The melt rheology of highly-purified ring polystyrenes in a wide range of molecular weights (10K ≤ Mw ≤ 240K g/mol) was investigated. All the rings revealed no obvious rubbery plateau and faster terminal relaxation compared to the linear counterparts. The rheological data obtained were compared with some theoretical models such as the Rouse ring model and the lattice-animal model. Moreover, two rheological parameters, zero-shear viscosities η0 and the steady-state recoverable compliances Je, were estimated, and their Mw dependence was discussed. From these data analysis, it was found that relaxation mechanisms of ring chains can be divided into three categories depending on their Mw as follows: (i) Smaller rings (Mw ≤ 20K) exhibit faster terminal relaxation than the predicted Rouse rings. This behavior is related to the difference of the local chain conformation from linear chains. (ii) Rings with the moderate molecular weight (40K ≤ Mw ≤ 90K) exhibit dynamic moduli similar to the Rouse ring prediction. (iii) A larger ring (Mw > 90K) also shows deviant behavior from the Rouse chain because its relaxation time is much longer than the Rouse ring prediction and also the lattice-animal model, where some intermolecular interactions are considered to occur.
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