Chain contraction and nonlinear stress damping in primitive chain network simulations

Area of Science:

• Polymer Physics
• Materials Science
• Rheology

Background:

• The Doi-Edwards (DE) model describes polymer relaxation in two independent steps: fast chain contraction and slow orientational relaxation.
• This decoupling is challenged by generalized convective constraint release (CCR) mechanisms and sliplink models where entanglements are not independent.

Purpose of the Study:

• To investigate chain contraction under step shear using primitive chain network simulations.
• To explore the molecular origins of the failure of time-strain separability in nonlinear polymer relaxation.

Main Methods:

• Primitive chain network simulations based on a multichain sliplink model.
• Analysis of nonlinear relaxation modulus G(t,γ) and its time-strain separability.
• Examination of chain contour length, subchain length, and subchain stretch relaxation dynamics.

Main Results:

• Simulations quantitatively reproduced experimental nonlinear relaxation modulus G(t,γ).
• Rigorous time-strain separability G(t,γ)=h(γ)G(t) was valid only near the terminal relaxation time.
• A delicate failure of separability was observed at times near the longest Rouse relaxation time (τ(R)).

Conclusions:

• Chain relaxation occurs in three steps: fast, intermediate, and terminal, governed by local force balance, Rouse relaxation, and reptation, respectively.
• Generalized CCR and slow entanglement reformation lead to retarded subchain length equilibration at the chain center.
• This retarded equilibration explains the failure of time-strain separability at t∼τ(R), a phenomenon not predicted by the original DE model.