Search

Abstract: Hydrogels, due to their soft and water abundant natures, highly resemble human skins and tissues and thereby have attracted extensive attention in recent years. Functional hydrogels showing responsiveness to various stimuli such as pH, temperature, strain and pressure are promising for a wide range of biomedical and electrical applications. However, traditional hydrogels crosslinked via permanent covalent bonds are generally weak, brittle and lack of functionalities. Incorporation of reversible and dynamic crosslinks into hydrogel networks can effectively improve their mechanical performances, introduce multiple responsiveness, and endow the hydrogels with self-healing capability. On the other hand, introduction of stiff nanomaterials into hydrogel matrix is an effective approach towards strong and functional gel materials. The combination of the nanofillers and appropriate interfacial reversible and dynamic interactions provides a promising method for the fabrication of high-performance multifunctional hydrogels to meet the increasing needs of modern materials. In this thesis, a review on hydrogels based on reversible and dynamic crosslinks, stimuli-responsive and self-healing functions, and nanocomposite hydrogels was presented first followed by three original research projects on developing nanocomposite hydrogels with stimuli sensitivity and self-healing property based on reversible interactions for biomedical and electrical applications. Injectable, self-healing and pH-responsive hydrogels are great intelligent drug delivery vehicles for controlled and localized therapeutic release. Hydrogels that show pH-sensitive behaviors in mildly acidic range are ideal to be used for the treatment of regions showing local acidosis like tumors, wounds and infections. In the first project, we present a facile preparation of an injectable, self-healing and super-sensitive pH-responsive nanocomposite hydrogel based on Schiff base reactions between aldehyde-functionalized polymers and amine-modified silica nanoparticles. The hydrogel shows fast gelation, injectability and rapid self-healing capability. Moreover, the hydrogel demonstrates excellent stability under neutral physiological conditions while a sharp gel-sol transition induced by faintly acidic environment. The pH-responsiveness of the hydrogel is ultra-sensitive, where the mechanical properties, hydrolytic degradation and drug release behaviors can alter significantly when subjected to a slight pH change of 0.2. The novel injectable, self-healing and sensitive pH-responsive hydrogel serves as a promising candidate as localized drug carriers with controlled delivery capability triggered by acidosis, holding great promise for cancer therapy, wound healing and infection treatment. Conductive hydrogels are of great significance for soft electronic devices. In the second project, we have developed a novel hydrogel ionic conductor by integrating nanofiller reinforcement with micelle cross-linking. The hydrogel was facilely prepared via one-pot polymerization of acrylamide and an amino-functionalized monomer in the presence of carbon nanotubes, aldehyde-modified F127 and LiCl. The dynamic chemical and physical interactions of the cross-linked network offers the hydrogel with a wide spectrum of properties, including excellent stretchability, toughness, exceptional elasticity, resistance to damage by sharp materials, self-healing property and high conductivity. In addition, the hydrogel demonstrated cooling-induced whitening optical behavior. When exploited as a strain and pressure sensor to monitor diverse human motions, the prepared hydrogel sensor showed excellent sensitivity and reliability. The hydrogel was further integrated with an eye mask to monitor human sleep and showed high reliability for the detection of rapid eye movement (REM) sleep. This work provides new insights into the fabrication of multifunctional, smart and conductive materials, holding great promise for a broad range of applications like wearable sensors, artificial skins, and soft robotics. In the third project, we have developed an ionic conductive nanocomposite hydrogel with ultra-stretchability and intelligent sensing functions. By leveraging the dynamic feature of multiple intermolecular interactions, polymer/carbon nanotube networks with excellent mechanical performances (i.e., tensile strength, stretchability and toughness up to 1.09 MPa, 4075% and 12.8 MJ/m3, respectively) were achieved. Additionally, the hydrogel is soft, elastic, transparent and self-healing. The rational combination of the mechanical and electrical properties renders the as-prepared hydrogel with excellent sensing performances and cycling stability, and therefore enables it to perform as a sensory unit of a complete platform for the recognition of some complicated human behaviors. Specifically, with the integration of machine learning module, the hydrogel-based platform exhibits great recognition accuracies to human handwriting motions from single letters to words and phrases after proper training. The combination of superior mechanical performances and intelligent sensing functions within this hydrogel-based ionic skin unlocks its potential as the intelligent human-device interface, which promotes the application of artificial intelligence in customized electronic devices.

