From polymer physics to macroscopic failure: Forces on polymer chains in nanofilled elastomers

Mechanochemistry is a powerful tool to map the stress distribution and quantify covalent bond scission in soft polymer networks, such as elastomers and gels^1,2. We incorporated a mechanochromic non-scission type mechanophore, spiropyran (SP), in nanosilica-filled crosslinked poly(ethylacrylate) nanocomposites (PEANC) containing different filler volume fractions and different levels of filler/matrix interaction strength³. The branched fractal-like morphology of the fillers (characterized by X-ray scattering, AFM, and SEM) and the mechanical properties of our samples in uniaxial tension are similar to industrial elastomer nanocomposites. Under cyclic tensile loading, the PEANC samples progressively change their color under load and subsequent unloading. The concentration of mechanophores activated during the loading cycle can be quantified from absorption spectra as previously demonstrated⁴. Results show that SP activation is governed by the peak nominal stress applied to the sample, resulting in a master curve of activated SP fraction as a function of maximum nominal stress, regardless of the filler volume fraction and level of interfacial coupling. The activation of mechanochromic molecules supports the hypothesis of a two-stages toughening mechanism of nanocomposites. At low strain, the load is mostly carried by fractal-like aggregates of silica filler, while at high strain, the load is transferred from the physical network of filler particles to the highly stretched polymer chains surrounding the aggregates. These results support the dominant role played by the finite extensibility of polymer chains in the nanocomposite stiffening rather than the jamming of nanofilelrs int he transverse direction. If time permits we will also discuss how similar materials labeled with scissile mechanophores can be damaged during fracture.

References: 1) Chen, Y., et al., Science Advances, 2020. 6(20): p. eaaz5093. 2) Slootman, J., et al., Physical Review X, 2020. 10(4): p. 041045. 3) Chen, Y., et al., Macromolecules 2023 Vol. 56 Issue 14 Pages 5336-5345 4) Chen, Y., et al., Chemical Science, 2021. 12: p. 1693-1701.