Entropy‐Driven Conformational Disorder Enables Outstanding High‐Temperature Energy Storage in Dielectric Polymers

ABSTRACT

The pursuit of high‐temperature polymer dielectrics is consistently hindered by the intrinsic tradeoff between ensuring robust electrical insulation and maintaining thermal stability. While aromatic polyimide (PI) has superior thermal resistance, its dense π–π stacking facilitates the formation of charge transfer complexes, causing significant leakage and capacitive failure at elevated temperatures. In this study, we developed an entropy‐driven conformational disorder strategy to maximize the conformational entropy of our designed ternary random copolymerized PI (R‐PI, ΔS _conf = 5.76 J/(mol·K)). The π‐conjugation decoupling and electron localization of the R‐PI were achieved by dynamic conformational flipping. Density functional theory and molecular dynamics calculations indicate that the structural randomized state generates a highly fluctuating electrostatic potential field. This field creates high‐density deep energy traps that effectively suppress the long‐range hopping transport of charge carriers. As a result, the optimal R‐PI‐0.5 delivers a discharged energy density of 6.12 J/cm³ (η = 91.1%) under an applied field of 650 MV/m at 200°C. This molecular‐level design paradigm leverages conformational entropy to exceed traditional dielectric limits, offering a robust pathway for next‐generation harsh‐environment energy storage.

An “entropy‐driven conformational disorder” strategy is proposed to engineer a stochastically entangled amorphous topology in polymer dielectrics. By maximizing conformational entropy, this design disrupts periodic π–π stacking, creating a rough potential landscape with exceptionally deep traps (depolarization peak at 241.3°C). This suppresses thermally activated conduction, achieving an exceptional energy density of 6.12 J/cm³ at 650 MV/m and 200°C.

1. Introduction

Polymer dielectric energy storage materials with high‐voltage and high‐temperature endurance are urgently needed in advanced electrical and electronic systems, such as advanced pulsed power systems, hybrid electric vehicles, and aerospace electronics [1, 2, 3, 4, 5]. Although polymer film dielectrics are recognized for their high breakdown strength (E _b), low dielectric loss, and self‐healing capability, their energy storage performance deteriorates sharply with increasing temperature [6, 7, 8]. For instance, the operating temperature of commercialized biaxially oriented polypropylene (BOPP) films is limited to 105°C, which falls short of meeting the demands of next‐generation wide‐bandgap semiconductor devices [9, 10]. To surmount this bottleneck, the development of high‐temperature polymer dielectrics capable of simultaneously delivering high energy density (U _d) and high discharge efficiency (η) in environments exceeding 150°C or even 200°C has emerged as a frontier challenge in the realm of materials science [11, 12, 13, 14].

Aromatic polymers such as polyimides (PIs) and polyetherimides (PEIs), owing to their exceptional thermal stability and mechanical properties, are regarded as one of the most promising candidate matrices for high‐temperature capacitors [15, 16, 17]. However, the energy storage performance of these aromatic polymers under high‐temperature and high‐field conditions remains unsatisfactory. A comprehensive study of this issue can be approached from two aspects: (i) within fully aromatic systems, polymer chains aggregate through π–π stacking interactions to form highly conjugated structures known as the interchain charge–transfer complexes (CTCs), which promote charge carrier migration between polymer chains; (ii) the presence of numerous aromatic rings in polymer backbones reduces bandgaps, leading to high electrical conductivity, especially at elevated temperatures [9, 18]. Such severe carrier transport not only results in substantial conduction loss but also precipitates thermal runaway, severely compromising the E _b and energy storage efficiency [14, 19]. Therefore, it is imperative to improve the dielectric energy storage performance of aromatic polymers to withstand the harsh conditions.

Early efforts to enhance high‐temperature polymer dielectric energy storage performance have focused on developing organic‐inorganic nanocomposite systems. Wide‐bandgap inorganic nanofillers (e.g., Al₂O₃, HfO₂, ZrO₂, Bi₄Ti₃O₁₂, h‐BN, etc.), which function as charge scattering centers, are incorporated into polymers to extend the breakdown path, thereby impeding the propagation of electrical branches [20, 21, 22, 23, 24, 25, 26, 27]. However, the substantial dielectric mismatch between inorganic fillers and the polymer matrix induces severe interfacial electric field distortion. Furthermore, high filler loadings inevitably precipitate nanoparticle agglomeration effects and significantly deteriorate the mechanical flexibility and processability of the films, constraining their application in large‐scale wound capacitors [23, 28]. To address the issue of interfacial compatibility, all‐organic composite strategies have emerged. Recent efforts demonstrate that molecular semiconductors (e.g., PCBM, ITIC) or organic small‐molecule fillers acting as charge traps can be integrated into the polymer [5, 19, 29]. Although encouraging, it still falls short of industrial‐scale production.

In comparison to extrinsic doping, intrinsic molecular‐level design rooted in polymer chain architectures is deemed a more promising approach. Traditional bandgap theory postulates that widening the bandgap of polymers is pivotal for elevating E _b [30, 31, 32]. However, recent investigations have illuminated that under the extreme operating scenarios, the control of carrier transport often assumes greater criticality than that of generation, wherein carrier transport at elevated temperatures is predominantly governed by microscopic chain conformations and the spatial distribution of electron wave functions [29, 33, 34, 35, 36, 37]. Locking molecular twisted conformations or constructing local electrostatic potential barriers, which can effectively entrap energetic electrons and thereby suppress charge multiplication, has been established as an efficacious strategy for curbing high‐temperature leakage current, significantly elevating the high‐temperature capacitive performance of polymers [16, 38]. For instance, employing donor‐acceptor (D‐A) molecular engineering, dipolar glass states, and incorporating strong electron‐withdrawing moieties into side chains or main chain backbones (e.g., trifluoromethyl, sulfonyl, or cyano groups) [35, 39, 40, 41].

