LEVERAGING A PEPTIDE-POLYMER HYBRID PLATFORM TO DEVELOP MECHANICALLY-TUNABLE AND STIMULI-RESPONSIVE MATERIALS

Abstract

Nature provides a myriad of sources of inspiration for the design of “smart” materials that can sense environmental changes and vary their physiochemical properties. Biological materials, including spider silk and the sea cucumber dermis, exhibit mechanical robustness by using a small set of natural building blocks and tailored interactions between these building blocks, enabling them to respond to their surroundings and to elicit a range of functions. One pathway to this responsive behavior and multifunctionality is the utilization of non-covalent interactions to control hierarchical arrangement. Applying this design principle, we aim to create mechanically-tunable and stimuli-responsive synthetic materials. Toward this goal, this dissertation focuses on developing hierarchy diversity in synthetic polymers via the modulation of non-covalent interactions between building blocks and probing how hierarchical ordering can be harnessed to tailor the mechanical and responsive behavior of the final materials. First, we use self-assembling peptide motifs, consisting of poly(β-benzyl-L-aspartate) (PBLA), to construct hierarchical structures in a non-chain extended poly(dimethyl siloxane) (PDMS)-based polyurea platform. These peptide-polyurea hybrid materials enable the tuning of thermo-responsive shape memory and mechanical properties through manipulating peptide secondary structures (e.g., α-helices and β-sheets). This peptidic ordering determines the degree of phase separation which significantly influences shape memory response. Furthermore, we utilize the ability of peptide blocks to alter secondary conformation as a function of temperature to demonstrate an additional route to modulate shape memory effect. This research reveals that secondary structures can be a facile handle to control mechanical shape memory behaviors. Extending the first research findings, cellulose nanocrystals (CNCs) are added to vary the self-assembled structures and tensile properties of the peptidic polyureas via matrix-filler interactions. The favorable interactions between PBLA blocks and CNCs are formed in a series of peptidic polyurea/cellulose nanocomposites, inhibiting intra-molecular hydrogen-bonded α-helical arrangements, but facilitating inter-molecular hydrogen-bonded β-sheet organization. PBLA and CNC contents are varied to tailor the extent of PBLA-CNC interactions, leading to morphological shifts and Young’s modulus changes from 10 MPa to 150 MPa. In this new type of nanocomposite platform, the mechanical response is defined by a balance of peptidic ordering, degree of matrix-filler interactions, and self-assembled morphology. These findings suggest that peptide-cellulose interactions can be leveraged to design materials with targeted mechanical performance for a specific application. Finally, PBLA motifs are incorporated into a passive, non-chain extended poly(ethylene glycol) (PEG)-based framework to induce water-responsive mechanically-adaptiveness. Peptidic ordering is shown to serve as physical crosslinking points, enabling aqueous swelling. In these hydrophilic peptidic polyureas, PBLA content can be used to modulate the microphase-separated morphology and mechanical response in the dry and wet states. In the peptidic polyureas, CNCs are added to diversify hierarchical structures, increase tensile properties, and broaden the magnitude of stiffness changes (i.e., water sensitivity). Depending on PBLA weight fraction (20 or 40 wt%) and CNC content (0 or 10 wt%), the water sensitivity of resultant materials is tailored from ~ 70 MPa to ~ 400 MPa, highlighting the potential of using the interplay of peptide assembly and peptide-cellulose interactions to generate mechanical tunability and water-adaptiveness in passive conventional polymers. My dissertation demonstrates that variations in hierarchical arrangements through control over non-covalent interactions can be a powerful engineering strategy to create stimuli-responsive materials with desired mechanical properties.