Hexaarylbiimidazole-based Dynamic Materials and their Utilization

Free radicals, atomic or molecular species with an unpaired valence electron, are common reactive species in many chemical reactions. Indeed, radicals often participate in many naturally-occurring biological processes as well as in polymerization reactions as the active propagating centers in both chain- and step-growth polymerizations. One particularly interesting class of radical-generating compounds consists of hexaarylbiimidazoles (HABIs) which have been the focus of significant research activity owing to their photo-, piezo-, and thermo-chromic nature. Upon irradiation, HABIs undergo homolytic cleavage to yield two strongly colored, 2,4,5-triarylimidazolyl radicals that are relatively unreactive with oxygen and show slow recombination rates, attributable to steric hindrance and electron delocalization, and thermally recombine to reproduce the HABI dimer. Because of their unique attributes, HABIs have found industrial utility as proofing papers, photoresists, and radical polymerization initiators. This dissertation details the design and synthesis of novel HABI derivatives to 1) address the deficiencies of conventional HABI photoinitiators, including their poor visible light absorption and low solubility, 2) elucidate the recombination mechanism of lophyl radicals and resolve the apparent disagreement between reported reaction orders, and 3) explore their utilization as a novel class of dynamic covalent bonds to effect the photo-mediated healing of cross-linked polymer networks. A novel, bis(hydroxyhexyl)-functionalized HABI was synthesized and employed as an efficient, visible light-active photoinitiator for thiol–ene resin formulations in the absence of any accompanying photosensitizer. Owing to the high reactivity of thiol functional groups with HABI-derived lophyl radical, in conjunction with the necessarily high thiol concentration in thiol–ene formulations, this HABI photoinitiator effectively initiated thiol–ene polymerizations upon visible light irradiation and exhibited improved visible light absorption and solubility in both organic solvents and resins relative to a commercial HABI photoinitiator. Additionally, the photo-mediated dissociation of HABIs and subsequent dark recombination of lophyl radicals were examined, where analysis of lophyl radical concentration curves revealed that the recombination reactions were well described as 3/2- and second-order reactions for the two respective parent HABIs. Finally, HABI-based functional groups were utilized as a new class of dynamic covalent bonds to demonstrate the photo-mediated healing of cross-linked polymer networks. Novel di- and tetra-functional, polymerizable HABI-based monomers were synthesized and subsequently incorporated into cross-linked polymer networks. The resultant HABI-incorporating cross-linked polymer networks were able to undergo photo-mediated backbone cleavage, temporarily affording reduced cross-link densities and dynamic connectivity rearrangement while under irradiation, then reverting back to stable, static networks upon irradiation cessation. This network stability in the dark, combined with a highly dynamic nature under irradiation, enabled rapid healing rates while precluding the creep that often accompanies the dynamic connectivity of intrinsically-healable polymer networks. Moreover, the cross-link density reduction upon irradiation and long half-life of the photo-generated lophyl radicals tethered to network strands provided sufficient time for effective, successful healing within HABI-incorporating, vitrified, cross-linked polymer networks. The findings described herein open up possibilities in the design and synthesis of novel, intrinsically-healable cross-linked polymer networks, and suggest the suitability of HABIs in advanced, responsive materials applications, including in stress-indicating and self-reinforcing materials.PHDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140853/1/ahndowon_1.pd

Additives for Ambient 3D Printing with Visible Light

With 3D printing we desire to be “limited only by our imagination”, and although remarkable advancements have been made in recent years the scope of printable materials remains narrow compared to other forms of manufacturing. Light-driven polymerization methods for 3D printing are particularly attractive due to unparalleled speed and resolution, yet the reliance on high energy UV/violet light in contemporary processes limits the number of compatible materials due to pervasive absorption, scattering, and degradation at these short wavelengths. Such issues can be addressed with visible light photopolymerizations. However, these lower-energy methods often suffer from slow reaction times and sensitivity to oxygen, precluding their utility in 3D printing processes that require rapid hardening (curing) to maximize build speed and resolution. Herein, multifunctional thiols are identified as simple additives to enable rapid high resolution visible light 3D printing under ambient (atmospheric O2) conditions that rival modern UV/violet-based technology. The present process is universal, providing access to commercially relevant acrylic resins with a range of disparate mechanical responses from strong and stiff to soft and extensible. Pushing forward, the insight presented within this study will inform the development of next generation 3D printing materials, such as multicomponent hydrogels and composites.We thank the Robert A. Welch Foundation (F-2007) and the Center for Dynamics and Control of Materials: an NSF MRSEC (DMR-1720595) for financial support. The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute, the Center for Dynamics and Control of Materials: an NSF MRSEC (DMR-1720595), and the NSF National Nanotechnology Coordinated Infrastructure (ECCS-1542159).Center for Dynamics and Control of Material