Drawing inspiration from the paradigms of high‐entropy alloys, topological complexity is introduced to induce electron localization [7, 29, 42]. Under this mechanism, whether stemming from high‐entropy chemical disorder or arising from the dynamic disorder of self‐healing networks [7], both can significantly amplify carrier scattering effects, inducing the spatial localization of electron wave functions. This suppresses electrical conductance and disrupts high‐temperature leakage pathways. For instance, utilizing non‐coplanar alicyclic structures to disrupt the planarity of molecular chains [4, 32, 43, 44], designing semi‐ladder dianhydride monomers [45], and constructing synergistic rigid‐flexible interpenetrating networks [46] have collectively demonstrated the immense potential of modulating electronic behaviors via topological structural design.

Herein, we propose an entropy‐driven conformational disorder strategy to decouple π‐conjugation and achieve electron localization, incorporating the concept of high‐entropy materials into the design of all‐organic polymer dielectrics. The influence of conformational entropy of polymer chains on polymer properties was systematically evaluated. The monomer dicyclohexyl alicyclic dianhydride (HBPDA) and the monomer 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were reacted with 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane (HFBAPP) to prepare a ternary random copolymerized polyimide (R‐PI‐n, where n represents the molar ratio of HBPDA to HFBAPP, n = 0.3, 0.5, 0.7). For comparison, two PI homopolymers, FPI and API were also prepared, respectively. This kinetic disorder mode, based on alicyclic conformational flipping, is thermally activated at elevated temperatures; the resulting substantial conformational entropy gain effectively disrupts the coherence of electron wave functions, decouples π‐conjugation, and establishes complex, randomly distributed traps and potential barriers to achieve electron localization. As illustrated in Figure 1a, the randomly distributed traps and potential barriers in R‐PI‐0.5 induce electron localization, effectively impeding both interchain and intrachain electron transport. From the perspective of electron transport physics, the high conformational entropy directly translates into a strongly disordered potential landscape. This observation is visually confirmed by the density functional theory (DFT)‐calculated electrostatic potential (ESP) surfaces shown in Figure 1b. Among the synthesized polymers, R‐PI‐0.5 exhibits the most complex and disordered charge distribution, characterized by intense alternating positive (red) and negative (blue) potentials. As depicted in Figure 1c, π‐conjugation in R‐PI‐0.5 is decoupled, and both interchain and intrachain charge transport are suppressed. This is attributed to the introduction of alicyclic HBPDA units, which disrupt the periodic arrangement of aromatic rings and sever the continuity of the π‐electron cloud, effectively decoupling the π‐conjugation. Furthermore, entropy‐driven conformational disorder engenders severe fluctuations in potential energy; consequently, electrons lack sufficient spatial coherence for long‐range tunneling or intra‐band transport, thereby inducing electron localization. Even under high‐temperature conditions with phonon assistance, owing to the energy mismatch between localized states, electrons cannot efficiently transport between these states via resonant tunneling or phonon‐assisted hopping. This effect directly corresponds to a reduction in hopping distance and an increase in E _b.

FIGURE 1.

Mechanism of entropy‐driven charge localization and transport regulation. (a) Schematic illustration of electron injection, excitation, trapping, and blocking processes in FPI, API, and R‐PI‐0.5. (b) ESP distribution maps of FPI, API, and R‐PI‐0.5, calculated using DFT. (c) Schematic representations of interchain and intrachain charge transport pathways in FPI, API, and R‐PI‐0.5.

Distinct from conventional inorganic doping or simple binary copolymerization, this ternary random system, benefiting from the dual mechanisms of entropy‐driven conformational disorder for π‐conjugation decoupling and electron localization. As a result, the R‐PI‐0.5 achieved a discharged energy density of 6.12 J/cm³ with an efficiency of 91.1% even at 200°C. This work not only presents a material exhibiting outstanding high‐temperature dielectric performance but also pioneers a universal strategy for designing high‐performance polymers via entropy‐driven disorder and electron localization, validating the immense application potential of conformational disorder design in dielectric materials for extreme environments.

2. Results and Discussion

2.1. Molecular Design of Conformational Disordered Copolymers

The chemical structures and synthetic routes for FPI, API, and R‐PI‐n are presented in Figure S1. When the HBPDA content reaches 50%, the calculated conformational entropy (ΔS _conf) attains a maximum value of 5.76 J/(mol·K), corresponding to the highest degree of chemical disorder. The ΔS _conf value not only reflects maximal disorder in chemical composition but also indicates that this stoichiometry enables the most intricate conformational twisting and the most stochastic distribution of traps (detailed calculation procedures are provided in the Supporting Information) [47]. To further investigate the influence of entropy‐driven conformational disorder on charge transport, we performed geometry optimizations on the molecular structures of FPI, API, and R‐PI‐0.5. These calculations were conducted using the B3LYP functional with the 6–31G(d) basis set, incorporating dispersion corrections for heavy atoms; the resulting optimized geometries are presented in Figure S2.

Figure 2a presents the HOMO and LUMO energy levels calculated via DFT. Notably, although the calculations reveal a narrower bandgap (3.30 eV) for R‐PI‐0.5, this does not necessarily imply compromised insulating properties. Conversely, this anomalous bandgap narrowing actually signifies the formation of deep‐level trap states. Conventionally, a narrower bandgap is perceived to facilitate electron transitions, thereby suggesting inferior insulating capability. However, current theoretical simulations regarding bandgaps predominantly employ simplified small‐molecule systems. Results derived from such simplified models often underestimate the extent of electron delocalization in polymers in the condensed state, thereby yielding underestimated bandgap values [48]. Experimentally, UV–vis spectroscopy revealed that the actual optical bandgap of R‐PI‐0.5 is 3.81 eV (Figure S3). As evidenced by the orbital distribution diagrams, the LUMO of FPI manifests as a continuous conduction band delocalized along the backbone, which is conducive to long‐range charge transport. In sharp contrast, the LUMO of R‐PI‐0.5 is topologically constituted by a random assembly of wide‐bandgap HBPDA and 6FDA units, engendering the most complex and disordered fluctuations in electrostatic potential energy.

FIGURE 2.

(a) HOMO and LUMO energy levels calculated via DFT. (b) ESP distributions of FPI, API, and R‐PI‐0.5, highlighting the potential energy differences. (c) Color‐coded molecular simulations illustrating the variations in molecular planarity. (d) ELF maps depicting electron distribution probabilities. (e) PL spectra and (f) PLQY of the FPI, API, and R‐PI‐0.5 films. (g) Two‐dimensional (2D) WAXS patterns and (h) the corresponding one‐dimensional (1D) integrated WAXS profiles of the FPI, API, and R‐PI‐0.5 films.

