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Bistable and Photoswitchable States of Matter

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Bistable and Photoswitchable States of Matter Corrected: Publisher correction ARTICLE DOI: 10.1038/s41467-018-05300-7 OPEN Bistable and photoswitchable states of matter Brady T. Worrell1, Matthew K. McBride1, Gayla B. Lyon1,7, Lewis M. Cox1,2,3, Chen Wang1,7, Sudheendran Mavila1, Chern-Hooi Lim1,8, Hannah M. Coley4, Charles B. Musgrave1, Yifu Ding3,4 & Christopher N. Bowman 1,4,5,6 Classical materials readily switch phases (solid to fluid or fluid to gas) upon changes in pressure or heat; however, subsequent reversion of the stimulus returns the material to their original phase. Covalently cross-linked polymer networks, which are solids that do not flow 1234567890():,; when strained, do not change phase even upon changes in temperature and pressure. However, upon the addition of dynamic cross-links, they become stimuli responsive, capable of switching phase from solid to fluid, but quickly returning to the solid state once the stimulus is removed. Reported here is the first material capable of a bistable switching of phase. A permanent solid to fluid transition or vice versa is demonstrated at room tem- perature, with inherent, spatiotemporal control over this switch in either direction triggered by exposure to light. 1 Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA. 2 Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, CO 80305, USA. 3 Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA. 4 Material Science and Engineering Program, University of Colorado, Boulder, CO 80309, USA. 5 BioFrontiers Institute, University of Colorado, Boulder, CO 80309, USA. 6 Department of Restorative Dentistry, University of Colorado, Anschutz Medical Campus, Aurora, CO 80045, USA. 7Present address: Formlabs Inc, 35 Medford St. #201, Somerville, MA 02143, USA. 8Present address: Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA. Correspondence and requests for materials should be addressed to C.N.B. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:2804 | DOI: 10.1038/s41467-018-05300-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05300-7 atter readily switches phases (solid to fluid or fluid to a b gas) upon changes in pressure or heat; however, sub- M - Bistable, photoswitchable sequent reversion of the stimulus returns the material to or–T Fluid material its original phase.1 Covalently cross-linked polymer networks + or+T + (i.e., thermosets), which are solids that do not irreversibly flow or OFF E E ON deform when a stress is applied, do not switch phase even with + Solid changes in temperature and pressure.2 Conversely, upon addition Fluid Solid of covalent cross-links with a dynamic character into the polymer network they, much like classical materials, switch from a solid to a fluid when the dynamic crosslinks are activated; however, due to c Switch to the types of dynamic chemistry that have hitherto been explored, solid removal of the stimulus inevitably returns the material to the + solid state (Fig. 1a).3–15 Covalently cross-linked polymers capable of switching phases + (solid to fluid and vice versa), a set of materials often referred to Switch to 15 fluid as covalent adaptable networks (CANs), are, as stated, con- Fluid Solid trollable by the application of an external stimulus such as heat or dynamic crosslinks permanent crosslinks light that activates the dynamic covalent chemistry (DCC) within Legend: d O O OH the network, temporarily switching the material from solid to HO S fl fi Free thiol Thioether Thioester uid (Fig. 1a). In the rst instance where heat acts as the stimulus, TE1 (scalable, modular core) O 3–5 6,7 techniques developed by Leibler, Du Prez, and others have Cut utilized degenerate exchange reactions with high kinetic barriers; e 106 f 3–7 fi 4 heating these so-called vitrimers to suf ciently high tempera- 2 tures overcomes these barriers and facilitates rapid bond 105 (Pa) 4 Virgin Cut exchange, effectively fluidizing the material, while cooling returns ″ G 2 material material , ′ Heal 8,9 4 the network to a solid state. Approaches pioneered by Bowman, G 10 10–12 Matyjaszewski, and others have used light to rapidly produce 4 2 thiyl or carbon centered radicals capable of adding to unsaturated 103 species within the cross-link to cause statistical fragmentation, 0.01 0.1 1 10 100 Healed Lens material temporarily forming a viscoelastic fluid. Turning off the light w (rad/s) Imprint results in rapid termination of radicals, thus eliminating the Fig. 1 Concept of a bistable, photoswitchable state of matter. a The addition-fragmentation bond exchange sequence which returns application of a given stimulus (i.e., heat or light) to a solid material the