Shear activation of mechanophore-crosslinked polymers

A new strategy is employed to impart productive mechanochemical response to crosslinked polymers. Force-sensitive molecules, termed mechanophores, are successfully incorporated as crosslinkers into poly(methyl methacrylate) through a free radical polymerization initiated with benzoyl peroxide and N,N-dimethylaniline. Evidence of a shear activated local chemical reaction (an electrocyclic ring opening) is provided by a color- and fluorescence-generating spiropyran mechanophore. Bulk polymer samples and controls were studied under shear loading. In situ full field fluorescence imaging is used to determine the threshold stress and strain required for activation as a function of shear rate and polymer architecture, both of which have a significant effect on mechanochemical activity in the bulk polymer. Increasing the shear rate leads to an increase in activation stress, similar to bulk polymer yielding. Increasing the length of the primary crosslinker with respect to the spiropyran leads to a decrease in activation stress, while the activation strain becomes more shear rate dependent with longer primary crosslinkers. These findings show that the molecular details of the network architecture can be altered to tune the mechanochemical response.