Density of states (DOS) analysis (Figure S4) reveals that both FPI and R‐PI‐0.5 exhibit similar discrete band tail states near the LUMO energy level, primarily originating from their shared electron‐withdrawing 6FDA structural unit. Although their calculated bandgaps are comparable, significant differences exist in their spatial topological structures. In the rigid FPI system, ordered intermolecular π–π stacking connects these states into coherent interchain transition pathways. In contrast, the pronounced conformational disorder in R‐PI‐0.5 disrupts the ordered stacking and decouples the intermolecular π‐conjugated network, resulting in spatially discrete and isolated electronic states. Possessing energy levels significantly below the mobility edge, these physically disconnected localized states function strictly as deep traps for capturing electrons, rather than serving as conduction channels [7]. This indicates that the suppression of electron transport in R‐PI‐0.5 is minimally dictated by the intrinsic band structure; instead, it is predominantly ascribed to the topological decoupling of π‐conjugation induced by conformational disorder and electron localization, which effectively mitigates electrical conduction [36]. Thermally stimulated depolarization current (TSDC) measurements (Figure S5) reveal a prominent peak for R‐PI‐0.5 at 241.3°C. This high‐temperature shift indicates increased thermal activation energy for charge detrapping, confirming the presence of deeper localized trap states. These conformation‐induced deep traps function as isolated barriers rather than transport sites, effectively immobilizing electrons and suppressing macroscopic leakage current under extreme electrothermal conditions.

To elucidate the microscopic physical origins of these experimentally observed deep traps, electrostatic potential calculations (Figure S6) substantiate the generation of substantial energetic roughness, constructing randomly distributed deep potential wells at the band edges. As displayed in Figure 2b, R‐PI‐0.5 exhibits the maximum electrostatic potential difference of 79.64 kcal/mol, implying that for electrons to transport within R‐PI‐0.5, they are compelled to continuously surmount formidable potential barriers or succumb to deep potential wells. This severe energetic disorder disrupts the coherence of electron wave functions, precluding long‐range transport via resonant tunneling and necessitating electron localization. This mechanism bestows the material with the most potent charge‐trapping capability. Collectively, these findings illuminate that the spatial distribution characteristics of traps, rather than mere quantitative metrics, constitute the dominant factor governing dielectric dynamics [49].

In amorphous polymer dielectrics, charge transport is predominantly governed by the hopping conduction mechanism, distinct from the band transport characteristic of crystalline semiconductors. Electrons undergo phonon‐assisted quantum tunneling between localized states. Within the framework of Miller‐Abrahams theory, the hopping rate V_ij from site i to site j is contingent upon two pivotal factors: the spatial separation (R _ij) and the energy difference (E _j— E _i) [50].

$${\nu }_{ij}={\nu }_0\exp (-2\gamma R_{ij})\exp \left(-\frac{E_j-E_i}{k_{B}T}\right),(E_j>E_i)$$ (1)

where γ denotes the wave function decay factor, which is intrinsically correlated with the localization length.

To suppress high‐temperature electrical conductance, it is imperative to simultaneously regulate both the spatial separation (R _ij) and the energy difference (E _j‐E _i). In this study, by incorporating bulky, non‐planar alicyclic moieties, we increased the interchain spacing and disrupted the spatial periodicity of the hopping sites (aromatic rings). This structural modification diminishes the overlap of wave functions and effectively increases and randomizes the hopping distance (R _ij) [51]. Simultaneously, the chemical disorder (stemming from the random chemical environment) and the conformational disorder (derived from chain distortion) induce severe fluctuations in the electrostatic potential between distinct hopping sites—referred to as energetic disorder. This phenomenon amplifies the energy mismatch (E _j‐E _i), thereby substantially suppressing charge transport.

Furthermore, polymers possessing an E _g greater than approximately 3.3 eV and those with an E _g below 3.0 eV exhibit distinct dependencies of leakage current density on the electric field under high‐temperature and high‐field conditions, signifying two divergent conduction mechanisms. In this context, the dominant conduction mechanism shifts from Poole‐Frenkel emission to hopping conduction. Consequently, the dihedral angle between adjacent conjugated planes plays a decisive role in determining capacitive performance at extreme temperatures [52]. Molecular planarity simulation serves as a pivotal metric for evaluating global molecular conjugation and visualizing the degree of spatial conformational distortion; specifically, the molecular planarity parameter (MPP) and the span of deviation from plane (SDP) critically influence carrier transport behaviors. As visibly discernible in Figure 2c, more extensive and intense red regions correspond to greater non‐planarity. Compared to FPI and API, the molecular backbone of R‐PI‐0.5 exhibits a substantially higher degree of distortion. Quantitative characterization of the polymer backbone planarity (Figure S7 and Table S1) reveals that the non‐planarity of R‐PI‐0.5 is markedly amplified (MPP = 1.821 Å, SDP = 7.400 Å), significantly exceeding that of FPI (MPP = 1.261 Å, SDP = 6.005 Å) and API (MPP = 1.538 Å, SDP = 6.078 Å). An elevated MPP value signifies reduced planarity, which corresponds to diminished electron delocalization along the molecular chain, increased hopping distances, and minimized π‐conjugation. These findings demonstrate that the conformational disorder in R‐PI‐0.5 maximizes molecular conformational distortion. The non‐coplanar dicyclohexyl structure of HBPDA disrupts the continuity of π‐conjugation, while the steric hindrance of the trifluoromethyl (‐CF₃) groups induces out‐of‐plane twisting. The resulting conformational disorder and decoupled π‐conjugation effectively impede polymer chain packing and the formation of CTCs, thereby suppressing both intra‐ and interchain charge transport. Consequently, this mechanism is conducive to enhancing high‐temperature energy storage performance and voltage endurance [4, 45, 48].

