Achieving Synergistic Improvement in Dielectric and Energy Storage Properties of All‐Organic Poly(Methyl Methacrylate)‐Based Copolymers Via Establishing Charge Traps

How to achieve synergistic improvement of permittivity (εr) and breakdown strength (Eb) is a huge challenge for polymer dielectrics. Here, for the first time, the π‐conjugated comonomer (MHT) can simultaneously promote the εr and Eb of linear poly(methyl methacrylate) (PMMA) copolymers. The PMMA‐based random copolymer films (P(MMA‐co‐MHT)), block copolymer films (PMMA‐b‐PMHT), and PMMA‐based blend films were prepared to investigate the effects of sequential structure, phase separation structure, and modification method on dielectric and energy storage properties of PMMA‐based dielectric films. As a result, the random copolymer P(MMA‐co‐MHT) can achieve a maximum εr of 5.8 at 1 kHz owing to the enhanced orientation polarization and electron polarization. Because electron injection and charge transfer are limited by the strong electrostatic attraction of π‐conjugated benzophenanthrene group analyzed by the density functional theory (DFT), the discharge energy density value of P(MMA‐co‐PMHT) containing 1 mol% MHT units with the efficiency of 80% reaches 15.00 J cm−3 at 872 MV m−1, which is 165% higher than that of pure PMMA. This study provides a simple and effective way to fabricate the high performance of polymer dielectrics via copolymerization with the monomer of P‐type semi‐conductive polymer.


Introduction
[3][4][5][6] Therefore, developing and updating the energy storage equipment is necessary to minimize energy waste.9][10][11][12] But the low permittivity (ε r ) of polymer also hinders the development of electrostatic capacitors with the demands of miniaturization and intellectualization. [13] Generally, the energy density of linear dielectric polymers can be calculated from the Equation (1): [14] where ε 0 is the vacuum permittivity.Unfortunately, the ε r and E b as the determining parameters of energy storage performance often run counter to each other.Consequently, how to realize the synergistic improvement of ε r and E b of the polymer is the key for developing high-performance dielectrics.23] The construction of the electron deep trap is an effective way to promote the energy storage properties of the polymer dielectrics.The breakdown enhancement mechanism is related to the inhibition of the conduction current, space charge injection, and accumulation.[25][26] For example, Yang et al. have found that the dielectric strength of the epoxy resin is increased by 22.08% when partial -CH 3 groups in the molecular structure of epoxy polymer are substituted by -CF 3 groups in which interface charge trap is established. [24]Huan et al. have prepared P(VDF-HFP)&PMMA-based Ba 0.4 Sr 0.6 TiO 3 /MnO 2 (BST/MnO 2 ) composites.The MnO 2 semiconductor with high affinity can improve the local field density on BST and can also trap the injected and excited electrons.Under the dual effect of BST and MnO 2 , the U e of 21.2 J cm −3 and η How to achieve synergistic improvement of permittivity (ε r ) and breakdown strength (E b ) is a huge challenge for polymer dielectrics.Here, for the first time, the π-conjugated comonomer (MHT) can simultaneously promote the ε r and E b of linear poly(methyl methacrylate) (PMMA) copolymers.The PMMA-based random copolymer films (P(MMA-co-MHT)), block copolymer films (PMMA-b-PMHT), and PMMA-based blend films were prepared to investigate the effects of sequential structure, phase separation structure, and modification method on dielectric and energy storage properties of PMMA-based dielectric films.As a result, the random copolymer P(MMA-co-MHT) can achieve a maximum ε r of 5.8 at 1 kHz owing to the enhanced orientation polarization and electron polarization.Because electron injection and charge transfer are limited by the strong electrostatic attraction of π-conjugated benzophenanthrene group analyzed by the density functional theory (DFT), the discharge energy density value of P(MMA-co-PMHT) containing 1 mol% MHT units with the efficiency of 80% reaches 15.00 J cm −3 at 872 MV m −1 , which is 165% higher than that of pure PMMA.This study provides a simple and effective way to fabricate the high performance of polymer dielectrics via copolymerization with the monomer of P-type semi-conductive polymer.
