•Increasing the chain-packing density through a highly scalable polymer blend strategy•The blends with dense chain packing exhibit >65% enhancement of breakdown strength•Polymer blends exhibit record-high breakdown strength over a broad temperature range Polymers are ubiquitous in our society because of their unique properties, such as light weight, pliability, corrosion resistance, ease of processing, and low cost. Eliminating sub-micro voids and free volumes in polymers is the major challenge for a variety of fields. For example, these weak points in high-temperature polymers can severely reduce their electric breakdown field (Eb), which is vital for electronic and electrical applications. Here, we present a general and scalable strategy for reducing weak points by exploiting interchain electrostatic forces in polymer blends. This previously unexplored strategy is efficient and straightforward in minimizing weak points and enhancing Eb in high-temperature polymers over a wide temperature. In addition, the results also pave a way to reduce weak points in many other applications, such as gas barrier polymers, used for packaging and enhancing thermal conductivity in polymers for electronics packaging and thermal interface. Polymers with high dielectric breakdown strength (Eb) over a broad temperature range are vital for many applications. The presence of weak points, such as voids and free volume, severely limit the Eb of many high-temperature polymers. Here, we present a general strategy to reduce these weak points by exploiting interchain electrostatic forces in polymer blends. We show that the strong interchain electrostatic interaction between two high-temperature polymers in blends of polyimide (PI) with poly(ether imide) (PEI) yields an extended polymer chain conformation, resulting in dense chain packing and a corresponding decrease in weak spots in the polymers. This leads to a greater than 65% enhancement of Eb at room temperature and 35% enhancement at 200°C. In conjunction with results from blends of PI/poly(1,4-phenylene ether-sulfone) (PSU) and blends of PEI/PSU, we show that this previously unexplored molecular engineering strategy is efficient and straightforward in minimizing weak points in dielectric polymers. Polymers with high dielectric breakdown strength (Eb) over a broad temperature range are vital for many applications. The presence of weak points, such as voids and free volume, severely limit the Eb of many high-temperature polymers. Here, we present a general strategy to reduce these weak points by exploiting interchain electrostatic forces in polymer blends. We show that the strong interchain electrostatic interaction between two high-temperature polymers in blends of polyimide (PI) with poly(ether imide) (PEI) yields an extended polymer chain conformation, resulting in dense chain packing and a corresponding decrease in weak spots in the polymers. This leads to a greater than 65% enhancement of Eb at room temperature and 35% enhancement at 200°C. In conjunction with results from blends of PI/poly(1,4-phenylene ether-sulfone) (PSU) and blends of PEI/PSU, we show that this previously unexplored molecular engineering strategy is efficient and straightforward in minimizing weak points in dielectric polymers. Polymers are ubiquitous in our society because of their unique properties, such as light weight, flexibility, corrosion resistance, ease of processing, and low cost. 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The Institution of Engineering and Technology, 1992Crossref Google Scholar,15White R.P. Lipson J.E.G. Polymyer free volume and its connection to the glass transition.Macrom. 