Force-induced mechanochemical reactions in polymeric systems

Chemical reactions traditionally rely on energy from stimuli such as heat, light, or electric fields to overcome activation barriers separating desired products from starting materials. In the field of mechanochemistry, the energy from mechanical force is harnessed to induce and direct chemical reactions. One focus of the research presented investigates the spiropyran (SP) mechanophore (force-sensitive molecule) incorporated into different polymer systems. The SP mechanophore converts to a colored and fluorescent merocyanine (MC) form upon the application of heat, UV light, or mechanical force, when linked into a polymer backbone. The SP to MC (activation) reaction is also reversible, and can be driven back to the colorless SP form by visible light. The SP mechanophore incorporated into polyurethane (PU) was demonstrated to be mechanochromic, and the conversion to the colored MC form was characterized by changes in absorption and fluorescence. PU’s optimized balance of mechanical toughness and elasticity also allowed for the change in SP-MC equilibrium to be studied. Segmented polyurethane (SPU) is a phase separated copolymer of a soft and hard segment PU. The incorporation of the SP mechanophore into PU by step growth polymerization allows for controlled placement of SP in either the soft or hard domain. Upon either tensile stretching or irradiation with UV light the SP-linked segmented polyurethanes (SP-SPU) adopts a deep purple coloration and is fluorescent, demonstrating the force and UV-induced activation to the open MC form of the mechanophore, respectively. Order parameters calculated from the anisotropy of the fluorescence polarization of MC were used to characterize the orientation of the mechanophore in each phase. Exploiting the ability of SP to be force activated, the SP-SPUs were also mechanically activated to track the force and orientation in each domain of segmented polyurethane during uniaxial tensile loading. The SP mechanophore was also used to investigate mechanical forces in crosslinked poly(methyl methacrylate) during swelling with common organic solvents. The SP was incorporated as a crosslinker and a correlation was observed between polymer swelling and fluorescence intensity; suggesting that the forces during swelling were sufficient to drive the electrocyclic ring-opening reaction of SP to its colored and fluorescent MC form. Control experiments and solvatochromic studies validated that activation was indeed due to swelling-induced mechanical forces, and not to solvent effects. Systematic studies varying solvents and crosslinking densities also provided insight on how these parameters influence mechanical forces at the molecular level during polymer swelling. The second research focus explores the use of phase separated polymers as new mechanochemical systems. Utilizing the nature of different polymers to segregate into microdomains, polymers with complimentary reactive functionalities, isolated in phase separated domains, could be brought in contact with each other by means of mechanical force. When brought in contact, the complimentary phases can react, changing the innate properties of the starting combination of copolymers or polymers. The investigation of the contact mechanochemistry concept will be discussed for a poly(acid) and poly(amine) block copolymer system and a poly(thiol) (initially masked as a disulfide) and poly(glycidyl methacrylates), a polymer with an epoxy side chain. Success in the demonstration of the epoxide ring-opening chemistry by the unmasked thiol side chained polymer, lays down a foundation for future research steps, involving the development of this chemistry into a solid state mechanochemical system