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

Abstract

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∝[M 2 exp(E a /kT)] 1 (1−n η) and D∝M[M 2 exp(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.