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Synthesis of Polyimides
Published in Malay K. Ghosh, K. L. Mittal, Polyimides Fundamentals and Applications, 2018
Among commonly available dianhydrides pyromellitic dianhydride (PMDA), 1,2,4,5-benzenetetracarboxylic dianhydride, is the most reactive. It possesses four carbonyl groups attached to the same benzene ring in a coplanar conformation and, therefore, shows the strongest tendency to accept an electron. The electrophilicity of carbonyl carbons of each dianhydride can be measured in terms of the electron affinity, a tendency to accept an electron, of the molecule. As shown in Table 1, earlier investigators [10] quantified electron affinity (Ea) for various dianhydrides by polarographic measurement and demonstrated that the rate of acylation reaction of 4,4′-diaminodiphenyl ether and a model compound 4-aminodiphenyl ether was closely correlated with the Ea value.
Preparation of polyimide alignment films with high photosensitivity and low solid content
Published in Liquid Crystals, 2021
Hailin Yu, Gang Zhu, Yinghan Wang
In our previous research [27], a photosensitive polyimide (PSPI) was successfully prepared by using 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (AHHFP) and 1,2,4,5-pyromellitic dianhydride as monomer and cinnamoyl group as photosensitive group via a two-step method. The obtained PSPI exhibits LC alignment after 60 min NPUVL irradiation. However, the strong electron absorption effect of -C(CF3)2 bridged group reduces the reactivity of AHHFP seriously and 10 wt % solid content is necessary to prepare PSPI with a viscosity of 20–30 mPa·s. The viscosity in this range is conducive to the control of optimum film thickness and good film-forming effect in industrial application. Under the premise of ensuring the alignment property, to reduce solid content can effectively decrease the amount of PSPI and cut down the cost of production. On the other hand, in the two-step method, acetic anhydride as dehydrating agent will cause partial loss of hydroxyl group, which will seriously affect the subsequent grafting rate of photosensitive group, reduce the photosensitivity and lengthen the irradiation time.
Deflection and Burst Properties of Polyimide Windows for High Pressures
Published in Fusion Science and Technology, 2018
T. Bernat, C. Castro, J. Hund, A. Pastrnak, N. Petta, J. Sin, O. Stein
Polyimide films were made by spin-casting precursor solutions onto clean 4-in.-diameter (10.16 cm) silicon wafers. After a soft-bake to remove solvents they were thermally imidized. Two PI formulations were prepared, which we identify as α and β. The α films are PIs of the Pyromellitic dianhydride/4,4ꞌ-oxydianiline (PMDA/ODA) type, similar to Kapton HN. The β films are PIs of the biphenyldianhydride/1,4 phenylenediamine (BPDA/PPD) type. The films had thickness variations less than 5%. They were prestretched flat onto support frames to remove any sagging or wrinkling and were mounted to one face of a washer using Epotek 302-3M epoxy adhesive. The adhesive was cured for more than 12 h at room temperature, after which the films were cut free of the support frames. The prestretching results in an initial stress built into the assembled window. The internal diameters of the washers determined the window diameters. Three washer materials were used in the experiments. The experimenters define the materials they require, but there is no indication that within the parameters we tested, the washer material makes a difference. The window assembly parameters are shown in Table II.
Mechanical properties of helical milling deep hole on QFRP using cryogenic cooling process
Published in Machining Science and Technology, 2020
As the experimental material, QRPC-245 type with the quartz content of 8% was chosen and used as the workpiece for each machining experiment. Meanwhile, the adhesive polyimide was fabricated by pyromellitic dianhydride and oxydimethyl aniline. The fiber width was 0.5 mm, the thickness was 0.12 mm and length was 5 mm with −5°＜λ＜5° bending angle of section fiber. Then it was cut into (100 mm ×50 mm × 10 mm and 100 mm × 50 mm × 40 mm) cubes perpendicular to the fiber direction by a saw machine. At last the rectangular and processed holes were numbered, respectively.