Polymerization-induced diffusion

Scheme 24 summarizes the possible processes that can occur during a free-radical polymerization induced via an intermolecular electron transfer process (PET) in the presence of aromatic amines kdii is the rate constant of diffusive encounters be-... 

The thermal diffusion of polymers induced by the applied temperature gradient, unlike polymer transport driven by sedimentation or electrical fields, is a poorly understood phenomenon. However, a good deal of information has accrued by virtue of thermal FFF experiments thermal FFF has now become the most useful and convenient tool for studying the mechanism of polymeric thermal diffusion [17-20]. 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

In region III, the discharge is maintained only by ionization in the gas phase without electron injection from the cathode. Because of the inertial effect of ions and electrons, only small part of charged particles in the gas phase can arrive on the electrode. Therefore, polymerization may be induced principally by diffused free radicals and/or ion-electron pairs ... 

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