APPLIED PHYSICS LETTERS 103, 262904 (2013)
Poly(vinylidene fluoride-co-hexafluoropropylene)-graft-poly(dopamine methacrylamide) copolymers: A nonlinear dielectric material for high energy density storage Mojtaba Rahimabady,1,2,a) Li Qun Xu,3,a) Saeid Arabnejad,1,2 Kui Yao,1,b) Li Lu,2 Victor P. W. Shim,2 Koon Gee Neoh,3 and En-Tang Kang3
1 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 2 Department of Mechanical Engineering, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 119260 3 Department of Chemical and Biomolecular Engineering, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 119260
(Received 2 October 2013; accepted 8 December 2013; published online 27 December 2013) A nonlinear dielectric poly(vinylidene fluoride-co-hexafluoropropylene)-graft-poly(dopamine methacrylamide) [P(VDF-HFP)-g-PDMA] graft copolymer with ultra-high energy density of 33 J/cm3 was obtained by thermally initiated radical graft polymerization. It was observed that the dielectric constant of the graft copolymer films was 63% higher than that of P(VDF-HFP), with a large dielectric breakdown strength (>850 MV/m). Theoretical analyses and experimental measurements showed that the significant improvement in the electric polarization was attributed to the introduction of the highly polarizable hydroxyl groups in the PDMA side chains, and the large breakdown strength arose from the strong adhesion bonding of the catechol-containing graft copolymer to the metal C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4858397] electrode. V
High energy density capacitors constitute a key component in various electrical power applications, for examples, electric guns, pulse lasers, power grids, power filters, hybrid electric vehicles, and particle accelerators. High quality capacitors made from linear polymer dielectrics, such as polypropylene, are often used for reliable high power operations. Arising from their low energy density, the sizes of these polymer capacitors are relatively large and their applications are limited by their volume and weight. To reduce the size of these capacitors, dielectric materials with high energy density and fast charging and discharging cycles are highly desirable. Among various polymeric materials, poly(vinylidene fluoride) (PVDF)-based polymers and copolymers with high dielectric constant and breakdown strength are suitable for applications in power storage devices.1–6 The optimized poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) has exhibited enhanced energy storage capabilities, with a high breakdown field strength of up to 700 MV/m and high energy density of up to 25 J/cm3.6 Further improvements in dielectric constant, breakdown field, and reduced dielectric loss, although very challenging, are desired. Blending of polymers is one of possible methods to modify the structure and property of the base polymers.3,7 Very recent study on P(VDF-HFP)/PVDF blends has showed that intermolecular interactions between polar groups in a polymer blend may contribute to improving the breakdown strength and energy density.8 However, preparation of homogenous films of polymer blends are often limited by polymer miscibility. a)
M. Rahimabady and L. Q. Xu contributed equally to this work. Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: 65-68745160.
b)
0003-6951/2013/103(26)/262904/4/$30.00
Modification of polymers by graft copolymerization is another potentially effective approach to tailor their structure and property.9,10 Through the introduction of new functional groups to fluoropolymer surfaces, changes in many properties, such as surface energy, morphology, conductivity, dielectric properties, and discharge speed can be attained.11–16 In an early work, PVDF was grafted with polystyrene (PS) via electron beam-induced free radical graft copolymerization. The dielectric constant of the graft copolymer reached 90, due to the increase in free charge carriers in the amorphous phase and/or charge accumulation at the amorphous/crystalline interface.17 In another similar study, poly(2-hydroxyethylmethacrylate)graft-PVDF copolymers with a high dielectric constant of >47 were synthesized via the free-radical graft copolymerization technique.18 However, the dielectric strength has to be maintained and even further improved for achieving competitive high energy density storage. For high energy density storage applications, graft copolymers containing highly polarizable groups in the nonlinear dielectric polymer side chains can effectively achieve large electrical polarization and dielectric constant. Furthermore, some relevant polar groups may also improve the affinity of polymer for metal to improve the interfacial interaction of the polymer film with the metal electrode, which will have a direct effect on improving the electrical breakdown strength. However, increasing the amount of the polar groups and bulky graft segments may increase the leakage current and decrease the crystallinity of the graft copolymer. Therefore, producing a graft copolymer containing an appropriate amount of selected polar groups to achieve a balance in high polarization, low dielectric loss and low charge leakage is an important strategy for achieving high energy storage density. Poly(dopamine methacrylamide) (PDMA) contains catechol (dual polar hydroxyl groups) moieties in its structure
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