2016; 49: 3987-4007https://doi.org/10.1021/acs.macromol.6b00215Crossref Scopus (224) Google Scholar However, experimental values in many widely used high-temperature polymers, such as polyimide (PI) and poly(ether imide) (PEI) are much lower than the intrinsic values.8Dissado L.A. Fothergill J.C. Electrical Degradation and Breakdown in Polymers. The Institution of Engineering and Technology, 1992Crossref Google Scholar,14Duponttm kapton® summary of properties. (2017). https://www.cmc-klebeband.de/media/files/DEC-Kapton-summary-of-properties-2017.pdf.Google Scholar In the past decade, extensive efforts have been made to improve the breakdown strength of high-temperature dielectric polymer films, which are polymers possessing high glass transition temperature Tg, across a wide range of temperatures.4Li Q. Chen L. Gadinski M.R. Zhang S.H. Zhang G.Z. Li H.Y. Iagodkine E. Haque A. Chen L.Q. Jackson T.N. Wang Q. Flexible high-temperature dielectric materials from polymer nanocomposites.Nature. 2015; 523: 576-579https://doi.org/10.1038/nature14647Crossref PubMed Scopus (1059) Google Scholar,10Zhu L. Exploring strategies for high dielectric constant and low loss polymer dielectrics.J. Phys. Chem. Lett. 2014; 5: 3677-3687https://doi.org/10.1021/jz501831qCrossref PubMed Scopus (380) Google Scholar, 11Zhang T. Chen X. Thakur Y. Lu B. Zhang Q. Runt J. Zhang Q.M. A highly scalable dielectric metamaterial with superior capacitor performance over a broad temperature.Sci. Adv. 2020; 6: eaax6622https://doi.org/10.1126/sciadv.aax6622Crossref PubMed Scopus (93) Google Scholar, 12Xu W. Liu J. Chen T. Jiang X. Qian X. Zhang Y. Jiang Z. Zhang Y. 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Electrical Degradation and Breakdown in Polymers. The Institution of Engineering and Technology, 1992Crossref Google Scholar,15White R.P. Lipson J.E.G. Polymyer free volume and its connection to the glass transition.Macrom. 2016; 49: 3987-4007https://doi.org/10.1021/acs.macromol.6b00215Crossref Scopus (224) Google Scholar,16Yong W.F. Shung T.-S. Mechanically strong and flexible hydrolyzed polymers of intrinsic microporosity (PIM-1) membranes.J. Poly. Sci. B: Polym. Phys. 2017; 55: 344-354https://doi.org/10.1002/polb.24279Crossref Scopus (24) Google Scholar The free volume, the part of internal volume unoccupied by polymer chains, consists of many small voids between polymer chains.15White R.P. Lipson J.E.G. Polymyer free volume and its connection to the glass transition.Macrom. 2016; 49: 3987-4007https://doi.org/10.1021/acs.macromol.6b00215Crossref Scopus (224) Google Scholar, 16Yong W.F. Shung T.-S. 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Mater. 2013; 3: 1051-1055https://doi.org/10.1002/aenm.201201110Crossref Scopus (52) Google Scholar From microstructure studies, one major reason is the strong hydrogen bonding between polymer chains in these specially designed polymers that results in compact chain packing, reducing weak points and leading to high Eb. On the other hand, most commercial high-temperature dielectric polymers important for broad market applications do not possess strong hydrogen bonding groups (see Table S1). Interchain electrostatic interactions exist in all polymers. Phenyl groups are key building blocks of many high-temperature polymers (Table S1). Strong interactions between phenyl groups of different polymer chains may be utilized to achieve high chain-packing density. Depending on the polymer's molecular structure, delocalized electrons in the phenyl groups may exhibit a partial positive or partial negative charge. Hence, blending appropriately matched high-temperature polymers may result in strong interchain electrostatic interaction between chains of different polymers, leading to blends with a tightly packed chain morphology and a reduction in weak points, resulting in a higher density when compared with pristine polymers. Moreover, the blending of different polymers is a general and highly scalable strategy that can be used to tune the conformation and packing behavior of polymer chains. It is widely used to improve electronic performance,19Gumyusenge A. Tran D.T. Luo X. Pitch G.M. Zhao Y. Jenkins K.A. Dunn T.J. Ayzner A.L. Savoie B.M. Mei J. Semiconducting polymer blends that exhibit stable charge transport at high temperatures.Science. 2018; 362: 1131-1134https://doi.org/10.1126/science.aau0759Crossref PubMed Scopus (101) Google Scholar thermal conductivity,20Xu J. Wang S. Wang G.J.N. Zhu C. Luo S. Jin L. Gu X. Chen S. Feig V.R. To J.W.F. et al.Highly stretchable polymer semiconductor films through the nanoconfinement effect.Science. 