The electron localization function (ELF) serves as a potent visualization method in quantum chemistry for delineating the degree of electron localization at distinct sites within polymer molecules; herein, ELF was employed to further investigate the impact of conformational disorder on electron localization. As exhibited in Figure 2d, the R‐PI‐0.5 molecular chain displays multiple discontinuous regions characterized by high ELF values (depicted in red). This indicates that electrons within the R‐PI‐0.5 backbone are strongly confined to specific atomic domains and are incapable of delocalizing along the chain, thereby manifesting pronounced electron localization. These randomly distributed zones of strong localization constitute intrinsic natural barriers that impede carrier transport. Collectively, these results demonstrate that conformational disorder induces charge localization to form randomly distributed traps, visibly breaking continuous delocalization pathways at multiple junctures. This localization disrupts the conjugated pathways extending along the chain, further corroborating that the decoupling of π‐conjugation and electron localization—driven by conformational disorder—effectively suppresses both intrachain and interchain charge transport [43, 53, 54].

Building upon the DFT insights, steady‐state photoluminescence (PL) and wide‐angle X‐ray scattering (WAXS) measurements were conducted to empirically corroborate the induction of electron localization and the disruption of π‐conjugation by conformational disorder. As illustrated in Figure 2e, the fluorescence emission spectra provide critical insights: typically, interchain charge transfer (CT) results in fluorescence quenching (attenuated intensity), whereas intrachain CT promotes a bathochromic shift (red shift). Relative to FPI, the emission peaks of both R‐PI‐0.5 and API exhibit a distinct hypsochromic shift (blue shift) [6, 36, 55]. Specifically, R‐PI‐0.5 manifests an emission peak at 519 nm with the highest fluorescence intensity, while API and FPI exhibit peaks at 524 and 528 nm, respectively. The observation that R‐PI‐0.5 possesses the shortest emission wavelength (minimum peak position) combined with the maximum fluorescence intensity compels the conclusion that conformational disorder‐driven decoupling of π‐conjugation and electron localization effectively inhibits both intrachain and interchain charge transfer. The photoluminescence quantum yield (PLQY) data, presented in Figure 2f, reveal a diverging trend: the PLQY of API increased marginally from 1.34% (FPI) to 1.50%, whereas R‐PI‐0.5 achieved a substantial elevation to 1.91%. In the case of API, although the non‐coplanar architecture and torsion induced by HBPDA curtail the effective conjugation length—resulting in a minor hypsochromic shift—the residual π–π stacking facilitates intramolecular charge coupling, thereby compromising the PLQY enhancement. In sharp contrast, the spatial conformational disorder synergistically induced by 6FDA and HBPDA in R‐PI‐0.5 effectively obliterates interchain π–π stacking and suppresses the formation of CTCs. Consequently, R‐PI‐0.5 exhibits a more pronounced hypsochromic shift alongside a superior PLQY. These results substantiate that entropy‐driven conformational disorder successfully decouples π‐conjugation and induces electron localization, ultimately ameliorating electrical insulation performance [54, 56].

The 2D WAXS patterns presented in Figure 2g reveal that R‐PI‐0.5 exhibits a markedly attenuated diffraction intensity, characterized by the palest hue and a minimized area of high‐intensity regions (orange or red). This suggests that conformational disorder effectively suppresses electron density, thereby mitigating the risk of high‐field breakdown [57]. The corresponding 1D integrated WAXS profiles (Figure 2h) show that the broad scattering halo of FPI is located at q = 1.139 Å⁻¹, corresponding to an interchain d‐spacing of 5.52 Å; for API, the peak appears at 1.114 Å⁻¹ (d = 5.64 Å). In contrast, R‐PI‐0.5 displays a peak at 1.147 Å⁻¹, corresponding to the smallest interchain distance of 5.48 Å. Strikingly, despite possessing the smallest interchain distance, no distinct peak is discernible in the region of q = 1.6–2 Å⁻¹ for R‐PI‐0.5. This absence, further corroborated by the extended conventional X‐ray diffraction (XRD) patterns up to 2θ = 80^° (Figure S8), these findings provide direct evidence for the significant suppression and structural frustration of ordered π–π stacking within the highly disordered matrix. These results collectively suggest that the stochastic orientation of the backbone segments, driven by conformational entropy, effectively prevents the local aggregation of phenyl rings into ordered stacking networks [54]. This phenomenon is attributed to the incorporation of HBPDA containing sp³‐hybridized carbon atoms. The inherent “chair‐boat” conformational flipping capability of the alicyclic rings, combined with the spatial disorder of 6FDA, physically disrupts the parallel alignment of benzene rings and severs the pathways for interchain electron transfer. Crucially, while R‐PI‐0.5 achieves a denser packing state (reduced free volume, which hinders chain motion), this densification does not stem from enhanced π–π stacking; rather, it translates macroscopically into enhanced mechanical properties.

Collectively, these findings from DFT, PL, and WAXS substantiate that the specific incorporation of 50% HBPDA maximizes the disruption of ordered π–π stacking. This structural modification realizes robust electron localization and inhibits the formation of CTCs, thereby mitigating high‐field conduction losses and effectively enhancing high‐temperature dielectric energy storage performance.

2.2. Structural Corroboration and Thermal Stability

To corroborate the successful fabrication of the polymers, all films were analyzed using Fourier transform infrared (FT‐IR) spectroscopy, as presented in Figure 3a. Copolymers containing HBPDA units, characteristic aliphatic C─H stretching vibration peaks were observed in the range of 2800–3000 cm⁻¹ (specifically at 2859 and 2932 cm⁻¹). Furthermore, the emergence of the C─N stretching band (1370 cm⁻¹), along with the symmetrical (1711 cm⁻¹) and asymmetrical (1781 cm⁻¹) C═O stretching vibrations, confirmed the complete imidization reaction and the successful synthesis of the polyimide backbone.

FIGURE 3.

(a) FT‐IR spectra confirming the chemical structures. (b) TGA curves indicating the 5% weight loss temperatures (T _d,5%). (c) DSC thermograms highlighting the T _g. (d) Equilibrium molecular structural models obtained from MD simulations with a DP of 10. (e) Calculated occupied volume, free volume, density, and FFV for FPI, API, and R‐PI‐0.5. (f) 2D SAXS patterns. (g) Representative stress‐strain curves. (h) Variations in Young's modulus and tensile strength as a function of HBPDA content.

