Diffusion of linear polymer melts in shear and extensional flows

Area of Science:

• Polymer physics
• Computational chemistry
• Fluid dynamics

Background:

• Understanding molecular behavior in flowing systems is crucial for materials science and rheology.
• Anisotropic diffusion and velocity correlations are key properties influencing macroscopic flow behavior.

Purpose of the Study:

• To investigate the anisotropic self-diffusion tensor and velocity autocorrelation functions of linear chain molecules under various flow conditions using molecular dynamics simulations.
• To provide the first simulation results for diffusion and velocity autocorrelation functions under planar extensional flow for these systems.
• To compare simulation results across a broad range of conditions for planar Couette flow.

Main Methods:

• Utilizing nonequilibrium molecular dynamics simulations.
• Employing two molecular models: finitely extensible nonlinear elastic (FENE) chain and freely jointed tangent sphere chain.
• Deriving the Green-Kubo expression for the diffusion tensor and its relation to mean-squared displacement.

Main Results:

• Presenting simulation data for anisotropic self-diffusion and velocity autocorrelation functions for linear chain molecules under planar Couette and extensional flow.
• Reporting novel simulation results for diffusion and velocity autocorrelation functions under planar extensional flow.
• Comparing simulation results for planar Couette flow across an extensive range of conditions.

Conclusions:

• The study provides fundamental insights into the molecular-scale dynamics governing diffusive transport in flowing polymer systems.
• The findings contribute to a deeper understanding of rheological properties influenced by molecular motion under flow.
• The derived Green-Kubo expression and its relation to mean-squared displacement offer valuable theoretical tools for diffusion analysis.