Mechanochemistry of Polymers

a division of polymer science concerning chemical transformations that take place in polymers under mechanical stress. The energy of mechanical action on polymers during processing is sufficient for scission of the chemical bonds in the macromolecules. Even under mild moderate processing conditions, the strain developed significantly exceeds the C—C bond strength [the C—C bonding energy is (4.8−5.5) X 10⁻¹² erg, or (4.8−5.5) X l0⁻¹⁹ joule].

The rupture of a macromolecule in a mechanical force field, called mechanodegradation, is accompanied by the appearance of free radicals, which under certain conditions are capable of activating and initiating chemical processes. The appearance of macroradicals is seen in crushing or rolling or under the action of ultrasound. The reduction in the energy of chemical bonds in polymer chains as a result of an increase in interatomic distances under the action of mechanical stresses may also activate oxidation and thermal and chemical degradation of polymers.

Mechanodegradation is accompanied by a significant change in the entire set of physicochemical properties of polymers—a decrease in molecular mass, the appearance of new functional groups, a change in solubility, and the appearance of a system of three-dimensional bonds. In addition, mechanical action on systems consisting of a number of polymers or a polymer and monomers makes possible the synthesis of new polymers and block and graft copolymers (mechanosynthesis) as a result of the interaction of macroradicals of various structures with each other or with monomers.

Mechanochemical transformations are also used to achieve controlled change in the properties of polymers (plasticization of rubber) to produce new polymers (impact-resistant polystyrene and poly vinyl chloride) and for restoration of three-dimensional structures (regeneration of vulcanizates). On the other hand, mechanochemical effects in many respects facilitate the development of fatigue and decomposition in polymers. These processes determine the feasibility of using parts made of plastic. Therefore, the stabilization of polymers is very important, particularly when the polymers are subjected to prolonged recurring stress (for this purpose, for example, antifatigue agents are added to rubber stock compounds).