Experimental Polymer Mechanochemistry and its Interpretational Frameworks

Abstract

Polymer mechanochemistry is an emerging field at the interface of chemistry, materials science, physics and engineering. It aims at understanding and exploiting unique reactivities of polymer chains confined to highly non‐equilibrium stretched geometries by interactions with their surroundings. Macromolecular chains or their segments become stretched in bulk polymers under mechanical loads or when polymer solutions are sonicated or flow rapidly through abrupt contractions. An increasing amount of empirical data suggests that mechanochemical phenomena are widespread wherever polymers are used. In the past decade, empirical mechanochemistry has progressed enormously, from studying fragmentations of commodity polymers by simple backbone homolysis to demonstrations of self‐strengthening and stress‐reporting materials and mechanochemical cascades using purposefully designed monomers. This progress has not yet been matched by the development of conceptual frameworks within which to rationalize, systematize and generalize empirical mechanochemical observations. As a result, mechanistic and/or quantitative understanding of mechanochemical phenomena remains, with few exceptions, tentative. In this review we aim at systematizing reported macroscopic manifestations of polymer mechanochemistry, and critically assessing the interpretational framework that underlies their molecular rationalizations from a physical chemist's perspective. We propose a hierarchy of mechanochemical phenomena which may guide the development of multiscale models of mechanochemical reactivity to match the breadth and utility of the Eyring equation of chemical kinetics. We discuss the limitations of the approaches to quantifying and validating mechanochemical reactivity, with particular focus on sonicated polymer solutions, in order to identify outstanding questions that need to be solved for polymer mechanochemistry to become a rigorous, quantitative field. We conclude by proposing 7 problems whose solution may have a disproportionate impact on the development of polymer mechanochemistry.

Forcing Ladderenes into Plastic Semiconductors with Mechanochemistry

“…Mechanical force has been known for many decades to break covalent bonds of polymers, causing their degradation. More recently, however, a number of research groups have harnessed the potential of mechanical force towards rationally designed “productive” mechanochemistry, in which chemical reactions of polymers, induced by mechanical force, yield desirable changes in properties or reactivity . In addition to fundamental insights in how force induces unusual chemical reactivity, mechanochemistry is of emerging importance in stimuli‐responsive materials: there are reports of polymers that change color, release acid, show chemiluminescence, or catalyze olefin metathesis reactions due to mechanochemical cleavage of specific chemical bonds …”

The Challenges and Opportunities of Contemporary Polymer Mechanochemistry

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Most polymeric materials are subjectt om echanical loads throughout their lifetimes, from manufacture to recycling. [1][2][3][4][5][6][7] Stretching, compression, twisting or other distortions of the macroscopic shapeo famaterial is accompanied by stretching of individual polymer chains or chain segments. The more ac hain is stretched, the more the kinetica nd thermodynamic stabilities of its constituent monomers changes.

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“…Reported extrapolations of force-dependent activation energies from strain free values vary from usefully approximate (estimated accelerations being within af actor of two of the explicitly calculated ones) [17,18] to qualitatively incorrect (e.g.,p redicted inhibition of reactions that are accelerated by force). [5] The known reasonsappear to reflect primarily strong and remarkably complex dependences of both the molecular geometries and reaction mechanisms [19,20] on force.…”

“…Sonicating dilute polymer solutions is by far the most popular technique to induce polymer mechanochemistry, but such experiments are as challenging to interpret mechanistically or quantitatively as they are easy to do technically. [5] SMFS is the only technique that allows control of the force acting on am acromolecule and remains the only means of measuring force/ratec orrelations. Despite the technical complexity of SMFS, the limited substrate scopea nd the lack of spectroscopic characterization of the products, SMF measurements continue to impact disproportionately the development of polymer mechanochemistry.…”

“…Yet, the long-term impact of overcoming these challenges seems equally substantial and is likely to extendb eyond polymer science. While we are probably many years away from achieving this goal, partial solutions to an umber of known mechanochemical problems [5,12] are likely to have far-ranging technological andf undamental implications.…”