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