Segmental diffusion solvent quality

Excluded volume and solvent quality. Up till here, the volume taken up by the polymer itself, i.e., n times the volume of a monomer, has been neglected. In other words, such an ideal random chain has no volume, which would imply that two different segments can occupy the same place in the solvent at the same time. This is, of course, physically impossible, which is why the statistics of a real chain are different from those of a random walk (diffusion). Instead of this, a self-avoiding random walk should be considered, and the average conformation then is different, rm being proportional to n to the power 0.6, rather than 0.5. This means that the molecule is more expanded than an ideal chain. 

Buback has proposed a termination model to account for the above-mentioned course of kt with conversion, considering segmental, translational and reaction diffusion processes [121], The model is somewhat crude as compared to the subtle solvent effects discussed above and doesn t take phenomena such as solvent quality and coil dimensions into account. Nevertheless, the model has been found to accurately describe a large set of experimental data over broad range of conversions [122-125, 127, 128] and provides insights in which diffusion mechanisms are dominant at which conversions. 

For translational diffusion, a is between 0.5 and 0.6, depending on the solvent quality, while segmental diffusion shows much less of a molecular dependence, with a 0.16 [41 6]. 

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