The thermal properties of the polymers were evaluated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As depicted in Figure 3b,c, the temperatures corresponding to 5% weight loss (T _d, 5%) for FPI, API, and R‐PI‐0.5 were 506°C, 388°C, and 456°C, respectively. While the fully aromatic FPI exhibited the highest thermal stability, the introduction of non‐conjugated alicyclic units resulted in a slight reduction in stability; nevertheless, R‐PI‐0.5 maintained a robust decomposition temperature exceeding 450°C. Despite the lower initial decomposition temperatures induced by flexible alicyclic units, R‐PI‐0.5 maintains a T _d,5% of 456°C, providing a safety margin exceeding 250°C above the 200°C operational threshold. Within this range, chemical degradation kinetics are negligible. This robust thermal stability ensures long‐term reliability, as evidenced by the exceptional high‐temperature fatigue endurance discussed subsequently. All polymer films exhibited T _g exceeding 240°C, demonstrating excellent thermal stability. Although R‐PI‐0.5 exhibits a lower T _g relative to FPI, the WAXS results confirm that the induced conformational disorder effectively disrupts π‐conjugation and inhibits organized π–π stacking, which serves as the dominant mechanism for leakage suppression. The T _g of R‐PI‐0.5 exhibits a localized minimum (244°C) corresponding to the maximum sequence randomness (ΔS _conf = 5.76 J/(mol·K)). Although this packing frustration marginally facilitates segmental relaxation, it fundamentally enhances high‐temperature energy storage. Specifically, the entropy‐driven conformational disorder effectively isolates localized electronic states and disrupts long‐range percolation pathways, establishing a robust structural barrier against high‐temperature conduction.

To further elucidate the impact of conformational disorder, molecular dynamics (MD) simulations were conducted to analyze the influence of incorporating 50% HBPDA into the polyimide matrix (Figure 3d). In the visualized molecular structure models, the blue regions represent free volume, while other colors indicate the polymer chains. A comparison of the calculated occupied volume, free volume, and fractional free volume (FFV), —defined as the ratio of free volume to the total volume, —is presented in Figure 3e and Table S2. Quantitatively, FPI possesses a free volume of 26246 ų with an FFV of 35%. API exhibits a free volume of 14745 ų and an FFV of 24%. Most notably, R‐PI‐0.5 demonstrates the lowest free volume (13166 ų) and the minimum FFV of 20%. Increasing the degree of polymerization (DP) from 10 to 22 (Figure S9 and Table S3), the simulated FFV trend remained highly consistent, with R‐PI‐0.5 maintaining a minimum FFV of 20.41%. Mechanistically, this intrinsic FFV minimization indicates that the copolymer achieves maximal conformational entropy. This entropy‐driven “dense‐yet‐disordered” packing enhances energetic disorder, creating deep traps that effectively immobilize charge carriers and suppress leakage current at 200°C.

Notably, R‐PI‐0.5 exhibits an apparent paradox: it possesses the lowest FFV yet a lower T _g (244°C) than rigid FPI (268°C). This phenomenon arises from the sp³‐hybridized HBPDA. While its flexible, non‐planar segments lower the intrachain rotational barrier (thereby reducing T _g), they concurrently enable highly contorted conformations. When stochastically arranged with planar aromatic rings, these contorted chains achieve significantly denser amorphous packing than rigid FPI, effectively minimizing interstitial voids. Crucially, the minimized FFV does not compromise the topological disorder essential for electron localization. Locked in during film solidification, this static conformational frustration preserves contorted chain geometries, effectively inhibiting ordered π–π stacking. Consequently, decoupled π‐conjugation networks and localized deep traps are robustly maintained despite the constrained free volume.

According to free volume theory, charge carriers can accelerate and accumulate kinetic energy within the free volume of the polymer under the influence of an electric field or thermal agitation. Dielectric breakdown is precipitated when the mean free path of the charge carriers extends to a critical length [58]. Consequently, the dielectric E _b of the polymers is intrinsically correlated with the magnitude of the free volume, a relationship mathematically articulated in the following equation:

$$E_b=\frac{E_{\mu }}{q{l}_E}$$ (2)

where q represents the elementary electron charge, and E _μ denotes the breakdown potential barrier of the polymer. Physically, the induced conformational disorder effectively curtails the free volume available for electron acceleration. This structural feature is highly conducive to impeding charge carrier migration, particularly under rigorous conditions of elevated temperatures and high electric fields. Consequently, this mechanism significantly bolsters the high‐field E _b and capacitive energy storage performance of the polymer.

Corroborated by MD simulations and WAXS, it is confirmed that conformational disorder facilitates intimate molecular chain entanglement and diminishes free volume. This structural optimization minimizes the genesis of defects within the polymer matrix and drastically curtails the mean free path of electrons, thereby effectively elevating the E _b and enhancing electrical breakdown resilience [59].

To further investigate the structural evolution of the materials at the mesoscale, small‐angle X‐ray scattering (SAXS) measurements were performed. As visualized in the 2D SAXS patterns (Figure 3f), R‐PI‐0.5 exhibits a reduced area of high‐intensity scattering (indicated by red or orange hues) compared to FPI and API. Furthermore, the corresponding 1D integrated profiles (Figure S10) reveal a featureless decay without any discernible scattering peaks or shoulders. This confirms the absence of microphase separation or periodic nanostructures within the random copolymer. It indicates that 6FDA and HBPDA achieve perfect miscibility at the nanoscale, establishing R‐PI‐0.5 as a homogeneous single‐phase system. Such homogeneity is critical as it prevents electric field distortion at phase interfaces and mitigates the risk of partial discharge under high electric fields [57]. These results attest that R‐PI‐0.5 exists as a uniform, conformationally disordered random copolymer, free from local aggregation. This highly uniform architecture eliminates nanoscale interfacial defects, a factor of paramount importance for preventing high‐voltage breakdown.