Effect of elevated atmospheric carbon dioxide on the allelopathic potential of common ragweed

Background Allelopathy has been suggested as one potential mechanism facilitating the successful colonisation and expansion of invasive plants. The impacts of the ongoing elevation in atmospheric carbon dioxide (CO2) on the production of allelochemicals by invasive species are of great importance because they play a potential role in promoting biological invasion at the global scale. Common ragweed (Ambrosia artemisiifolia var. elatior), one of the most notorious invasive exotic plant species, was used to assess changes in foliar mono- and sesquiterpene production in response to CO2 elevation (389.12 ± 2.55 vs. 802.08 ± 2.69 ppm). Results The plant growth of common ragweed significantly increased in elevated CO2. The major monoterpenes in the essential oil extracted from common ragweed leaves were β-myrcene, dl-limonene and 1,3,6-octatriene, and the major sesquiterpenes were β-caryophyllene and germacrene-D. The concentrations of 1,3,6-octatriene (258%) and β-caryophyllene (421%) significantly increased with CO2 elevation. Conclusions These findings improve our understanding of how allelochemicals in common ragweed respond to CO2 elevation.The authors would like to acknowledge Dr. Samsik Kang (College of Pharmacy, Seoul National University) and Dr. Jonghee Kim (Department of Biology, Gyeongnam University) for the technical advice and supportive discussion and Dr. Changsuk Kim (National Institute of Agricultural Science and Technology) for the seed collection. This work has been supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)(2018R1C1B6005351). We are grateful to NRF (2016R1D1A1A02937049, 2017096A001719BB01) for the financial support

Additives for Ambient 3D Printing with Visible Light

With 3D printing we desire to be “limited only by our imagination”, and although remarkable advancements have been made in recent years the scope of printable materials remains narrow compared to other forms of manufacturing. Light-driven polymerization methods for 3D printing are particularly attractive due to unparalleled speed and resolution, yet the reliance on high energy UV/violet light in contemporary processes limits the number of compatible materials due to pervasive absorption, scattering, and degradation at these short wavelengths. Such issues can be addressed with visible light photopolymerizations. However, these lower-energy methods often suffer from slow reaction times and sensitivity to oxygen, precluding their utility in 3D printing processes that require rapid hardening (curing) to maximize build speed and resolution. Herein, multifunctional thiols are identified as simple additives to enable rapid high resolution visible light 3D printing under ambient (atmospheric O2) conditions that rival modern UV/violet-based technology. The present process is universal, providing access to commercially relevant acrylic resins with a range of disparate mechanical responses from strong and stiff to soft and extensible. Pushing forward, the insight presented within this study will inform the development of next generation 3D printing materials, such as multicomponent hydrogels and composites

Rapid High Resolution Visible Light 3D Printing

Light-driven 3D printing to convert liquid resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resolution of any additive manufacturing process. However, the reliance on high energy UV/violet light derived from decades of photolithography research, limits the materials scope due to degradation and attenuation (e.g., absorption and/or scattering). Chemical innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those containing biological compounds and multi-material structures. Photochemistry at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high resolution visible light 3D printing. The combination of electron deficient iodonium and rich borate co-initiators were critical to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradiation zones, improving spatial resolution. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin composition to resolution, cure rate, and mechanical performance. Ultimately, a versatile and general visible light-based printing method was shown to afford 1) stiff and soft objects with feature sizes < 100 μm, 2) build speeds up to 45 mm/h, and 3) mechanical isotropy, rivaling modern UV-based 3D printing technology and providing a foundation from which bio- and composite-printing can emerge.</p

Rapid High-Resolution Visible Light 3D Printing

Light-driven 3D printing to convert liquid resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resolution of any additive manufacturing process. However, the reliance on high-energy UV/violet light limits the materials scope due to degradation and attenuation (e.g., absorption and/or scattering). Chemical innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those containing biological compounds, nanocomposites, and multimaterial structures. Photo- chemistry at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high-resolution visible light 3D printing. The combination of electron-deficient and electron-rich coinitiators was critical to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradiation zones, improving spatial resolution. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin composition to resolution, cure rate, and mechanical performance. Ultimately, a versatile and general visible-light-based printing method was shown to afford (1) stiff and soft objects with feature sizes <100 μm, (2) build speeds up to 45 mm/h, and (3) mechanical isotropy, rivaling modern UV-based 3D printing technology and providing a foundation from which bio- and composite-printing can emerge.We thank the ARO STIR program of the Department of Defense (W911NF1910310) and Robert A. Welch Foundation (F-2007) for financial support. The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute, the Center for Dynamics and Control of Materials, an NSF MRSEC (DMR-1720595), and the NSF National Nanotechnology Coordinated Infrastructure (ECCS- 1542159).Center for Dynamics and Control of Material

