Molecular Rheology of Nanoconfined Polymer Melts

We use non-equilibrium atomistic molecular dynamics simulations of unentangled melts of linear and star polymers (${\mathrm{C}}_{25}{\mathrm{H}}_{52}$) to study the steady-state viscoelastic response under confinement within nanoscale hematite $\left(\alpha -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)$ 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.