  For the first time, we carry out molecular dynamics (MD) simulation of
shear-induced martensitic phase transitions between the orthorhombic and
non-orthorhombic (triclinic and monoclinic) phases of crystalline polyethylene
(PE) in the framework of a realistic all atom model of the polymer. We show
that the variation of the shear rate allows observing on a nano-sample both a
strongly nonequilibrium phase transition occurring by random nucleation and
irregular growth of a new phase ('civilian' way, for rapid deformations) and
the coherent, or 'military', kinetics (generally considered as usual for
martensitic transformations). We induce transitions from the orthorhombic to
the triclinic phase according to two transformation modes observed in
experiment on PE single crystals. Rapid deformation favors the transition
directly to the triclinic phase, slow deformation - first to the intermediate
monoclinic, and only then - to the triclinic phase. The second way corresponds
to the experiment on extended chain PE. We explain this result and analyze the
competition between different transformation and plastic deformation modes.
Rotations of PE chains around their axes necessary for the transition between
the orthorhombic and non-orthorhombic phases are executed by short twist
defects diffusing along the chains. The transition between the monoclinic and
triclinic phases occurs through half-chain-period translations of the chains
along their axes, mostly collectively, as crystallographic slips.