The E _b of polymers is also significantly governed by the electromechanical breakdown mechanism. Theoretically, E _b can be quantified using the following equation: $E_{\mathrm{b}}\approx 0.606\sqrt{\frac{Y}{{\epsilon }_0{\epsilon }_r}}$, where Y denotes Young's modulus. According to this relationship, E _b scales positively with Y; consequently, the mechanical modulus of the polymer film serves as a pivotal determinant of its E _b. Structural densification typically yields a substantial enhancement in the modulus of modified films. As evidenced by Figure 3g,h, R‐PI‐0.5 exhibits the highest tensile strength of 117.62 MPa, markedly exceeding that of FPI (70.15 MPa) and API (70.12 MPa). Furthermore, R‐PI‐0.5 possesses a superior Young's modulus of 3.67 GPa, significantly surpassing both FPI (1.63 GPa) and API (2.40 GPa). This mechanical reinforcement effectively mitigates the susceptibility to electromechanical breakdown [17]. Operating well below its T _g of 244°C, R‐PI‐0.5 retains its intrinsic glassy state at 200°C. Crucially, its conformationally disordered architecture forms a densely entangled network (FFV = 20%), in contrast to the rigid, sliding‐prone FPI (FFV = 35%). This pronounced topological entanglement restricts segmental mobility and disperses localized stress. Consequently, it mitigates high‐temperature mechanical softening while maintaining the robust modulus required to withstand intense electromechanical breakdown.

The interchain spacing characteristics derived from XRD results (Figure S8) are highly consistent with the WAXS analysis, where R‐PI‐0.5 demonstrates the smallest d‐spacing. Furthermore, the incorporation of bulky ‐CF₃ groups imposes significant steric hindrance and structural distortion. This geometric obstruction, coupled with the entropy‐driven conformational disorder, further diminishes the probability of organized molecular chain stacking [45]. This stochastic arrangement effectively diminishes the free volume, concomitantly resulting in a substantial elevation of the Young's modulus.

Collectively, these results demonstrate that R‐PI‐0.5 exhibits a unique dense yet disordered packing paradigm. The incorporation of non‐planar alicyclic units facilitates efficient space filling (yielding a minimum fractional free volume of 20%) and significantly elevates the Young's modulus; simultaneously, the stochastic nature of the conformation eradicates periodic π–π stacking signals. This synergistic interplay between structural densification and electronic disorder fortifies the resistance to electromechanical breakdown while effectively severing electron delocalization pathways. The thermal stability of this structural state is supported by in situ infrared and Raman spectroscopy (Figures S11 and S12). Anomalous spectral changes at elevated temperatures confirm a highly confined local environment with restricted segmental mobility, demonstrating that this stochastically entangled topology effectively preserves π‐conjugation decoupling and electronic localization well below the T _g. Furthermore, the role of chemical electronegativity in this disordered paradigm warrants specific consideration. Although ‐CF₃ groups provide essential electronegative sites for charge localization, their absolute density does not dictate insulating performance. Despite having a lower fluorinated monomer ratio (75%) than FPI (100%), R‐PI‐0.5 significantly suppresses conduction. While periodic π–π stacking in rigid FPI bridges these sites into continuous percolation pathways, the dense‐yet‐disordered packing of R‐PI‐0.5 spatially isolates them, disrupting long‐range hopping. Consequently, spatial connectivity governed by conformational entropy exerts a more decisive influence on high‐temperature dielectric strength than absolute electronegative group density.

2.3. Dielectric Properties and Breakdown Strength

The dielectric constant (ε _r) and dielectric loss tangent (tan δ) constitute fundamental parameters for evaluating dielectric performance, directly dictating the energy storage capability of the material. As illustrated in Figure 4a, the room‐temperature frequency spectra reveal that all polymer films exhibit remarkable dielectric stability across a broad frequency range (10^0–10⁶ Hz). Specifically, at 1 kHz, R‐PI‐0.5 possesses a ε _r of 3.11, which is intermediate between that of the fully aromatic FPI (2.82) and the alicyclic API (3.26). Figure 4b further delineates the dependence of dielectric properties on the HBPDA content at 1 kHz. The results further reveal that R‐PI‐0.5 exhibits the lowest tan δ (0.00229), markedly lower than that of FPI (0.00366) and API (0.00317). This minimization of energy dissipation is pivotal for significantly enhancing charge–discharge efficiency. Furthermore, the dielectric stability of the polymer films at elevated temperatures was evaluated over a temperature range from 30°C to 200°C at 1 kHz (Figure 4c). Notably, R ‐ PI ‐ 0.5 consistently maintains the lowest tan δ from 30°C to 200°C.

FIGURE 4.

(a) Frequency‐dependent ε _r and tan δ. (b) Variation of ε _r and tan δ as a function of HBPDA content at 1 kHz. (c) Temperature‐dependent ε _r and tan δ. (d) Weibull distribution of E _b at 150°C. (e) Weibull distribution of E _b at 200°C. (f) Temperature‐dependent Weibull E _b. (g–i) Leakage current density versus electric field curves measured at RT, 150°C, and 200°C, respectively.

The Weibull plots and corresponding E _b for FPI, API, and the random copolymer R‐PI across varying temperatures are presented in Figure 4d,e and Figure S13. Notably, β for all polymer films exceeds 13, indicative of high data reliability and excellent film uniformity. As clearly summarized in Figure 4f, R‐PI‐0.5 exhibits an E _b significantly superior to that of both FPI and API films at both ambient and elevated temperatures. Specifically, as the temperature rises from 25°C to 200°C, the E _b of the FPI film undergoes a precipitous decline from 596 to 432 MV/m. In stark contrast, R‐PI‐0.5 boasts an impressive initial E _b of 729 MV/m at 25°C. Although it experiences a slight reduction at 200°C, it retains a remarkably high value of approximately 634 MV/m. This corresponds to a substantial enhancement of 46.6% compared to FPI (432 MV/m) and 14.8% compared to API (552 MV/m) under identical thermal conditions.

