学术报告： Dynamics and Rheology of Supramolecular Polymer Networks Formed by Telechelic Chains in Equilibrium and Under Flow
报告题目：Dynamics and Rheology of Supramolecular Polymer Networks Formed by Telechelic Chains in Equilibrium and Under Flow
报 告 人： Zuowei Wang PhD （Associate Professor/University of Reading）
Supramolecular polymer networks are formed by physical association of linear or branched polymers via reversible and highly directional non-covalent bonds. The potential applications of these materials have inspired strong interests in understanding the physical mechanisms underlying their structural, dynamical and mechanical properties. In this talk we present hybrid molecular dynamics/Monte Carlo simulations of supramolecular networks formed by unentangled telechelic chains with sticky end monomers of finite functionality. We study the kinetics of sticky monomer association, the topological structure and the resulting dynamic and rheological behavior of the supramolecular systems as a function of the sticker bonding energy and the parent polymer chain length in equilibrium and under shear and planar extensional flows.
In equilibrium state percolated transient networks are formed above a threshold bonding energy around 4.3kBT. At high bonding energies (³10kBT), the majority of the stickers are fully reacted and the fraction of open stickers is less than 1%. We find the dynamical and rheological behavior of such strongly associated supramolecular networks are dominated by a partner exchange mechanism in which the stickers exchange their associated partners, and so release the imposed topological constraints, through the association and disassociation of sticker clusters. The characteristic time of the partner exchange events grows exponentially with the bonding energy and is up to 2 orders of magnitude longer than the average sticky bond lifetime. As a result, three relaxation regimes, namely the initial Rouse, intermediate rubbery and terminal relaxation regimes, can be clearly distinguished in the stress and chain end-to-end vector relaxation functions. A phantom chain hopping model based on the microscopic understanding is proposed to describe the chain relaxation dynamics in the supramolecular networks.
A shear hardening behavior of the strongly associated supramolecular networks is observed in startup shear with shear rates above the inverse of the average sticky bond lifetime. These systems also demonstrate much stronger extensional hardening behavior than their polymer melt counterparts in the startup planar extensional flow. The startup flow hardening can be well interpreted by the non-Gaussian stretching of the polymer chains in the transient networks. On the other hand, the supramolecular polymer systems we studied only exhibit shear thinning behavior in the steady states owing to the decrease in the total number of elastically effective strands in the transient networks with the increase of flow rate.
Zuowei Wang School of Mathematical，Physical and Computational Sciences, University of Reading, Associate Professor. Areas of Interest:.Multiscale computer simulation and theoretical modelling of soft matters:1.Entangled linear and branched polymers: dynamics and rheological properties2.Charged block copolymers (polyelectrolytes and polyampholytes): conformational transition and self-assembly behaviour3.Surfactant micelles: micellar shape transition, interaction with polymers and additives, rheological properties4.Dipolar colloidal suspensions, including ferrofluids, electro- (ER) and magneto-rheological (MR) fluids: field-induced structure formation, electrostatic and magnetic properties5.Computational and theoretical algorithms: atomistic and coarse-grained molecular dynamics, Monte Carlo, scaling theory, tube theory, Ewald summation, first-principle electronic structure calculation, etc.