Shear-Induced Nanorod Dynamics in Unentangled Polymer Melts

The orientation of nanofillers under shear flow during processing plays a decisive role in determining the macroscopic properties of polymer nanocomposites. However, a clear understanding of the shear-induced dynamics of individual nanofillers remains elusive, limiting the establishment of a mechanistic link between microscopic polymer chain dynamics and mesoscopic nanofiller organization. Here, we investigate the shear-induced orientational dynamics of a single nanorod in unentangled polymer melts using nonequilibrium coarse-grained molecular dynamics simulations. By systematically varying the shear rate and polymer chain length, we examine the dynamic response over regimes ranging from thermal fluctuation-dominated conditions, characterized by low Weissenberg and Péclet numbers (Wi, Pe < 1), to shear-dominated regimes (Wi, Pe ≫ 1). As the shear strength increases, polymer chains undergo a conformational transition from isotropic coils (Wi < 1) to stretched and flow-aligned configurations (Wi ≫ 1), with long chains exhibiting a characteristic stretching-contraction-tumbling cycle. Concurrently, the nanorod exhibits a transition from a randomized orientation (Pe < 1) to pronounced shear-induced alignment (Pe ≫ 1). Quantitative analysis of the nanorod flipping period and residence time based on shear-plane projections reveals that enhanced orientational stability primarily arises from an increase in the residence time within each flipping cycle. Moreover, while both the nanorod and polymer chains align passively with the applied shear flow in the absence of intrinsic orientational coupling, the extent and stability of the nanorod alignment are strongly modulated by the viscoelastic properties of the polymer matrix.