Smart materials allow for feedback devices to be built into existing engineering systems. Force sensitive polymeric materials enable visualization of mechanical stress in prototypes and applications. Spiropyran (SP) mechanophores are prime examples for mechanophores and isomerize under stress to their colored merocyanine (MC) form. For tailor-made smart materials control of the force-dependent activation barrier is crucial. Electronic effects induced by different substituents attached to SP change the Gibbs free energy of the isomerization reaction and most likely the associated barrier as well. We were able to realize substituent effects by varying both the regiochemistry as well as the substituent of SP mechanophores incorporated into polymers. We incorporated different SP derivatives into semicrystalline PCL and demonstrate instantaneous as well as delayed color formation after the yield point. These observations are corroborated by DFT calculations of activation energy barriers under simulated molecular force. Additionally, by using a high molecular weight, high Tg, amorphous and tough polyarylene with covalently incorporated SP main chain units, we show that the electronic effect exerted by the phenyl-based comonomer lowers the half-life time of the colored merocyanine to 3 seconds, leading to instantaneous discoloration after force release. This enabled transient mechanochromism, which demonstrates that SP copolymers can be used as online force sensors.

Transient mechanochromic response and stereochemical effects of spiropyran copolymers

2019 Fall National Meeting

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