High-Temperature Dielectric Polyimide Films for Energy Storage Applications

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High-Temperature Dielectric Polyimide Films for Energy Storage Applications David H. Wang,a,b Brian A. Kurish,a,b Imre Treufeld,c Lianyun Yang,c Lei Zhu,c and Loon-Seng Tan a,* a Materials & Manufacturing Directorate, Soft-Matter Materials Branch (AFRL/RXAS), Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7750 b UES Inc., Dayton OH, 45432 c Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106 *Email: [email protected] ABSTRACT 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. INTRODUCTION Mechanically and thermally robust polymer dielectrics are needed to increase the operating temperature range up to 300C, and mitigate thermal management issues in compact pulsed power applications.[1] Arguably an important class of high temperature polymers with versatility in properties and processing, polyimides (PIs) would be one of the most promising candidates for such applications.[2,3] They have found utility in high performance films, coatings, microelectronics, optoelectronics, adhesives, aerospace structures, and liquid crystal displays. A special class of polyimides that contain nitrile units has been studied for the piezoelectric and other dielectric applications due to the high polarity of nitrile groups. [4,5,6] The pendant nitrile groups promoted the adhesion of the polymers to many substrates, possibly through polar interaction with other functional groups.[7] Polymers with high dielectric constants are required for further miniaturization of electronics.[8] Kakimoto et al reported that attaching of nitrile pendants into polyimides would enhance their dielectric constants.[9] Two polyimides prepared from an unsymmetrical diamine with nitrile groups showed poor solubility in aprotic solvents and only became soluble after the nitrile groups were hydrolyzed. Their dielectric properties were not reported.[10] Hybrid films based on nitrile-containing polyimides/inorganic particles (pyrite ash, barium and titanium oxides) was prepared and their nano-actuation was investigated.[11] Polyimides containing one and two nitrile groups per repeat unit were synthesized and their piezoelectric behavior has been analyzed. The polymer containing two nitrile groups per repeat unit showed higher value of remnant polarization than the ones containing only one per repeat unit due to its polarity increase. However, their values are low compare with commercial piezoelectric polymers such as PVDF.[12]

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