Isolating the Effects of Morphology and Chain Architecture on the Mechanical Properties of Triblock Copolymers

This study extends the understanding of the effects of morphology and chain architecture on mechanical properties of block copolymers by isolating the effect of morphology from other factors, such as composition and molecular weight. Using a single poly (isoprene-styrene-isoprene) triblock copolymer (ISI), we have used solvent casting with solvents of varying selectivity to produce three morphologies (cylinders, gyroid, and lamellae). The elastic moduli of these as-cast morphologies were measured using microtensile tests and found that at fixed molecular weight and composition (38 wt % PS) the moduli increased with increasing glassy domain continuity, in the order of PS cylinders to lamellae to gyroid morphology. At higher PS composition this trend continued in the order of lamellae to PI cylinders. Finally, with the same total molecular weight, composition, casting solvent, and morphology, SIS triblocks exhibit higher elastic moduli than ISI because both ends of the rubbery PI blocks are tethered in SIS.