Probing relationship between structure and viscosity of unentangled polymers in steady shear flow

Structural responses of unentangled polymers of various architectures, including linear, ring, star, asymmetric star, and H-shaped polymers, in steady shear flow are investigated via nonequilibrium molecular dynamics simulations. As observed in our previous simulations, these polymers, when having an identical equilibrium mean-square radius of gyration 〈R2g0〉 exhibit the same viscosity curve in both the linear and nonlinear regimes. In this article, the polymer orientation and deformation and the rate of deformation are calculated as a function of shear rate. The differences between different architectures in most of these properties are found to be related to the polymer extensibility, the number of free ends and the ability of folding. The unique architecture-independent quantity is the gradient component 〈Gyy〉 of the radius of gyration tensor. We find that a simple relation η=C,〈RZg0〉（3〈Gyy〉/〈R2g0〉）3/2 gives a near-perfect fit to shear viscosity η/data, where C, is a constant. The results suggest that this relation between chain thickness and viscosity is universal for unentangled polymers of various architectures.

Structural responses of unentangled polymers of various architectures, including linear, ring, star, asymmetric star, and H-shaped polymers, in steady shear flow are investigated via nonequilibrium molecular dynamics simulations. As observed in our previous simulations, these polymers, when having an identical equilibrium mean-square radius of gyration <R_(g0)~2> exhibit the same viscosity curve in both the linear and nonlinear regimes. In this article, the polymer orientation and deformation and the rate of deformation are calculated as a function of shear rate. The differences between different architectures in most of these properties are found to be related to the polymer extensibility, the number of free ends and the ability of folding. The unique architecture-independent quantity is the gradient component <G_(yy)> of the radius of gyration tensor. We find that a simple relation η=C_η<R_(g0)~2>(3<G_(yy)>/<R_(g0)~2>)^(3/2) gives a near-perfect fit to shear viscosity η data, where C_η is a constant. The results suggest that this relation between chain thickness and viscosity is universal for unentangled polymers of various architectures.