(187g) Crystalline Precursor Formations under Steady-State Isothermal Planar Elongational Stretching of N-Eicosane: a Comparison between Simulation and Experiment

Introduction

The
crystallization of polymer melts under flow has generated a tremendous amount of
interest over the years. Understanding crystallization mechanisms, kinetics and
crystallite morphologies are just a few of the problems that present an ongoing
interest among research communities. The research interest dedicated to solving
these problems has been almost exclusively experimental. With the rapid
advancements in computation capabilities and the development of new simulation
strategies, simulation techniques are playing an increasingly important role in
elucidating these problems. Short and long chain n-alkanes have been
extensively used to model the behavior of polyethylene in particular and
polymers in general. Crystallization of long chain molecules through
equilibrium molecular simulation techniques is particularly difficult, due to
long simulation times needed to observe such phenomena [1, 2]. However, driving
the system away from equilibrium has been shown to reduce such simulation times
by a few orders of magnitude [3, 4]. Alternative techniques to induce the
crystal formation have been developed, which include crystallization induced by
the presence of a surface in the system [2],crystallization induced by uniaxial
stretching of the chains, followed by quenching [3, 4] etc. To the best of our
knowledge, we are the first to report crystalline-like structure formation from
purely steady-state elongational stretching under isothermal conditions. In the
present work we investigate the structures of a series of n-eicosane at
high elongation rates.  

Approach

The
system under investigation consisted of linear n-eicosane. Non
Equilibrium Molecular Dynamics (NEMD) simulations have been performed, using an
in-house developed algorithm. The oriented structures were generated via
steady-state isothermal planar elongational stretching at constant volume. The
interactions between atoms were modeled using the united atom approach of Siepmann
et al [5].

Results

The
simulated structures are compared to experiment using the structure factor (s(k))
 approach. Structure factors are computed as the Fourier transforms of the
total pair correlation functions. Excellent agreement has been found between
the simulated liquid structure factors and the x-ray diffraction determined
ones under the same conditions of pressure and temperature. The structure factor
for the elongated melt can be decomposed into an intra-molecular and an
inter-molecular region. In the intra-molecular region we found evidence of
molecules adopting the all-trans stretched conformation, which is in
excellent quantitative agreement with the experimental x-ray diffraction
investigation also performed for this work. In the inter-molecular region, we
found a shift of the first inter-molecular peak towards higher k values,
which is indicative of closer chain side packing. This shift is also in
excellent agreement with experimental x-ray diffraction data for crystalline C₂₀.
 

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