Deformation and Fracture of Lamellar and Cylindrical Block Copolymers with Unentangled Glassy Matrices: Effect of Chain Architecture and Microdomain Orientation

We investigate the influence of chain architecture and microdomain orientation on the deformation and fracture properties of relatively thick films (600-800 nm) of poly(cyclohexylethylene)-poly(ethylene) (PCHE-PE) block copolymers with various architectures. Optical microscopy, transmission electron microscopy, and scanning force microscopy are used to study the domain structure as well as examine the details of deformation and fracture of films stretched on ductile copper grids. For lamellar block copolymers the deformation and fracture properties depend significantly on the chain architecture. Annealed diblock copolymer films containing 48% PE show 100% failure at very low strains of 5% with a median strain to failure of 2%, whereas the lamellar triblock (CEC) and pentablock copolymer (CECEC) films do not fail at strains up to 27%. However, a 50/50 blend of diblock (CE M w = 22 kg/mol, f PE = 0.48) and triblock (CEC M w = 45 kg/mol, f PE = 0.48) copolymers shows a median strain to failure of about 6.4%. A 50/50 blend of the same diblock and pentablock (CECEC M w = 66 kg/mol, f PE = 0.48) copolymers, on the other hand, does not show any failure at strains up to 27%. Experiments on a cylinder-forming pentablock copolymer (CECEC M w = 60 kg/mol, f PE = 0.25) with slow ordering kinetics show that the maximum ductility with no failure at the maximum strain of 27% is obtained if the PE cylinders are randomly oriented (spun-cast films). Films having cylinders that are aligned parallel as well as perpendicular to the tensile direction (annealed for 3 days with cylinders lying down in the plane of the film) show a reduction in ductility with 30% failure at the maximum strain. As the proportion of cylinders aligned perpendicular to the tensile direction is increased (by increasing the annealing time to 7 days, the top half of the film shows cylinders that align normal to the film plane), 100% of the film squares fail at the maximum strain of 27% with a median strain to failure of about 19%. Moreover, in a film half as thick (∼300 nm), after a 7 day anneal, the perpendicular orientation persists throughout the film thickness. This sample shows complete failure at about 22% strain with a median strain to failure of about 15.7%. These results indicate that the orientation of the cylinders in the films has a strong effect on the mechanical properties of this pentablock copolymer, similar to conclusions reached previously for the triblock copolymer architecture. The results obtained in these studies are compared with those reported in previous works for the same system to ascertain the relative effect of chain architecture, microdomain orientation, and composition on the mechanical behavior of the block copolymer thin films.