[35][36] For instance, Wen et al. have added poly(vinylidene fluoridechlorotrifluoroethylene)-double bond (P-DB) into PMMA to prepare blended films via the solution casting method. [37]In contrast to pure PMMA films, the polarizability and E b of the blended films are significantly improved.At 400 MV m −1 , the U e value of 40 vol% p-DB/ PMMA composite reaches 9.3 J cm −3 with an efficiency of 86% at room temperature.In addition, at 90 °C, the composite can produce a high energy density of 8.7 J cm −3 at 350 MV m −1 .Besides acting as a substrate, PMMA as a filler can be used to regulate the properties of ferroelectric polymers.Luo et al. have manufactured PMMA/poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) composite films by hot molding. [33]Theoretically, the Gibbs energy, miscibility, and phase composition of binary mixtures were studied by using the extended Florye-Huggins model.The results show that P(VDF-HFP) has good interaction and compatibility with PMMA.Experimentally, compared with the original P(VDF-HFP) film, the ε r and tanδ of the PMMA/P (VDF-HFP) blend film are reduced, and the polarization-electric loop of the PMMA/P(VDF-HFP) blend film is much thinner.Unsurprisingly, under the electric field of 475 MV m −1 , the U e of PMMA/P(VDF-HFP) blend film with 42.6 vol% PMMA is 11.2 J cm −3 , and the efficiency is 85.8%.
][36]38] However, there are few reports to research the relationship between the molecular structure and the E b of the PMMA-based copolymers, which is related to chemical modification. [39][42][43] Inspired by the properties of semi-conductive organic fillers and molecular structures design, in the paper, we introduced πconjugated unit into the PMMA main-chain by randomly copolymerizing MMA (the monomer) with MHT (the comonomer of P-type semiconductor polymer) in different feeding ratios.The poly (2-((3,6,7,10,11-pentakis(hexyloxy)triphenylen-2-yl)oxy)ethyl methacrylate) (PMHT) is P-type liquid crystalline semiconductor polymer and the corresponding monomer is easily synthesized and shows high reactivity.Wishfully, the MHT unit in the PMMA-based random copolymers (P(MMA-co-MHT)) can construct the electron trap in the nanoscale to improve the E b .On the other hand, uniform and thin film can be obtained in PMMA-based copolymers, which is conducive to large-scale preparation.To understand the dielectric and breakdown mechanism, block copolymers (PMMA-b-PMHT) and PMHT/PMMA blends were fabricated for comparison.As expected, the largest ε r of 5.8 is realized in P(MMA-co-MHT)7 with 7 mol% MHT at 1 kHz.Moreover, under the dual effect of excellent ε r and E b , P(MMA-co-MHT)1 with 1 mol% MHT exhibits a superhigh U e of 15.00 J cm −3 at 872 MV m −1 with an efficiency of 80%.The research work provides a good reference value for the preparation of PMM-based all-organic dielectrics with excellent performance via introducing π-conjugated constitutional unit into polymer chains.

Structure Characterization of Monomers and Polymers
To obtain pure polymers, the prerequisite is the high purity of the monomer before polymerization.The purchased monomer MMA and the synthesized monomer MHT were verified by 1 H NMR (CDCl 3 ) as shown in Figure 1a,b.Obviously, all protons of monomers MMA and MHT are identified in the 1 H NMR, which confirms that the refined MMA and the synthesized MHT have high purity.Homopolymer (PMMA, PMHT) and random copolymer (P(MMA-co-MHT)) were obtained by free radical polymerization, and block copolymer (PMMAb-PMHT) was prepared by RAFT polymerization. 1 H NMR spectrum (CDCl 3 ) of these polymers is illustrated in Figure 1c.The characteristic peaks of the vinyl group at 6.2-5.5 ppm disappeared in 1 H NMR of all polymers, suggesting that the polymers were successfully synthesized.For the copolymers, characteristic peaks of monomer MHT appeared, such as benzene ring characteristic peaks at 8.0-7.5 ppm, meaning that MHT structural unit was introduced into the PMMA chain.Moreover, this characteristic peak intensity increased gradually in the random copolymer with the increase of the feed ratio of monomer MHT and MMA, implying that the proportion of MHT structural unit is increased.The molar content of the MHT repeating unit is calculated by 1 S1, Supporting Information), which coincide with the monomer feeding ratio of MHT/MMA.
