Poly 2,2 m-phenylene -5-5 bibenzimidazole PBI blends

One of the most highly studied high temperature polymers in miscible blends is poly (2,2 (m-phenylene)-5-5 bibenzimidazole) or PBI, the chemical structure of which is shown in Rg. 4.1. The fundamental reason for the observation of miscibility in PBI-based systems is the presence of the N-H functional group that can interact with the functional groups which are present on the backbone of other polymers. Thus, miscibility in these types of systems is an example of a specific interaction that leads to a negative enthalpy of mixing. 

The most extensively studied PBI blend system is PBI blended with a polyetherimide, originally available from GE, Ultem 1000. The chemical structure of Ultem is shown in Fig. 4.2. The early work on that blend led to the phase separation diagram shown in Fig. 4.3. This figure shows that a processing window exists for blends that contain in excess of 75% PBI and also for blends that contain less than about 20% PBI. In this case, the processing window is defined as those temperatures between Tg and the phase separation temperature. For example, for an 85/15 PBI/Ultem blend ratio, the glass transition temperature is 400°C and the processing window is 25°C. 

The phase behavior observed with the PBI/Ultem blends is to be contrasted with the phase separation observed in blends of PBI with another polyimide, XU218 from Ciba Geigy. That system showed phase separation only above 400°C. The actual temperatures are determined by the blend composition. Thus, the phase behavior observed in PBI/Polyimide blends is dependent both on the type of polyimide and the thermal history of the blends. 

In further investigations of PBI blends, Jaffe et alP showed that PBI is miscible with certain polyimides that contain the hexafluoroisopropylidene or 6F moiety, i.e. CF3-C-CF3. The miscibility and phase behavior in these blend systems is dependent on the overall structure of the polyimide. In addition, the amount of 6F chemical group affects the of the polyimide itself and, hence, the of any miscible blends. 

Those studies of blends of PBI with various polyimides were subsequently extended to include other polymers. For example, it was shown that PBI and polysulfone form immiscible mixtures. However, it was later shown that the introduction of functional groups, such as sulfonate groups, into the polysulfone polymer chain resulted in the formation of miscible blends with 

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