Nadic imide

The pre-polymer has a very low DP (x < 3). The norbornene-ended polyimide (norbornene-2,3-dicarboxylic imide also known as nadic imide) is further polymerized by heating it at about 300 C. The thermosetting process of norbornene-ended polyimide can be investigated by Py-GC/MS. For example, the crosslinking of norbornene-ended polyimide PMR-II, was investigated using Py-GC/MS [20] (the pre-polymer PAR-11 structure is shown below) ... 

Addition reaction Pis are also based on aromatic diamines and dianhydrides, but in addition have Nadic imide end groups. These systems also contain solvents to make the adhesive film pliable. Curing of these adhesives initially proceeds via a condensation reaction at low pressure. However, unlike the conventional condensation reaction Pis, after completion of the condensation reaction and removal of solvent, these adhesives are still thermoplastic. At this point the temperature and pressure are increased to density the bond line and the... 

To improve the processability of PI and to alleviate or eliminate volatiles, work began in the late 1960 s under NASA Lewis sponsorship (23), to end-cap oligomers with reactive groups. The first report involved the use of 3,6-endomethylene-l,2,3,6-tetrahy-drophthalic anhydride (5-norbornene-2,3-dicarbo ylic anhydride, nadic anhydride) and alkyl derivatives thereof (e.g. citraconic anhydride) and 1,2,3,4-tetrahydrophthalic anhydride (4-cyclohexene-1, 2-dicarboxylic anhydride) as the reactive end group on imide oligomers (23-25). This initial effort led to the development of P13N [with T for polyimide, 13 for Mn of 1300g/mole and N for nadic end cap] whose structure is shown below. 

The structures and corresponding calculated MI values are given in Table 7 (nos. 1-5). Mesophases were observed for scores of MI=12 and 13 but not for the odd structures (MI range 9.5-11). In addition, a series of imide-amide oligomers were synthesised with molecular weights above 1000, in which the terminal groups were nadic anhydride. Mis ranged from 8.3 to 10.7 but no positive identification of mesophases were made in any of these (Table 7, nos. 6-9)... 

Citraconic anhydride Nadic anhydride Figure 15.2 Modifiers for Poly(imide)s... 

The studies of the crosslinking reaction of a nadic end-capped imide, N, N -(ox-ydi-3,4 -phenylene)bis(5-norbornene-2,3-dicarboximide) in thermal and micro-wave processes were performed by Scola at al. (Fig. 14) [80]. The investigated starting resin (RP-46) consisted of polyimide precursors, 3,3, 4,4 -benzophenonetet-racarboxylic acid methyl ester (BTDE), 3,4 -oxydianiline (3,4 -ODA), and end capping reagent, 5-norbornene-2,3-dicarboxylic acid monomethyl ester (NE). The cure process proceeds in two steps imidization (1) and a thermally induced (retro-diene Diels-Alder) decomposition-recombination crosslinking step (2) (Fig. 14). 

Two types of reactive groups, ethynyl and nor-bomene (Nadic), have been placed on the ends of imide oligomers in an attempt to develop more processable high temperature PI adhesives. The thermal reaction of the ethynyl groups occurs without the evolution of volatiles, whereas the thermal reaction of the nor-bomene group may involve the release of a small amount of cyclopentadiene from the reverse Diels-Alder reaction (depending on con-... 

Figure 48 Crosslinking reaction of nadic end-capped imides.

Nadicimide end-capped oligomers were usually prepared in two steps namely preparation of amine terminated nadicimide and chain extension with a dianhydride. Appropriate quantities of nadic or methyl nadic anhydride and diamine in glacial acetic acid were refluxed for several hours and the amine terminated nadicimide was recovered by precipitation. Then chain extension with 6FDA was carried out in acetone at 60 C. Chemical imidization of the amic acid to imide was carried out using sodium acetate and acetic anhydride [130,131]. Instead of diamines, triamines can also be used [132-134]. Chemical structure of nadicimide end-capped resins prepared from di and triamine (Figure 23) is given below. 

Figure 10 Reactions in the formation of LARC-13 polyimide adhesive. The first reaction is the addition of the three ingredients to create a polyamic acid. The acid further condenses to yieid the imide. Further heating causes condensation of the terminal nadic groups to yield a crosslinked structure.

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