The equilibrium dynamics and nonlinear rheology of unentangled polymer blends remain inadequately understood, especially regarding the influence of short-chain matrix length NS on the structure and rheological behavior of dispersed long chains. Using molecular dynamics simulations based on the Kremer-Grest model, we systematically explore the NS-dependence of static conformations, equilibrium dynamics, and nonlinear shear responses in unentangled long-chain/short-chain polymer blends. Our results demonstrate a decoupling between the static and dynamic sensitivity to NS: while the static chain size, Rg, follows Flory theory with slight swelling at small NS due to incomplete excluded volume screening, the diffusion coefficient, D, and the relaxation time, τ0, exhibit a strong, non-monotonic NS-dependence, transitioning from monomeric friction dominance at small NS to collective segmental rearrangement at large NS. Additionally, we observe partial decoupling between the viscous and normal stress responses: while the zero-shear viscosity, η, is strongly NS-dependent, the first and second normal stress coefficients, Ψ1 and Ψ2, collapse onto universal curves when scaled by the dimensionless shear rate, <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$\dot{\gamma}\tau_{0}$</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mover> <mi>γ</mi> <mo>˙</mo> </mover> </mrow> <msub> <mi>τ</mi> <mrow> <mn>0</mn> </mrow> </msub> </math></EquationSource> </InlineEquation>, suggesting a common mechanism of orientation and stretching. Under shear, long chains compress in the vorticity direction λz ∼ Wi−0.2, which reduces collision frequency and contributes to shear thinning, while the scaling of weaker orientation resistance mG ∼ Wi0.35 reflects hydrodynamic screening by the short-chain matrix. These findings highlight the limitations of single-chain models and emphasize the necessity of considering NS-dependent matrix dynamics and flow-induced structural changes in understanding the rheology of unentangled polymer blends.

Short-chain Length Dependence of Equilibrium Dynamics and Nonlinear Rheology in Unentangled Long-chain/Short-chain Polymer Blends

期刊论文
发表日期：

2026年1月22日

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Xiao-Yang Wang,
Bo Liu,
Li-Jia An,
Zhen-Hua Wang,
Yu-Yuan Lu

摘要

The equilibrium dynamics and nonlinear rheology of unentangled polymer blends remain inadequately understood, especially regarding the influence of short-chain matrix length N_S on the structure and rheological behavior of dispersed long chains. Using molecular dynamics simulations based on the Kremer-Grest model, we systematically explore the N_S-dependence of static conformations, equilibrium dynamics, and nonlinear shear responses in unentangled long-chain/short-chain polymer blends. Our results demonstrate a decoupling between the static and dynamic sensitivity to N_S: while the static chain size, R_g, follows Flory theory with slight swelling at small N_S due to incomplete excluded volume screening, the diffusion coefficient, D, and the relaxation time, τ₀, exhibit a strong, non-monotonic N_S-dependence, transitioning from monomeric friction dominance at small N_S to collective segmental rearrangement at large N_S. Additionally, we observe partial decoupling between the viscous and normal stress responses: while the zero-shear viscosity, η, is strongly N_S-dependent, the first and second normal stress coefficients, Ψ₁ and Ψ₂, collapse onto universal curves when scaled by the dimensionless shear rate, $\dot{\gamma}\tau_{0}$ $\dot{γ} τ_{0}$ , suggesting a common mechanism of orientation and stretching. Under shear, long chains compress in the vorticity direction λ_z ∼ Wi^−0.2, which reduces collision frequency and contributes to shear thinning, while the scaling of weaker orientation resistance m_G ∼ Wi^0.35 reflects hydrodynamic screening by the short-chain matrix. These findings highlight the limitations of single-chain models and emphasize the necessity of considering N_S-dependent matrix dynamics and flow-induced structural changes in understanding the rheology of unentangled polymer blends.