Gels with thermoreversible physical crosslinks show great promise for designing materials with tuneable rheology and self-healing properties. However, many questions remain to be answered before the goal of tailoring a gel’s macroscopic properties by controlling molecular scale parameters, can be achieved. We show that considerable progress in this direction can be made by examining the behaviour of physical gels near the gel transition with the help of Brownian dynamics simulations. Due to the scale-free and semidilute character of critical gels, fully capturing their structure and dynamics requires the inclusion of associative interactions between sticky monomers, solvent-mediated hydrodynamic interactions between all the monomers in a large simulation volume, and time scales spanning several orders of magnitude. We have adapted Jim Swans’ algorithm for the efficient computation of hydrodynamic interactions in colloids to polymer chains, making the simulation of the dynamics of physical gels at the transition point tractable for the first time. Rheological properties such as the zero-shear viscosity and relaxation modulus are investigated systematically as functions of polymer concentration and binding energy between associative sites. We show the structural emergence of a gel as a power law distribution of chain cluster sizes, indicating a divergence of the average cluster size. It is shown that a system-spanning network can form regardless of binding energy at sufficiently high concentration. However, the contribution to the stress sustained by this physical network can decay faster than other relaxation processes, even single chain relaxations. If the polymer relaxation time scales overlap with short-lived associations, the mechanical response of a gel becomes “evanescent”, decaying before it can be rheologically observed, even though the network is instantaneously mechanically rigid. In our simulations, the concentration of elastically active chains and the dynamic moduli are computed independently. This makes it possible to combine structural and rheological information to identify the concentration at which the sol-gel transition occurs as a function of binding energy. Further, it is shown that the competition of scales between the sticker dissociation time and the single-polymer relaxation time determines if the gel is in the evanescent regime. Finally, we compare the prediction of the concentration at the sol-gel transition by a variety of different static and dynamic signatures of gelation.
Ravi Prakash Jagadeeshan is currently a Professor in the Department
of Chemical Engineering at Monash University where he has been since
2001. Before joining Monash, he was an Associate Professor at the
Indian Institute of Technology, Madras, and did postdoctoral work on
Sandpile dynamics with Prof. S. F. Edwards at Cavendish Laboratory in
Cambridge, and on Polymer solution rheology with Prof. H. C. Öttinger
at ETH Zürich. He was a Humboldt Fellow in the Techno-Mathematik
Department at the University of Kaiserslautern in 1999/2000. Ravi’s
research interests revolve around understanding the interaction of
flow and micro-structure in complex fluids predominantly with the
help of nonequilibrium Brownian Dynamics simulations. He is also
interested in applying methods of soft matter physics to studying
problems in biology. He was elected a Fellow of the American Society
of Rheology in 2019 and was awarded the Medallion of the Australian
Society of Rheology in 2020.