Styrene−Butadiene Gradient Block Copolymers for Transparent Impact Polystyrene

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 ( Macromolecules 2007, 40, 2432. ). 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.