Differences in the molecular weight and the temperature dependences of self-diffusion and zero shear viscosity in linear polyethylene and hydrogenated polybutadiene

Within the context of a generalized coupling model we can support the hypothesis that, while the mode of relaxation for self diffusion (D) and shear flow (η) are the same, the entanglement interactions are different. We assume that there are two distinct coupling parameters n_D and n_η for self diffusion and shear flow respectively. The model predicts the molecular weight and temperature dependences to be scaled by the relevant coupling parameters as: η∝[M²exp(E_a/kT)] ^{1 (1-n_η)} and D∝M[M²exp(E_a/kT)] ^{-1 (1-n_D)}. for melts with Arrhenius temperature dependences. We have found that n_n=0.43 and 0.42 for polyethylene (PE) and hydrogenated polybutadiene (HPB) which scale η as M^3.5 and M^3.4. Also the apparent flow activation energies E*_a of 6.35 kcal mole^-1 for PE and 7.2 kcal mol^-1 for HPB scale to primitive activation energies E_a of 3.6 and 4.2 kcal mole^-1 for PE and HPB respectively. On the other hand the M^-2 dependence of D results in n_D=1/3. Then the reported activation energies for self-diffusion in PE and HPB of 5.49 and 6.2 kcal mole^-1 scale to primitive activation energies of 3.7 and 4.1 kcal mole^-1, respectively.