Light-Mediated Control of Polymeric Materials

The synthesis and characterization of precision polymeric materials represents a broad research area with significant value to society. Recently, light-mediated approaches to controlled polymer synthesis have attracted significant interest due to the low cost and tremendous tunability of modern light sources. Through manipulating these various properties (wavelength, intensity, etc), researchers have made considerable progress in controlling a wide range of important polymeric properties such as molecular weight distribution, comonomer composition, and molecular architecture. However, the systems developed to date have largely focused on homogenous targets and have lacked in-depth photophysical interpretation. This dissertation describes the development of an in-situ approach to studying light-mediated chemical reactions and efforts to better understand the temporal control of state-of-the-art photopolymerizations. Furthermore, the lessons learned from these studies led to the development of a new approach to 3D printing, wherein specific wavelengths of visible light were used to independently and simultaneously define local materials properties throughout a part in a single step. This breakthrough in additive manufacturing greatly expands the potential of multi-material printing techniques. The findings of these works have broad implications for using light as a tool for the future development of synthetic approaches to advanced polymeric materials

Visible light-responsive DASA-polymer conjugates

A modular synthesis of Donor-Acceptor Stenhouse Adduct (DASA) polymer conjugates is described. Pentafluorophenyl-ester chemistry is employed to incorporate aromatic amines into acrylate and methacrylate copolymers, which are subsequently coupled with activated furans to generate polymers bearing a range of DASA units in a modular manner. The effect of polymer glass transition temperature on switching kinetics is studied, showing dramatic rate enhancements in going from a glassy to a rubbery matrix. Moreover, tuning the DASA absorption profile allows for selective switching, as demonstrated by ternary photopatterning, with potential applications in rewriteable data storage

Dynamic-bond-induced sticky friction tailors non-Newtonian rheology

Frictional network formation has become a new paradigm for understanding the non-Newtonian shear-thickening behavior of dense suspensions. Recent studies have exclusively focused on interparticle friction that instantaneously vanishes when applied shear is ceased. Herein, we investigate a friction that emerges from dynamic chemical bridging of functionalized particle surfaces sheared into close proximity. This enables tailoring of both friction magnitude and the time release of the frictional coupling. The experiments use dense suspensions of thiol-functionalized particles suspended in ditopic polymers endcapped with benzalcyanoacetamide Michael-acceptors. The subsequent room temperature, catalyst-free dynamic thia-Michael reactions can form bridging interactions between the particles with dynamic covalent bonds that linger after formation and release in the absence of shear. This chemical friction mimics physical friction but is stickier, leading to tunable rheopexy. The effect of sticky friction on dense suspension rheology is explored by varying the electronic nature of the benzalcyanoacetamide moiety, the molecular weight of the ditopic polymers, the amount of a competitive bonding compound, and temperature. These results demonstrate how dynamic-bond-induced sticky friction can be used to systematically control the time dependence of the non-Newtonian suspension rheology

Enhancing the Equilibrium of Dynamic Thia-Michael Reactions through Heterocyclic Design

Although the catalyst-free dynamic thia-Michael (tM) reaction has been leveraged for a range of significant applications in materials science and pharmaceutical development, exploiting its full potential has been limited by relatively low equilibrium constants. To address this shortcoming, a new series of catalyst-free, room-temperature dynamic thia-Michael acceptors bearing an isoxazolone motif were developed and utilized to access both dynamic covalent networks and linear polymers. By leveraging the generation of aromaticity upon thiol addition and tuning the electronic-withdrawing/donating nature of the acceptor at two different sites, a wide range of equilibrium constants (Keq ∼1000 to ∼100,000 M–1) were obtained, constituting a 2 orders of magnitude increase compared to their noncyclic benzalcyanoacetate analogues. Integration into a ditopic isoxazolone-based Michael acceptor allowed access to both bulk dynamic networks and linear polymers; these materials not only exhibited tailorable thermomechanical properties based on thia-Michael acceptor composition, but the higher Keq tM bonds resulted in more mechanically robust materials relative to past designs. Furthermore, solution-state formation of linear polymers was achieved thanks to the increased Keq of the isoxazolone-based acceptors

