The rheology of chain molecules under shear

Author

Jonathan D. Moore

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Peter T. Cummings

Committee Members

Paul Phillips, H. D. Cochran, Charlie F. Moore

Abstract

The rheology of chain molecules is a subject that comprises a wide variety of complex physical phenomena, challenging scientific questions, and fundamentally important practical applications. Computer simulation, molecular dynamics in particular, is a powerful tool to provide insight into these subjects on a molecular level. In this work, nonequilibrium molecular dynamics (NEMD) is employed to study linear and branched alkane chains in the melt state under transient and steady-state shearing conditions. This study focuses on three isomers of C₃₀H₆₂ (n-triacontane, squalane, and 9-n-octyldocosane) as well as a linear short-chain polyethylene (C₁₀₀H₂₀₂)- A transferable united atom potential is used to model these alkane chains, and the simulations of planar Couette flow are performed using the SLLOD algorithm and a multitimestep simulation technique implemented on massively parallel computer architectures. The strain rates studied in this work (10⁸-10¹² s^-1) are extremely difficult to study experimentally yet typical of the severe conditions commonly found in engines and other machinery.

Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accurately predict the values for squalane (within 15%) at temperatures of 311 and 372 K. In addition, the predicted kinematic viscosity index values for both 9-n-octyldocosane and squalane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. Thus, this same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the possibility of simulation guided lubricant design.

Simulations of C¹⁰⁰H²⁰² under steady-state shearing conditions reveal a pronounced minimum in the hydrostatic pressure at an intermediate strain rate that is associated with a minimum in the intermolecular potential energy as well as transitions in the strain-rate-dependent behavior of several other viscous and structural properties of the system. Upon onset of shear, the stress overshoot curves calculated for C¹⁰⁰ are in good quantitative agreement with Doi-Edwards theory if the terminal relaxation time is assumed to have the same strain-rate dependence as the calculated self-diffusion coefficient in the flow direction. This shear-enhanced diffusion offers a possible mechanism for strain-rate-dependent relaxation times in the fast flows of polymers.

Recommended Citation

Moore, Jonathan D., "The rheology of chain molecules under shear. " PhD diss., University of Tennessee, 1999.
https://trace.tennessee.edu/utk_graddiss/8886