Towards reversible mechanochemically triggered strengthening, toughening and assembly of soft materials

Abstract

The field of mechanochemistry is on many levels inspired by biology, where mechanical force regulates the flux of ions through channels, triggers electrochemical impulses, and activates the rearrangement of protein complexes. A key attribute of many biochemical reactions is reversibility, as this enables the machinery of cells to perform their functions many times. With the exception of color switching mechanophores, the majority of mechanochemical reactions developed in the lab are completely irreversible, thus limiting their function to a one time use. Spiropyran (SP) is a unique mechanophore that reversibly switches from a ring-closed state to a ring-open state, merocyanine (MC), in response to a variety of stimuli, including UV light, heat, and mechanical force. Herein, a strategy is proposed to reversibly crosslink polymer chains using SP’s well documented ability to bind to metal cations in its MC state.

A proof of concept is demonstrated, whereby viscous solutions containing transition metal ions and a SP functionalized polymer spontaneously switch from a viscous fluid to a viscoelastic gel in response to heat. Exposure to visible light dissociates polymer crosslinks, switching the gel back to a viscous fluid state. Rheological studies using small amplitude oscillatory shear demonstrates reversible crosslinking over several cycles. The data strongly suggest that crosslinking is due to intermolecular bridging of MC units via coordination to divalent metal ions.

A methodology is developed for incorporating metal ions into spiropyran mechanophore linked poly(dimethylsiloxane) elastomers. By removing moieties that stabilize MC, as well as incorporating additives that competitively solvate metal ions in PDMS, force-triggered MC-metal complexation is demonstrated for the first time with autonomous dissociation of the metal complexes after removal of the applied stress. Force-triggered complexation is demonstrated over many cycles without any sign of hysteresis. Though spontaneous activation is not completely inhibited it is well controlled. An inverse relationship between the thermodynamic stability of MC-metal complexes measured in solution and the ratio of mechanochemical to thermal activation in the SP-PDMS/metal ion composite is revealed. Though force-triggered crosslinking is not demonstrated in this system, these results are a significant step towards engineering systems with reversible mechanochemical functionalities that extend beyond color indication.

2D diffusion NMR is employed to gain a deeper understanding of the molecular mechanisms that result in metal-ion-mediated gelation of SP polymer solutions. A number of complexities are revealed, and a method for decoupling the effects of shape, size, solvation and charge on the relative diffusivity of MC and SP is proposed.

Implications of our findings towards the possibility of engineering SP polymers that mechanochemical strengthen and toughen are heavily emphasized. This dissertation concludes with two rational strategies for the engineer or chemist who wishes to pursue these ideas further.