Tunable visible and near infrared photoswitches

The article of record as published may be found at http://dx.doi.org/10.1021/jacs.6b07434A class of tunable visible and near-infrared donor−acceptor Stenhouse adduct (DASA) photoswitches were efficiently synthesized in two to four steps from commercially available starting materials with minimal purification. Using either Meldrum’s or barbituric acid “acceptors” in combination with aniline-based “donors”, an absorption range spanning from 450 to 750 nm is obtained. Additionally, photoisomerization results in complete decoloration for all adducts, yielding fully transparent, colorless solutions and films. Detailed investigations using density functional theory, nuclear magnetic resonance, and visible absorption spectroscopies provide valuable insight into the unique structure−property relationships for this novel class of photoswitches. As a final demonstration, selective photochromism is accomplished in a variety of solvents and polymer matrices, a significant advantage for applications of this new generation of DASAs.National Science Foundation (MRSEC program DMR 1121053)California NanoSystems Institute (CNSI) Challenge Grant Progra

Controlling dark equilibria and enhancing donor–acceptor Stenhouse adduct photoswitching properties through carbon acid design

A novel library of tunable negative photochromic compounds, donor-acceptor Stenhouse adducts (DASAs), is reported. Tailoring the electron deficient "acceptor" moiety yielded DASAs that can be activated with mild visible and far red light. The effect of acceptor composition on reactivity, absorption, equilibrium, and cyclability is exploited for the design of high performance photoswitches. The structural changes to the carbon acid acceptor also provide access to new, more structurally diverse DASA derivatives by facilitating the ring-opening reaction with electron deficient amine donors

A Versatile Approach for In Situ Monitoring of Photoswitches and Photopolymerizations

A simple, inexpensive, and modular method to directly illuminate NMR samples for in situ analysis of photochemical transformations is reported. The versatility of this technique is demonstrated by analyzing the light-induced propagating front for small-molecule photoswitches and the kinetics of photocontrolled living radical polymerizations. In situ measurements allow oxygen-sensitive and rapid photoevents to be studied in detail, leading to reliable determination of photoswitching quantum yields and polymerization rates. By systematically tuning light intensity, a direct relationship between propagation rate and intensity is revealed. Of particular note is the facile translation of the conditions identified through this NMR analysis to analogous benchtop experiments with insight into the nature of the photoreactive species

Light-controllable ionic conductivity in a polymeric ionic liquid

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light-responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipulation, spatiotemporal control, and low environmental impact. To address this potential, responsive PIL materials based on diarylethene units were designed to undergo light-mediated conductivity changes. Key to this modulation is tuning of the cationic character of the imidazolium bridging unit upon photoswitching. Irradiation of these materials with UV light triggers a circa 70 % drop in conductivity in the solid state that can be recovered upon subsequent irradiation with visible light. This light-responsive ionic conductivity enables spatiotemporal and reversible patterning of PIL films using light. This modulation of ionic conductivity allows for the development of light-controlled electrical circuits and wearable photodetectors

Solvent-free synthesis of high-performance polyhexahydrotriazine (PHT) thermosets

Polyhexahydrotriazines (PHT) are promising high-performance thermosets exhibiting enhanced thermal and mechanical properties. Here, we demonstrate a new solvent-free approach for the fabrication of PHT based on low-melting-point diamines enabling the production of adhesives with comparable properties to well-established epoxy adhesives. Structure property relationships were supported by small molecule model reactions that mimic the cross-linking hexahydrotriazine (HT) cores and allowed the unique reactivity of benzylamine building blocks to be demonstrated. Through a variety of spectroscopic and physical techniques, this increased molecular level understanding of monomer design led to the ability to tune acidic degradation and thermal properties