Imide oligomer

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. 

About 1970, research was initiated under Air Force funding on acetylene-terminated imide oligomers (ATI) which could be thermally chain extended through the acetylenic end-groups (33, 34). This effort resulted in the development of HR-600 (Eq. 9) and subsequent commercialization by Gulf Oil Chemicals Company in the form of Ther-mid -600. Neat resin properties of HR-600 are presented in Table IV while preliminary composite properties are given in Table V. 

The saiq>les studied were a BCB terminated aromatic imide oligomer and a BCB terminated imide monomer blended with a compatible bismaleimide (BMI) resin. Neat properties studied on both resin systems included the thersial and rheological properties of the uncured specimens subsequently used to determine appropriate cure conditions. Thermal and mechanical properties of the cured materials are also discussed. 

Addition poly(imide) oligomers are used as matrix resins for high performance composites based on glass-, carbon- and aramide fibers. The world wide market for advanced composites and adhesives was about 70 million in 1990. This amounted to approximately 30-40 million in resin sales. Currently, epoxy resins constitute over 90% of the matrix resin materials in advanced composites. The remaining 10% are unsaturated polyester and vinylester for the low temperature applications and cyanate esters and addition poly(imides) for high temperatures. More recently thermoplastics have become important and materials such as polyimides and poly(arylene ether) are becoming more competitive with addition polyimides. 

Within this chapter, a review is given on the classical approaches to introduce polymerizable endgroups into the (imide) oligomer backbone. The... 

Following these patent disclosures, research organizations involved in air-craft/aerospace material developments recognized the potential of thermosetting imide oligomers as resins for high temperature composites and adhesives. 

A novel and very attractive approach to cure imide oligomers via benzocyclobutene endgroups was first published in 1985 (17). Benzocyclobutene endgroups are particularly attractive because they are neat hydrocarbons (provide low dielectrics), cure thermally around 200-250 °C with no volatile evolution and copolymerize with other functional groups, for instance, male-imide. 

Recently a convenient method for the synthesis of maleimide-terminated imide oligomers has been described (38). Aromatic diamine, biphenyl tetracar-boxylic dianhydride and maleic anhydride are reacted in DMAc/Xylene at 50 °C to form the amic acid oligomer which was subsequently cyclodehydrated by refluxing in the presence of pyridine as a catalyst. Water is removed azeo-tropically over a period of three hours. The maleimide terminated imide oligomer is isolated by precipitation in water or a non-solvent. The molar ratio of the monomers can be varied widely to tailor the molecular weight and... 

Fig. 12. Chemical structure of maleimide-terminated imide oligomer...

The idea of synthesizing imide oligomers which carry acetylenic terminations appeared attractive because homopolymerization through acetylenic endgroups occurs without any volatile evolution and provides materials with good properties. Landis et. al (8,9) published the synthesis of such acetylene terminated imide oligomers from benzophenone tetracarboxylic anhydride, aromatic diamine and 3-ethynylaniline via the classical route. As usual, the amide acid is formed as an intermediate which, after chemical cyclodehydration, provides the polymide. Since ethynyl-terminated polyimide is used as a matrix resin for fiber composites, processing is possible via the amide acid, which is soluble in acetone, or via the fully imidized prepolymer, which is soluble in NMP. The chemical structure of the fully imidized ethynyl-terminated polyimide is provided in Fig. 44. 

The key to acetylene terminated polyimides is the availability of the end-capper which carries the acetylene group. Hergenrother (130) published a series of ATI resins based on 4-ethynylphthalic anhydride as endcapping agent. This approach first requires the synthesis of an amine-terminated amide acid prepolymer, by reacting 1 mole of tetracarboxylic dianhydride with 2 moles of diamine, which subsequently is endcapped with 4-ethynylphthalic anhydride. The imide oligomer is finally obtained via chemical cyclodehydration. The properties of the ATI resin prepared via this route are not too different from those prepared from 3-ethynylaniline as an endcapper. When l,3-bis(3-aminophenox)benzene was used as diamine, the prepolymer is completely soluble in DMAc or NMP at room temperature, whereas 4,4 -methylene dianiline and 4,4 -oxydianiline based ATIs were only partially soluble. The chemical structure of ATIs based on 4-ethynylphthalic anhydride endcapper is shown in Fig. 45. 

