Mechanical Properties of Binary Polymer Blends

There have been many investigations of the mechanical properties of binary blends. If the two polymers are miscible, the mechanical behavior of the blend is generally intermediate between the two components. If the structures of the two polymers are different and they are immiscible, the mechanical behavior is usually found to be significantly decreased. 

Another example is poly p-phenylene oxide (PPO) and polystyrene. The General Electric Noryl is a blend of PPO with rubber modified polystyrene (i.e., high impact polystyrene, often known as HIPS) (see Section 6.8). HIPS is much tougher than glassy polystyrene but HIPS miscibility with PPO (Formula 5.13) is based upon HIPS polystyrene content. The improvement in toughness of the brittle PPO by HIPS makes this blend a commercial success. 

A third example is blends of isotactic polypropylene with ethylene-propylene copolymer (EPM) (or terpolymer EPDM). While these polymers are immiscible there is near miscibility of the amorphous regions of crystalline polypropylene with EPM. The addition of EPM lowers the modulus and increases the toughness of polypropylene. 

The examples given above are, however, exceptions. In general, polymer blends are immiscible. Immiscible polymer blends usually exhibit ultimate mechanical properties such as elongation at break inferior to either of the pure components. This is notably the case in polyolefin-polystyrene blends. Another example of this inferiority of ultimate properties is found in studies of the tensile test energy to break in blends of polyethylene and polyamides [89]. 

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