The number-average molecular weight (M n ) and polydispersity index (Ð_) of the synthesized polymers were detected by gel permeation chromatography (GPC), and the results were listed in Figure S2a, Supporting Information and Table 1.Apart from block copolymers, all polymers exhibit high M n and similar Ð_.In the preparation of block copolymer, PMMA-based macromolecular chain transfer agent with M n of 5.7 × 10 4 g mol −1 and Ð_ of 1.5 was reacted with monomer MHT to obtain PMMA-b-PMHT with M n of 6.3 × 10 4 g mol −1 and Ð_ of 1.4.The variation trend of M n and Ð_ further proved that the block copolymer was successfully prepared.The UV-Vis absorption spectra of polymers were recorded using Agilent Cary 60 UV-visible spectrophotometer, as shown in Figure S2b, Supporting Information.The process of sample tests is as follows: 1 wt% polymer solution was completely covered on the quartz glass and run at 11.1 × g for 10 s.Then, all samples were dried at 80 °C for 8 h.Except for PMMA, the absorption peaks at 278 nm were observed in the other tested polymer films, mainly from the UV-Vis curve of the triphenylene group π-π transition, which also indicated the presence of the MHT structure unit in the copolymers.
As illustrated in Figure 2, the chemical structure of polymer films was investigated by infrared spectroscopy (FT-IR).The absorption peaks at 3200 ~3000 cm −1 and 1500 ~1450 cm −1 are assigned to C-H stretching vibration and benzene stretching vibration of the MHT structure unit, respectively.The sharp absorption peak at 1255 cm −1 can be attributed to the C-O-C stretching vibration of the MHT structure unit.The signal of C=O stretching vibration is examined at 1725 cm −1 .Compared with PMMA, characteristic bands at 3200 ~3000 cm −1 , 1500 ~1450 cm −1 and 1255 cm −1 are found in P(MMA-co-MHT) films, PMMA-b-PMHT films, and PMMA/PMHT blend films, indicating that MHT structure unit was successfully introduced into PMMA.

Thermal Properties, Phase Transitions and Phase Structures of the Polymers
Thermogravimetric analysis was executed to check the thermal properties of the synthetic polymers under the N 2 atmosphere at a rate of 20 °C min −1 .The test results are shown in Figure 3 and given in Table 1.The thermal decomposition temperature of 5% weight loss (T d ) of PMMA, PMHT, P(MMA-co-MHT)1, P(MMA-co-MHT)3, P (MMA-co-MHT)5, P(MMA-co-MHT)7, and PMMA-b-PMHT are 229, 422, 266, 281, 311, 316, and 322 °C, respectively.Because of the presence of aromatic benzene rings, homopolymer PMHT has a higher T d than PMMA.Compared with PMMA, the T d value of P (MMA-co-MHT) and PMMA-b-PMHT increased significantly with the increase of the MHT molar ratio, which also substantiates that the presence of the MHT structure unit is beneficial to achieve better thermal stability.
The thermal transition temperature of polymers was investigated by differential scanning calorimetry (DSC) under N 2 atmosphere at a rate of 10 °C min −1 .As can be seen from Figure 4; Table S1, Supporting Information, the glass transition temperature (T g ) of pure PMMA reaches up to 120 °C due to semi-rigid main-chain and dipole-dipole interaction between polar ester groups.For PMHT, only a clear point (the transition temperature from liquid crystalline phase to isotropic phase, T i ) is observed at 120 °C.As depicted in Figure 4a,b, there is only one T g without T i in random copolymer films.From P(MMA-co-MHT)1 to P(MMA-co-MHT)7, the T g decreased from 117 to 90 °C because the long alkane of MHT structure unit causes the internal  The results come from 1H NMR spectra. b) Data from GPC (THF as eluent and PS as standard).plasticization and the dipole-dipole interaction between the polar ester groups in random copolymers is reduced.In the block copolymer PMMA-b-PMHT, the T g of the PMMA block and T i of the PMHT block were observed at 122 and 82 °C, respectively.The decrease of T i may be attributed to the low M n of the PMHT block.As shown in Figure 4c, PMMA/PMHT blends contain one exothermic peak and a weak secondorder transition.The former is associated with the transition from the isotropic phase to liquid crystalline and the latter is assigned as a glass transition.Moreover, PMMA/PMHT blends basically maintain the same two transition temperatures no matter how the loading of PMHT filler varies, signifying the incompatibility of the two polymers.Interestingly, at the second heating curve (see Figure 4d), there is only one wide transition peak for PMMA/PMHT blends, which is attributed to the same transition temperature (T i = 120 °C for PMHT and T g = 120 °C for PMMA).