It is well‐established that the E _b is governed by a complex interplay of factors, primarily involving three critical aspects. First, the Young's modulus plays a decisive role, typically exhibiting a positive correlation with the E _b of polymers. Under identical conditions, a higher Young's modulus corresponds to a greater E _b. Indeed, a strong correlation is observed between the E _b values presented in Figure 4f and the modulus data in Figure 3h, confirming that the mechanical reinforcement in R‐PI‐0.5 contributes significantly to its high‐voltage resilience. Second, the leakage current density significantly impacts E _b. Leakage currents resulting from electrode charge injection exert a detrimental effect, tending to reduce the breakdown threshold. As illustrated in Figure 4g–i, FPI exhibits a leakage current density more than an order of magnitude higher than that of the R‐PI‐0.5 film at both ambient and elevated temperatures, severely compromising its E _b. Particularly at high temperatures, the leakage current of FPI increases drastically with the applied electric field, leading to a significant deterioration in energy storage performance and rendering it unsuitable for high‐temperature film capacitor applications. Subsequently, to elucidate the underlying charge transport physics, the conduction mechanisms were examined. While various models—such as Ohmic conduction, Schottky emission, and hopping conduction—have been established to describe the relationship between leakage current density and electric field, our data fitting analysis reveals a high degree of consistency with the hopping conduction model. As is evident from Figure 4g–i, the coefficients of determination (R²) for all polymer films fall within the range of 0.992–0.999. This exceptional goodness of fit signifies a strong correlation between the experimental data and the hopping conduction model, validating its applicability. Furthermore, based on these fitting results, the hopping distances (λ) at RT, 150°C, and 200°C were derived.

λ is intrinsically inversely proportional to the depth of the charge carrier traps. Consequently, a larger λ value signifies shallower traps, while a smaller λ suggests the presence of deeper energetic traps. As discerned from Figure 4g, the R‐PI‐0.5 film exhibits the minimum λ value (0.50 nm) at RT, which is lower than that of both FPI (0.54 nm) and API (0.53 nm). This distinction becomes particularly pronounced under high‐temperature operating conditions, as shown in Figure 4i. Upon heating to 200°C, the λ for FPI escalates drastically from 0.54 to 1.32 nm, and for API from 0.53 to 1.20 nm. In stark contrast, the λ for R‐PI‐0.5 increases only moderately from 0.50 to 1.00 nm, providing compelling evidence for the abundance of deep‐level traps within the R‐PI‐0.5 matrix. Benefiting from the conformational disorder—which decouples π‐conjugation and induces robust electron localization—R‐PI‐0.5 effectively deepens the intrinsic trap energy levels. This mechanism significantly inhibits charge transfer, thereby mitigating conduction losses. While intense thermal agitation at 200°C typically reduces the critical onset field for nonlinear hopping conduction, the conformation‐induced deep traps in R‐PI‐0.5 serve as a robust thermodynamic buffer. Due to their high activation energy requirement, these localized states effectively confine energetic electrons and suppress thermally assisted hopping. Consequently, R‐PI‐0.5 mitigates thermal acceleration, significantly delaying nonlinear conduction and maintaining a high critical electric field under extreme electrothermal conditions. The consistent manifestation of the lowest leakage current density and the smallest hopping distance across the entire temperature range underscores the superior voltage endurance of R‐PI‐0.5, directly enhancing its high‐temperature E _b.

Furthermore, TSDC measurements were conducted to unequivocally verify the existence of deep traps (Figure S5). R‐PI‐0.5 exhibits a prominent depolarization peak at 241.3°C, indicating high detrapping activation energy and confirming the presence of significantly deeper localized states. These conformation‐induced deep traps function exclusively as isolated capture centers rather than hopping sites, thereby immobilizing energetic electrons. This characteristic fundamentally inhibits conduction, reducing leakage current by an order of magnitude and enhancing robust high‐temperature insulation performance. Consistent with this microscopic trap mechanism, the macroscopic temperature‐dependent volume resistivity (Figure S14) further demonstrates the exceptional thermal runaway resistance of R‐PI‐0.5. While the resistivity of rigid FPI decreases sharply near 200°C, R‐PI‐0.5 maintains values several orders of magnitude higher. This robust resistivity retention confirms that conformational disorder effectively restricts thermally activated long‐range charge migration, thereby suppressing thermal runaway under extreme electrothermal conditions. Collectively, these findings are in excellent agreement with the theoretical predictions from DFT, as well as the structural insights derived from PL and WAXS analyses.

2.4. High‐Temperature Capacitive Energy Storage Performance

The capacitive energy storage performance of the specimens was systematically evaluated at both ambient and elevated temperatures via unipolar polarization‐electric field (P‐E) hysteresis loop measurements. The representative P–E loops of the polymer films measured at 100 Hz are depicted in Figures S15–S17, serving as a direct reflection of their dielectric energy storage behavior. Most notably, as vividly illustrated in Figure 5a–c and Figure S18, R‐PI‐0.5 exhibits significantly augmented energy storage performance under high‐temperature conditions.

FIGURE 5.

(a) U _d and η as a function of the applied electric field at 200°C. (b) Variation of discharge energy density (maintaining η > 90%) with temperature. (c) P–E hysteresis loops at 200°C for FPI, API, and R‐PI‐0.5 (under electric fields where η > 90%). (d) Radar chart comparing the key dielectric, mechanical, and thermal characteristics of FPI, API, and R‐PI‐0.5. (e) Comparison of discharge energy density (at η > 90%) for R‐PI‐0.5 versus recently reported state‐of‐the‐art dielectric polymers at 200°C [5, 14, 43, 44, 46, 53, 59, 60, 61, 62, 63]. (f) Homogeneity evaluation: P–E loops, U _d, and η measured from random distinct regions of the R‐PI‐0.5 film at 200°C and 500 MV/m. (g) Fatigue endurance test of R‐PI‐0.5 measured at 200°C under 300 MV/m over extended cycles. (h) Steady‐state thermal simulations of wound film capacitors based on FPI, API, and R‐PI‐0.5 films.

