A Dissipative Particle Dynamics Study On The Morphological Behavior Of Diblock Copolymer Microphases Under Steady Shear

In this study, we use the Dissipative Particle Dynamics (DPD) technique tostudy the morphological behavior of diblock copolymer microphases, such asLAM, PL, HEX, and BCC, under steady shear. In our simulations of the LAM microphases, we have successfullyreproduced many important phenomena that have been reported by variousresearch groups, such as the parallel and perpendicular reorientation of LAM,parallel-perpendicular transition. By analysing the snapshots of the systems atdifferent time-evolution stages, as well as the time-evolution of the orderparameter of the systems, we have proposed two different reorientationmechanisms under weak and strong shear rates, respectively. When the shearrate is small, the reorientation is achieved by the rotation of the LAM layers,and the layered structure is retained during the reoritation process, thoughsome deformations may take place. When the shear rate is large, however, thereorientation proceeds via a two-stage mechanism, i.e. microdomainbreakup-reformation mechanism. The shear-induced reorientations take placein HEX phases, too.The shear-induced undulation instabilies are observed in the LAM, as wellas HEX microphases. In the WSL, the undulation instabilities take thesinusoidal shapes, while in the SSL, chevron shapes are prefered.The PL-LAM and PL-HEX phase transition are both found in our simulationsof the PL phases under shear. We postulate that whether the PL phase transfersto a LAM phase or a HEX phase depends on which phase is closer to theoriginal PL phase in the phase diagram.Shear-induced melting is also observed in our simulations of the BCCphases.