Molecular engineering of mechanophore activity for stress-responsive polymeric materials

Abstract

Force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for sensing and countering stress-induced material failure. The general utility of this strategy is limited, however, because the levels of mechanophore activation in the bulk are typically low and observed only under large, typically irreversible strains. Strategies that enhance activation are therefore quite useful. Molecular-level design principles by which to engineer enhanced mechanophore activity are reviewed, with an emphasis on quantitative structure-activity studies determined for a family of gem-dihalocyclopropane mechanophores.

Fig. 1. (a) Stress in polymers localize along individual polymer subchains, resulting in chain scission that can lead to material failure. Mechanophores can be embedded within these overstressed subchains to trigger a constructive, rather than a destructive response. For example: (b) gem-dibromocyclopropane will ring open in response to high forces of tension, releasing stored length that provides local stress relief in the overstressed chains, and (c) the 2,3-dibromoalkene products of the ring opening are cross-reactive toward mild nucleophiles such as carboxylates, and that reactivity has been exploited to generate in situ cross-linking and stress-strengthening.

Fig. 2. (a) Compression mechanically activates the gDBC, but only very low levels of activation are observed despite the dramatic, irreversible deformation of the bulk material. (b) Tensile strain applied to a gDBC–poly(butadiene) cast film to the point of failure does not lead to detectable gDBC ring opening (by ¹H NMR). Adapted from ref. 18 with permission from The Royal Society of Chemistry.

Fig. 3. gem-Dihalocyclopropane undergoes a force-induced ring opening to a 2,3-dihaloalkene, and serves as a platform to quantitatively compare mechanical reactivity as a function of force-free reactivity, the geometry of attachment, and polymer backbone effects. The change in length along the vector of applied force on going from reactant to transition state, Δx, is depicted here as a local change, but must be considered in the context of where the force is applied.

Fig. 4. gDBC and gDCC mechanophores embedded along a poly(butadiene) backbone are activated at forces of 1210 pN and 1330 pN respectively under single molecule force spectroscopy on the time scale of ∼0.1 s. The lower force required for gDBC relative to gDCC mirrors the force-free activity.

Fig. 5. Plot of experimentally determined rate constants of polymer cleavage as a function of initial polymer molecular weight for trans dicyano-substituted cyclobutanes (DCT), trans monocyano-substituted cyclobutane (MCT), and trans cyclobutanes having no cyano substituents (NCT). Reprinted with permission from ref. 35. Copyright 2011 American Chemical Society.

Fig. 6. Under applied force, cis- and trans-gDFC open to the same s/trans-s/trans 1,3-diradical, which is a minimum on the force-modified potential energy surface, at f* ∼ 1290 pN and f* ∼ 1820 pN on the ∼0.1 s time scale of an SMFS experiment, respectively. When force is removed, the 1,3-diradical becomes a transition state for the disrotatory inversion path from trans- to cis-gDFC.

Fig. 7. The force-free conrotatory ring opening of trans-BCB is much faster than the force-free conrotatory ring opening of cis-BCB, and yield different isomer products. But under the influence of mechanical force, the disrotatory ring-opening pathway of cis-BCB becomes more favourable than the conrotatory pathway and even occurs at a lower force on the ∼0.1 s time scale of SMFS experiments than the conrotatory ring-opening of trans-BCB.

Fig. 8. Analysis of force transduction in BCB-C_n as a function of chain length n. The red line shows the dependence of the breaking force F _max on the polymer length n. Reprinted with permission from ref. 36. Copyright 2011 American Chemical Society.

Fig. 9. A simple change in polymer backbone from poly(butadiene) (PB) to poly(norborne) (PNB) increases Δx by ∼0.3 Å in the case of gDCC mechanophores. The red highlighted portion of the backbone depicts how the connected atoms become misaligned with the direction of the pulling force in the case of PB, but remain aligned in the case of PNB. Note, not all gDCC mechanophores are adjacent as depicted.

Cameron L. Brown

Stephen L. Craig