2017; 64: 59-64https://doi.org/10.1126/science.aah4496Crossref Scopus (641) Google Scholar, 21Xu Y. Wang X. Zhou J. Song B. Jiang Z. Lee E.M.Y. Huberman S. Gleason K.K. Chen G. Molecular engineered conjugated polymer with high thermal conductivity.Sci. Adv. 2018; 4: 1-7https://doi.org/10.1126/sciadv.aar3031Crossref Scopus (116) Google Scholar, 22Shanker A. Li C. Kim G.H. Gidley D. Pipe K.P. Kim J. High thermal conductivity in electrostatically engineered amorphous polymers.Sci. Adv. 2017; 3: e1700342https://doi.org/10.1126/sciadv.1700342Crossref PubMed Scopus (67) Google Scholar dielectric properties,23Kim G.H. Lee D. Shanker A. Shao L. Kwon M.S. Gidley D. Kim J. Pipe K.P. High thermal conductivity in amorphous polymer blends by engineered interchain interactions.Nat. Mater. 2015; 14: 295-300https://doi.org/10.1038/nmat4141Crossref PubMed Scopus (344) Google Scholar processability, and mechanical properties of polymers. Here, we show that such a blending strategy, enabled by strong interchain electrostatic interactions, can be effectively applied to several widely used high-temperature dielectric polymer films, including PI and PEI (and poly(1,4-phenylene ether-sulfone) [PSU]), to reduce the weak points and consequently improve their breakdown strengths over a broad temperature range, e.g., from room temperature to 200°C. PI is an essential high Tg (>310°C) dielectric polymer (see Figure 1A for the chemical structure) that is widely used for a variety of applications, such as electrical insulation, circuit boards, and interlayer dielectrics in the microelectronics industry.24Dang Z.M. Zhou T. Yao S.H. Yuan J.K. Zha J.W. Song H.T. Li J.Y. Chen Q. Yang W.T. Bai J. Advanced calcium copper titanate/polyimide functional hybrid films with high dielectric permittivity.Adv. Mater. 2009; 21: 2077-2082https://doi.org/10.1002/adma.200803427Crossref Scopus (360) Google Scholar, 25Liaw D.J. Wang K.L. Huang Y.C. Lee K.R. Lai J.Y. Ha C.S. Progress in polymer science advanced polyimide materials: syntheses, physical properties and applications.Prog. Polym. Sci. 2012; 37: 907-974https://doi.org/10.1016/j.progpolymsci.2012.02.005Crossref Scopus (1396) Google Scholar, 26Min Y.J. Kang K. Kim D. Development of polyimide films reinforced with boron nitride and boron nitride nanosheets for transparent flexible device applications.Nano Res. 2018; 11: 2366-2378https://doi.org/10.1007/s12274-017-1856-0Crossref Scopus (31) Google Scholar However, the breakdown strength reported for commercial PI is <500 MV/m at room temperature and decreases with temperature,14Duponttm kapton® summary of properties. (2017). https://www.cmc-klebeband.de/media/files/DEC-Kapton-summary-of-properties-2017.pdf.Google Scholar which significantly limits its performance and application space. PEI is another essential high-temperature (Tg ∼ 217°C) dielectric polymer (see Figure 1B for the chemical structure).3Tan D. Zhang L. Chen Q. Irwin P. High-temperature capacitor polymer films.J. Elect. Mater. 2014; 43: 4569-4575https://doi.org/10.1007/s11664-014-3440-7Crossref Scopus (131) Google Scholar,13Li H. Ai D. Ren L. Yao B. Han Z. Shen Z. Wang J. Chen L.Q. Wang Q. Scalable polymer nanocomposites with record high-temperature capacitive performance enabled by rationally designed nanostructured inorganic fillers.Adv. Mater. 2019; 31: 1-7https://doi.org/10.1002/adma.201900875Crossref Scopus (134) Google Scholar As illustrated in Figures 1A and 1B, PI has two strongly positively charged phenyls, while PEI has three relatively strongly negatively charged phenyls. Blending these two polymers leads to more extended chain-packing morphology and a near 10% increase in chain-packing density, resulting in decreased voids and free volume. Consequently, the PI/PEI blend exhibits a significantly enhanced breakdown strength over a wide temperature range. That is, the PI/PEI 50/50 blend at room temperature exhibits Eb = 1,000 MV/m, compared with the pristine PI and PEI of Eb = 600 MV/m. At 