A computational and experimental study of the linear and nonlinear response of a star polymer melt with a moderate number of unentangled arms

Abstract

We present from simulations and experiments results on the linear and nonlinear rheology of a moderate functionality, low molecular weight unentangled polystyrene (PS) star melt. The PS samples were anionically synthesized and close to monodisperse while their moderate functionality ensures that they do not display a pronounced core effect. We employ a highly coarse-grained model known as Responsive Particle Dynamics where each star polymer is approximated as a point particle. The eliminated degrees of freedom are used in the definition of an appropriate free energy as well as describing the transient pair-wise potential between particles that accounts for the viscoelastic response. First we reproduce very satisfactorily the experimental moduli using simulation. We then consider the nonlinear response of the same polymer melts by implementing a start-up shear protocol for a wide range of shear rates. As in experiments, we observe the development of a stress overshoot with increasing shear rate followed by a steady-state shear stress. We also recover the shear-thinning nature of the melt, although we slightly overestimate the extent of shear-thinning with simulations. In addition, we study relaxations upon the removal of shear where we find encouraging agreement between experiments and simulations, a finding that corroborates our agreement for the linear rheology.

Mesoscale Simulations of the Rheology of Filled Styrene–Butadiene Compounds

“…We do not expect to recover agreement at this large frequency range, i.e., ω > 100 rad s −1 , as RaPiD is unable to resolve this range due to the coarse‐graining of all degrees of freedom associated with a high frequency response. This behavior has been observed in previous studies with RaPiD . In conclusion, the model provides an adequate description of the linear rheological response of an unfilled SBR melt over the time and frequency ranges of interest in this study.…”

“…In the RaPiD model, polymers are highly coarse grained to spherical point particles. The coordinates of the particles represent the center of mass positions of the polymers.…”

“…As described in more detail below, this is realized by introducing auxiliary dynamical coordinates whose deviations from their equilibrium values creates a transient viscoelastic response. RaPiD has been successfully applied for the simulation of shear banding effects, particle alignment in sheared viscoelastic fluids, entangled polymer melts, nonlinear flow rheology in polymer solutions and star polymer melts, flow of polymer solutions near solid interfaces and anomalous polymer chain diffusion . Since RaPiD is new to the rubber field, we present a brief overview of the RaPiD algorithm; the reader is referred to earlier work for further details.…”

“…One such approach explicitly models only the filler particles while the rubber–filler interactions are included through micromechanical considerations or in the interaction potential for the rubber itself . In this feasibility study, we apply, for the first time, the highly coarse‐grained approach known as responsive particle dynamics (RaPiD) to simulate filled SBR compounds. In RaPiD, each elastomer or polymer is represented as a point particle with the dynamical effects of the eliminated degrees of freedom retained to describe the viscoelastic response within and between particles.…”

Deciphering the dynamics of star molecules in shear flow

“…4 More recently, the field is flourishing (see citations in ref. 5 and 6) with new simulations under shear flow, 6,7 response under oscillatory perturbation, 8 experiments to elucidate their rich spectra of relaxation times 5 and studies to elucidate their dual behavior, between polymer and particles. 9 Star polymers consist of several linear polymeric chains attached to a central monomer.…”