Smart polyolefins feeling the force: Color changeable poly(ethylene-vinyl acetate) and poly(ethylene-octene) in response to mechanical force

Spiropyran (SP) mechanophore cross-linker was covalently incorporated into two widely used polyolefins, poly(ethylene-vinyl acetate) (EVA) and poly(ethylene-octene) (EOC), through facile cross-linking by peroxide under hot press. It was found that (1'-(2-(methacryloyloxy)ethyl)-3′,3′ dimethylspiro[chromene-2,2'-indolin]-6-yl)methyl methacrylate (SP3) could not be thermally driven to merocyanine (MC) in polyolefins during high temperature cross-linking, which is superior to other types of SP mechanophores used for polymer processing. The force-induced ring-opening reaction of SP-to-MC was demonstrated on SP3-cross-linked EVA. It was found that increasing the SP content resulted in earlier activation and that more MC was driven from SP at a slower strain rate. When held at constant strain, MC gradually reverted to SP. The mechanoactivation of SP was also investigated for SP3-cross-linked EOC. This work represents the first example of color-changeable polyolefins in response to mechanical force and demonstrates the feasibility of applying mechanophores to widely-used commercial polyolefins for stress sensing.