Microphase separation of star-diblock copolymer melts studied by dissipative particle dynamics simulation

The microphase-separation behaviors of two types of star-diblock copolymers (A)(4)(B)(4) and (AB)(4) are investigated through the dissipative particle dynamics (DPD). The simulated phase diagrams show similar ordered mesostructures as those of corresponding linear and cyclic ones, such as lamellas (LAM), perforated lamellas (PL), hexagonal (HEX) cylinders and body-centered-cubic (BCC) spheres, besides, a series of gyroid (GR) morphologies predicted by theoretical research have been identified. In the regions between the totally ordered and disordered phases, we have found some melted morphologies that can be thought as locally ordered, such as micelles (M), liquid rods (LR) and random network (RN), which have not been identified in relevant theoretical predictions. The simulated threshold for a totally ordered mesostructure to appear is higher than theoretical predictions, which can be ascribed mainly to the increasing fluctuation with finite chain length, and the star architecture can facilitate microphase separation, which is in agreement with the theoretical predictions. In addition, it is easier for the (A)(n)(B)(n) copolymers than for corresponding (AB), ones to trigger microphase separation under the same conditions. The relations between the root-mean-square radius of gyration (RMSGR) and the composition f(A) in the two types of star copolymers are almost contrary, which can be attributed to the differences in their structural characteristics.