DASA was introduced in 2014 and its reversible switching under visible light has attracted many applications including drug delivery. However, reports on DASA synthesized from a functionalized starting material have been very limited and standardized methodology for its synthesis is not available. Moreover, the effect of polymer flexibility, chain length and glass transition temperature (Tg) on switching has not been investigated for micellar DASA in water. Systematic approach to study the DASA from a simple model molecule to a more complex homopolymer and block copolymer was performed to determine its solubility, stability and switching in various solvents. The effect of polymer flexibility was studied by comparing methacrylate (hard) to acrylate (soft) DASA polymers. Long and short polymers were synthesized for each materials to compare the effect of the chain length. And finally, the effect of Tg was investigated by synthesizing a block copolymer with a hydrophilic block and a statistical copolymer of DASA and butyl acrylate (BA, Tg = -53 °C) or DASA and adamantyl acrylate (ADA, Tg = > 200 °C). The study reveals that conjugating DASA onto polymer provides stabilization for the linear isomer in water. Methacrylate polymers shows better stability in water, thus result in a reversible switching. Acrylate polymers undergo hydrolyzation in water, leading to decomposition during switching. In addition, shorter chain length shows better reversibility than longer chain length due to coiling effect which restricts DASA isomerization. Block copolymer with statistical copolymerization of DASA and high or low Tg polymers resulted in irreversible switching in water (self-assembled into micelle), thus a block copolymer where the second block is DASA (conjugation > 99%) is a better candidate for light responsive drug delivery vehicle. Methacrylate DASA block copolymer (DP = 20 units) has shown successful light-responsive ellipticine release on A2780 human ovarian carcinoma (IC50 = 31 μM). These findings have provided valuable fundamental information for choosing the optimum materials for DASA drug delivery vehicles which is currently not available in the literature.

ABSTRACT Micro to Macro: Translating the behavior of small molecules to functional materials through the use of external stimuli by Kyle Darrin Clark As the scientific community continues to develop a vast array of functional materials, the demand for improved control over a broad range of properties becomes all the more prevalent. Stimuli such as light and temperature have been employed consistently throughout history as easily accessible tools for chemists, and in recent years they have shown great promise as a means to control polymer property on-demand. The discovery of novel photo-responsive small molecules and temperature triggered reactions and applications of the materials are presented. Self-immolative polymers (SIPs) are a sub-class of stimuli-responsive materials that, when exposed to an appropriate stimulus, degrade end-to-end into their composite monomers and release a given cargo. One challenge with current SIPs is that once initiated, unraveling of the polymer typically occurs in an uncontrolled fashion. To address this challenge, we designed a SIP system capable of dual-stimulus activation, allowing on/off control of depolymerization. Specifically, furan-endcapped polyurethanes were prepared and the controlled depolymerization was studied with both acid and heat triggers. From these SIPs, a new class of temporally controlled degradable materials could be accessed which hold potential for on/off drug release. Photo-responsive polymers represent another class of stimuli responsive material with myriad of applications that range from controlled micelle rupture and surface patterning to mechanical actuators. Advances in this area has been fueled by photoswitching molecules – compounds that change molecular conformation and numerous properties in response to light. Donor–acceptor Stenhouse adducts (DASAs), a novel class of photochromic developed in our group, exhibit large changes in color, polarity and molecular volume upon visible light irradiation. The first generation of these photochromic molecules were limited to alkyl amine donors, absorbing green light. To expand their capabilities, we developed second and third generations DASA derivatives, vastly expanding the scope of both structure and properties. Second generation DASAs were synthesized and using aniline derivatives, yielding compounds with red-shifted absorption windows. Although advantageous, we discovered that these new derivatives lost control over the thermodynamic equilibrium between the open and closed state in the dark. As such, a third generation DASA derivative was developed. By modifying the acceptor unit, we demonstrated both tunable equilibria between the open and closed form and an expanded absorption window (550 to 750 nm). With optimized DASA derived photochromic material in hand, we focused on demonstrating the ability convert light into mechanical work. To accomplish this, we developed a new approach to liquid crystal elastomers (LCEs) based on mild, additive free Diels–Alder step-growth polymerization reaction. This approach expands the functional-group tolerance of current click chemistry platforms, enabling Michael- acceptor-bearing compounds such as DASAs to be incorporated into LCE networks. In addition, this new method provides a platform to study the performance of a multitude of photoswitches under a unified polymer system. This holds the potential to enable for the first time structure-property relationships between the photochromic material and LCE architecture that are needed to advance this field.

