UC Irvine

Drawing on inspiration from nature, transient, dynamic interactions can encode function into synthetic materials. Reversible interactions, such as dynamic covalent chemistry and transient noncovalent interactios can provide mechanisms for a host of functional applications such as self-healing, malleability, and energy dissipation. The effect of dynamic interactions on the emergent functional properties is a common thread throughout the thesis. The dissertation begins with an extensive introduction to self-healing polymer materials. Here, three main self-healing mechanisms are discussed including physical, vascular, and intrinsic based healing. A short exposition of polymerization techniques used in this thesis is also discussed briefly in the beginning.

In Chapter 2, I discuss my efforts toward the development of tunable boronic ester transesterification mediated self-healing and malleable materials. Small molecule model studies show that introduction of an amino-neighboring group accelerates the rate of transesterification of aryl boronic esters over five orders of magnitude. Telechelic boronic ester cross-linkers, one with the neighboring group and one without, are introduced to a poly-ol matrix and showed dramatic differences in dynamic properties. This study is the first self-healing system where the dynamicity of the healing motif has been systematically tuned based on small molecule experiments which directly relate to the emergent dynamic properties. In Chapter 3, the boronic ester transesterification reaction is further tuned to access to rates that are within the five orders of magnitude described in Chapter 2. Here, tuning the electronics on the aryl boronic ester and the sterics on the neighboring amine are varied and can be used successfully to finely tune the rate of transesterification. We believe that this fine tuning will provide a method by which bottom up approaches to dynamic materials can be pursued.

In Chapter 4, I then discuss polycyclooctene based thermoplastic elastomers. Here, by introducing different amounts of dihydroxylated cyclooctene in a polycyclooctene based polymer, the melting temperature and therefore crystallinity can be adjusted. The bulk mechanical properties show that the materials with intermediate diol are thermoplastic elastomers with interesting mechanical properties. By TEM, phase collapse is observed.

In Chapter 5, I discuss a hydrogen bonding-based self-healing nanocomposite system. Here, polymer grafted silica nanoparticles that contain amides are prepared. Three different approaches to controlling the graft density on particle surface are explored. Silica nanoparticles could indeed be functionalized with polymerization initiators and subsequent polymerizations could be completed. All three approaches, however, led to irreproducible and inconsistent results. This chapter largely serves as a cautionary tale about the intricacies of nanocomposite materials.

Finally, in chapter 6, I describe my contributions to a multi-university collaboration where nanocomposite polymer coating materials for blast-induced shockwave mitigation is explored. First, nanoparticles with acrylate polymers grafted from the surface are blended into a host polyurea matrix. When the acrylate polymers have urea units that can hydrogen bond to the host matrix, the materials are able to significantly dissipate energy. The second half of the chapter discusses the development of superlattice nanocomposite materials that have controlled center to center spacing for targeted frequency absorption. Throughout this thesis, I hope to convey that programming subtle molecular interactions can have dramatic impacts on the dynamic properties of bulk polymer materials.