The hydrodynamic characteristics and weight-average molecular weights of some samples of poly-(4,4′-oxydiphenylene) pyromellitamic acid (
I) and poly (4,4′-phenylene) pyromellitamic acid (
II) in DMF have been obtained by sedimentation, viscometry and light scattering. For (
I) the dependence of
S0 on
M was obtained and it was found that the hydrodynamic parameters
depend on
M up to
M = 10
5. The experimental results were treated by using two models, viz. a Yamakawa-Fujii persistent chain and a rotational isomeric chain with free rotation about valence bonds. It was shown that the hydrodynamic behaviour of macromolecules of (
I) with a pin-joint oxygen atom in the main chain is satisfactorily described by the rotational isomeric chain model with free rotation about valence bonds. The dependence of [η] on
M was obtained and the constants K and a in an equation of the Mark-Kuhn-Houwink type were determined for (
II). The flexibility of the chain was estimated and the Kuhn segment was found to be 200 Å using the Yamakawa-Fujii theory. The flexibilities of the chains of (
I) and (
II) are compared; it is shown that the rigidity of the chain of (
II) without a pin-joint oxygen atom in the monomer unit is much higher than that of the chain of (
I). The dependence of
S0 on
M obtained for (
II) suggests that its macromolecules show a more pronounced tendency for formation of aggregates than those of (
I).
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