Transient Supramolecular Interactions for Templating Peptide Folding and Designing New Self-Healing Polymers

Engineering transient interactions is a powerful method that can be used to encode structure and function in chemical systems. This is the common theme in all three chapters of this dissertation, despite the slight differences in initial motivations. I begin my dissertation by describing my efforts to direct peptide folding in organic environments using supramolecular amino acids (SAAs). The incorporated supramolecular motif has a strong driving force to dimerize in a sequence- and orientation-specific manner. By introducing SAAs into the primary sequence of peptides, we envisioned to perturb the native hydrogen bond patterns and facilitate the folding of the peptides through specific and directional dimerization of the supramolecular units.In Chapter 2, I describe the synthesis and characterization of a new self-healing multiphase polymer, in which, a pervasive network of dynamic metal-ligand (zinc-imidazole) interactions are programmed in the soft matrix of a hard/soft two-phase brush copolymer system. Following mechanical damage, these thermoplastic elastomers show excellent self-healing ability under ambient conditions without any intervention. The mechanical and dynamic properties of the materials can be tuned by varying a number of molecular parameters (e.g., backbone/brush degree of polymerization and brush density) as well as the ligand/metal ratio. This chapter concludes by noting an attempt to correlate the observed mechanical properties to the small molecule parameters of metal-ligand complexes.Synthesis of multiphasic self-healing polymers prepared from simple commercially-available monomers in a facile and scalable manner was the motivation behind the work described in the Chapter 3. The mechanical properties of these new polymers are easily tunable across a broad range (from soft rubber to hard plastic) by changing several molecular parameters. Self-healing ability was investigated at mild conditions, and most notably, most samples demonstrate excellent self-healing abilities with minimal intervention

Transient Supramolecular Interactions for Templating Peptide Folding and Designing New Self-Healing Polymers

Engineering transient interactions is a powerful method that can be used to encode structure and function in chemical systems. This is the common theme in all three chapters of this dissertation, despite the slight differences in initial motivations. I begin my dissertation by describing my efforts to direct peptide folding in organic environments using supramolecular amino acids (SAAs). The incorporated supramolecular motif has a strong driving force to dimerize in a sequence- and orientation-specific manner. By introducing SAAs into the primary sequence of peptides, we envisioned to perturb the native hydrogen bond patterns and facilitate the folding of the peptides through specific and directional dimerization of the supramolecular units.In Chapter 2, I describe the synthesis and characterization of a new self-healing multiphase polymer, in which, a pervasive network of dynamic metal-ligand (zinc-imidazole) interactions are programmed in the soft matrix of a hard/soft two-phase brush copolymer system. Following mechanical damage, these thermoplastic elastomers show excellent self-healing ability under ambient conditions without any intervention. The mechanical and dynamic properties of the materials can be tuned by varying a number of molecular parameters (e.g., backbone/brush degree of polymerization and brush density) as well as the ligand/metal ratio. This chapter concludes by noting an attempt to correlate the observed mechanical properties to the small molecule parameters of metal-ligand complexes.Synthesis of multiphasic self-healing polymers prepared from simple commercially-available monomers in a facile and scalable manner was the motivation behind the work described in the Chapter 3. The mechanical properties of these new polymers are easily tunable across a broad range (from soft rubber to hard plastic) by changing several molecular parameters. Self-healing ability was investigated at mild conditions, and most notably, most samples demonstrate excellent self-healing abilities with minimal intervention

Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds

Polymers that repair themselves after mechanical damage can significantly improve their durability and safety. A major goal in the field of self-healing materials is to combine robust mechanical and efficient healing properties. Here, we show that incorporation of sacrificial bonds into a self-repairable network dramatically improves the overall mechanical properties. Specifically, we use simple secondary amide side chains to create dynamic energy dissipative hydrogen bonds in a covalently cross-linked polymer network, which can self-heal via olefin cross-metathesis. We envision that this straightforward sacrificial bonding strategy can be employed to improve mechanical properties in a variety of self-healing systems