Rapid, Photomediated Healing of Hexaarylbiimidazole-Based Covalently Cross-Linked Gels

The intrinsic healing of covalently cross-linked polymer networks is commonly effected via the utilization of backbone-borne functional groups able to reversibly cleave or rearrange, thereby enabling mixing and coreaction of network strands that bridge contacted interfaces; however, such materials often exhibit slow healing rates and are susceptible to creep under load. To address these deficiencies, we incorporated hexaarylbiimidazole (HABI) functionalities, groups that are homolytically cleavable, to yield relatively low reactivity lophyl radicals under UV or visible light irradiation and which, in the absence of light, spontaneously recombine without significantly participating in deleterious side reactions, into the backbone of poly­(ethylene glycol)-based polymeric gels. Whereas the network connectivity of these HABI-incorporating gels was stable in the dark, they exhibited significant creep upon irradiation. The influence of swelling solvent on the reaction kinetics of backbone-borne HABI photolysis and lophyl radical recombination was examined and revealed that gels swollen with 1,1,2-trichloroethane (TCE) exhibited higher radical concentrations than those swollen with either acetonitrile or water under equivalent irradiation conditions, attributable to the relative solvent affinity for the hydrophobic HABI functionalities affording more rapid HABI cleavage and slower radical recombination rates in TCE than in water. The fastest healing rates for cleaved samples brought into contact and irradiated with visible light were observed for TCE-swollen gels, although rapid restoration of mechanical integrity was achieved for gels swollen with any of the solvents examined where tensile strengths approached those of the pristine materials after 1 to 3 min of light exposure

Re-examining the Photomediated Dissociation and Recombination Kinetics of Hexaarylbiimidazoles

The recombination of lophyl radicals generated by the photodissociation of hexaarylbiimidazole (HABI) compounds has been previously reported to proceed as either first-, 3/2-, or second-order reactions. Here, we re-examine the recombination of HABI-derived lophyl radicals to resolve these disparate reported recombination reaction orders. EPR spectroscopy was used to monitor the radical concentration for two HABI-based compounds in solution both during irradiation until steady state was achieved, then in the dark, where only the radical recombination reaction proceeded. Over short dark periods, lophyl radical recombination could be adequately described by second-order reaction kinetics. To better evaluate these reactions, UV–vis spectrophotometry measurements were performed over longer dark recombination periods. The molar absorptivities of the lophyl radical species were determined and used to express UV–vis absorbance data as radical concentrations. Analysis of these radical concentration curves over extended dark periods revealed that the recombination reactions at low initial HABI concentrations and incident irradiation intensities were well fit as 3/2- and second-order reactions for the two respective parent HABI compounds; however, raised initial HABI concentrations and irradiation intensities progressively increased deviation from the reaction order fits. The fitted recombination and corresponding photodissociation rate constants were validated by predicting radical concentration curves using stepped irradiation intensity profiles, which were compared against experimentally determined radical concentrations obtained under identical reaction conditions

Wavelength-selective light-matter interactions in polymer science

Light has emerged as a prominent stimulus to both generate and manipulate polymeric materials across multiple length scales. Compared with other external stimuli, light-mediated approaches enable unprecedented control over when and where chemical transformations occur (i.e., spatiotemporal control). To date, the majority of established protocols rely on individual wavelengths of light (∼monochromatic), which does not harness the full potential of light-matter interactions. This review summarizes the nascent progress in utilizing multiple discrete wavelengths of light as a tool to create and alter soft matter. The concepts are structured in an effort to provide a roadmap to foster new directions in light-based polymer materials chemistry. The physical organic nature of wavelength selectivity is first detailed in the introduction to provide key mechanistic insight and lay a foundation for further developments. Next, an overview of chromophores that undergo various light-driven transformations is presented, followed by their utility in polymer platforms for controlled synthesis, property manipulation, and advanced manufacturing. The review concludes with a summary and outlook on the exciting future of wavelength-selective light-matter interactions in polymer science.</p

폴리(비닐 신나메이트)와 올리고머 신나메이트 블렌드를 기반으로 한 그루브 패턴 표면의 광배향막

Photo-alignment property of groove patterned surface prepared from blend of poly (vinyl cinnamate) (PVCi) and oligomeric dicinnamate was investigated for the application for alignment layer of liquid crystal display. The study of the photoreaction kinetics using UV-vis spectrum with the irradiation time showed that the reaction rate of oligomeric cinnamate was enhanced compared to that of PVCi. Blend where PVCi was main component showed a slight improvement on the photoreaction rate. It was unable to obtain groove patterned surface only using oligomeric cinnamate itself owing to the high crystalline character. However, blending of PVCi made it possible to obtain clear surface pattern. Molecular orientation could be confirmed from the polar plot data. It can be suggested that blend of oligomeric cinnamate and polymeric cinnamate is promising material for the photoalignment layer.1