Polyethylene, diffusion radicals

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

Results from irradiations in polyethylene films. Given these analyses, the data from irradiations of (/ )-3b in polyethylene films will now be discussed (Table 13.7). As mentioned above, the ability of the radical pairs from (/ )-3b to diffuse translationally within a cage is related qualitatively to the 2-BN/4-BN ratios whereas the ability of the 1-phenylethyl radical to tumble along the translational course that brings it to combine at either the 2- or 4-position of its 1 -naphthoxy partner is related to or %ee4B. Although a 1-phenylethyl radical center is attracted... 

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation . Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer . Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air . These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments... 

The decay reaction of the alkyl radical, R -i- R R-R is very simple because the activation energy of the reaction in stage (b) is nearly equal to zero and the probability of the back reaction R-R R-i-Ris also nearly equal to zero. In the case of polyethylene, the products, R-R contain inter chain cross-links and double bonds. Therefore, we can analyze the decay reaction with a scheme involving a diffusion-controlled bimolecular reaction. 

The evaluation of molecular mobility (molecular motion) of polymer chains, (a) The decay of the radicals trapped in polyethylene can be interpreted in terms of a diffusion controlled reaction. The decay reaction is closely related to the molecular motion of polymer chains in the crystalline region, attributed to so-called a-relaxation because the time constants of the molecular motion agree with those of the diffusion, (b) The high molecular mobility of isolated polyethylene chains tethered on polytetrafluoroethylene surfaces has been identified. 

---