  The nucleation process of polyethylene under quiescent and shear flow
conditions are comparatively studied with all_atom molecular dynamical
simulations. At both conditions, nucleation are demonstrated to be two_step
processes, which, however, proceed via different intermediate orders. Quiescent
nucleation is assisted by local structure order coupling conformational and
local rotational symmetric orderings, while flow_induced nucleation is promoted
by density fluctuation, which is a coupling effect of conformational and
orientation orderings. Flow drives the transformation from flexible chains to
rigid conformational ordered segments and circumvents the entropic penalty,
which is the most peculiar and rate_limited step in polymer crystallization.
Current work suggests that flow accelerates nucleation in orders of magnitude
is not simply due to flow_induced entropic reduction of melt as early models
proposed, which is mainly attributed to the different kinetic pathway via
conformational/orientational ordering_density fluctuation_nucleation.
