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Engineering high-k dielectric materials based on copolyimides for energy-storage devices
Project status: Ongoing
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Stage I (year 2022) of High-k-coPI project had as main objective the synthesis of semiaromatic copolyimides containing different polar groups with the aim to be used as film capacitors in energy storage devices. To achieve this objective, two aromatic diamines with polar nitrile groups were synthesized with the role of increasing the dielectric constant of the corresponding copolymers, since higher values are necessary for the intended electronic applications. The structure of these monomers was confirmed by using FTIR and 1H-NMR spectral techniques.

By using as monomers an aromatic dianhydride with polar carbonyl groups and a mixture of two diamines (an aromatic one with polar nitrile groups and meta-phenoxy segments and an aliphatic diamine with polar polyethylene (PE) and polypropylene oxide (PEO) groups selected from a series of 3 diamines) a series of 3 semiaromatic copolyimides with polar groups in a mass ratio of 70:30 between the aromatic and the aliphatic diamine was subsequently prepared. In order to analyze the influence of nitrile groups on the properties of the obtained copolymers, 3 benchmark copolymers were also synthesized by using the same aromatic dianhydride with polar carbonyl groups and a mixture consisting of an aromatic diamine without nitrile groups and the 3 Jeffamines by using the same mass ratio of 70:30. All copolymers were structurally characterized by FTIR and 1H-NMR spectral techniques that fully confirmed the obtaining of the initially designed copolymers.

The optimization of the copolyimide synthesis process was achieved through two parallel activities: changing the molecular mass of the aliphatic jeffamine diamine (by using 3 diamines with molecular masses of 600, 900 and 2000 Da) and, respectively, the use of different mass ratios between the aromatic diamine and the aliphatic diamine. Thus, 2 more series of copolyimides were synthesized, obtaining at the end of the stage a total of 12 semiaromatic copolyimides with various polar groups with/without nitrile units which will be analyzed from the physico-chemical point of view in the next stage of this project.

Part of the results obtained during the first stage of High-k-coPI project were disseminated in form of 4 presentations at international scientific events that acknowledge the financial support offered by this project.


STAGE II (year 2023) of High-k-coPI project aimed to synthesize semi-aromatic copolyimides with para-phenoxy units and nitrile groups along polyethylene (PE) and polypropylene oxide (PEO) groups, for use as film capacitor in energy storage devices. To fulfill this objective, 9 copolyimides with p-phenoxy and various polar groups were synthesized, with the aim to increase the mechanical resistance and the value of the dielectric constant. To evaluate the influence of the aliphatic segment on the properties of the copolymers, 3 series of copolymers were synthesized with different molar ratios between the aromatic diamine with p-phenoxy and CN units, and 3 aliphatic diamines with different molecular weights of 600, 900 and 2000 Da. The structure of the obtained copolyimides was confirmed by using FTIR and 1H-NMR spectral techniques. These compounds were analyzed from the point of view of solubility, molecular weight and the ability to form free-standing films. The optimization process towards free-standing, flexible film formation represented a milestone, due to its importance in the manufacture of suitable films for the targeted application. The copolyimide films were subsequently evaluated regarding the surface morphology, optical, crystallinity, mechanical and thermal resistance, as well as the dielectric behavior. The obtained results indicated that the copolyimide films had a homogeneous surface, without defects, were amorphous and had  high mechanical and thermal resistance, in close connection with the aliphatic segment introduced in the corresponding copolymer. The incorporation of aliphatic diamines in the macromolecular chain led to the possibility of modulating the values of the dielectric constant and dielectric losses. 

Overall, an obvious correlation was observed between the structure and the properties of the corresponding copolyimide, the defining element in their tailoring being represented by the nature and quantity of the aliphatic diamine. 

Part of the results obtained during the second stage of High-k-coPI project were disseminated in 3 scientific articles in journals with IF>3, 1 book chapter and 5 presentations at international symposia that acknowledged the financial support provided by this project.


