AbstractVarious amounts of a styrene‐butadiene‐based triblock copolymer (SEBS) was used to compatibilize immiscible blends of high density polyethylene (HDPE) and an amorphous glassy phase consisting of either pure polystyrene (PS) or a miscible blend of PS and a polyether copolymer (PEC). PEC is structurally similar to poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO). Mechanical properties were determined for blends fabricated by injection and compression molding. The inherently brittle two‐phase HDPE/(PEC/PS) blends show significant increases in ductility and impact strength resulting from addition of SEBS. These improvements coincide with a slight loss in modulus and yield strength. If the amount of HDPE and SEBS is held constant, impact strength and ductility increase with the amount of PEC in the glassy phase. These trends evidently result from the added ductility of glassy phases containing PEC and perhaps from better interfacial adhesion in blends after adding SEBS. The latter stems from the thermodynamic miscibility between PEC and PS endblocks of SEBS which provide an enthalpic driving force for compatibilization. Differences between the properties of compression and injection‐molded blends can be attributed to the degree of crystallinity and orientation induced d
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