The ethynyl terminated imide oligomers are very attractive because their cured polymers are thermally stable (131). However, improvements are required in processability. An interesting approach to this problem was the synthesis and use of ethynyl-terminated isoimide (132). If the cyclodehydration of the amide acid intermediate is performed chemically with dicyclohexylcarbodiimide, isoimide is formed in almost quantitative yield. (Fig. 46). It is claimed that the isoimide provides better flow and solubility compared with the corresponding imide. At elevated temperatures, during cure, the isoimide rearranges into the... 

The most widely used endcapper for ATI resins is 3-amino-phenylacetylene. This endcapper, however, polymerizes at relatively low temperature (200 °C) and therefore is responsible for the narrow processing window. Attempts to overcome this deficiency resulted in the use of 3-phenylethynyl aniline and 3-(3-(phenylethynyl)phenoxy)-aniline as endcappers for imide oligomers (134). The polymerization onset temperature, measured via DSC analysis, is dramatically shifted to higher temperatures, as shown in Table 12. For improved processing,... 

Of all the thermosetting imide oligomers discussed in this article, the totally aromatic acetylene terminated imides are the most promising because their... 

A novel cure chemistry employed for addition poly(imides) has recently been published. The successful preparation of 4-aminobenzocyclobutene allowed the synthesis of benzocyclobutene-terminated imide oligomers and bisfbenzocylobutenes) (17). The benzocyclobutene group is a latent diene which isomerizes to o-guinodimethane at temperatures of about 200 °C and may homo- and/or co-polymerize for example with bismaleimide (83). Details on the benzocyclobutene chemistry are described in chapter I of this book. 

St. Clair [36] synthesized and evaluated the properties of a semi-2-IPN comprising Thermid 600 (an acetylene terminated imide oligomer from National Starch and Chemical Company) and LaRC-TPI (a thermoplastic polyimide with a Tg of 257 °C). The composition having the ratio of 65 35 of thermosetting Thermid 600 to LaRC-TPI showed the best flexural strength at ambient temperature [36]. 

Starting materials and solvents were purchased from Aldrich Chemical Co. acetonitrile (ACN), N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) were obtained anhydrous in Sure/Seal bottles and used as received. The polyamic acid of PMDA-ODA (2545 Pyralin) was supplied by DuPont. The soluble polyimide XU-218, derived from 3,3, 4,4 -benzophenone tetracarboxylic dianhydride (BTDA) and diamino-1,1,3-trimethyl-3-phenylindan isomers (DAPI) was purchased from Ciba-Geigy Corp. The acetylene terminated imide oligomer powder (Thermid MC-600) derived from BTDA, aminophenylacetylene, and 1,3-bis (2-aminophenoxy) benzene (APB) was obtained from National Starch and Chemical Company. Kapton Type II (PMDA-ODA) films were obtained from DuPont Co., Apical polyimide films were obtained from Allied Corp., and Upilex Type-S and Type-R polyimide films derived from 3,3, 4,4 -biphenyl tetracarboxylic dianhydride (BPDA) plus p-phenylenediamine (PDA) and ODA, respectively were obtained from ICI Americas Inc. 

Similarly, mixtures of imide oligomers containing BCB groups and those containing maleimide groups were shown to polymerize at moderate temperatures of 200-250 °C to give thermally stable polymers [95]. 

Sefcik and coworkers [80] were the first to use solid-state NMR to study the curing of poly(imide) oligomers terminated with reactive groups. The authors observed a clearly-resolved peak at 84 ppm in the spectrum of the unreacted poly(imide) resin, Thermid 600, due to terminal acetylene groups. On curing at 450-640 K, the decrease in intensity of this peak allowed facile... 

Catalytic cyclopolymerization or polycyclotri-merization of imide oligomers containing nitrile groups with or without graphite fibers is described. 

Catalytic cyclopolymerization of imide oligomers containing nitrile groups is a convenient method for synthesizing TSTR cross-linked polyimides. The TSTR cross-linking density can be controlled by addition of excess nitrile. 

Thermoset molecular composites reported by Chuah etal. [1989] investigated poly(p-phenylene benzobisthiazole) reinforcement of bisbenzocy-clobutene (BCB) terminated imide oligomers. Samples were prepared via extrusion of a solution of the blend followed by coagulation. Samples were compression molded to advance the thermosetting constituent. Phase separation (domain size of 100-200 nm) occurred during coagulation... 

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