200°C, the blend can maintain a record high of Eb = 550 MV/m. We fabricated a series of PI-PEI dielectric films with different ratios of PI/PEI (wt %/wt /%) and systematically studied the nanostructure and chain-packing morphology of the blends. The PI/PEI blends' miscibility was investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM), which are standard analytical tools widely used for characterizing blend films. As shown in Figure S1A, the PI/PEI blends at all compositions show a single glass transition temperature (Tg), and the Tg values for the blends are in good agreement with the theoretical values predicted by the Flory-Fox equation (Figure 1C),27Fox T.G. Influence of diluent and of copolymer composition on the glass temperature of a polymer system.Bull. Am. Phys. Soc. 1956; 1: 123-132Google Scholar indicating that the blends are miscible.28Lin A.A. Kwei T.K. Reiser A. On the physical meaning of the Kwei equation for the glass transition temperature of polymer blends.Macromolecules. 1989; 22: 4112-4119https://doi.org/10.1021/ma00200a052Crossref Scopus (128) Google Scholar The tapping mode AFM data, as presented in Figures 1D, 1E, and S2, show no sign of phase separation of the two polymers in the blends.19Gumyusenge A. Tran D.T. Luo X. Pitch G.M. Zhao Y. Jenkins K.A. Dunn T.J. Ayzner A.L. Savoie B.M. Mei J. Semiconducting polymer blends that exhibit stable charge transport at high temperatures.Science. 2018; 362: 1131-1134https://doi.org/10.1126/science.aau0759Crossref PubMed Scopus (101) Google Scholar,23Kim G.H. Lee D. Shanker A. Shao L. Kwon M.S. Gidley D. Kim J. Pipe K.P. High thermal conductivity in amorphous polymer blends by engineered interchain interactions.Nat. Mater. 2015; 14: 295-300https://doi.org/10.1038/nmat4141Crossref PubMed Scopus (344) Google Scholar The conformations of the polymer chains and the packing behavior in the blends were evaluated experimentally. X-ray diffraction (XRD) data reveal that the 50%–50% PI/PEI blend exhibits a broad amorphous scattering peak at the 2θ angle of 17.8° (see Figure S3). This corresponds to the smallest interchain spacing as shown in Figure 1F,29Halasa A.F. Wathen G.D. Hsu W.L. Matrana B.A. Massie J.M. Relationship between interchain spacing of amorphous polymers and blend miscibility as determined by wide-angle X-ray scattering.J. Appl. Polym. Sci. 1991; 43: 183-190https://doi.org/10.1002/app.1991.070430115Crossref Scopus (62) Google Scholar and a reduction in the interchain spacing of about 10% compared with the averaged interchain spacing of the pristine PI and PEI. The density of 50%–50% blend films was also measured, which is 1.35 g/cm3, higher than 1.3 g/cm3 for PI films and 1.27 g/cm3 for PEI films used in this study. Figure 1G reveals that the change in specific heat capacity (ΔCp) during the glass transition is the lowest as the content of PEI approaches 0.5. In general, a change of polymer chain conformation from a highly self-entangled state to an extended backbone state is the primary contributor to the ΔCp during a polymer's glass transition, as observed in polystyrene and poly(N-acryloyl piperidine)/poly(acrylic acid) blends.23Kim G.H. Lee D. Shanker A. Shao L. Kwon M.S. Gidley D. Kim J. Pipe K.P. High thermal conductivity in amorphous polymer blends by engineered interchain interactions.Nat. Mater. 2015; 14: 295-300https://doi.org/10.1038/nmat4141Crossref PubMed Scopus (344) Google Scholar,30Rong W. Fan Z. Yu Y. Bu H. Wang M. Influence of entanglements on glass transition of atactic polystyrene.J. Polym. Sci. Part B Polym. Phys. 2005; 43: 2243-2251https://doi.org/10.1002/polb.20513Crossref Scopus (30) Google Scholar Therefore, the minimum ΔCp of the blend with PI/PEI 50%–50% ratio (Figure 1G) implies that both PI and PEI's backbones are more extended for this blend compared with pristine polymers and other blends before the glass transition at 248°C. Our results show that the blend film of 50%–50% PI/PEI ratio has the most extended polymer chain conformation among all blend compositions of PI and PEI studied, which leads to the lowest ΔCp and the smallest interchain average space as observed in XRD. Clearly, the deformed extended chain-packing morphology improves the packing density. Increased thermal conductivity and elastic modulus of the blend, compared with PI and PEI, are also observed (see Figures 1H and 1I), consistent with more extended chain-packing morphology and the increase in density.20Xu J. Wang S. Wang G.J.N. Zhu C. Luo S. Jin L. Gu X. Chen S. Feig V.R. To J.W.F. et al.Highly stretchable polymer semiconductor films through the nanoconfinement effect.Science. 