The birefringence of the PMMA-based copolymers and PMMA/ PMHT blends was observed by polarizing optical microscope (POM).In order to maintain the consistency with DSC analysis, all polymers were heated to 130 °C and subsequently cooled to room temperature.The POM results are shown in Figure 5 and the polymers behaved differently depending on the microstructure of copolymers and the method of modification.No birefringence is found in the P(MMA-co-MHT) random copolymers (see Figure 5a), revealing that the random copolymers cannot form the order structure because the number of MMA structure units disturbs the order stacking of triphenylene mesogen.In contrast to the P(MMA-co-MHT) random copolymers, the color texture is detected in the block polymer PMMA-b-PMHT and PMMA/PMHT blends (see Figure 5b,c), suggesting that the PMHT portion can independently form liquid crystalline phase.
To further identify the phase structure of copolymer films and polymer blend films, wide-angle X-ray diffraction (WAXD) is performed, as shown in Figure 6.For PMMA, the typical amorphous diffraction peak occurs at a high-angle of 13°.For PMHT, there is a sharp peak in the low-angle region of 4.8°and two wide diffraction peaks in the high-angle region of 20°and 24.5°.Combining the previous report with experiment results, [43,44] the prepared neat PMHT exhibited the hexagonal columnar phase (4.8°) and strong π-π stacking interaction (24.5°).However, there is no diffraction peak appeared in the low-angle region and high-angle region for the random copolymers P(MMA-co-MHT) (see Figure 6a).In the block copolymer PMMA-b-PMHT, the diffraction peak of the lowangle region is similar to that of PMHT, indicating that PMMA-b-PMHT has the same phase structure as PMHT.It is worth mentioning that the diffraction peak of 24.5°disappeared in PMMA-b-PMHT, which may be caused by the low mole fraction of the MHT structure unit.The PMMA/PMHT polymer blend film has a diffraction peak in the low-angle region (see Figure 6b), demonstrating that PMHT can still hold the ordered structure in the blend film.Besides, the diffraction peak strength increases with the increase of PMHT content, showing the increased loading of PMHT filler in PMMA/PMHT polymer blend film.The results of WAXD are in agreement with the DSC and POM results.

Dielectric Properties of Polymer Films
All polymer films used for dielectric and energy storage properties testing were prepared by solution casting.Scanning electron microscopy (SEM) was executed to detect the thickness and quality of the polymer films.As shown in Figure 7, the thickness of the films was between 10 and 15 μm.The intrinsic PMMA, P(MMA-co-MHT), and PMMA-b-PMHT polymer films show in uniform cross-sections (see Figure 7a-f   For example, the ε r of PMMA decreases from 3.8 to 2.6 with the increment of frequency from 40 Hz to 10 7 Hz.This change stems from the fact that the orientation polarization decreases with the increase of frequency.In Figure 8b, the effect of frequency on the tanδ of all samples is consistent with that of frequency on ε r , and the tanδ of all films is kept at a relatively low level. For the sake of better studying, the influence of composition and modification methods on the ε r of PMMA-based films, the ε r value of all polymer films with the varied molar fraction of MHT units at 1 kHz is summarized in Figure 8c.Firstly, the ε r value of 3.2 at 1 kHz is obtained for PMMA thanks to the contribution of polar ester groups.Secondly, the ε r increases linearly with the increase of MHT/ MMA units mole ratio in the random copolymer.For example, the MHT unit content increased from 0 to 7 mol% in the random copolymer, and the ε r value increased from 3.2 to 5.8 at 1 kHz.One side, the increase of orientation space and the decrease of T g lead to the increase of orientation polarization. [39,45]Another side, the enhancement of electron polarization from triphenylene group also contributes to the improvement of ε r .Thirdly, the contribution of the interfacial polarization between the amorphous region and the liquid crystal region and the electron polarization from the PMHT block improve the dielectric properties of the block copolymer PMMA-b-PMHT, which gives PMMA-b-PMHT a ε r value of 4.3 at 1 kHz. [41,43]Fourthly, the ε r value of PMMA/PMHT blend films shows a trend of first increasing and then decreasing with the increased mole fraction of the PMHT fillers.Such as the ε r value of PMMA/PMHT (97/ 3) composite film reaches a maximum ε r value of 4.0 at 1 kHz, while the ε r value of PMMA/PMHT (93/7) drops to 3.8 at 1 kHz.The increase of ε r is attributed