THE DECAY OF FREE RADICALS iN POLYMER

Kinetic laws and particularities of migration mechanisms of free valence in the processes of decay of free radicals in polymers are dealt with. The basic examples correspond to the results for polyethylene and polymethyl methacrylate. The kinetics of the decay reactions are discussed from the viewpoint of Waite's cage diffusion theory and Levedev's model. From the kinetic data of various authors the cage radius and diffusion coefficient are calculated. With increasing temperature the cage radius in polyethylene increases from 5.3A (120°K) to 40—60 A (360°K) and it has to be looked upon as an effective kinetic parameter. The diffusion coefficient corresponding to free valence displacement at long distances is in the range 10— 16 to 10— 18 cm2 s'. In amorphous polymers at temperatures exceeding the glass temperature diffusion proves to be of major importance for long-distance migration. In a series of other examples, chemical reactions play a more important role, such as hydrogen atom transfer and radical decomposition. Results of measurement of the corresponding elementary reaction compared with the rate constants of decay are mentioned. Under real conditions, migration is affected simultaneously by both diffusion and chemical mechanisms. The investigation of free radical decay proves to be a complicated problem closely associated with several fields of the chemistry and physics of polymers, reactivity theory and reaction kinetics in the solid state etc. The radicals are formed in chemical, irradiation, mechanical, photochemical and other effects on the polymer. Each of these processes provokes deep changes in the material. Therefore the overall complex of investigations of the laws of decay should consist of a great many phenomena: The central problem of free radicals in polymers (their structure, conformation, intrinsic mobility, radical ractivity in fundamental kinds of reactions at low temperatures etc.); The supermolecular structure of polymers (transition and relaxation effects, intrinsic mobility, diffusion and the like); Radiation and photo chemistry (main types of defects and distortions, radical and ion-molecular reactions, distribution of distortion centres according to their distances and depths of 'traps', structural changes in polymers and so on);