2017; 64: 59-64https://doi.org/10.1126/science.aah4496Crossref Scopus (641) Google Scholar, 21Xu Y. Wang X. Zhou J. Song B. Jiang Z. Lee E.M.Y. Huberman S. Gleason K.K. Chen G. Molecular engineered conjugated polymer with high thermal conductivity.Sci. Adv. 2018; 4: 1-7https://doi.org/10.1126/sciadv.aar3031Crossref Scopus (116) Google Scholar, 22Shanker A. Li C. Kim G.H. Gidley D. Pipe K.P. Kim J. High thermal conductivity in electrostatically engineered amorphous polymers.Sci. Adv. 2017; 3: e1700342https://doi.org/10.1126/sciadv.1700342Crossref PubMed Scopus (67) Google Scholar We characterized the breakdown strength of the dielectric films (films on glass substrate) from room temperature to 200°C, just below the PEI films' glass transition, Tg = 217°C. The data were analyzed by the two-parameter Weibull formula (see Figure S4A).31IEC/IEEE Guide for the Statistical Analysis of Electrical Insulation Breakdown Data. (IEEE Std 930-2004), (2007). https://doi.org/10.1109/IEEESTD.2007.4288250.Google Scholar As shown by Figure 2A, the blends display higher Eb than those of pristine PI and PEI. Notably, the blend with the 50%–50% ratio exhibits an ultrahigh Eb of 1000 MV/m, with an increase of more than 65% compared with the pristine PEI and PI. In dielectric films, it is well known that Eb increases with reduced film thickness. The Eb = 1000 MV/m of the PI blend is the highest reported for high-temperature polymer films of 10 μm thick. This is presumably due to the much-reduced voids and free volume in the blend films, stemming from the compact packing of extended polymer chains, which reduces the probability of dielectric breakdown at high electric fields. The dielectric breakdown strengths of the 50%–50% blend as well as of pristine PI and pristine PEI at 200°C are presented in Figure 2B. The blend films exhibit an Eb of 550 MV/m, a more than 35% increase compared with Eb = 400 MV/m of the PI and PEI films. In dielectrics, it is well known that film fabrication conditions have great influence on Eb. Any damage introduced in the fabrication process of dielectric films reduces the measured Eb. The data presented in Figures 2A and 2B were measured for solution-cast polymer films, which are 5–10 μm thick and supported on a Pt-coated silicon substrate. The films were peeled off from the glass substrate to evaluate the effect of damage in the fabrication process on Eb. Subsequently, the Eb of free-standing films was measured. The room temperature results are presented in Figures S4B and S4C. The Eb of 50%–50% blend is 650 MV/m, a near 45% increase compared with PI films. At 200°C, as shown in Figure S4D, the Eb of the 50%–50% blend is 450 MV/m, a 50% increase compared with PI films. The results show that despite the damage induced by the film fabrication conditions that reduce Eb, the blend films maintain marked enhancement in Eb over a broad temperature range compared with the pristine polymers, due to dense chain packing that leads to improved intrinsic quality. The significantly enlarged Eb of the 50%–50% blend films will lead to a substantial increase in the energy density Ue. This was indeed observed experimentally, as presented in Figure 2C for the discharged energy density and Figure 2D for the charge/discharge (C/D) efficiency at room temperature for films on substrate. Figure S5A illustrates how the discharged energy density and C/D are deduced from the C/D curves. Figure S5B presents a comparison of the C/D curves of 50%–50% blend, PI, and PEI films under 600 MV/m at room temperature, which