Short-chain Length Dependence of Equilibrium Dynamics and Nonlinear Rheology in Unentangled Long-chain/Short-chain Polymer Blends
简介：The equilibrium dynamics and nonlinear rheology of unentangled polymer blends remain inadequately understood,especially re-garding the influence of short-chain matrix length Ns on the structure and rheological behavior of dispersed long chains.Using molecular dynam-ics 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 vol-ume 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 be-tween the viscous and normal stress responses:while the zero-shear viscosity,η,is strongly Ns-dependent,the first and second normal stress co-efficients,φ1 and φ2,collapse onto universal curves when scaled by the dimensionless shear rate,γτ0,suggesting a common mechanism of orien-tation 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-in-duced structural changes in understanding the rheology of unentangled polymer blends.展开
