Spiropyran Mechano-Activation in Model Silica-Filled Elastomer Nanocomposites Reveals How Macroscopic Stress in Uniaxial Tension Transfers from Filler/Filler Contacts to Highly Stretched Polymer Strands

Mechanochemistryhas proved to be a powerful tool to map the stressdistribution and quantify covalent bond scission occurring when modelpolymer networks, such as elastomers and gels, are deformed and fractured.In the current work, we incorporate a mechanochromic non-scissiontype mechanophore, spiropyran (SP), in the elastomeric matrix of nanosilica-filledcross-linked poly(ethylacrylate) nanocomposites (PEANC) containingdifferent filler volume fractions and different filler/matrix interfacialproperties. The branched fractal-like morphology of the fillers (characterizedby X-ray scattering, AFM, and SEM) and the mechanical properties ofour samples in uniaxial tension are similar to the industrially usedelastomer nanocomposites. Under tensile loading, the PEANC sampleschange their color and the concentration of mechanophores activatedinto merocyanine can be quantified from absorption spectra. Resultsshow that, in uniaxial tension, SP activation is governed by the peaknominal stress applied to the sample resulting in a master curve ofactivated SP fraction as a function of stress, independent of thefiller volume fraction and interfacial coupling. Upon several loadingcycles to the same stretch level, the concentration of activated SPdecreases moderately, especially at high stretches. The activationof mechanochromic molecules supports the hypothesis of a two-stagestoughening mechanism of nanocomposites. At low strain, the load ismostly carried by fractal-like aggregates of silica filler, whileat high strain, the load is transferred from the physical networkof filler particles to the highly stretched polymer chains surroundingthe aggregates. This scenario supports the dominant role played bythe limited extensibility of polymer chains in the nanocomposite stiffening.The labeling of polymer networks with mechanosensitive molecules provideshere a clear visual pathway to the mechanisms responsible for stiffening,and ultimately toughening, of nanocomposites.