Thermostability, polyimide

As mentioned earlier PMR polyimide thermosts are used as matrix resins for glass- and carbon fiber composites, mainly in aeroengine applications. At this point it has to be mentioned that the thermal oxidative stability of a PMR composite is dependent on the type of fiber used (113) and the cure conditions (time/temperature/atmosphere) employed for molding. Very interesting is the observed higher thermal oxidative stability of PMR-II composites when cured/-... 

The thermal stability of the polymer is an interesting criterion for one to develop because several authors have considered photoablation as proceeding through a purely thermal degradation. Polyimide and polyphenylquinoxaline, two polymers of the thermostable family, are stable up to a temperature of approximately 500. As seen in Figure 3a, they display quite... 

All the polyimides are thermostable. They do not melt before decomposition at high temperature. In thermogravimetric analysis, the decomposition starts at more than 400°C. 

Thermostability requirement for microelectronic applications basically involves only the thermo exposure during processing. Since the devices are not expected to operate at anywhere near the processing temperature. At 400°C in air, even with very thin films polyimide do not show any sign of degradation within the time (30-60 min) processing take place. We, therefore, conclude that fully aromatic polyimide is thermally sufficient for this application. 

Conditions have been defined for applying polyimide coatings onto the silicon wafer as passivation and/or dielectric. Processing variables studied included the critical areas of adhesion, cure cycle and thermostability. Aminosilane was shown to be effective adhesion promoter. The rate of imidization was followed by F.T.I.R. employing time lapse technique. 

In Fig. 14.12 we show two TSC curves one for a typical commodity polymer (PVC, PP, PMMA, etc. and one for a thermostable polymer (aromatic polyimide). Users are essentially interested in the maximum temperature at which the material works for a long period. This temperature (for a duration of about 10 years) will be 50 100°C for the commodity polymer and 200-250°C for the thermostable polymer. Processing practitioners are rather interested in the maximum temperature at which the material can be processed for a duration of, typically, a few minutes. This temperature is of the order of 180 250° C for commodity polymers and 350 150° C for ther-... 

Figure 14.12 Shape of thermal stability ceiling curves for a commodity polymer of the PVC-PP type (com) and a thermostable polymer of the aromatic polyimide type (ther).

At the same time, the fluorinated chains have found applications in various thermostable polymers such as polyquinazolones [398] and polyimides [399]. In this latter case, the presence of the fluorinated group decreases both the water absorption and the dielectric constant. 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

In addition, the Japanese Companies, the Hitachi Chemical Co and the Mitsui Toatsu Chemical Inc. have developed interesting applications with polyimides semi-conductors, liquid crystals, poorly colorable thermostable films [103-107], Finally, let us mention monomers obtained from diamines first the synthesis of a diacetylenic compound [7] ... 

Polycondensation methods for thermostable polymer synthesis 86MI12. Polyimides from heterocyclic monomers 80UK2418. 

PADS containing siloxane polyimide compositions were evaluated for adhesion to silicon wafers according to the identical test protocol employed by Davis (7). As shown in Table VIII, these polymers had adhesive properties similar to the GAP derived siloxane polyimides however, the PADS containing materials are of higher thermostability than the GAP derived materials. Thus, the PADS class of materials offers a balance of adhesive and thermal qualities not hitherto attainable. 

Thermoset materials such as polyimides behave quite differently from thermoplastic materials. They usually do not exhibit noticeable temperature transitions. The term thermoset is applied to materials that, once heated, react irreversibly so that subsequent applications of heat and pressure do not cause them to soften and flow [3], This property leads to good thermostability. 

Many polymers, such as PE, PP, PVC, PS, PA-6,6, PC, due to their durability, PEEK (polyetherethreketone), by its thermostability, and polyepoxy and polyimide resins are widely used as matrix in polymer composites. 

The materials based on polyimides have valuable mechanical and chemical properties [45]. Currently, these thermostable polymers are used extensively in aerospace engineering. Investigations of the thermal and thermo-oxidative stability of polyimides have been carried out in detail [49]. However, the mechanism of action of such aggressive gases as nitrogen oxides on these materials remains practically unexplored. The efficiency of initiation by the ion-radical mechanism (Equation... 

M.I. Bessonove, M.M. Koton, V.V. Kudryavtsev and L.A. Laius, Polyimides - A Class of Thermostable Polymers, Nauka, Leningrad, Russia, 1983. [in Russian]... 

For example, the structure of the polyimide that is shown below meets these criteria it is approximately thermostable up to 450°C ... 

Depending on the nature of the dianhydride and the diamine, the resulting polyimide exhibits variable characteristics in addition to common basic properties thermomechanical and chemical thermostabilities, along with chain stiffness... 

The few properties listed in this table illustrate the main advantage of these aromatic polyimides, namely an outstanding thermostability combined with low sensitivity to oxidation. These properties come close to the limits of soluble organic polymers. Probably the most widely used representative of this class of polyimides is poly(4,4 -diaminodiphenyl ether pyrromellitimide), which was... 

Finally, it should be mentioned that the aromatic polyimides were the forerunners of a diversified group of thermostable polymers having heterocyclic moieties in their repeat units, for example, poly(benzimidazole)s, poly(benzox-azole)s, poly(benzthiazole)s, poly(benzobisthiazole)s, poly(l,3,4-oxadiazole)s. 

---