Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups-so-called mechanophores-that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.
Polymeric materials exhibit an extraordinary range of mechanical responses (Figure 1a), which depend on the chemical and physical structure of the polymer chains. 3 Polymers are broadly categorized as thermoplastic, thermoset, or elastomer. Thermoplastic polymers consist of linear or branched chains and can be amorphous or semicrystalline. The mechanical response of thermoplastic polymers is highly influenced by the molecular mass, chain entanglements, chain alignment, and degree of crystallinity. Thermosetting polymers consist of highly cross-linked three-dimensional networks. The mechanical properties of these amorphous polymers depend on the molecular mass and the cross-link density. Elastomers (e.g., rubber) are highly deformable elastic networks that are lightly cross-linked by chemical or physical junctions. Mary M. Caruso (center) was born and raised in Tampa, FL. She received a B.S. degree in Chemistry from Elon University (Elon, NC) in 2006, where she worked under the direction of Prof. Karl D. Sienerth. Her research included the synthesis and electrochemical characterization of a novel palladium complex. She is currently pursuing her Ph.D. in Organic Chemistry at the University of Illinois at Urbana-Champaign under the guidance of Prof. Jeffrey S. Moore and Prof. Scott R. White. Her research interests include the development of new catalyst-free self-healing polymers, microencapsulation, and mechanical testing of bulk polymers. Douglas A. Davis (second from left) was born in Martin, TN. In 2004, he received his B.S. degree in Chemistry from the University of Tennessee, Knoxville, where he worked on surface enhanced Raman spectroscopy (SERS) in an electrospray plume with Professors Charles Feigerle and Kelsey Cook. He joined Prof. Jeffrey Moore's group at the University of Illinois, Urbana-Champaign in 2005 to pursue his Ph.D. in Organic Chemistry. His current research interests include designing and synthesizing mechanophores, which can be induced to undergo chemical reactions with mechanical force. Qilong Shen (third from left) was born in 1974 in Zhejiang Province, China. He received his B.S. degree in Chemistry (1996) from Nanjing University, China, and his M.S. in Chemistry (1999) from Shanghai Institute of Organic Chemistry, the Chinese Science Academy, China, under the supervision of Prof. Long Lu. After a two-year stay at the University of Massachusetts at Dartmouth with Prof. Gerald B. Hammond, he moved to Yale University, where he received his Ph.D. under the guidance of Prof. John F. Hartwig in 2007. He is currently a postdoctoral researcher with Prof. Jeffrey S. Moore at University of Illinois at Urbana-Champaign. His research interests include the discovery, development, and understanding of new transition metal-catalyzed reactions, and the mechanochemistry of polymers.
There is growing interest in the use of mechanical energy to alter the molecular and supramolecular structure of polymers to create stress-responsive materials. 1a-l Chemical reactions that are accelerated by force remain poorly understood, and there is a need for the rapid discovery of new mechanophores (i.e., stress-sensitive units). Screening putative mechanophores, however, is a slow process that requires a high molecular weight polymer having a single testable unit positioned near the midpoint of the chain, the location where stress under elongation is greatest. Here we show that the required mechanophore-linked addition polymers are easily prepared using bifunctional initiators and a living polymerization method. The approach is demonstrated with benzocyclobutene 1k and spiropyran mechanophores that undergo stress-induced 4π and 6π electrocyclic ring opening, respectively. Mechanophore-linked addition polymers thus show considerable promise for rapidly identifying new mechanophores and will lead to a greater, molecular-level understanding of mechanochemical transduction in polymeric materials.Single electron transfer living radical polymerization (SET-LRP) 2 was employed for the synthesis of mechanophore-linked polymers, as this method has been shown to generate high molecular weight macromolecules with narrow polydispersity indices (PDIs). cis-1,2-Bis(R-bromopropionyloxy)-1,2-dihydrobenzocyclobutene (1), capable of initiating bidirectional SET-LRP, was synthesized and used to produce a series of benzocyclobutene (BCB)-linked PMAs (Scheme 1). Polymerizations were performed at room temperature in DMSO with Cu(0) catalyst and a hexamethylated tris(2-aminoethyl)amine (Me 6 TREN) ligand. Low (18 kDa), medium (91 kDa), and high (287 kDa) molecular weight BCB-linked PMAs (PMA-BCB-PMA) with PDIs around 1.3 were synthesized and used to investigate the ultrasound-induced electrocyclic ring opening reaction. Mechanochemical activation was analyzed by trapping the intermediate ortho-quinodimethide with UV-active N-(1-pyrene)-maleimide via cycloaddition (Scheme 1). 1k PMA end-functionalized with a BCB unit (PMA-BCB) was prepared as a mechanochemical control polymer since ultrasound-generated forces at the chain ends are minimal. Specifically, the monofunctional initiator cis-1-acetoxy-2-(R-bromopropionyloxy) 1,2-dihydrobenzocyclobutene was used to produce a PMA-BCB with a PDI of 1.3 and molecular weight of 190 kDa. This control polymer dispels the notion that the chemical changes are thermally induced, rather than the result of mechanical force.The BCB-containing polymers and PMA homopolymer were subjected to an acoustic field to probe for mechanical activity. Each polymer was dissolved in CH 3 CN with a large excess of N-(1-pyrene)maleimide and radical trap 2,6-di-tert-butyl-4-methylphenol (BHT) and exposed to pulsed sonication 3 for 45 min under Ar at 6-9°C. Aliquots were withdrawn at the beginning and end of each experiment and analyzed by analytical gel permeation chromatography (GPC) using a refractive index (RI) ...
This work presents a method for adapting a single, fixed deep neural network to multiple tasks without affecting performance on already learned tasks. By building upon ideas from network quantization and pruning, we learn binary masks that "piggyback" on an existing network, or are applied to unmodified weights of that network to provide good performance on a new task. These masks are learned in an end-toend differentiable fashion, and incur a low overhead of 1 bit per network parameter, per task. Even though the underlying network is fixed, the ability to mask individual weights allows for the learning of a large number of filters. We show performance comparable to dedicated fine-tuned networks for a variety of classification tasks, including those with large domain shifts from the initial task (ImageNet), and a variety of network architectures. Unlike prior work, we do not suffer from catastrophic forgetting or competition between tasks, and our performance is agnostic to task ordering. Fig. 1: Overview of our method for learning piggyback masks for fixed backbone networks. During training, we maintain a set of real-valued weights m r which are passed through a thresholding function to obtain binary-valued masks m. These masks are applied to the weights W of the backbone network in an elementwise fashion, keeping individual weights active, or masked out. The gradients obtained through backpropagation of the task-specific loss are used to update the realvalued mask weights. After training, the real-valued mask weights are discarded and only the thresholded mask is retained, giving one network mask per task.
Spiropyran (SP) mechanophores (mechanochemically reactive units) can impart the unique functionality of visual stress detection to polymers and have potential for use in smart materials with self-sensing capabilities. These color-generating mechanophores were incorporated into polyurethane via step growth polymerization. Polyurethane, which is inherently a versatile engineering polymer, possesses an optimized balance of mechanical toughness and elasticity to allow for investigation of the kinetics of the mechanochemical response of the SP mechanophore in the bulk polymer via fluorescence and absorbance measurements. The stress-induced 6-π electrocyclic ring-opening to the colored merocyanine (MC) form of the mechanophore was quantified by measuring the change in absorbance of the polymer, while it was held at constant strain. The closing kinetics of the mechanophore was also studied by fluorescence imaging. Finally, the effects of mechanical strain on the equilibrium between the SP and MC forms are reported and discussed.
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