Cured acetylene-terminated imide

Table IV. Cured Acetylene-Terminated Imide (HR-600) Properties1 2...

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... 

Murphy et al. [73] studied the cure and degradation of an acetylene-terminated N-labelled poly(imide) using N CPMAS NMR. Initially, the conversion of the amic acid to the imide precursor was followed. Four resolved peaks are observed due to amide and imide either attached to a phenyl ring or at the terminal position. Measurements of the rate of crosspolarization, and the dipolar dephasing experiment, assisted in the assignment to the spectra. Very different rates of cross-polarization (1/Tnh), and values of Ti, were measured for the various structures. Imidization was incomplete after heating to 670 K for 1 h, a result at variance with the results of... 

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... 

In a later study. Swanson and coworkers [81] studied the cure of acetylene-terminated poly(imide)s selectively labelled at various positions with nuclei. Curing of the sample, labelled at the imide carbonyl group, confirmed the completion of the imidization reaction on heating. The product of addition onto the carboxyl group was not observed. Four new peaks were identified in the spectrum of the cured sample labelled at the Ci-acetylene group, while a similar result was obtained for the sample labelled at the C2-acetylene position. Analysis of these results rules out the participation of coupling reactions and the biradical mechanism, which would produce triple-bond structures, but confirms the presence of the product of cyclotrimerization and Friedel-Crafts reactions. The latter mechanism is confirmed from the presence of small peaks due to aliphatic carbons in the spectra of the materials labelled at the acetylene groups. 

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. 

Polyimides have been cured by synthesizing acetylene-terminated oligomers containing finished imide groups, and these have shown excellent heat resistance, as discussed above (Sec. 3.1.6.3.2). 

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