Long Versus Short Chain Covalent Functionalization in the Preparation of Mechanochromic Polymers

Mechanochromic polymers that translate deformation into optical changes can allow the direct visualization of stress distribution and predict mechanical failure. Designing functional spacers for covalently integrated mechanophores can affect both mechanical performance and optical signaling; however, this is seldom addressed systematically in the literature. Here, we prepared a polymeric perylene bisimide (PBI) macromechanophore bearing poly(ϵ-caprolactone) spacers and terminal hydroxy functionalities (M-PBI), and covalently incorporated it into maleic anhydride-grafted styrene-ethylene-butylene-styrene (SEBS-g-MAH) and linear low-density polyethylene (LLDPE-g-MAH). The optical, mechanical, and thermal properties of the films were benchmarked against an analogous short-spacer PBI (P-PBI) and a physically blended PBI (S-PBI). We found that covalent incorporation promoted aggregate emission (640–660 nm), and, under deformation, films comprising M-PBI and P-PBI showed strain-induced disaggregation, whereas S-PBI blends remained nonmechanochromic. Notably, the incorporation of the macromechanophore M-PBI yielded crosslinked films without compromising the mechanical properties of the polymeric matrices significantly, whereas the use of short-chain P-PBI crosslinker could trigger premature failure in semicrystalline LLDPE at higher loadings. Overall, this study suggests that long, compliant spacers improve force transduction and mechanochromic robustness, enabling reversible optical strain indicators in technologically relevant plastics without affecting their mechanical behavior.