We examine the influence of steady shear on structural relaxation in a simulated coarse-grained unentangled polymer melt over a wide range of temperature and shear rates. Shear is found to progressively suppress the α-relaxation process observed in the intermediate scattering function, leading ultimately to a purely inertially dominated β-relaxation at high shear rates, a trend similar to increasing temperature. On the basis of a scaling argument emphasizing dynamic heterogeneity in cooled liquids and its alteration under material deformation, we deduce and validate a parameter-free scaling relation for both the structural relaxation time τα from the intermediate scattering function and the “stretching exponent” β quantifying the extent of dynamic heterogeneity over the entire range of temperatures and shear rates that we can simulate.

Understanding the interactions between polymer-grafted nanoparticles is imperative to predict the macroscale mechanical properties of the nanocomposites they form. Molecular dynamics simulations capture the interfacial effects of grafting on structure and mobility, but directly linking these features to macroscale constitutive relations for nanocomposites remains challenging. As a step toward addressing this challenge, we develop a computational framework to predict the effective pairwise interparticle interactions between polymer-grafted nanoparticles with different design parameters, that is, polymer chain length, grafting density, and polymer chemistry. Using coarse-grained molecular dynamics simulations, we evaluate the potential of mean force between two nanoparticles by varying their radial distance under uniaxial deformations, from which an effective interaction can be derived. We find that the repulsive part of the interaction can be expressed as an exponential repulsion term, whereas the attractive part is best captured using a sigmoidal form. The empirical constants of these equations depend linearly on the chain length and quadratically on the grafting density. To ensure that the finite rate of deformation does not affect our conclusions, we also take into account the strain rate dependence of the effective interaction, using a Cowper–Symonds model to extrapolate the zero-rate limit of our results. With the development of this interatomic potential between the nanoparticles, we propose a mesoscopic model for nanoparticle assemblies that circumvents the need to explicitly simulate polymer chains, significantly improving the computational efficiency by extending the spatiotemporal scales by 6–7 orders of magnitude.

The isothermal, isobaric spontaneous crystallization of a supercooled polymer melt is investigated by molecular-dynamics simulation of an ensemble of fully-flexible linear chains. Frustration is introduced via two incommensurate length scales set by the bond length and the position of the minimum of the non- bonding potential. Marked polymorphism with considerable bond disorder, distortions of both the local packing and the global monomer arrangements is observed. The analyses in terms of: i) orientational order parameters characterizing the global and the local order and ii) the angular distribution of the next-nearest neighbors of a monomer reach the conclusion that the polymorphs are arranged in distorted Bcc-like lattice