Mechanochemical Reactions of Polymers

Abstract

An external mechanical force is able to activate covalent bonds. Grinding, milling, or extrusion of polymers and polymeric materials leads to polymer degradation, because covalent bonds are broken. Mechanoradicals are generated by homolytic rupture of covalent bonds, but bond hydrolysis or other bimolecular reactions may as well be mechanically activated. Rubber mastication, thermomechanical treatment of food polymers, and recycling of postconsumer plastic waste are technological applications of mechanochemical reactions of polymers. The design of mechanophores opens new possibilities for mechanoresponsive materials. Weak linkages introduce defined rupture points in polymers. Mechanochromic mechanophores lead to color change of polymer solids when exposed to excessive mechanical stress, even before macroscopic damage becomes visible. Further examples are chemiluminescent mechanophores, cross‐linking mechanophores, or mechanophores acting as mechanocatalysts. A quantitative understanding of mechanochemical reactions is reached with a wide variety of experimental and theoretical methods. Defined force can be applied to the bulk material, as well as to individual molecules embedded in macrocycles, or single polymer chains manipulated with an atomic force microscope. The latter method forms the basis for single molecule force spectroscopy, which is performed either in the force‐ramp or force‐clamp mode. The theoretical framework ranges from one‐dimensional description of covalent bonds with a Morse potential via the constrained geometries simulate external force method to full‐dimensional quantum chemical methods like external force explicitly included or the force‐modified potential energy surface. Ab initio molecular dynamics simulations, transition state optimizations, or intrinsic reaction paths following are routinely performed on molecules exposed to a defined mechanical force.

Polymer Fracture

“…This subject is covered in this Encyclopedia by Beyer (72). A detailed account of earlier studies is given by C. Vasiliu-Oprea (73).…”

Theoretical simulation of the infrared signature of mechanically stressed polymer solids

“…Mechanical stress on polymer solids leads to conformational changes, bond elongation and widening of bond angles on the molecular level [ 1 – 4 ]. If the local force on an individual polymer strand reaches values in the range of nN, rupture of covalent bonds becomes possible, leading to irreversible changes and the destruction of the molecule [ 5 – 8 ].…”

“…Computational chemistry has proven to be an indispensable tool in the analysis of mechanochemical phenomena of organic molecules, polymers and mechanophores [ 5 – 6 31 – 74 ]. A variety of theoretical approaches have been developed to model external force using methods of quantum chemistry [ 9 , 75 – 76 ], including constrained geometries simulate external force (COGEF) [ 4 ], external force is explicitly included (EFEI) [ 61 , 77 ] and force modified potential energy surface (FMPES) [ 45 ]. Within the EFEI method, force is applied along the direction defined by two atoms in the molecule, which modifies the potential energy surface, closely resembling FMPES.…”

Revealing the Dependence of Molecular-Level Force Transfer and Distribution on Polymer Cross-Link Density via Mechanophores

“…A mechanophore is a molecule that exhibits a specific chemical or physical response to mechanical force. , When chemically incorporated into a polymeric material, mechanophores impart it with the ability to map stress (or strain) or self-detect damages. , In particular, when covalently incorporated into polymers, the molecular-level response of mechanophores makes them attractive and informative as spatially sensitive force probes. In this respect, spiropyran (SP) is one of the most intensively studied mechanophores. , Although SP has been studied for several decades because of its reaction to light, , heat, or solvents, its mechano-response upon the application of force was only reported for the first time in SP-linked poly­(methyl methacrylate) (PMMA) in 2009 .…”