Effect of end-block asymmetry on the packing of ABA triblock copolymers

We examine the packing of polystyrene-b-poly(ethylene-alt-propylene)-b-polystyrene (SEPS) copolymers at constant composition (25% polystyrene) but varying the ratio of the length of one styrene block to the other, from 0 (i.e., a diblock) to 1 (a symmetric triblock). Samples were prepared in the EP-selective solvent squalane, with polymer concentrations ranging from 10% up to the melt. Morphologies were characterized by SAXS, supplemented by viscoelastic measurements. For the diblock and symmetric triblock, the expected phase behavior was found: body-centered cubic from 10% to 30%, hexagonal from 50% to 80%, and lamellae at 90% and 100%. Remarkably, the asymmetric triblocks show no long-range order up to at least 30% concentration, even after considerable annealing. Clear form-factor features and structure factor peaks suggest a liquid-like packing. Several possible explanations for this suppression of lattice formation will be evaluated.