Temperature-dependent undulation and kink formation of layered block copolymers: A coarse-grained molecular-dynamics study

Multilayer polymer solids with different mechanical properties in each layer demonstrate significant nonlinearity and anisotropy in elastic and plastic deformation. This framework provides a fundamental basis for exploring composite materials with novel mechanical responses. However, the underlying deformation mechanisms are not yet adequately understood, particularly at the microscopic level. In this study, a coarse-grained particle approach combining the Kremer-Grest-type bead-spring model and dissipative particle dynamics method was used to construct microphase-separated lamellar structures consisting of alternating hard (glassy) and soft (rubbery) layers formed by triblock copolymers. The triblock-copolymer lamellae exhibited undulation instability when subjected to tensile strain along the lamellar normal owing to Poisson's contraction of the soft layers, provided that the lamellae were sufficiently long along the in-plane major axis. The lamellae evolved into a chevron-like kinked morphology owing to further strain beyond the instability caused by the buckling of the hard layers. The deformation behavior depended significantly on the temperature relative to the glass-transition temperature (Tg) of the two layers. At temperatures higher or lower than the Tg values of both layers, layer dilation occurred based on the applied strain. However, at temperatures between the two Tg values, lamellar kinking occurred because of the formation of chevrons. The results also indicate that kink deformation in layered triblock copolymers is primarily driven by the competition between two instabilities: the buckling of hard layers and cavitation of soft layers. This study elucidates the previously unknown mechanical properties of polymeric layered materials that comprise layers of varying strengths.