(757b) Tuning the Saw-Tooth Tensile Response and Toughness of Multiblock Copolymer Diamond Networks

Authors: 
Nowak, C., Cornell University
Escobedo, F., Cornell University
Liquid crystalline elastomers consisting of block copolymer chains end-linked into a diamond network have recently been shown to have a saw-tooth tensile response to uniaxial deformation, which can provide the basis to design super-tough, highly stretchable materials. When deformed, these systems are forced to increase the interfacial area between blocks, increasing the stress which is only released when chain hairpins unfold and new segregated domains form, leading to the saw-tooth tensile response. We use Molecular Dynamics to study different chain designs to predetermine the number of â??teethâ? and the strain-orientation dependence of the tensile response. Two chain designs having distinct tensile behaviors have been identified. First, in a ABA (15%-70%-15%) chain with lower backbone stiffness, the stress response is found to be independent of the extensional direction by virtue of morphological rearrangements that occur at low strains, before the saw-tooth behavior ensues. Second, a pentablock chain ABACA (20%-20%-20%-20%-20%) is found to exhibit a sawtooth response with twice as many â??teethâ? for a given strain. This pentablock architecture significantly decreases the hairpin length, which has the effect of creating two separate sets of unfolding chains in the network, and decreasing the relaxation of the network when new domains form. Unlike other architectures, the number of hairpins in these pentablocks does not monotonically decrease upon deformation, but can plateau or even increase due to hairpin regeneration events. We also explore the realistic synthesis of these diamond networks, with the goal of enhancing the saw-tooth behavior, network extensibility and cross-link mobility.