Blending or Bonding? Mechanochromism of an Aggregachromic Mechanophore in a Thermoplastic Elastomer

Abstract

A straightforward way for the preparation of mechanochromic polymers consists of incorporating a suitable content of a mechanophore in the polymeric matrix either by physical dispersion or via covalent functionalization. Although covalent incorporation may require demanding chemical efforts, this approach can offer significant advantages over physical dispersion. In this work, a common thermoplastic elastomer, styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer grafted with maleic anhydride (SEBS-MAH), was covalently functionalized with 1-aminomethylpyrene (AMP). MAH functional groups are covalently linked to the ethylene-co-butylene blocks, thus allowing a precise and selective confinement of the chromogenic AMP units in the soft block. Flat, fully conjugated pyrene units undergo the reversible formation of π−π aggregates, readily distinguishable by their red-shifted emission. These aggregates were heavily affected by the application of mechanical stimuli. Despite the low degree of mechanophore functionalization (about 1 wt %), uniaxial deformation of the polymer was reliably monitored via fluorescence and a clear drop in the excimer to monomer emission ratio (I E /I M ) was observed starting from 50% of strain. The marked mechanochromism was confirmed by emission lifetime measurements and also by near-field investigations. In addition, the mechanoresponse showed good reversibility after repeated stress−relaxation cycles. Control experiments performed on formulations comprising a physical dispersion of pyrene in unfunctionalized SEBS showed faint excimer emission and a negligible mechanochromic response up to 5 wt % of doping, in substantial agreement with the scanning near-field optical microscopy analysis. An evident drop of the I E /I M ratio occurred for 10 wt % of pyrene, albeit the excimer emission remained predominant even at the highest deformation, being a smaller fraction of pyrene moieties involved. Overall, the covalent approach appeared as an elegant procedure to confine the chromogenic unit in the soft phase of block copolymers and thus to provide an elastomeric film showing a detectable and reversible mechanochromic response with a modest (i.e., ∼1 wt %) amount of pyrene molecules, i.e., 10 times smaller compared to the dispersed system.

Comparative Mechanochromic Performance of Perylene Diimide-Doped Polyurethanes: Blending vs Bonding

“…31 In a recent paper, we addressed the comparison between blending and bonding, employing an elastomeric SEBS triblock copolymer. 23 In that study, we achieved a discernible and reversible mechanochromic response with a moderate dye concentration when the dye was covalently linked to the polymer chain. In contrast, achieving a detectable mechanochromism through physical dispersion necessitated a dye concentration 10 times higher.…”

“…Weder and co-workers synthesized mechanochromic polymers in which the dyes were cleverly linked to the chain by fixing the distance between the chromophore . Differently, Pucci and co-workers spatially confined the dye through phase separation . By exploiting these kinds of molecular designs, it was possible to overcome the reduced tendency to aggregation of covalently bound dyes and to obtain the aggregation degree required to achieve a relevant mechanochromic response.…”

“…Additionally, their applications are restricted because the mechanical activation of such mechanophores is usually not immediately reversible without the use of an external stimulus. 23 Polymers containing dyes whose mechanochromic response is based on noncovalent interactions have the potential to overcome these significant limitations. The development of this last strategy favors the industrial acceptance of mechanochromic probes based on cost-andbenefit analysis, especially in terms of preparation steps and times.…”

“…23 Until now, a limited number of works present a thorough comparison between the blended and the corresponding covalently functionalized systems. 23,27,31 In these cases, the latter always provided a better mechanochromic response than its dispersed analogous. This result is attributed to the different disaggregation mechanisms that occur in covalently functionalized and blended systems.…”

Sub‐Micrometer Mechanochromic Inclusions Enable Strain Sensing in Polymers

“…In amorphous polymer matrices, limited nucleation and relatively high molecular mobility generally lead to the formation of larger dye crystals that are mechanically inert. [ 32 ] It therefore remains difficult to endow, e.g., elastomers and related classes of amorphous polymers with MRL properties, unless the dyes are covalently incorporated [ 33–35 ] or contained within protective microcapsules. [ 24,36,37 ]…”