Acetylene-terminated resins structure

Recent advances in acetylene terminated resin technology have provided a family of new high performance structural materials (U2). One member of this family, the acetylene terminated sulfone ( TS) 3) has been made available in sufficient quantities to allow research efforts on characterization and determination of the effects of structure on physical and mechanical properties ( - ). In a companion paper (t) the effects of environment on the cure of ATS was reported. This paper reports... 

NMR spectroscopy and in particular solid-state NMR spectroscopy proved to be a powerful method for studying the mechanism and extent of reaction in complex poly(imide) materials. In particular, during the cure of BMI resins, careful use of C CPMAS NMR indicated that measurements of the extent of cure by DSC were significantly overestimated [86, 87]. This article demonstrates that NMR spectroscopy has been able to characterize the structure of condensation poly(imide)s and, more successfully, the cure of BMI, PMR and acetylene-terminated resins. 

HR-602 - This system represents a different resin chemistry through use of acetylene-terminated polyimide structure. The cure mechanism consists of an addition reaction with no release of volatiles during cure. The intermediate acetylene-terminated polyimide structure is shown below. Reaction to the final cured structure is proposed as a trimerization of the acetylene groups to form an aromatic structure. 

A substantial effort in our laboratory has been directed toward the synthesis and characterization of acetylene-terminated (AT) matrix resins. The most significant feature and driving force for the effort is that the thermal induced addition reaction provides a moisture Insensitive cured product. This technology offers a wide variety of thermoset resins for various high temperature applications. Backbone structural design for use temperature capabilities, processing characteristics and mechanical performance has demonstrated the versatility of the AT type systems. 

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. 

Films of the polyisoimides were cast from DMAC at 55 °C under reduced pressure (0.1 mm). A study of the isomerization reaction was conducted by FTIR and showed that the isomerization began at approximately 100 °C and was complete after 3 h at 250 °C. In all cases the thermally treated films were insoluble in all solvents tested. Composite films were produced with XVII and three commercial matrix systems a polyarylsulfone (Radel), a polysulfone (Udel), and an acetylene terminated isoimide thermosetting resin (IP-600). Films of the matrix and XVII were cast from DMAC. Slightly cloudy films, indicating some phase separation, resulted with both the Radel and Udel systems. Composite films cast with IP-600, however, were completely clear and showed no signs of phase separation. The structural similarity of the IP-600 resin and XVII may account for the greater homogeneity of the system. Property assessment of these films before and after thermal treatment is currently underway. 

Acetylene terminated (AT) resins are being studied as a new technology to form the matrix of high performance structural composites (1). This class of resin cures by an addition reaction mechanism. Because of the absence of volatile by-product during cure, these resins are easy to process to yield void-free components. These resins also have the advantage over epoxy resins in terms of resistance to deterioration of properties at elevated temperature due to exposure to humid environments (2). 

A family of acetylene-terminated phenyl quinoxalines have been synthesized by the Polymer Branch of the Materials Laboratory. ( 1) These phenyl quinoxalines are remarkable for their thermooxidative stability and resistance to moisture. These materials have potential for structural applications as adhesives or composite matrix resins.(2) The feature of moisture resistance makes the materials especially attractive for bonding aluminum. However, problems arise from the fact that aircraft aluminum alloys (and their surface oxiges) are altered by exposures to temperatures above 177 C (350 F) and this is much lower than the polymerization temperatures of the acetylene-terminated oligomers. 

The polyimide matrix resin employed in all composites was THERMID 600, a thermosetting resin polymerized from acetylene-terminated oligomers. Its chemical structure is shown below (9). 

Figure 3.11 Acetylene-terminated polyimide resin structure.

The ethynyl terminated imide oligomer work was initially reported in 1974. Neat resin and composite " properties for cured acetylene terminated imide oligomers have been reported. These materials were initially designated HR-600 and later Thermid -600 (60). The ethynyl terminated imide oligomer in structure 8 is representative of this class... 

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