Flow induced orientational ordering of linear alkanes from NEMD
Tahir Cagin, Khalid A. Mansour, and William A. Goddard, III
In recent years there is a great deal of interest in understanding the
molecular structure, conformations, segmental alignment, reorientation
mechanisms, reorinetation rates and spatial organization of polymers.
The experimental methods in this field include high resoltion and wide line,
1- and 2-dimensional spectroscopies as well as NMR imaging. Aiming at
developing an understanding synthetic polymers, proteins and surfactants,
giving a molecular perspective on macroscopic material properties.
In this quest molecular level simulations using Non Equilibrium Molecular
Dynamics (NEMD) can play a crucial role.
We will present a molecular level analysis of shear induced orientational
ordering of linear alkanes using these atomistic simulations.
The orientational distribution functions and orientational correlation
functions are the center piece in this analysis.
Degreee of orientation is defined in terms of both end to end vectors and
principal moments of radius gyration tensor and also N-monomer long vectors.
We investigated the degree of orientational ordering both as a function of
strain rate and the molecular structure (chain length). We also utilized
the time correlation analysis of orientational vectors to determine the
the relaxation times.
Molecular Rheology of Alkanes
The simulations on normal alkanes, hexane, octane and decane are carried
out at 300 K using a united atom representation of CH2 and CH3 groups.
In simulations Dreiding II force fields parameters are employed.
The movie of shear flow in octane is the first 20 picosecond long portion of
simulation with a shear rate of 0.1 (1/ps). Simulation starts from
an equilibrated liquid.
Octane at 300 Octane (larger picture)
5.8 MB 16.5 MB