to the double support of interfacial polarization and electron polarization, while the decrease of ε r is due to the decrease of interfacial polarization caused by serious microphase separation of PMHT fillers. [43]Finally, the ε r value of random copolymer is significantly better than that of PMMA/PMHT composite dielectric with the same MHT unit mole content, and the tanδ of all copolymers and composite films remains at the same level as that of PMMA.This proves that chemical modification can play a better role in improving the dielectric properties of PMMA.However, something quite interesting happens in the chemical modification, that is, at the same mole fraction of MHT, random copolymerization can achieve greater improvement in dielectric properties than block copolymerization, which may be due to the maximization of orientation polarization of polar ester groups after random copolymerization of MHT and MMA.This also confirms that the ingenious dispersion of MHT monomer in PMMA is an excellent means.In addition, the conductivity of all polymer films was studied at 40 to 10 7 Hz using an impedance analyzer, as displayed in Figure 8d.The conductivity of all polymer films

Breakdown Strength of Polymer Films
To explore the optimal MHT unit molar content and modification methods, the breakdown strength (E b ) of all films was tested at 10 Hz and room temperature using TF analyzer 2000 ferroelectric polarization tester.Double-parameter Weibull statistics were conducted on the E b of polymer film according to Equation (2) to verify the reliability of the data, as illustrated in Figure 9a.
In Equation ( 2), [9] E is the applied electric field, β is the shape However, when the mole ratio of MHT structure units increased to 3 mol% in the random polymers P (MMA-co-MHT), the E b began to decrease.For example, the E b of random copolymers decreased from 647 to 451 MV m −1 when the content of MHT units changed from 3 mol% to 7 mol%.The E b of block copolymer PMMA-b-PMHT with 5 mol% MHT is only 514 MV m −1 , which is significantly lower than that of PMMA (696 MV m −1 ).In the PMMA/PMHT blend system, the highest E b of 756 MV m −1 was found in the PMMA/PMHT (99/1).Nevertheless, as the mole ratio of PMHT filler continues to improve, the E b decreases rapidly.For instance, the E b of PMMA/PMHT (93/7) with 7 mol% MHT is only 109 MV m −1 .It is worth mentioning that the E b of all random copolymer films is superior to the PMMA/PMHT blended films at the same mole content of MHT units.Such as the E b of P (MMA-co-MHT)3 is up to 647 MV m −1 , while that of PMMA/ PMHT (97/3) is only 243 MV m −1 .At the same MHT molar content, the E b of the block copolymer PMMA-b-PMHT was higher than that of PMMA/PMHT composite film but slightly lower than that of the random copolymer P(MMA-co-MHT) (see Figure 9b).This absolutely demonstrates that high E b can be obtained via chemically modifying PMMA with the monomer of semi-conductive polymers.Energy Environ.Mater.2024, 7, e12577

Breakdown Mechanism of Polymer Films
The influence of composition and PMHT filler on the E b of PMMAbased films is analyzed according to the literature report and experimental results.As shown in Figure 10a, a high E b can be achieved in PMMA due to strong inter-chain interactions and close inter-chain stacking. [45][29][30][31][32] Therefore, the E b of P (MMA-co-MHT)1 bearing 1 mol% MHT is 125% higher than that of PMMA.Thereafter, the E b of random polymers decreased when MHT increased to 3 mol% and more.The reason can be interpreted by two aspects: on the one hand, the interaction between molecular chains is weakened and the brittleness of thin films is increased.On the other hand, the increase of MHT unit content leads to π-π stacking, which is beneficial to electron transport. [41]So, the content of MHT units has to be precisely regulated to hold the charge capturing effect but avoid the charge conduction effect.As for the block copolymer PMMA-b-PMHT, the E b is significantly lower than that of PMMA, which is related to the strong π-π stacking between MHT units from the PMHT segment (see Figure 10c).Furthermore, the E b of the block copolymer is inferior to that of the random copolymer at the same MHT content, which proves that the skillful dispersion of MHT structure unit in PMMA can provide great assistance in improving E b .[29][30][31][32] However, when the mole ratio of MHT continues to increase, the E b of PMMA/PMHT composite film decreases rapidly, which is caused by serious electronic conduction of the excited charge carriers, microphase separation, filler aggregation, and holes (see Figure 7h-j).
