Molecular Rheology of Nanoconfined Polymer Melts

TL;DR

This study uses molecular dynamics simulations to explore how unentangled polymer melts behave under nanoscale confinement, revealing unique viscoelastic effects linked to chain alignment near surfaces.

Contribution

It provides new insights into the molecular-level viscoelastic response of confined polymer melts, highlighting the role of surface interactions and chain alignment.

Findings

Negative first normal stress difference observed

Presence of viscoelastic tension at low shear rates

Chain alignment influences elastic force transmission

Abstract

We use non-equilibrium atomistic molecular dynamics simulations of unentangled melts of linear and star polymers ($\mathrm{C_{25}H_{52}}$) to study the steady-state viscoelastic response under confinement within nanoscale hematite $\left ( \mathrm{\alpha-Fe_2O_3} \right )$ channels. We report (i) the negative (positive) first (second) normal stress difference and (ii) the presence of viscoelastic tension at low shear rates. We link these effects to bond alignment such that chains near the surface can carry the elastic force exerted on the walls, which decays as the chains become more aligned in the flow direction as the shear rate increases.