STAGE III (year 2024) of High-k-coPI project had as objectives the study of breakdown strength and the evaluation of energy storage density, structure-property correlations and the assessment of the applicative potential of copolyimide films for energy storage. The results showed that the copolyimides from the second series (based on p-phenoxy units) obtained higher values for breakdown strength than those from the first series (containing m-phenoxy linkages). Consequently, the values calculated for the storage energy were higher compared to that of the copolyimides with m-phenoxy segments from the first series. An increase in the storage capacity of these materials was observed in the case of copolyimides that incorporated a higher amount of the aromatic component and shorter Jeffamine segments (e.g. a mass ratio aromatic diamine: aliphatic = 70: 30 and ED-600), due to an increased rigidity of the macromolecular chain. Based on the results obtained in this project, the correlations between the structure and properties of the copolyimides were established. In each case, the influence of the structural design on the characteristics of the copolymers was observed, the defining element being the position of the phenoxy unit linked to the benzonitrile nitrile group in the hard component, as well as the amount and molecular weight of the aliphatic diamine in the soft component of the copolymers.

The comparison of the performances obtained by these materials with those reported in the literature allowed the evaluation of the efficiency of the copolyimides and the establishment of the applicative potential. Thus, the best performing copolymers in terms of storage energies were coPI4 (Ue= 1.36 J/cm3) and coPI7 (Ue= 1.22 J/cm3) from the first series based on CN-m-phenoxy fragments, and coPI14 (Ue = 2.70 J/cm3) and coPI16 (Ue= 2.59 J/cm3) from the second series based on CN-p-phenoxy fragments. Moreover, since the selected copolymers have high thermal stability (Tg = 72–152°C, Tonset = 310–399°C), formed free-standing flexible films, without defects, with excellent or good mechanical properties, they can be successfully used as dielectric film capacitors for energy storage in flexible electronics. Still, for their use in flexible/stretchable electronics, additional studies are needed to overcome the trade-off between the excellent mechanical properties (obtained in the coPI4–coPI12 series) and the appealing electrical characteristics (dielectric breakdown and storage energy obtained in the case of the coPI13–coPI21 series) to obtain materials that exceed the performances of current polymers. New strategies have been proposed to create copolyimides with high dielectric constant and superior storage energies.

The dissemination of the results obtained within the third stage of the High-k-coPI project consisted in a scientific article under evaluation in a journal with FI>3, which brings acknowledgments to the financial support offered by this project.


Final resume of the project

High-k-coPI project aimed to deliver solutions to some challenges addressed by Horizon Europe - Cluster 5: Climate, Energy and Mobility by focusing its research activity on advanced materials engineering with target on film capacitors for energy storage devices. The project approach can influence the progress of dielectric materials with high permittivity, which are more suitable in high electric fields compared to inorganic materials. The idea of High-k-coPI was to find the optimal combination between the chemical structure of copolyimides and the attractive properties determined by it. In order to obtain competitive polymeric materials, the synergistic effect between the aromatic and aliphatic segments was exploited. The originality of the project consisted in the use of solution-processable copolyimide flexible films, thermally stable, with increased permittivity, breakdown strengths and high storage energies as viable and cheap alternatives to current electronic components. To achieve the objectives, the activities were divided into 3 stages, which included various sub-activities from the synthesis and detailed characterization of copolyimides, to their structural optimization with emphasis on their processability into free-standing flexible films. The properties of these films were then evaluated from the morphological, optical, mechanical, thermal, dielectric, breakdown strength and energy storage points of view.

The project results led to a multidisciplinary impact: in scientific area of high-performance materials (copolyimides were designed and obtained with various physico-chemical properties that can be modulated, which were published in ISI prestigious journals), in applied research domain (the obtained flexible films have excellent properties that have attractive characteristics for electronic applications) and in educational area (by disseminating the knowledge obtained in the project at scientific events, as well as by consolidating the research group in an attractive and challenging field).

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