Mechanical properties have a great influence on the electromechanical breakdown of materials, especially the deformation and fracture of polymer materials.Therefore, Young's modulus of the samples is measured to better estimate the mechanical properties of the material and the E b , and the results are shown in Figure 11a.The relationship between Young's modulus and E b is based on the following Equation (3): [14,42] where Y is Young's modulus.In random copolymer films and blend films, Young's modulus accords well with the changing trend of E b .For instance, P(MMA-co-MHT) 1 exhibits the maximal Young's modulus of 3.18 GPa (see Figure 11b), which is more than 12% higher than that of pure PMMA.It is should be noted that a high Young's modulus does not always have a high E b for the polymer dielectrics because the E b is controlled by multiple factors. [5]For example, at same content of 5 mol% MHT structural unit, the E b of P(MMA-co-MHT)3 and PMMA/PMHT(95/5) are 647 and 243 MV m −1 ; however, their Young's modulus is 2.51 and 2.71 GPa, respectively.
Density functional theory (DFT) was performed to compute the energy band structure, state density, the lowest unoccupied molecular orbital (LUMO), the highest occupied molecular orbital (HOMO) and electrostatic potential of PMMA and P(MMA-co-MHT) to further demonstrate the trap sites of MHT unit and the influence on the carrier transport and capture in the copolymer.It is well-known that  below the HOMO level is the valence band and above the LUMO level is the conduction band.The difference between the HOMO level and the LUMO level is the band gap.The states located at the band gap of the polymer substrate will act as charge trapping sites.The LUMO level respects the electron affinity and the HOMO energy level implies the ability to lose electrons or capture holes.The LUMO level difference and the HOMO level difference between pure polymer chains and copolymer chains can be defined as the electron trap depth (E trap ) and the hole trap depth (H trap ), respectively. [42,46]Furthermore, the model of PMMA was simplified as a molecular chain contained the 5 MMA units and one MHT unit was inserted into the molecular chain of PMMA in the P(MMA-co-MHT) model.As displayed in Figure 12a, compared with neat PMMA, the LUMO level of P(MMA-co-MHT) decreases to −0.51 eV from 0.29 eV and the HOMO level increases to −6.38 eV from −7.69 eV.Therefore, the E trap and H trap are 0.80 and 2.04 eV, respectively, suggesting that P (MMA-co-MHT) has a stronger ability to capture both holes and electrons than PMMA.Based on the density of states spectrum (see Figure 12b), P(MMA-co-MHT) also shows two energy states in the region near the valence band and the conduction band, representing hole traps and electron traps, respectively.The electrostatic potential is the key to studying and predicting molecular interactions, so DFT simulation of PMMA and P(MMA-co-MHT) is helpful to analyze the distribution of positive and negative electrostatic potential regions.The electrostatic potential was calculated on the electron density plane of 0.001 a.u., and the quantitative results of the electrostatic potential distribution of PMMA and P(MMA-co-MHT) are shown in Figure 12c,d.Consistent with the results of energy band structure and state density, the electrostatic potential distribution of the P(MMA-co-MHT) copolymer shows a more prominent attraction to positive and negative charge carriers than that of PMMA.These simulation results provide strong support for the trap role of semiconductor MHT units in copolymers. [46]

Storage Energy Density of all Samples
Figure 13a shows unipolar polarizationelectric field curves of polymer films at ambient temperature and 10 Hz, in which all data are derived from the Weibull analysis curve.The maximum polarization value (P max ) highlights the importance of achieving high energy storage density, and the highest P max of 4.1 μC cm −2 at 872 MV m −1 is found in P (MMA-co-MHT)1.The MHT repeating unit in random copolymer changed from 1 to 7 mol %, the P max value decreased from 4.1 to 2.8 μC cm −2 .For the block copolymer P (MMA-b-MHT) with 5 mol% MHT, the P max value is 3.3 μC cm −2 .In the PMMA/PMHT blend system, the molar content of PMHT filler increased from 0 to 1 mol%, and the P max value increased from 3.3 to 4.0 μC cm −2 , but the molar content of PMHT filler reaches 7 mol%, the P max value is only 0.51 μC cm −2 .The P max value of polymer film is decided by both ε r and E b .
