Mechanochemical strengthening of a synthetic polymer in response to typically destructive shear forces

Abstract

High shear stresses are known to trigger destructive bond-scission reactions in polymers. Recent work has shown that the same shear forces can be used to accelerate non-destructive reactions in mechanophores along polymer backbones, and it is demonstrated here that such mechanochemical reactions can be used to strengthen a polymer subjected to otherwise destructive shear forces. Polybutadiene was functionalized with dibromocyclopropane mechanophores, whose mechanical activation generates allylic bromides that are crosslinked in situ by nucleophilic substitution reactions with carboxylates. The crosslinking is activated efficiently by shear forces both in solvated systems and in bulk materials, and the resulting covalent polymer networks possess moduli that are orders-of-magnitude greater than those of the unactivated polymers. These molecular-level responses and their impact on polymer properties have implications for the design of materials that, like biological materials, actively remodel locally as a function of their physical environment.

Figure 1. The mechanochemical self-strengthening concept

a, A gDBC mechanophore within a polymer chain under tension undergoes a ring-opening reaction from 1_closed to 1_open. 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 crosslinker (blue) to form an active crosslink (purple) that overcomes the damage.

Figure 2. Shear-induced mechanochemical crosslinking in solution and the bulk

a, The ARM-1 system responds to extrusion in the bulk (left) and sonochemical shearing in solution (right) by forming covalent crosslinks in the regions of high force along the polymer backbone. Polymer chain scission also occurs, but is not shown for clarity. The fraction of PB monomer (subscript a in the chemical formulae) remained constant throughout either experiment, and the fraction of gDBC mechanophore (b) decreases to (d) because of activation to the 2,3-dibromoalkene (e). A portion of the activated mechanophores reacts to form crosslinks (c). b, FTIR spectra overlay of the initial ARM-1 polymer (ARM-1^a, blue), the ARM-1 polymer extruded (ARM-1^b, purple) and the same ARM-1 polymer one week post-extrusion (ARM-1^c, black). The dotted lines designate the precursor carboxylate absorbance peak at 1,571 cm⁻¹ (blue) and the product ester absorbance peak at 1,721 cm⁻¹ (purple). c, Sonication of ARM-1 leads to fronting in the GPC trace, indicative of the early stages of network formation. a.u., arbitrary units.

Figure 3. Chemistry and response of a single-component ARM system

a, Treatment of 2 with TBA⁺ OH⁻ leads to the ARM-2 system. Sonication of 2 leads to mechanochemical ring-opening of the mechanophore, but does not cause the carbonyl absorbance to shift from 1,701 cm⁻¹ in FTIR (b, red), which matches the model compound absorbance (b, black). Sonication of ARM-2, however, leads to covalent crosslinking and gelation through ester formation as indicated by the carbonyl absorbance at 1,724 cm⁻¹ (c, purple), in agreement with a small molecule model compound (c, black). Shown for contrast in c is the infrared spectrum of the TBA carboxylate of the model compound (dashed red line). Polymer main-chain scission also occurs in both cases, but is not shown for clarity.