Onset of Mechanochromic Response in the High Strain Rate Uniaxial Compression of Spiropyran Embedded Silicone Elastomers

Abstract

The molecular processes that accompany dynamic mechanical response to large deformations at high strain rate (≈1000 s−1 or higher) underlie the early stages of damage in materials, but understanding of material response in this regime is typically limited to macroscopic constitutive equations. Here, spiropyran mechanophores are embedded in very short, stress‐bearing strands in silicone elastomers, and their mechanochromic response to uniaxial compression is explored in a Split Hopkinson Pressure (or Kolsky) Bar. At strain rates of 1000 s−1, the onset of mechanochromism occurs at lower strains, but higher stresses, than in the same materials under quasi‐static loading. Similar to quasi‐static loading, however, a negligible effect of mechanophore structure on the critical strain for colorimetric onset is observed. The results suggest that nonequilibrium, inhomogeneous local tension distributions in the elastomers lead to greater stress in individual strands than at the same strains under equilibrium loading, but that within the regions of force concentration, mechanochromic onset is determined primarily by a limiting local strain threshold.

Mechanochromophore‐Linked Polymeric Materials with Visible Color Changes

“…High strain rate compression was also employed to study its effect on activation of SP cross‐linked elastomers. [ 156 ]…”

“…Exceptionally, a more recent study suggested that an extremely high strain rate (1000 s −1 ) of compression led to the onset activation of SP‐PDMS elastomer at a lower strain but a higher stress. [ 156 ] It was hypothesized that the critical strain achieved on the bulk matrix dominated the colorimetric onset because the stress was concentrated in a small region of SP containing strands.…”

“…High strain rate compression was also employed to study its effect on activation of SP cross-linked elastomers. [156] The deformation caused by shearing has been shown to induce the ring-opening of SP in polymers. [103] Here torsion testing was performed using a rheometer equipped with a fluorescence imaging set-up.…”

Mechano‐Optical Response of Novel Polymer Composites Based on Elastic Polyurethane Matrix Filled with Low‐Molar‐Mass Cholesteric Droplets

“…[ 2 ] For example, in spiropyran‐containing polymers, mechanochemical ring opening reaction causes the transition of colorless spiropyran form into a colored merocyanine one, resulting in significant mechano‐optical response. [ 11,12 ] The photoinduced Z‐form of the azobenzene groups of azobenzene‐containing materials [ 13 ] can be successfully transformed into the initial E‐form upon stretching of the polymer film. [ 14 ] This process is accompanied by changes the ordering of molecules in the material and leads to a modification in its optical properties.…”

Mechanochromic Polymers

“…[ 9 ] In their communication, Stephen L. Craig and co‐workers report how the strain rate affects the activation of such spiropyrans in silicone elastomers. [ 10 ] Guillaume De Bo and co‐workers report a new mechanoresponsive fluorescent hydrogen‐bonded rotaxane based on a maleimide dye and demonstrate its force‐sensing properties in a synthetic model of living tissue. [ 11 ] Ester Verde‐Sesto, José A. Pomposo and co‐workers review on techniques that can be used to manipulate and mechanically activate single‐chain molecules and thus allow one to elucidate information on the stress‐induced response of individual mechanophores and polymer molecules.…”