Self-Healing Multiphase Polymers via Dynamic Metal–Ligand Interactions

A new self-healing multiphase polymer is developed in which a pervasive network of dynamic metal–ligand (zinc–imidazole) interactions are programmed in the soft matrix of a hard/soft two-phase brush copolymer system. The mechanical and dynamic properties of the materials can be tuned by varying a number of molecular parameters (e.g., backbone/brush degree of polymerization and brush density) as well as the ligand/metal ratio. Following mechanical damage, these thermoplastic elastomers show excellent self-healing ability under ambient conditions without any intervention

Control of hierarchical polymer mechanics with bioinspired metal-coordination dynamics

In conventional polymer materials, mechanical performance is traditionally engineered via material structure, using motifs such as polymer molecular weight, polymer branching, or block copolymer design. Here, by means of a model system of 4-arm poly(ethylene glycol) hydrogels crosslinked with multiple, kinetically distinct dynamic metal-ligand coordinate complexes, we show that polymer materials with decoupled spatial structure and mechanical performance can be designed. By tuning the relative concentration of two types of metal-ligand crosslinks, we demonstrate control over the material's mechanical hierarchy of energy-dissipating modes under dynamic mechanical loading, and therefore the ability to engineer a priori the viscoelastic properties of these materials by controlling the types of crosslinks rather than by modifying the polymer itself. This strategy to decouple material mechanics from structure is general and may inform the design of soft materials for use in complex mechanical environments. Three examples that demonstrate this are provided.National Science Foundation (U.S.) (Award DMR-0819762)National Science Foundation (U.S.) (Award DMR-1419807

Tuning Dynamic Mechanical Response in Metallopolymer Networks through Simultaneous Control of Structural and Temporal Properties of the Networks

Tunable mechanical response under dynamic and static loading is desirable for many technological applications. Traditionally, mechanical performance of polymeric materials is controlled by modulating structural (i.e., molecular weight, chain packing, or cross-link density) or temporal parameters (such as kinetics of the exchange of dynamic cross-linkers). Metal–ligand interactions are uniquely suited to control both structural and temporal parameters as the thermodynamics and kinetics of mechanically active cross-linkers can be varied by careful selection of metal without significant synthetic modification of the polymer backbone. Here, we have demonstrated that it is possible to engineer desired mechanical properties in a metallopolymer with a high degree of tunability by simply changing the type and amount of added metal. Specifically, we cross-linked an imidazole-containing brush copolymer system with the divalent cations of zinc, copper, and cobalt. Using rheology and tensile experiments, we have correlated the emergent mechanical properties to the stoichiometric ratio of ligand to metal as well as the coordination number and ligand exchange mechanism of the imidazole–metal cross-links. In contrary to the general view that unbound free ligands are normally regarded as mechanically inactive dangling chains in metallopolymer networks, this study clearly shows that they can play a critical role in stress distribution and chain relaxation. Importantly, this work shows for the first time that it is possible to simultaneously control both the structure of networks and the temporal response of bulk materials using dynamic association of weak and monodentate ligands with transition metals

Control of hierarchical polymer mechanics with bioinspired metal-coordination dynamics

In conventional polymer materials, mechanical performance is traditionally engineered via material structure, using motifs such as polymer molecular weight, polymer branching, or copolymer-block design(1). Here, by means of a model system of 4-arm poly(ethylene glycol) hydrogels crosslinked with multiple, kinetically distinct dynamic metal-ligand coordinate complexes, we show that polymer materials with decoupled spatial structure and mechanical performance can be designed. By tuning the relative concentration of two types of metal-ligand crosslinks, we demonstrate control over the material’s mechanical hierarchy of energy-dissipating modes under dynamic mechanical loading, and therefore the ability to engineer a priori the viscoelastic properties of these materials by controlling the types of crosslinks rather than by modifying the polymer itself. This strategy to decouple material mechanics from structure may inform the design of soft materials for use in complex mechanical environments