This dissertation focuses on the design and fabrication of different cellulose nanocrystals (CNCs) polymer nanocomposites, with the goal of impacting the structure-property relationship between CNCs and the CNCs/matrix interactions through the surface functionalization of the CNCs with different chemical functional groups. Chapters 2-4 focus on how CNCs from sea tunicates (t-CNCs) functionalized with different chemical moieties affect the mechanical properties of the resulting nanocomposite. First (Chapter 2), t-CNCs were functionalized with lower critical solution temperature (LCST) responsive poly(oligoethylene glycol)monomethyl ether (meth)acrylates, which were incorporated into a poly(vinyl acetate) (PVAc) matrix to create reversible, thermal stiffening nanocomposites. When placed in water below the LCST the nanocomposites are soft, however, when placed in water above the LCST the nanocomposites stiffened as a result of the collapse of the grafted polymer chains allowing the engagement of t-CNCs nanorods. Secondly (Chapter 3), t-CNCs were used as fillers by functionalizing the surface with carboxylic acid moieties which aided in its dispersion in solvents such as N-methyl-2-pyyrolidone (NMP). The dispersion was further used in the synthesis of polyimide aerogels which demonstrated improved physical and mechanical properties as well as thermal stability with the incorporation of t-CNCs as a filler. Lastly (Chapter 4), by functionalizing the surface of t-CNCs with carboxylic acid and amine moieties, t-CNCs were demonstrated to be electrically active. Applying electric current across aqueous solutions of such t-CNCs, resulted in the fabrication of aligned micron-sized t-CNC fibers. Electrically aligned fiber composites with collagen were fabricated by matching the carboxylic acid/amine ratio of t-CNC and collagen. These aligned nanocomposite fibers demonstrated improved mechanical properties with higher contents of t-CNCs.Chapter 5 highlights the isolation of CNCs from a new scalable source of CNCs, Miscanthus x. Giganteus (MxG), and how properties of CNCs from this new source compares to current biosources for CNCs such as wood. Chapter 6 demonstrates the use of CNCs from MxG in a cross-linked polymer nanocomposite adhesive and how its adhesive properties are affected. MxG-CNCs were isolated from stalks of MxG through base hydrolysis, bleaching and acid hydrolysis. After acid hydrolysis, MxG-CNCs showed a relatively high crystallinity (>90%) and high aspect ratios of 60-70 confirmed by Transmission Electron Microscopy (TEM). Comparing mechanical properties of PVAc nanocomposites containing CNCs from wood to CNCs from MxG showed at least as good if not slightly better properties for MxG-CNC/PVAc nanocomposites. Following the successful isolation of MxG-CNCs, a new class of MxG-CNC related nanocomposite adhesive was designed. The surface of MxG-CNCs was functionalized with a thiol group (MxG-CNC-SH) which were used as a cross-linking moiety in a polydisulfide network. The utility of the MxG-CNC-SH nanocomposite adhesive was demonstrated to bond to different substrates such as hydrophilic and hydrophobic glass slides and metal, with the high surface energy substrates (metal and hydrophilic glass slides) exhibiting stronger adhesive shear strength with higher MxG-CNC content. On the other hand, bonding to low surface energy hydrophobic glass showed a maximum adhesive shear strength with composites that contained only 1 wt% MxG-CNC-SH.