Crystal nucleation mechanism in melts of short polymer chains under quiescent conditions and under shear flow

We present a molecular dynamics simulation study of crystal nucleation from undercooled melts
of n-alkanes, and we identify the molecular mechanism of homogeneous crystal nucleation under
quiescent conditions and under shear flow. We compare results for n-eicosane (C20) and npentacontahectane
(C150), i.e., one system below the entanglement length and one above, at 20%–
30% undercooling. Under quiescent conditions, we observe that entanglement does not have an effect
on the nucleation mechanism. For both chain lengths, the chains first align and then straighten locally,
then the local density increases and finally positional ordering sets in. At low shear rates the
nucleation mechanism is the same as under quiescent conditions, while at high shear rates the chains
align and straighten at the same time. We report on the effects of shear rate and temperature on the
nucleation rates and estimate the critical shear rates, beyond which the nucleation rates increase with
the shear rate. In agreement with previous experimental observation and theoretical work, we find
that the critical shear rate corresponds to a Weissenberg number of order 1. Finally, we show that
the viscosity of the system is not affected by the crystalline nuclei.