Simulating Stretch-Induced Crystallization of Polyethylene Films: Strain Rate and Temperature Effect on the Kinetics and Morphology

A well-known process used to produce recyclable polymer films by
stretching is machine direction orientation (MDO). During the drawing
stage of MDO, a semicrystalline morphology is developed, which ultimately
determines the final properties of the film. Predicting this morphology
is demanding, as it is affected by numerous factors such as the strain
rate, the temperature, the molecular weight of the drawn polymers,
and its distribution. Atomistic simulations are valuable for studying
the development of semicrystalline morphology upon stretching, as
they can shed light on crystal nucleation and growth and their dependencies
on molecular characteristics and processing conditions. In this work,
beginning from linear monodisperse polyethylene melts of high molecular
weight, we perform molecular dynamics drawing simulations. Applying
a planar extension protocol that mimics the drawing stage of a real
MDO process, we investigate the dependence of the developed semicrystalline
morphology on the strain rate and on temperature. In particular, using
a home-built algorithm to estimate the evolution of the degree of
crystallinity over time and applying a mean first-passage time analysis,
we show that increasing the strain rate affects both crystal nucleation
and growth mechanisms, accelerating the development of the semicrystalline
morphology. On the other hand, a more complicated dependence on temperature
is found. At deep supercoolings, crystal growth is rate-controlling,
whereas at high temperatures, crystal nucleation is limiting. As a
consequence, an optimal temperature where the overall crystallization
rate exhibits a maximum is found. Our results are compared against
experimental data available in the literature, and a very good qualitative
agreement is found. Finally, we develop a theoretical model for both
the strain rate and temperature dependence of PE nucleation. Beginning
from the simple geometric idea that a crystal nucleus can be envisaged
as a cylindrical bundle and assuming a purely elastic response to
deformation up to the induction time, we extract useful relations
for the critical nucleus size and the nucleation rate as functions
of the temperature and the strain rate that provide an accurate description
of our simulation results.