|Abstract:||Mechanophores are force sensitive molecules that undergo productive chemical transformations under a mechanical force including color change, small molecule release, and cross-linking. Productive chemical transformations depend on the intrinsic molecular structure as well as the extrinsic environment of the mechanophore. On the molecular level, the mechanochemical activity is influenced by regiochemistry, electronic structure, ring strain, etc. Extrinsic factors that influence activity include environmental stimuli like temperature and applied stress, any phases that the mechanophore is covalently linked with, and other molecules or ions able to interact with the mechanophores. This dissertation investigates the influence of both intrinsic molecular and extrinsic environmental variables on two different color-changing mechanophores, mechanochromic spiropyran (SP) and recently developed naphthopyran (NP), in polymeric matrices.
First, the effect of force on the rate constants and activation energies for SP–merocyanine (MC) transition is characterized in bulk polymers. Under different values of a macroscopic tensile stress, the change in fluorescence intensity is measured, indicating the evolution of merocyanine. Assuming first-order reaction kinetics, both forward and reverse rate constants are obtained and corresponding activation energies are calculated. Above a threshold stress level, the potential energy surface for SP–MC transition is modified toward the ring-opened MC species.
The regiochemical effects on mechanical activity of SP and NP isomers are investigated in a polydimethylsiloxane (PDMS) matrix. For SP mechanophores, two known isomers are chosen and in situ fluorescence measurements are performed under tensile and shear activation. In contrast to simulation predictions, both isomers require similar threshold stress and strain for mechanical activation, indicating that the mechanophore activation in bulk polymeric materials is largely governed by the characteristic properties of the polymeric matrix. The regiochemical effect is further studied for six NP regioisomers, each with different pulling points on the naphthalene ring. Computational simulations predict that only two regioisomers are mechanochromic. Tensile testing of polymers containing these two regioisomers confirmed mechanochemical activity. However, one additional regioisomer, not predicted by simulation, also demonstrated mechanochromism in the experiments. The mechanical reactivity of the three active NP mechanophores is further investigated by measuring the absorbance change at different stress values. The mechanical reactivity of NP mechanophores somewhat correlates with the orientation between the C–O pyran bond and the externally applied force.
Both SP and NP mechanophores are combined and cross-linked in the same PDMS matrix. In contrast to polymers incorporating one type of mechanochromic molecule, the PDMS with two mechanophores develops multiple color changes depending on the stress levels and deformation rate.
Finally, the mechanical activity of mechanophores functionalized at the interface between SiO2 particle and a polymer matrix is compared with that of mechanophores linked into bulk polymers. Asymmetrically functionalized SP mechanophores are designed for incorporation at the interfaces between the SiO2 particles and a poly(methyl acrylate) (PMA) matrix. Surface initiated living radical polymerization is performed to grow linear PMA from SP-functionalized SiO2 particles. The active SPs activate (change color) in both solution and in a bulk polymer. Cross-linked PMA is also prepared using cross-linkable SP-linked SiO2. Compared with the SP-linked only to the polymer, the interfacial mechanophores exhibit more efficient mechanical activation under tensile loading. In addition, the SP-functionalized particles increase the toughness of the polymer.