reveals a much lower loss of the blend, e.g., the area II in Figure S5A, than PI and PEI. The 50%–50% blend films exhibit a discharged energy density of 8 J/cm3, compared with less than 5 J/cm3 of the pristine PI and PEI films. Figure S6 presents discharged energy density Ue and C/D efficiency at 100°C to 200°C of free-standing films. Even at 200°C, the 50%–50% blend films deliver a discharged energy density Ue of 1.4 J/cm3 at the breakdown field of 450 MV/m. The leakage currents of the free-standing films of blends, PI, and PEI films at room temperature were characterized and are presented in Figure S7. The blend films, especially 50%–50% blend, exhibit much lower leakage currents, compared with PI and PEI. This is consistent with the higher C/D efficiency of 50%–50% blend films shown in Figures 2D and S6. The dielectric constants K and the dielectric losses of blends with different compositions were studied as a function of frequency at room temperature. The results are presented in Figure 2E, which show that the blend films exhibit slightly reduced dielectric constants (from K = 3.2 of PI and PEI to K = 3 of 50%–50% blend) and dielectric losses compared with PI and PEI. The 50%–50% blend also shows the same temperature dependence of dielectric constant and dielectric loss as PI and PEI from 10°C to 200°C (Figure 2F). These results can be understood by noting that although there is much room to enhance the dielectric breakdown strength by eliminating voids and free volume, the dielectric constant of dipolar polymers, such as PI and PEI, which consist of monomer units with large permanent dipole moments, cannot be much reduced even with a 10% increase in the chain-packing density. The Ue values of the blend are superior to those of PI and PEI due to a significantly higher dielectric breakdown strength. In addition, the C/D efficiency of the blend is higher than those of the pristine PI and PEI, especially when subjected to high electric fields and high temperatures. As shown in the literature, the conduction loss in dielectric polymers can become quite large under high electric fields, resulting in lowered C/D efficiency and reduced discharge energy density. Hence, it is essential to reduce the conduction loss for dielectric polymers exposed to high electric fields for use in practical applications of capacitors.4Li Q. Chen L. Gadinski M.R. Zhang S.H. Zhang G.Z. Li H.Y. Iagodkine E. Haque A. Chen L.Q. Jackson T.N. Wang Q. Flexible high-temperature dielectric materials from polymer nanocomposites.Nature. 2015; 523: 576-579https://doi.org/10.1038/nature14647Crossref PubMed Scopus (1059) Google Scholar,10Zhu L. Exploring strategies for high dielectric constant and low loss polymer dielectrics.J. Phys. Chem. Lett. 2014; 5: 3677-3687https://doi.org/10.1021/jz501831qCrossref PubMed Scopus (380) Google Scholar Furthermore, the conduction loss at high electric fields becomes more severe at increased temperatures owing to thermally assisted conduction mechanisms.11Zhang T. Chen X. Thakur Y. Lu B. Zhang Q. Runt J. Zhang Q.M. A highly scalable dielectric metamaterial with superior capacitor performance over a broad temperature.Sci. Adv. 2020; 6: eaax6622https://doi.org/10.1126/sciadv.aax6622Crossref PubMed Scopus (93) Google Scholar The PI/PEI blends show much lower conduction loss and a higher C/D efficiency than the pristine PI and PEI at high electric fields and high temperatures. In the experimental results presented, wt % (and density) is used for the blend compositions. As shown in Figure S8, wt % is nearly the same as mol % of blend compositions. We used molecular dynamics (MD) simulations to examine the molecular structure for the specifically designed PI/PEI blends. Models of coiled and deformed extended chains were simulated and assembled into blends with various starting geometries and annealing conditions to simulate polymer blends' formation and morphology. The results, presented in Figures 3A and 3B , show that in the majority of starting configurations and annealing conditions, the blends in both deformed extended and