Rubbery−glassy block copolymer dispersions are an attractive solution for toughening rigid thermoplastics like polystyrene without affecting optical transparency. An interesting facet of the copolymers used is molecular disorder, artificially introduced during anionic synthesis through composition gradients along the copolymer chain and/or blending and partial coupling of different copolymers. In particular, this level of disorder is apparently a key to achieve the desired PS/copolymer blend morphologies and properties in short processing times. In this work, we investigate the role of these “synthesis imperfections” on self-assembly of styrene-rich asymmetric gradient triblock copolymers, denoted S1−G−S2, where Si are pure polystyrene blocks and G is a gradient copolymer of styrene and butadiene. Kinetic modeling of conversion data is used to predict gradient composition profiles for the anionic copolymerization conditions used. Self-assembly, dynamic viscoelastic behavior, and experimentally determined mesoscopic composition profiles across microdomains are discussed in light of the particular copolymer structure.

The mechanical behavior of styrene/butadiene triblock copolymers and their blends with high molecular weight polystyrene (PS) homopolymer was studied. Symmetric triblock copolymers with two PS end blocks of equal length (SBS) were compared with asymmetric triblocks with end blocks of different lengths (S1BS2) and with asymmetric triblocks comprising a styrene/butadiene composition gradient in the middle block (S1GS2). The precise molecular architecture of these copolymers and its role on morphology and properties were described elsewhere (Macromolecules2007402432.). Here, immiscible blends of these copolymers with long PS chains are discussed. While solvent casting yields the expected phase separation, melt extrusion produces tough and transparent blends with metastable submicrometer scale dispersions of soft microdomains highly controlled by processing. The particular molecular architecture of gradient asymmetric triblocks significantly improves ductility of these nanostructured blends. The volume fraction of soft microdomains extracted from dynamic mechanical data is identified as a universal parameter that controls tensile behavior of the different blends studied.