Polyimides basic properties

TABLE 61.12 Basic Properties of Adhesive-Based Copper-Qad Laminates (Polyimide-Based)... 

TABLE 61.14 Basic Properties of Rim Coverlay (Polyimide Rim Base)... 

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 thermal polymerization of reactive polyimide oligomers is a critical part of a number of currently important polymers. Both the system in which we are interested, PMR-15, and others like it (LARC-13, HR-600), are useful high temperature resins. They also share the feature that, while the basic structure and chemistry of their imide portions is well defined, the mode of reaction and ultimately the structures that result from their thermally activated end-groups is not clear. Since an understanding of this thermal cure would be an important step towards the improvement of both the cure process and the properties of such systems, we have approached our study of PMR-15 with a focus only on this higher temperature thermal curing process. To this end, we have used small molecule model compounds with pre-formed imide moieties and have concentrated on the chemistry of the norbornenyl end-cap (1). 

There are two basic types of polyimides condensation and addition resins. The condensation polyimides are based on a reaction of an aromatic diamine with an aromatic dianhydride. A tractable (fusible) polyamic acid intermediate produced by their reaction is converted by heat to an insoluble and infusible polyimide, with water being given off during the cure. Generally, the condensation polyimides result in products having high void contents that detract from optimal mechanical properties and result in some loss of long-term heat resistance. 

Two basic types of polyimides are of commercial significance. In the first type, which are unmodified polyimides, imide is the principal functional group present. In the second type, which are modified polyimides, other functional groups are also present modified polyimides include poly(amide-imide)s, polyesterimides, polyetherimides and polybismaleimides. The development of this large variety of modified polyimides has been stimulated by the desire to find materials which are more readily processable than the unmodified polyimides whilst retaining the same superior properties. 

This procedure is not well suited for aromatic diamines, because their basicity is 5-6 orders of magnitude lower than that of aliphatic diamines. This problem prompted the elaboration of a new approach which became the standard procedure for the technical production of polyimides (see Formula 6.2, bottom). At first the diamine is directly reacted with the dianhydride yielding a polyamic acid which in most cases is soluble in the reaction medium (e.g., dimethyl acetamide or NMP) [43, 47, 48]. This solution which is useful for casting films, may be stored at — 10 °C for several months. Heating of poly(amic acid)s to 200 °C or better to 250-300 °C promotes the inmidization process the progress of which increases with time and temperature. Stmcture and thermal properties of several polyimides prepared by the DuPont group before 1964 are compiled in Table 6.3. 

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