Precyclized polymers

Most polyphenylquinoxalines show no crystallinity in wide-angle X-ray diffraction studies. Their amorphous nature is primarily due to the configurational disorder carried out in the backbone of the polymer chains by the presence of different isomers. Glass transition temperature values decrease from 360 °C for polymer 15 to 320 °C for 16 and 290 °C for 17. However, the Tg significantly increases when the polymers are heated at 

If thermal imidization of polyamic acids remains the predominant technique to make polyimide films, the main drawback of this method is the production of water molecules during the processing stage. This, 

Chow et al. investigated the isoimide to imide conversion process with commercial Larc-TPI samples. As illustrated in Fig. 9.11, the reaction of 3,3, 4, 4 -benzophenonetetracarboxylic acid dianhydride 18 and 3,3 -carbonylbisbenzeneamine 19 leads to polyamic acid 20. Dehydrocyclization to poly-isoimide 21 is then performed in solution by adding 

21 via the intermediate polyamic acid 20. Isoimide-imide conversion predominantly occurs by heating polymer 21 at 300 °C, followed by a final excursion at 350—400 °C. 

Larc TPI-ODA 23 is a copolyimide containing repeating units 22 (95 mol%) and 23 (5 mol%) with a Tg of 250 °C and a processability better than that of 

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In electronics, polyimides are now extensively used in the form of self-standing films for flexible circuitry, deposited films for interlayer dielectrics, passivation and buffer coatings, moulding thermoplastic powders for PCBs, and adhesive pastes or tapes. The basic polyimide chemistry has been adapted to fulfill the specific requirements of these applications. A series of books provides complete information not only on the chemistry of polyimides but also on their utilization in electronics [4,23,24]. The following figures summarize the chemical formulae of the most important categories of polyimide precursors or precyclized polymers that are commonly used in electronics. 

Another method used to improve the fracture energy of BMI resins consists in mixing the thermosetting material with linear thermoplastic polymers. This can be illusirated by the behavior of mixtures containing Compimide 796 and TM 123 BMI resins with SABIC Ultem 1000 , poly(ether-imide) 26 (Fig. 9.13). The critical stress intensity factor ATic of the Unear polymer is six times higher than that of the BMI matrix and does follow the mixture law for all BMI/Ultem combinations. The linear polyimide can also be added as 20—40 p.m spherical particles to the BMI resin before it is polymerized. In another example, particles of a soluble precyclized polyimide (Ciba Geigy XU 218 ) are dispersed in BMI monomer, which is subsequently melted at 177 °C and cured at 232 °C. The two-phase material obtained by this means exhibits a glass transition temperature of 348 °C compared with only 260 °C for the BMI resin without polyimide additive. 

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