A set of 12 new polyimides (PIs) with one or three polar CN dipoles directly attached to the aromatic diamine part were synthesized and their electric energy storage properties were studied using broadband dielectric spectroscopy (BDS) and electric displacement-electric field (D-E) loop measurements to determine their potential for high temperature film capacitors for aerospace applications. It was found that adding highly polar nitrile groups to the PI structure increased permittivity and thus electrical energy storage, especially at high temperatures, and 3 CN dipoles were better than 1 CN dipole. Below the glass transition temperature (T g ), a weak g transition was observed around À100 C and a broad b transition was observed between 100 and 150 C. It was the b (i.e., precursor dipolar motion before long-range segmental motion, or glass transition), rather than the g sub-T g transition that substantially increased the permittivity of PIs. From the BDS results on PIs having 3 nitrile groups, the enhancement in permittivity from permanent dipoles decreased with dianhydride in the order of pyromellitic dianhydride (PMDA) > 4,4 0 -oxydiphthalic dianhydride (OPDA) > 1,1,1,3,3,3hexafluoropropane dianhydride (6FDA) > 4,4 0 -benzophenonetetracarboxylic dianhydride (BTDA). Meanwhile, the increase in permittivity also decreased in the order of para-para, meta-para, and meta-meta linkage in the diamine, suggesting that the para-para linkage favored easier dipole rotation than the meta-meta linkage.From the D-E loop study, the PIs with a combination of PMDA dianhydride and a para-para linkage exhibited the highest discharged energy density and a reasonably low loss. † Electronic supplementary information (ESI) available: Syntheses of monomers and polymers, reection and transmission X-ray diffraction proles, details of calculation, and BDS results for sample 2a. See

Three new isomeric diamines containing three, oxylinked benzonitriles (3BCN), one of which is asymmetric (meta, para, or m, p), are synthesized in a 3-step sequence. Polycondensation of these diamines and four common dianhydrides (6FDA, OPDA, BTDA, and PMDA) in N,N-dimethylacetamide via poly(amic acid) precursors and thermal curing at temperatures up to 300 C lead to three series of tough, creasable polyimide (PI) films (tensile moduli 5 1.63 2 2.86 GPa). Among these PIs, two PMDA-based PIs possess relatively high crystallinity and two OPDA-based PIs, low crystallinity, whereas all 6FDA-and BTDA-based PIs, and m,m-3BCN-OPDA-PI are amorphous, readily soluble in common polar aprotic solvents. Thermally stable and having high T g (216 2 341 C), these PIs lose 5% weight around 493-503 C in air and 463-492 C in nitrogen. Dielectric properties have been evaluated by broadband dielec-tric spectroscopy (BDS) and electric displacement-electric-field (D-E) loop measurements. D-E loop results show an increase in high temperature permittivity (at 190 C/1 kHz) from 2.9 (for parent PI CP2 with no nitrile group) to as high as 4.9 for these PIs, while keeping their dielectric loss relatively low. Thus, an increase in dipole moment density by the presence of three neighboring CN per repeat unit can increase the overall permittivity, which could be further enhanced by sub-T g mobility of para-phenylene linkages (BDS results). Published 2014. † J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 422-436

Two new diamines containing three nitriles are synthesized via a 3-step route. They are polymerized with four commercial dianhydrides (i.e. 6FDA, OPDA, BTDA and PMDA) in N,Ndimethylacetamide (DMAc) to afford poly(amic acid)s, which are thermally cured at temperatures up to 300 C to form tough, creasable films. Most of these polyimides are soluble in common solvents. Their glass transition temperatures range from 216 to 341 C. The polyimides are stable up to 400 C. The dielectric constants of these OPDA-based polyimides increase from 2.9 (CP2) to 4.7 as measured by the D-E loops.

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