Tribochemical Transformations in Polymers

Destruction of macromolecules and distortion of the permolecular structure lead to changes in their properties, especially solubility, resistance to chemical agents, strength, fatigue and impact viscosity, as well as elasticity and plasticity. From the practical viewpoint, variations in polymer properties induced by the tribochemical destruction raise interest in two respects first, as unavoidable phenomena that accompany any mechanical effect on the polymers during their treatment or operation and, second, as a deliberate change of properties of solid pol3uners to obtain materials with specific characteristics. 

Elementary stages of tribochemical transformations. Tribochemical reactions in polymers usually follow a free-radical mechanism [78,80]. As a result of mechanical interactions pol3mier or low-molecular radicals appear, taking part in substitution, addition, decomposition or other reactions. On breakage of a macromolecule, two primary radicals with free valence at the chain ends are formed. In the vinyl series these radicals are CH2 and CXY (where X and Y are hydrogen atoms and other side substitutes) in 

During interaction with neighboring macromolecules end radicals may convert into central ones [79]  

Secondary radicals with free valence in the chain center ( CX ) participate as a rule in the reactions of macromolecular decomposition and the transfer of H atom. Decomposition is the inverse to the reaction of addition. That is why, the lower the polymerization heat, the lower the activation energy of decomposition and the stronger the probability of macromolecular destruction on free valence localization. Polymers characterized by low polymerization heat of their monomers (e.g. the polymerization heat of PMMA is about 55kJ/mole) are strongly decomposing, not only under mechanical effects, but also in other processes of radical formation. 

The probability of decay of CX radicals in polyethylene and other polymers whose polymerization heat of monomers reaches 88-92 kJ/mole is less than that of other possible transformations of radicals. 

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A.P. Krasnov, LA. Gribova and A.N. Chumayevskaya. Chemical structure of polymers and tribochemical transformations in polymers and filled systems. 

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