Distinct Viscoelasticity of Hierarchical Nanostructures Self-Assembled from Multiblock Copolymers

Abstract

We employed a nonequilibrium dissipative particle dynamics method to study viscoelastic properties of hierarchical lamellar structures self-assembled from heptablock copolymers. Three ways of shearing with respect to lamellar planes, including parallel, vertical, and transverse shearing, were imposed to study the viscoelasticity of ordered systems. In parallel shear, with the morphology transformation from disorder to general lamellae to parallel lamellae-in-lamella, both the storage and loss moduli of multiblock copolymer melt show a remarkable improvement. In addition to the parallel lamellae-in-lamella, the multiblock copolymers can form perpendicular lamellae-in-lamella with different viscoelastic behaviors. In vertical shear, parallel lamellae-in-lamella shows terminal behavior, while a low-frequency plateau in the storage modulus exists for the perpendicular lamellae-in-lamella. In transverse shear, the storage moduli for both perpendicular and parallel lamellae-in-lamellae with strong separation of small-length-scale structure exhibit a solid-like plateau at low frequency. The physical origin underlying the distinct viscoelasticity of various hierarchical lamellae was revealed by monitoring the motions of polymer blocks at different length scales. The present work could provide information for preparing advanced materials based on packed lamellae.