Shear forces self-strengthen mechanochemically active polymers
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The high binding energies of PVP with Li2S and Li2Sn allowed for uniform dispersion of the active material and carbon within the electrode and minimized loss of polysulfides into the electrolytes during cycling. These batteries also exhibited an initial specific capacity of 760 mA h g–1 and 94% capacity retention in the first 100
cycles. Even after 500 cycles, the PVP cells retained up to 69% of their initial capacity. The researchers envisage that their simple strategy of rational binder selection could be extended to the identification of new binders for other promising high-capacity electrode materials. Dominica H.C. Wong
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Shear forces self-strengthen mechanochemically active polymers
Stress activation
Cross-linking
hen polymeric materials are subjected to shear stresses, they typically suffer from bond breaking, which reduces their molecular weight and viscosity. Indeed, this is a problem that plagues oils and lubricants in high-performance engines. Now A.L. Black Ramirez of Duke University, J.A. Orlicki of the Army Research Laboratory, M. Champhekar of North Carolina State University, and their colleagues have developed synthetic polymers where the same forces responsible for the destructive processes of bond scission and chain disentanglement are channeled into constructive, bond-forming reactions. As reported in the September issue of Nature Chemistry (DOI: 10.1038/ nchem.1720; p. 757), the researchers inserted “mechanophores” (i.e., functional groups that respond to mechanical perturbation in a controlled manner) in the covalent backbone of the polymers, which enable remodeling and self-repair under mechanical stress. This concept of “activated remodeling through mechanochemistry” (ARM) is illustrated in the figure. Gemdibromocyclopropanes (gDBCs, 1closed) were embedded within a poly(butadiene) (PB) backbone as mechanophores, which provide the basis for self-repair through covalent cross-linking. When mechanically triggered, these undergo a ring opening polymerization, which leads to a 2,3-dibromoalkene product (1open) open to nucleophilic substitution. The parent gDBC mechanophore is inert toward nucleophilic substitutions, so the repair reaction only occurs when sufficient forces are MRS BULLETIN
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1closed
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VOLUME 38 • OCTOBER 2013
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Stress activation and chain scission
Cross-linking
Mechanochemistry self-strengthening concept: (a) A gDBC mechanophore within a polymer chain under tension undergoes a ring-opening reaction from 1closed to 1open. This increases the contour length and provides an allylic bromide that is capable of self-strengthening through nucleophilic displacement reactions. (b) System-wide force causes chain scission, but also activates the mechanophore (black triangle to red dot), which subsequently reacts with a cross-linker (blue) to form an active cross-link (purple) that overcomes the damage. Reproduced with permission from Nature Chem. 5 (2013), DOI: 10.1038/nchem.1720. © 2013 Macmillan Publishers Ltd.
experienced within the polymer to cause the
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