Abstract
Incorporating hidden length into polymer chains can improve their mechanical properties, because release of the hidden length under mechanical loads enables localized strain relief without chain fracture. To date, the design of hidden length has focused primarily on the choice of the sacrificial bonds holding the hidden length together. Here we demonstrate the advantages of adding mechanochemical reactivity to hidden length itself, using a new mechanophore that integrates (Z)-2,3-diphenylcyclobutene-1,4-dicarboxylate, with hitherto unknown mechanochemistry, into macrocyclic cinnamate dimers. Stretching a polymer of this mechanophore more than doubles the chain contour length without fracture. DFT calculations indicate that the sequential dissociation of the dimer, followed by cyclobutene isomerization at higher forces yields a chain fracture energy 11 times that of a simple polyester of the same initial contour length and preserves high energy-dissipating capacity up to ∼3 nN. In sonicated solutions cyclobutene isomerizes to two distinct products by competing reaction paths, validating the computed mechanochemical mechanism and suggesting an experimental approach to quantifying the distribution of single-chain forces under diverse loading scenarios.
Highlights
Polymer hidden length is a portion of a polymer backbone that does not contribute to the end to end distance because it is contained within a macrocyclic loop held together by a chemical bond between two nonadjacent backbone atoms
When a polymer chain containing hidden length is stretched, the backbone within each loop remains strain-free until and unless the bond that holds the loop together (“sacrificial bond”) dissociates.[1−4] This dissociation abruptly increases the contour length of the stretched chain and redistributes the load away from overstretched segments
The kinetics of sacrificial bond dissociation is a key determinant of the mechanical behavior of polymers with hidden lengths.[1]
Summary
Polymer hidden length is a portion of a polymer backbone that does not contribute to the end to end distance because it is contained within a macrocyclic loop held together by a chemical bond between two nonadjacent backbone atoms. Backbone loops held by ionic or H bonds are common in biological structural materials and are well recognized to contribute to the remarkable mechanical properties of the bone and shells, muscle, and biological adhesives.[4,9] These weak sacrificial bonds dissociate in response to modest loads (singlechain force 1 nN) They are designed to compete with mechanochemical chain fracture and to occur only in overstressed regions of loaded materials,[15] which are at the highest risk of macroscopic failure.
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