Mechanical Degradation of Unentangled Polymer Melts under Uniaxial Extensional Flow

TL;DR

This study uses molecular dynamics simulations to investigate how unentangled polymer melts degrade mechanically under uniaxial extensional flow, revealing flow-induced conformational changes and bond-breaking behaviors relevant for manufacturing.

Contribution

It provides new molecular-level insights into polymer chain deformation and bond breaking under flow, aiding the design of manufacturing processes for polymeric materials.

Findings

Identified shear thickening-thinning-thickening stages with increasing flow rates.

Discovered bond-breaking potential leading to flow thinning at high extension rates.

Revealed fracture kinetics transition from first-order to more complex behavior.

Abstract

Complex flow fields govern the deformation of polymers in various manufacturing processes. However, high flow rates may trigger reaction events (i.e., bond breaking or undesirable reaction of mechanophores) in raw polymeric materials, leading to the mechanical or functional debasement of manufactured structures. Additionally, it is difficult to fully characterize such molecular-level flow in the laboratory due to time- and length-scale limits. In this study, we perform non-equilibrium molecular dynamics (NEMD) simulations to explore the rheological and mechanical degradation of unentangled polymer melts under uniaxial extensional flow (UEF), allowing for chain breaking. Our simulations demonstrate shear thickening-thinning-thickening stages with the increase of UEF extension rates, resulting from flow-induced changes of chain conformation. With further increasing UEF extension rates, a bond-breaking potential leads to another flow thinning stage. Interestingly, fracture kinetics is originally first-order owing to the need for highly stretched polymer chains before bond fracture. It is no longer first-order when bond fracture is instigated before chains are stretched. Our computational work provides insight into the optimal design of the manufacturing process for polymeric materials.