Bisnadimides

BMI polymers have glass transition temperatures in excess of 260°C and continuous-use temperatures of 200-230°C. BMI polymers lend themselves to processing by the same techniques used for epoxy polymers. They are finding applications in high-performance structural composites and adhesives (e.g., for aircraft, aerospace, and defense applications) used at tem-peratrues beyond the 150-180°C range for the epoxies. Bisnadimide (BNI) polymers are similar materials based on bisnadimides instead of bismaleimides. 

Two classes of reactive secondary amines (XX and XXI) were synthesized [90] for the thermoset version of the In Situ Molecular Composite concept. The thermoset chemistry of the bisnadimide (XX) involved that of PMR systems (polymerization of monomeric reactants) [95], The secondary amine (XXI) embodied the more recent ringopening polymerization of benzocyclobutene (BCB) [96]. Polymerization exotherms for both thermoset amines occurred with an onset at 225 °C, maximizing at approximately 260 °C as evidenced by DSC at a heating rate of 10 °C/m. 

Structural modifications were envisioned early to overcome these limitations. A first improvement was outlined by preparing copolymers, which were soluble in the state of full imidation, mainly poly(ester-imide)s and poly(amide-imide)s [2,4, 5]. As an alternative to these conventional copolymers, addition polyimides were developed in the 1970s as a new class of thermosetting materials. Thus, bismaleimides, bisnadimides, and end-capped thermocurable polyimides were successfully developed and marketed [6,7]. These resins were the precursors of the modern PMR (polymeric monomer reactants) formulations [8]. 

Bisnadimide-Linear High Tg Thermoplastic Semi-IPNs.172... 

In this chapter a comparative study of the bismaleimide and bisnadimide oligomers will be presented with the following items ... 

Fig. 4. Chemistry of the first bisnadimide end-capped oligomer (theoretical n=1.67 Mn= 1300 Da)...

Model compounds (mono- and bisnadimides) as well as nadimide end-capped oligomers have been extensively studied. In this section we will examine first the characterization of BNIs, then the monitoring of their thermal polymerization and finally the characterization of polymers. 

Thus, the oligomerization can be followed accurately to obtain an entirely imidized material. Moreover, at this last stage, it is possible to determine the relative amounts of lowest polycondensation degrees (n=0, n=l and n>2). In this way, it was determined that n=0 (bisnadimide of MDA, 35-50%) and n=l oligomers (15-25%) are the largest constituents of the oligoimide [26]. The same kind of observations were done with oligobenzhydrolimides [31,32]. 13C NMR spectroscopy also allows qualitative determinations [24,25,27,28]. 

Although this work was carried out with a model compound, the results are quite consistent with the mechanisms suggested by the BP team [86] which was working within a bisnadimide. However, the work of Meador et al. [42] presents some diverging conclusions by minimizing the role of the reverse Diels-Alder reaction in the crosslinking mechanism. 

The role of the miscibility of semi-IPN components on the mechanical properties has been discussed. The linear bisnadimide was a benzhydrol bisnadimide (Fig. 33). Three polyimides prepared from the same diamine and three different dianhydrides (Fig. 37) were used as linear components. The blends were cured up to 300 °C in a similar fashion to the bisnadimide alone. The results for the blend containing 20% by weight of linear polymers are summarized in Table 9. The non-miscible character of the components gives a phase segregation leading to the best toughness [121]. 

The semi-IPN concept has also been used in order to improve the high-temperature adhesive properties of a flexible polyimide. Polyimide prepared with isophthaloyldiphthalic anhydride and mefa-phenylenediamine presents good adhesive properties but is limited to 270 °C. By blending it with a bisnadimide... 

Fig. 37. High Tg polyimides blended with a benzhydrol bisnadimide (Fig. 33) [121]...

Fig. 38. Linear polyimide and bisnadimide obtained with isophthaloyl diphthalic anhydride and raefa-phenylenediamine...

To the thermoset type belong bismaleimides and bisnadimides as well as oligomeric end capped imides. End capping occurs with reactive phen-ylethyl groups. These types are used for reactive injection molding and related techiuques. The chemistry of formation of the imide moiety is quite similar for both the thermoset type and the thermoplastic type. There are several monographs on PIs. Bismaleimides are a separate subclass... 

Fig. 15. Structure of the model compound, bisnadimide. Reprinted from (1998) J Polym Sci A Polym Chem 36 2653 [80] with permission...

At the same time, kinetic studies of a model compound, bisnadimide (Fig. 15), in thermal and microwave processes were carried out in order to simulate the crosslinking reaction of polyimide RP-46. 

Figure 18 Chemical sequences involved in the polymerisation of monomer reactants (PMR) process. The average molecular weight of intermediate oligomer 43 is controlled by the relative amounts of monoester 41, diester 42 and diamine 34. Chain extension of bisnadimide 43 occurs at high temperature via the reverse Diels-Alder polymerisation process.

Addition-type polyimides, which are thermosetting resins, were developed to improve the processability of polyimides, but their thermal stability is severely degraded by the presence of aliphatic bonds in place of the aromatic nuclei. However, the adhesive strength has been evaluated for the bismaleimide- (Section 4.3.4.1), bisnadimide- (Section 4.3.4.2) and acetylene-terminated (Section 4.3.4.3) imide oligomers. 

From a practical viewpoint, the compounds most commonly used in the fabrication of addition polyimides are bismaleimides, bisnadimides and ethynyl terminated oligoimides [275]. Poly (bismaleimides) have actually achieved special importance as technical polymer materials [276]. Due to the aliphatic-type linkages, which appear as a consequence of polyaddition, polymers of this type are not as thermally stable as aromatic polyimides are. However, their good processability, the polymerization without volatiles release and the relative low cost of raw materials have helped them to become a real alternative for long term uses up to 200-250 °C. 

Bisnadimides are obtained usually in a two-step process. The monomers are dissolved in a polar solvent (NMP, DMF, dyglime, etc.) and made to react at moderate temperature (80 °C). In the second step, the imidization is performed by raising the temperature to around 160 °C. This is the main difference with bismaleimides, where thermal imidization is not possible because of their low-onset curing temperatures. The synthesis of bisnadimides is shown in Scheme (63). 

In order to avoid the problems associated to high cross-linking density, that affect adversely the properties of the final materials, bisnadimides are usually obtained as imide oligomers terminated on nadimide groups. In general, a nadimide terminated oligoimide is... 

Torrecillas and co-workers [a.359, a.360] established that, during thermal degradation of bismaleimide and bisnadimide networks, water, carbon monoxide and carbon dioxide are the most important products formed (Table 10) the major organic compounds detected were aniline, polycyclic molecules and isocyanate products, as shown in Scheme 35. 

Figure 49 Comparison of the reaction rate for microwave and thermal cure of the bisnadimide model compounds at 280 °C. Reproduced with permission from Liu, Y. Sun, X. D. Xie, X.Q. Scola, D. A. J. Polym. Sci. Part A Polym. Chem. 1998, 36, 2653." ...

---