directs the self-assembly process leading to control over the geometry of the material. Since the product is a living polymer, ad infinitum polymerization continuously occur on the precipitate. Control over the rate of polymerization (Kp) over the rate of self-assembly (KL) control the aspect of the ratio with high-aspect ratio materials obtained for Kp>>KL and vice versa.
Finally, besides introducing new paradigms in polymer synthesis, polymer matrix composites—especially for autonomous self-responsive polymers, new fabrication paradigms are desired. Research in soft materials has recently gained impetus in part due to quest for biomimetic smart autonomous responsive materials. Specifically, mechanically responsive polymer composites have garnered most interest. The main component in designing such materials is presence of mechanically sensitive kinetic barriers in the reaction coordinate. Metastable phase change materials offer a unique approach for rapid and reaction-free mechanical response. Metastable materials are, however, challenging to synthesize. Studies in metastable materials also tend to be heuristic, hence it is challenging to develop design rules for such materials. The advent of a reliable, high-yield route to liquid metal undercooled particles, however, present a chance to develop new class or composite materials. Mechanical-triggered solidification of these undercooled particles implies that an autonomously stiffening material can be made by using the particles as filler. Thermodynamic potential of each particle relates to its size, hence, asymmetric responses can be engineered through size polydispersity. This approach introduces a thermodynamic-governed stiffening, a new paradigm in autonomously responsive composite materials.
Boyce Sek Koon Chang
Chang, Boyce Sek Koon, "Non-classical approaches to the synthesis of polymeric materials" (2018). Graduate Theses and Dissertations. 17153.