Physics Colloquium: Ring polymers as rheology modifiers and strong gelling agents
Prof. Dimitris Vlassopoulos, Foundation for Research and Technology Hellas (FORTH), Institute of Electronic Structure & Laser, and University of Crete
Abstract: Ring polymers represent a unique class of macromolecules because of the absence of free ends that affect their conformational entropy and are responsible for their rearrangements in a topological network. In the melt, these macromolecules exhibit power-law stress relaxation, in the absence of entanglement plateau and have a lower viscosity compared to their linear counterparts. Here, we discuss two distinct features of rings, that are consequences of their loopy structure.
Adding small amounts of ring polystyrenes to entangled linear matrices yields viscosity enhancement, which can be understood as the consequence of ring-linear threading, by invoking the (coherent) constraint release process on the rings due to the escape of linear chains. The ring-linear molar mass ratio provides a means to tailor the viscosity through the constraint release effect. In the same vein, the addition of single chain polystyrene nanoparticles, which are also (crosslinked) loopy structures, to linear matrices yields similar viscosity enhancement effect, because of the action of loop threading.
We provide experimental evidence that ring polymers are stronger depleting agents in colloidal suspensions than their linear counterparts. We use PMMA hard sphere colloidal suspensions and add linear or ring non-adsorbing polystyrene of the same molar mass or the same size. The experimental results reveal that in the presence of rings, gels are formed at smaller concentrations and possess a larger storage modulus and larger yield stress in comparison to those induced by the linear chains. These findings are in agreement with theoretical calculations and point to the crucial role of macromolecular architecture on the properties of colloidal gels.
This presentation reflects the work of D. Parisi (Groningen), K. Peponaki (Crete) C. Pyromali (Athens), E. Moghimi (Georgetown), and longstanding collaboration with M. Rubinstein (Duke), G. Sakellariou (Athens), A. Moreno (San Sebastian) and C. Likos (Vienna). Support from the Hellenic Foundation for Research & Innovation (project 4631) is gratefully acknowledged.