Rozlyn Chambliss, Ph.D. (2013)

Dissertation Topic:  A Molecular Dynamics Study of Tensile and Compression Deformation of Polyethylene and Polyethylene Composites with Varying Temperature and Strain Rates
Major Professor:  Dr. Melissa Reeves, Associate Professor of Chemistry
B.S.:  Chemistry, Tuskegee University, Tuskegee, AL
M.S.:  Chemistry, Tuskegee University, Tuskegee, AL
Employment: State of Alabama, Auburn, AL
 
Dissertation Abstract:

Polyethylene (PE) has been studied quite often both experimentally and with molecular dynamics (MD) techniques.  The purpose of this research work was to use MD simulations to determine how the tensile and compression properties of PE and PE-carbon nantubes (CNTs) composites are altered through changes in temperatures and strain rate.  This may result in predictive equations for realistic strain rate and temperature dependent mechanical properties of PE-CNT composites.  The MD was carried out with Daresbury Laboratory's software DL_POLY.  The neat PE-like system consisted of a 100-chain polymer system, with each chain containing ten CH2 units, which was examined using Clarke's intra-polymer potential.  The composite system consisted of an 82-chain polymer system, each chain containing ten CH2 units, and an end-capped (10,0) 20A long SWNT containing 204 carbon atoms.  The composite systems were examined with the Clarke's intrapolymer and Tersoff-Brenner potential.  These systems were subjected to a uniaxial strain and compression in the z-direction.

The Tg  average for neat PE was found to be 125 K.  The average Tg  for PE-CNT composite was found to be 140 K.  There was an improvement in the Tg  when the nanotube was added to PE.  This was expected since the CNT makes the system stiffer.  Additionally, in the radial distribution curves for PE Trial 1 and Trial 2, the first peak in the neat PE Trial 1 was slightly lower than in Trial 2. There was little difference shown between Trials 1 and 2 second peak.  These results were expected since both PE trials had the same amount of PE chains.

From the experimental literature, the trend that was expected was as the temperature decreases, the Young's modulus should increase, but in theses directly simulated results the reverse was found.  For this system, it was inferred that as the temperature of the system increased, the Young's modulus values increased.  Similarly, it was inferred that the slower the strain rate, the lower the Young's modulus value.  This is true because the system has more time to relax, resulting in more realistic values.

The results from the MD study were used to determine an extrapolated fit for Young's modulus that relates the simulated strain rate results to the experimental strain rate.  These extrapolations were accomplished  from a plot a Young's modulus versus the logarithmic function of strain rate.  The MD simulated results, when extrapolated to experimental strain rate, showed the expected increase in the Young's modulus of PE-CNT composite over the Young's modulus of the neat PE.

There were large fluctuations seen both the neat PE and PE-CNT composites.  These fluctuations were seen because polymers are soft systems and they were not allowed to have a complete relaxation during the fast strain rates.  With long relaxation times, the short decane chains had time to realign to the preferred crystalline state.  It may be possible to correct some of the fluctuations that were seen in this system by having longer chains of PE or using a stiffer polymer matrix such as polystyrene.  Nevertheless,  this work shows for the first time that it is possible to simulate the Young's modulus of soft-systems with large fluctuations and extrapolate the results to experimentally feasible strain rates to still deduce the correct trends.