Molecular Damage Detection in an Elastomer Nanocomposite with a Coumarin Dimer Mechanophore

Abstract

resistance that result from dispersing stiffer particulates or fibers into the softer polymer matrix. [2,3] In high performance polymer composites, mechanical performance is often determined by how stress transfer from the matrix to the filler either prevents or facilitates the formation of voids, depending on the mechanism of toughening. [4,5] Stress overload can lead to the scission of covalent bonds or chain slippage and induce irreversible structural changes at the molecular, mesoscopic, and macroscopic levels, [6-8] ultimately leading to catastrophic material failure. Therefore, damage detection in polymer composites has long been an active area of study from both fundamental and applied perspectives. Currently, damage detection in composites is mostly achieved through techniques such as ultrasonics, computerized vibro-thermography, x-ray tomography and infrared thermography. [9] These techniques are non-destructive, and in some cases they have been applied in combination with methods for repairing the damage, such as resin injection and the use of reinforcing patches. [9] In addition, higher resolution techniques such as scanning electron microscopy and transmission electron microscopy have been used in post-mortem damage analysis in composites. [10,11] These methods have been quite successful, but they require that material damage has progressed to the point of nanoscale to mesoscale defects in order to be detected, and they do not directly probe the nature of the specific molecular events that are responsible for damage initiation and/or propagation. Polymer mechanochemistry offers exciting opportunities for stress-sensing and damage detection in polymer composites through incorporating mechanophores, which are molecular units that react in response to force. [12,13] By embedding different mechanophores, materials can be endowed with stress-responsive properties that include changes in modulus, [14,15] color, [16-19] and electronic properties. [20] Recent years have seen increased efforts to utilize mechanophores in bulk composites, [21-27] and mechanophores that provide irreversible optical responses to mechanical events are promising candidates to characterize the molecular stresses caused by wear or fatigue (Figure 1a). [12,28] But, to the best of our knowledge, mechanochromophores have Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are mol...

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

“…[ 25 ] Sources of relaxation dynamics include dangling chain ends and the interactions of network chains with nano‐silica filler, and mechanophores might be useful in probing the molecular origins of high strain rate behavior. In particular, the placement of mechanophores within the elastomers [ 26 ] (e.g., cross‐links versus mid‐chain versus at the interface of fillers) can be controlled through synthesis, which might be useful in elucidating the molecular details of the various processes. We suggest that this understanding might be especially relevant to the early stages of material damage, because the tension required for mechanophore activation (100s of pN) corresponds to polymer chains (or partial subchains) that are fully stretched beyond the conformational entropy dominated regime, [ 18 ] to a point where chain scission is increasingly likely.…”

Semi‐IPNs Reinforced with Silica Janus Nanoparticles and Their Stress Sensing with Mechanoluminescent Probe

“…What is already known is that the stress transfer within the complex composites determines the overall mechanical properties, a build-in sensitive stress probe thus would be helpful for understanding their complex mechanical behaviors. [26][27][28] Herein, with the dispersion of different surface-modified Janus nanofillers (sheets or hollow spheres) into the 1,2-dioxetane containing Semi-IPNs, a new kind of mechanochemiluminescent Semi-IPNs nanocomposites with enhanced mechanical properties and stress-reporting capability were developed. Comprehensive characterizations based on fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), scanning electronic microscope (SEM), and tensile tests demonstrated that the hierarchically crosslinked structures, including H-bond interactions mediated by Janus nanoparticles and chemical crosslinking within Semi-IPNs, jointly contributed to excellent mechanical properties of the composites.…”

“…What is already known is that the stress transfer within the complex composites determines the overall mechanical properties, a build‐in sensitive stress probe thus would be helpful for understanding their complex mechanical behaviors. [ 26–28 ]…”

Mechanochromic Polymers

“…[8] These examples illustrate that mechanically induced color changes in polymers can be achieved by very different mechanisms that include physical at the molecular scale in such polymers. [13] Hideyuki Otsuka's team demonstrate that multicolor mechanochromism in mixtures of two mechanochromic polystyrene samples containing different mechanochromophores allows one to detect the duration mechanical stimulation. [14] In their communication on semi-interpenetrating elastomer network nanocomposites containing Janus nanoparticles and a mechanoluminescent motif, Yulan Chen and co-workers further raise the bar with respect to complexity.…”

“…Stephen L. Craig's group shows that mechanophores based on coumarin dimers exhibit a strength that is comparable to that of sulfur‐sulfur bonds that represent the weakest bonds in vulcanized rubbers and allowed them to investigate how macroscopic mechanical stress is transferred at the molecular scale in such polymers. [ 13 ] Hideyuki Otsuka's team demonstrate that multicolor mechanochromism in mixtures of two mechanochromic polystyrene samples containing different mechanochromophores allows one to detect the duration mechanical stimulation. [ 14 ] In their communication on semi‐interpenetrating elastomer network nanocomposites containing Janus nanoparticles and a mechanoluminescent motif, Yulan Chen and co‐workers further raise the bar with respect to complexity.…”