Candidate Lecture: Dr. Yuval Vidavsky, SibleySchool of Mechanical and Aerospace Engineering Cornell University, The Importance ofBreaking Bonds

Abstract

Application of force on polymeric materials ultimately causes chemical bond scission and usually results in destructive behaviors such as failure, fatigue and abrasion. Hence, it is often desirable to prevent processes that result in bond scission. However, with the proper material design, bond breaking can generate beneficial features with tailor-made behaviors.

In this talk, I will present the incorporation of spiropyran mechanochromic functionality in bisphenol A
polycarbonate that is synthesized by the addition of dihydroxyspiropyran as bisphenol monomer. This
copolymer demonstrates the high glass transition temperature, toughness and stiffness that are associated with polycarbonate, while boasting the desired mechanochromic activity at room temperature. The mechanochemical bond scission and isomerization of spiropyran to merocyanine are thoroughly
investigated by measuring the fluorescence in situ while stretching the material. Evaluation of the
mechanochromic response to changes in stress, deformation and time shows a complex dependence on
the combination of these factors and provides new insights regarding the stress accumulation in polymer
chains.

In addition, I will present a polymeric system with metal-ligand crosslinking, which is based on the ionic
interaction of divalent metal cations with carboxylate anions grafted on polymer backbones. The
metal-ligand crosslinking increases the stiffness and strength and reduces ductility. In addition, by attaching different types or number of neutral ligands to the metal center, the metal-carboxylate interactions could be modulated, thus allowing to fine-tune the mechanical properties of the materials.

Insights gained from this work can be used for the rational design of mechano-active polymers with wide
range of targeted mechanical properties and application.