At 150°C, the R‐PI‐0.5 film delivers a discharge energy density of 6.92 J/cm³ with a high efficiency of 91.5% under an electric field of 690 MV/m. This represents a 55.1% improvement over FPI (4.46 J/cm³, η = 87.3%) and a 27.0% increase compared to API (5.45 J/cm³, η = 88.0%). This performance superiority becomes even more pronounced at 200°C. Under 650 MV/m, R‐PI‐0.5 yields an U _d of 6.12 J/cm³ (η = 91.1%). Notably, this corresponds to a staggering 180.7% enhancement relative to FPI (2.18 J/cm³, η = 92.7%) and a 101.3% improvement over API (3.04 J/cm³, η = 90.2%). As visually summarized in the radar chart (Figure 5d), R‐PI‐0.5 exhibits comprehensive superiority over both FPI and API across key performance metrics, including ultra‐high discharge energy density (U _d, η > 90%), E _b, and Young's modulus. Furthermore, by benchmarking against state‐of‐the‐art polymer dielectrics reported recently (Figure 5e), R‐PI‐0.5 demonstrates exceptional high‐temperature energy storage capabilities that outperform the majority of existing counterparts at 200°C while maintaining an efficiency exceeding 90% [5, 14, 43, 44, 46, 53, 59, 60, 61, 62, 63].

Moreover, the film exhibits exceptional flexibility and film‐forming capability. Random sampling from nine distinct regions of a large‐area film (Figure 5f) further corroborates this uniformity. Even under harsh conditions of 200°C and 500 MV/m, the discharge energy density remains stable at approximately 3.60 J/cm³. This outstanding performance stability and film homogeneity underscore the potential for scalable industrial production. The reliability of the material was further assessed through fatigue endurance tests at 200°C and 300 MV/m. As shown in Figure 5g, R‐PI‐0.5 demonstrates superior cyclic stability, sustaining its performance without degradation over 10⁵ charge‐discharge cycles, thereby ensuring long‐term operational reliability at elevated temperatures.

To validate the feasibility of these materials in practical applications, the steady‐state internal temperature distribution of a wound film capacitor was simulated using COMSOL Multiphysics. As illustrated in Figure 5h, under harsh operating conditions of 200°C and 300 MV/m, the R‐PI‐0.5‐based capacitor demonstrates superior thermal stability compared to FPI and API. Specifically, the maximum core temperature (T _max) of the FPI capacitor surges to 335°C, a critical level that would likely precipitate thermal runaway and device failure. In sharp contrast, the internal temperature of the R‐PI‐0.5 capacitor remains significantly lower, reaching only 207°C—a mere 7°C rise above the ambient temperature. This exceptional thermal management is attributed to the minimized Joule heating resulting from the suppressed energy loss in the R‐PI‐0.5 dielectric. Consequently, these results suggest that R‐PI‐0.5 films hold great promise for deployment in high‐temperature power electronics, potentially eliminating the need for bulky active cooling systems.

3. Conclusion

In summary, we have developed a ternary R‐PI that effectively bridges the gap between high‐temperature thermal stability and robust electrical insulation. By incorporating nonplanar alicyclic units into a fluorinated aromatic backbone, we successfully implemented an entropy‐driven conformational disorder strategy to engineer the polymer's topological morphology. DFT and molecular dynamics calculations indicate that the increased conformational entropy will induce profound energetic disorder and electrostatic potential fluctuation. This specific conformation physically disrupts the long‐range π–π stacking of aromatic rings, thereby suppressing the formation of charge transfer complexes. As a result, the designed random copolymer, R‐PI‐0.5, delivered an ultrahigh discharge energy density of 6.12 J/cm³ with a charge‐discharge efficiency of 91.1% under extreme conditions of 200°C. This performance significantly surpasses that of most existing high‐temperature polymers, demonstrating the potency of molecular‐level disorder in controlling macroscopic electronic properties. This study establishes a universal design paradigm that utilizes entropy‐induced disorder to simultaneously enhance electron localization and structural densification, offering a reliable approach for polymer‐based dielectrics operating in extreme environments.

4. Experimental Section

4.1. Key Experimental and Computational Methods

4.1.1. Materials and Film Fabrication

The random copolyimide (R‐PI‐0.5) and control samples (FPI, API) were synthesized via a standard two‐step polycondensation and gradient thermal imidization method. The precursor polyamic acid (PAA) solutions were cast onto glass substrates and subjected to a controlled gradient heating process (up to 260°C) to obtain free‐standing dielectric films with a thickness of approximately 11 µm.

4.1.2. Dielectric and Energy Storage Characterization

Temperature‐dependent dielectric properties (permittivity and loss) were measured using a broadband dielectric spectrometer. The macroscopic P–E loops and leakage current densities were evaluated using a ferroelectric testing system combined with a high‐voltage amplifier in a silicone oil bath heating stage.

4.1.3. Computational Methods

Density functional theory (DFT) calculations were performed to determine the HOMO/LUMO energy levels and DOS. MD simulations were conducted with a degree of polymerization of 22 to calculate the FFV.

Detailed synthetic routes, comprehensive characterization techniques, and specific computational parameters are provided in the Supporting Information.

4.2. Statistical Analysis

4.2.1. Data Preprocessing and Presentation

Raw data obtained directly from instrumental outputs were used for all physical analyses without excluding any data points as outliers. For theoretical simulations, such as fractional free volume (FFV), values were extracted directly from equilibrium trajectory frames. Spectroscopic, thermal, and current‐voltage characterizations (e.g., TSDC, XRD, P–E loops) are presented as acquired from representative high‐quality specimens to demonstrate intrinsic material properties.

4.2.2. Sample Size (n) and Statistical Methods

As this study examines fundamental materials physics rather than comparative biological or clinical trials, traditional significance testing (e.g., p‐values) is not applicable. However, for evaluating macroscopic dielectric breakdown strength, statistical reliability is paramount. Consequently, dielectric breakdown tests were conducted on multiple independent specimens (n = 10) for each polymer group. The dielectric breakdown strength data were statistically assessed using a standard two‐parameter Weibull distribution model: $\mathrm{P}(\mathrm{E})=1-\exp \left[-{\left(\frac{\mathrm{E}}{{\mathrm{E}}_{\mathrm{b}}}\right)}^{\beta }\right]$, where P(E) is the cumulative probability of failure, E _b represents the scale parameter (the breakdown strength at a 63.2% failure probability), and β is the shape parameter, reflecting data dispersion and reliability.

4.2.3. Software

All macroscopic data plotting, Weibull statistical modeling, and linear and nonlinear regressions were performed using OriginPro software.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73875‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding authors upon reasonable request.