Molecular Dynamics Simulation of Entangled Melts at High Rates: Identifying Entanglement Lockup Mechanism Leading to True Strain Hardening

Abstract

In the present work, molecular dynamics simulations are carried out based on the bead‐spring model to indicate how the entanglement lockup manifests in the late stage of fast Rouse‐Weissnberg number (WiR>>1) uniaxial melt stretching of entangled polymer melts. At high strains, distinct features show up to reveal the emergence of an increasingly tightened entanglement network. Chain tension can build up, peaking at the middle of the chain, to a level for chain scission, through accumulated interchain interactions, as if there is a tug‐of‐war ongoing for each load‐bearing chain. Thanks to the interchain uncrossability, network junctions form by the pairing of two or more hairpins. It is hypothesized that the interchain entanglement at junctions can lockup through prevailing twist‐like interchain couplings as long as WiR > 9. In this limit, a significant fraction of chains act like cyclic chains to form a network held by interchain uncrossability, and appreciable chain tension emerges.

Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance

“…The length scale between two locked entanglements seems to be cross-linking density dependent. This hypothesis of incomplete chain pull-out suggests that the tearing energy of E1 would eventually saturate if N p keeps increasing while keeping cross-linking density constant because the effective contour length between locked entanglements would limit the toughening effect ( 25 ). The leveling off of tearing energy for E1 observed in Fig.…”

“…3F) suggests that the primary chains cannot be completely pulled out before they break, because if tearing energy is dominated by complete chain pull-out (breaking all the 3 of 5 C1 at least along one direction), the tearing energy would increase while the modulus increases because there are more cross-linkers to break to dissipate energy. This incomplete primary chain pull-out might originate from entanglement lockup under higher tension (25). The locked entanglements act like strong crosslinker C2, which force the primary chain to break instead of being pulled out.…”

Effective Entanglement and Constraint Release in Deformed Polymer Melts

“…With several key relaxation mechanisms including reptation, contour length fluctuation (CLF), and constraint release (CR), − the tube model has been remarkably successful in describing the linear rheology of entangled polymers . However, the modeling for the responses to large and rapid deformations is still challenging. , For example, the generalized tube model, which incorporates convective constraint release , and chain stretch and retraction, fails in predicting the nonlinear rheology in the extensional flow. − Particularly, the roles of chain conformations, , entanglement behaviors, − and dissipative properties , in determining the nonlinear rheology remain in debate.…”