Molecular Simulations Revealing Effects of Non-concatenated Ring Topology on Phase Behavior of Symmetric Diblock Copolymers

The distinctiveness of nonconcatenated ring polymers, as manifested in their fractal globular conformations and self-similar dynamics with no long-lived entanglement network, propels the idea of using ring topology to transform the phase behavior of block copolymers. With limited experimental studies of high-molecular-weight diblock ring polymers, large-scale molecular simulations of symmetric diblock copolymers are used to investigate the effects of nonconcatenated ring topology on their phase behavior. The absence of an entanglement network facilitates the phase-separation kinetics, suggesting relative ease in processing diblock ring polymers. The more compact globular conformations of ring polymers with respect to the Gaussian random-walk conformations require a higher enthalpic repulsion to drive the lamellar phase separation. Compared with the mean-field theory for the order–disorder transition in Gaussian diblock ring polymers, the simulations demonstrate the necessity for a new theory incorporating both the effects of fluctuations and the topological invariance of nonconcatenation. In the strong segregation regime, diblock ring polymers are stretched near the lamellar interface but with the globular conformational statistics preserved at larger length scales. The lamellar spacing d increases with enthalpic repulsion between the two blocks as well as the molecular weight of the diblock ring. The scaling argument for d of diblock linear polymers is modified by accounting for the globular conformations and predicts well the dependencies of d on the enthalpic repulsion and molecular weight. The lamellar interface becomes sharper as the enthalpic repulsion increases. While the theory predicts that the intrinsic interfacial width does not depend on the polymer molecular weight or topology, the apparent interfacial width w, which is broadened by the capillary wave, exhibits slight variation with the molecular weight and topology.