2013-11-20
11:15 at HCI J 574

Experimental study of the viscoelasticity of critically purified ring polymers

Dimitris Vlassopoulos

Polymer Physics, Department of Materials, ETH Zurich

Since free ends play an important role in the motion of entangled polymers (reptation, fluctuations), examining the behaviour of ring polymers is a challenge. In fact, the topic has attracted the attention of scientific community in the 1980s but no final conclusions could be drawn. The controversial results were attributed to the presence of impurities in the ring samples. To overcome this problem, and in particular the unlinked linear side products from their anionic synthesis, ring polymers were purified via liquid chromatography at the critical condition. Recently, it has been shown that this approach yields reasonably pure ring polystyrenes which at a molar mass corresponding to 12 entanglements do not exhibit a rubbery plateau, in contrast to their linear counterparts. This is suggestive of a (new) self-similar mechanism of stress relaxation of these macromolecules, which reflects the conformation they adopt in the entanglement state (lattice animal). We have extended this work in different directions: we have used different chemistries, polyisoprene, polystyrene and poly(ethylene oxide); molar masses extend from unentangled regime to about 15 entanglements. We have compared the as-synthesized and purified ring polymers, and blended the latter with linear polymers of the same molar mass over a wide range of concentrations. Moreover, we have compared results against scaling predictions as well as recent molecular dynamics simulations. Results are critically discussed in view of the emerging picture of the dynamics of ring polymers, whereas open issues and new challenges are outlined. This work is part of a huge international effort involving the groups of Jülich (A. Bras, A. Wischnewski, W. Pyckhout-Hintzen, J. Allgaeir, D. Richter), Athens/KAUST (G. Sakellariou, N. Hadjichristidis), Pohang (Y. C. Yeong, T. Chang), North Carolina (M. Rubinstein) and Crete (R. Pasquino, D. Vlassopoulos). It is supported in part by the Greek General Secretariat for Research and Technology (ESPA ARISTEIA-RINGS).