PMR polyimides

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/-... 

An approach for improved processing of PMR polyimide is the addition of jV-phenylnadimide to the precursor solution of the monomeric reactants. After the in-situ condensation, a PMR-15 resin is obtained which is diluted with JV-phenylnadimide in order to improve the rheological properties of the system (118). The amount of JV-phenylnadimide (PN) added was in the range of 4 to 20 mol %. Just 4 mol % caused a significant and disproportionate reduction of the minimum viscosity with no concomitant loss of thermal stability. 

One of the major problem areas with PMR-15 polyimide is the high final cure temperature required (188-320 °C) to fully develop the high temperature properties of the resin. Serafini and coworkers (119) showed that the use of rw-aminostyrene as an endcapper instead of NE lowered the final cure temperature of the PMR polyimide from 320 to 260 °C. However, the (Tg) was lowered to 260 °C and thus limited the use temperature to 260 °C. The use of equimolar amounts of NE and p-aminostyrene in a PMR resin (120) helped to overcome this problem, however the flow properties suffered. The flow problem could again be overcome through the use of JV-phenylnadimide as a reactive diluent (121). The effect increased when the endo-isomer of the JV-phenylnadimide was used, because it melts at a lower temperature than the exo-isomer (122). 

Lindenmeyer PH, Sheppard CH (1984) Characterization of PMR Polyimide Resin and Prepreg NASA Report N84-20695... 

Jenkins SD, Emmerson GT, McGrail PT, Robinson RM, J Adhesion, 45(1-4), 15-27, 1994. Labronici M, Ishida H, Dynamic mechanical characterization of PMR polyimide/carbon fiber composites modified by fiber coating with silicones. Composite Interfaces, 5(3), 257-275,1998. Labronici M, Ishida H, Effect of the silicone interlayer on mechanical properties of carbon fiber reinforced PMR-15 polyimide composites. Composite Interfaces, 5(2), 87-116, 1998. 

T.T. Serafini, R.D.Vannncci and W.B. Alston, Second Generation PMR Polyimides, NASA Technical Memorandum No.TMX-71894, NASA, WAShington, DC, USA, 1976. 

Recall that clay 1 is a commercial clay with an aliphatic quaternary ammonium ion while clay 2 contained an aromatic surfactant possessing the same norbomene end group as the PMR polyimide resin. In the latter case, the norbomene end group can potentially participate in crosslinking reactions with the PMR resin. Such reactive clay treatments have been reported to... 

Figure 2. Rate of growth of G and ti values for varying molecular weight PMR polyimides.

Bis-maleimide resins composed of BMI and diamines have been reported in the early 1960s in the patent literature. Since that time, a number of patents have appeared describing improvements in their properties and uses [3]. Although many bis-maleimide resins are commercially developed, relatively few reports of their use as adhesives are to be found in scientific journals [4-10]. Improvements of maleimide resins are mirrored in the improvements of thermosetting polyimides. For example, the method of in situ polymerization of monomer reactants (PMR method) was developed [6]. 

Polyimide by the PMR method should also belong to this category, however, before reaching the final stage of curing, elimination of the alcoholic component seems to be causing reduction of performance. 

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). 

More recently PMR resin modifications were developed because MDA used in PMR-15 is potentially hazardous. PMR-15 which is the most widely used addition polyimide in composites, is under pressure because of the new OSHA regulations. The target is to employ diamines which are not toxic or to prereact the diamines to avoid the presence of free diamines. 

In summary, improved thermal oxidative stability in PMR-type polyimides is achieved by tailoring a stable backbone structure through ... 

Strength retention of polyimide (PMR-15)-silicon carbide fiber composites at 315°C... 

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]. 

LARC TPI is a linear polyimide prepared with BTDA and 3,3 -diaminobenzo-phenone. The polyamide acid intermediate was used to prepare a semi-IPN with the PMR-15 reactants. After curing the blend exhibited a twofold Tg and an improved value of Glc (Table 8). 

On the other hand, water uptake is a continuous problem for polyimides and particularly for polynadimides [133]. Dynamic mechanical spectrometry (viscoelastic measurements) have been used to investigate the network degradations due to hydrolytic process [134]. It was shown on a special PMR resin (AFR 700 B) that network reformations are possible through post-curing. In addition, for fluorinated systems, 19F NMR can be used to follow the hydrolysis of the im-ide groups [135]. 

Scola DA (1985) Some chemical characteristics of the reverse Diels-Alder polyimide, PMR-15. In Gupta MR, Weber WD (eds) Polyimides synthesis, characterization and application (Proceedings of the Second International Conference on Polyimides). Society of Plastics Engineers, Inc, Hopewell Jet, p 247... 

Hay JN, Boyle JD, James PG, Walton JR, Wilson D (1989) Polymerization mechanism in PMR-15 polyimide. In Feger C, Khojasteh MM, McGrath JE (eds) Polyimides materials, chemistry and characterization. Elsevier, Amsterdam, p 305... 

Parker SS, Doyle ND, Walton JR (1991) Infrared studies of the cure of PMR-15. In Abadie MJM, Sillion B (eds) Polyimides and other high-temperature polymers (Proceedings of the 2nd European Technical Symposium on Polyimides and High Temperature Polymers, STEPI 2). Elsevier, Amsterdam, p 301... 

---