Influence of Tie and Loop Molecules on the Mechanical Properties of Lamellar Block Copolymers

We use coarse-grained molecular dynamics simulation to study the influence of molecular architecture and conformations on the mechanical response of lamellar nanostructured polymers. For this purpose, a recently developed generation method (radical like polymerization method) has been optimized to generate lamellar triblock copolymer samples with alternate glassy and rubbery stacks. Several systems, with various rate of loop (both ends of the chain are in the same glassy block), cilia (cut triblock, effectively a diblock chain) and tie molecules (TMs) (chain that bridges two subsequent glassy blocks through the intermediate rubbery block) were generated. Uniaxial tensile tests were performed, the tensile strain being applied in the normal direction of the lamellae. This situation can be understood as a simple way of mimicking the deformation of the equatorial stack in semicrystalline polymers, with the hard phase being the glassy (rather than crystalline) one. The resulting constitutive laws reveal the key role of the tie molecules in the transmission of stress between glassy blocks. Decreasing the amount of tie molecules with respect to cilia chain leads to a decrease in the entanglement density, which affects the yield and the strain hardening behavior. It is also demonstrated that loop chains play the exact same role as tie molecules because they are strongly entangled.