coiled chain morphologies have lower energies (negative mixing energies) than the corresponding states in the pristine polymers, which confirm the miscibility of the PI and PEI at the molecular level. Notably, the deformed extended chain-packing morphology results in a significant drop in the energy of mixing and a major increase in the blend's packing density, which confirm the experimental results of Figures 1F and 1G. Comparison of the lower panels of Figures 3A and 3B for the blend chain-packing morphologies reveals that the higher packing density of the blends with deformed parallel chains reduces the microvoids and free volume present in amorphous polymers and should thus lead to significantly increased dielectric breakdown strength and thermal stability, which further confirms the experimental results. Figure S9 provides a schematic representation of our simulation strategy of the blend assembly process. Figure 3C presents such blending strategy in engineering chain packing, which leads to a marked increase in Eb. As presented in Figure S10, PSU, a high-temperature dielectric polymer with Tg of 210°C, has the same negative charge on phenyl groups in the polymer chain as PEI. Hence, PI/PSU blends should also exhibit similar Eb enhancement as observed in PI/PEI blends. Free-standing films of PI/PSU blends were fabricated and studied. The results are presented in Figures 4A and 4B for the Eb and dielectric constant versus blend composition at room temperature, respectively. In analogy with the PI/PEI blends, the 50%–50% PI/PSU blend also exhibits an enhancement of Eb from room temperature to 200°C (see Figure 4C). Specifically, the PI/PSU 50%–50% blend films exhibit a 45% enhancement in Eb at room temperature and 30% enhancement at 200°C. The change of the dielectric constant with blend composition is also similar to that in PI/PEI blends. In contrast, the blends of PEI/PSU, which do not have strong electrostatic interactions between the two polymer chains, exhibit a reduced breakdown strength Eb and an increased dielectric constant, as shown in Figure S12. Presumably, the lack of strong PEI-PSU interchain interactions leads to an increased interchain spacing in the blend, as shown in Figure S13 of XRD data, and thus a reduced Eb and increased dielectric constant, analogous to that observed earlier in blends of PEEU and an aromatic polythiourea.18Burlingame Q. Wu S. Lin M. Zhang Q.M. Conduction mechanisms and structure-property relationships in high energy density aromatic polythiourea dielectric films.Adv. Energ. Mater. 2013; 3: 1051-1055https://doi.org/10.1002/aenm.201201110Crossref Scopus (52) Google Scholar In conclusion, we present a general and scalable strategy for reducing weak points, such as voids and free volume, and enhancing the breakdown strength in high-temperature dielectric polymers over a wide temperature range, which are enabled by strong interchain electrostatic interactions. The polymer chains in the PI/PEI blend are more extended and aligned through this simple blending strategy due to favorable electrostatic interactions between the two polymer chains compared with those in pristine PI and PEI. The increase in chain-packing density decreases voids and free volume, which leads to enhancement of the dielectric breakdown strength of the blend from room temperature to 200°C. In addition to dielectric polymers, finding a way to reduce free volume is also important in a variety of polymer fields, e.g., in gas barrier polymers used for packaging.32Lagaron J.M. Catalá R. Gavara R. Structural characteristics defining high barrier properties in polymeric materials.Mater. Sci. Tech. 2005; 20: 1-7https://doi.org/10.1179/026708304225010442Crossref Scopus (205) Google Scholar,33Ulbricht M. Advanced functional polymer membranes.Polymer. 2006; 47: 2217-2262https://doi.org/10.1016/j.polymer.2006.01.084Crossref Scopus (1550) Google Scholar These results pave the way for molecular engineering of high-temperature polymer dielectrics to reduce weak points and achieve high breakdown strength over a broad temperature range.