Computer Simulations of Crystallization Mechanism in Polymeric Materials

In this work, we have studied crystallization in short polymer chains using molec-
ular dynamics simulations. We use a realistic united atom model which is able to
reproduce the physical quantities related to phase transitions. We present a study
of crystal nucleation from undercooled melts of n-alkanes and identify the molec-
ular mechanism of homogeneous crystal nucleation under quiescent conditions and
under shear flow. We choose n-eicosane (C20) the length of which is below the en-
tanglement length and n-pentacontahectane (C150) the length of which is above the
entanglement length so that we can compare results for unentangled and entangled
polymer chains. We also provide the crystal growth mechanism of n-eicosane under
quiescent conditions. For C150, we present crystal lamellae structure and compare
our results with published simulation results. We use a mean first passage time
analysis and a committor analysis to determine the critical nucleus size and then to
compute the nucleation rate. We observe that the critical nucleus is of cylindrical
shape. We report on the effects of shear rate and temperature on the nucleation
rates and estimate the critical shear rates, beyond which the nucleation rate in-
creases with the shear rate. We show that the critical shear rate corresponds to a
Weissenberg number of order unity which is in agreement with previous experimen-
tal observation and theoretical work. We also show that the power law behaviour
between nucleation rate and shear rate is in agreement with experiments and theory.
We compute the viscosity of the system during the formation of crystalline nuclei
and we show that the viscosity of the system is not affected by the crystalline nuclei.
Finally, we present results of crystallization in the polyethylene (C500) melt under
quiescent conditions.