material to an elastic solid state. In either instance switching overcomes the unfavorable solid to fluid phase transition in covalent of the phase, from a solid to a fluid, only occurs while the sti- adaptable networks; removal of the stimulus returns the material to the mulus is being applied to the material as high energy inter- favored solid phase. b Equivalent energetic favorability of the fluid and solid mediates are required to be formed for fluidic behavior to persist. phase of a covalent adaptable network with a small barrier allows for Thus, a platform which enables thermosetting polymers, without permanent, bistable switching of phase with light. c A cartoon of how any other changes to their chemical structure, to switch phase light can effectively switch the state of matter in a network polymer. permanently and in a bistable manner following a short, transient d Rheometry shows that thioester crosslinked materials act as fluids when exposure to an external stimulus has not been developed or thiol, thioester, and a base catalyst (PMDETA) are present (G′ = black filled explored. circle; G″ = black open circle), and as solids when the base catalyst is Here, we demonstrate responsive materials based on similar removed (G′ = grey filled square; G″ = grey open square). e The ability of chemistry with switches phases, solid to fluid or vice versa, at thioester crosslinked materials while in the fluid phase to undergo large room temperature upon exposure to light; this phase change is changes in structure at room temperature is indicated by the ability to bistable, permanent, and removal of the stimulus does not return shred and subsequently heal the material into a defect free, optically active the materials to their previous state (Figs. 1b, c).16 This room material temperature change in phase is feasible due to the incorporation of a thioester functional group placed throughout the polymer network which engages in a remarkably robust dynamic exchange reaction with free thiol as promoted by a base catalyst.17–19 This be achieved if the strengths of both thermal and light-induced dynamic exchange was found to be promoted or halted by mild bond exchange approaches were combined. Namely, we sought to basic or acidic catalysts, respectively, which are released by develop a degenerate anionic exchange reaction with a low kinetic light.20,21 Furthermore, the photo-mediated release of these cat- barrier (active at room temperature, ~23 °C) that requires a alysts has inherent spatial and temporal control over the phase of persistent catalyst which is readily generated or consumed using the material, allowing for a singular material to have a controlled light without altering the structure of the network. Here, we have multitude of volumes which act as fluids or solids in a bistable developed a unique approach using a thioester containing manner as directed by light (Fig. 1c). Our results demonstrate monomer which is seamlessly incorporated into standard thiol- that light can be employed to permanently transition where and X22 and other polymerizations to rapidly form cross-linked net- when this switchable material acts as either a fluid or a solid work polymers where a residual amount of thiol remains (Fig. 1b). unreacted. Such networks were shown to exhibit rapid ambient temperature fluidic properties in the presence of a base catalyst via the thiol-thioester exchange (Supplementary Figure 7, the – Results mechanism of the thiol-thioester exchange).17 19 Within these Development of a photoswitchable material. Considering these networks, the exchange was found to be chemoselective and previously reported systems, we postulated that a bistable mate- proceed rapidly at room temperature. As the exchange is pro- rial capable of permanent, photoinduced switching of phase could moted with very mild catalysts, light was employed to temporally 2 NATURE COMMUNICATIONS | (2018) 9:2804 | DOI: 10.1038/s41467-018-05300-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05300-7 ARTICLE and instantaneously provide or deprive the material of these Although weak bases, such as PMDETA, were effective reagents. Spatial fluidization (turning ON exchange) or solidifi- catalysts in promoting fluidization in these networks, other, cation (turning OFF exchange) was further evidenced in macro- potentially more effective catalytic reagents were explored to scopic and microscopic demonstrations towards applications in affect the thiol-thioester exchange with the goal of offering higher photonics, smart coatings, and bulk materials. exchange rates, reduced loading, enhanced stability and other To form suitable thiol-thioester exchange-based polymer net- beneficial attributes. As these networks were formed
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