Literature data on the self-diffusion coefficient D
s of linear polymers in bulk and solution have been reexamined. The major conclusions are as follows:
(a) The currently accepted relation for D
s as a function of polymer molecular weight M and concentration c
p, i. e., D
s∝M
-2,p
-1.75 (i) is not exactly followed by the data, though it is usually taken as evidence for the reptation motion of a polymer chain assumed in the tube theory and also for the scaling theory.
(b) The experimental data examined suggest that D
s tends to obey the relation D
s∝M
-2.5 (ii) for M>M
c, the critical molecular weight for the onset of entanglement, and D
s∝M
-1 (iii) for M<M
c, with a smooth transition between the two regions. Equation (ii) is consistent with the relation expected from viscoelastic data on polymer concentrates.
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