As an index to evaluate the energy storage performance, the discharge energy density (U e ) and efficiency (η) values are obtained by P-E integration of Figure 13a, which are calculated by Equations ( 4) and ( 5). [10]e ¼ Z EdD (4) In Equation ( 4), D represents the electric displacement and D is equal to P when ε r > 1.In Equation ( 5), U c represents the charging energy  density.The U e and η of all samples at different applied electric fields are shown in Figure 13b, c.The U e of 9.06 J cm −3 and η of 78% are obtained for PMMA at an electric field of 670 MV m −1 .The maximum U e value of P(MMAco-MHT)1 with 1 mol% MHT is up to 15.00 J cm −3 at 872 MV m −1 , which is 1.65 times than that of PMMA film.Meanwhile, the η remains at 80%.The copolymerization ratio of MMA and MHT changed from 99:1 to 93:7, and the U e value of random copolymer P (MMA-co-MHT) decreased from 15.00 to 6.48 J cm −3 .The U e of 6.57J cm −3 and η of 76% at 500 MV m −1 are exhibited in block copolymer PMMA-b-PMHT at an electric field of 500 MV m −1 .In the blend system of PMMA/PMHT, the best sample PMMA/PMHT (99/1) at 758 MV m −1 has the U e of 11.53 J cm −3 , but the η is only 71%.With the same MHT unit content, its energy storage performance is distinctly inferior to P(MMAco-MHT) (see Figure 13d).Figure 14a and Table S2, Supporting Information show a of ε r , E b , U e , and η of this work with recent dielectric polymers.As a result, the P(MMA-co-MHT)1 exhibits outstanding U e and η. [21,27,37,[47][48][49][50][51][52][53][54][55][56][57] In addition, the ferroelectric analyzer is used to test the cyclic stability of P(MMA-co-MHT)1 film.As displayed in Figure 14b, the number of cycles increased from 1 to 10 6 at 200 MV m −1 , and the P max only changed from 0.948 to 0.943 μC cm −2 , indicating that the prepared polymer film has good stability and application value.These experimental results manifest that chemical modification of PMMA via using comonomer with triphenylene group is an important measure to achieve high energy storage performance of PMMA-based dielectric polymers.

Conclusion
In summary, via using p-type liquid crystalline semiconductive polymer PMHT and corresponding monomer MHT, all-organic PMMA-based composites and copolymers are prepared for capacitive energy storage.The results showed that the PMMA-based copolymer can form high-quality and homogeneous films.The P(MMA-co-PMHT)7 with 7 mol% MHT structure units achieves the highest ε r value of 5.8 at 1 kHz and room temperature due to the enhancement of electron polarization and orientation polarization.An outstanding U e value of 15.00 J cm −3 at an electric field of 872 MV m −1 was found in P(MMA-co-MHT)1 with 1 mol% MHT structure units  with the predominant η of 80%, which is attributed to that the triphenylene group can immobilize charge through strong electrostatic attraction.This work provides a good idea for improving the existing polymer dielectric to achieve high energy storage polymer dielectric.
Synthesis of homopolymer and random copolymers: The 2-((3,6,7,10,11pentakis(hexyloxy)triphenylen-2-yl)oxy)ethyl methacrylate (MHT) was synthesized using methods mentioned in previous literature and the synthetic route is shown in Scheme S1, Supporting Information. [43,44]The monomer MMA was refined by vacuum distillation.Homopolymer (PMHT) and random copolymer (P (MMA-co-MHT) were prepared by different feeding ratios of MHT and MMA (100/0, 1/99, 2/98, 3/97, 5/95, 7/93, 0/100), as illustrated in Scheme 1a-c.The detailed operation steps are as follows: First, according to the feed ratio, the monomers MHT and MMA were added to polymerization tube.Next, a certain amount of refined THF and AIBN were poured in the tube.After experiencing three freeze-vacuum-thaw cycles, the polymer reaction was carried out at 70 °C for 12 h.Subsequently, the reaction solution was diluted with lots of THF solvent and precipitated twice in anhydrous methanol to remove the unreacted monomer.Finally, the collected polymers were dried in the vacuum at 50 °C for 12 h: poly(2-((3,6,7,10,11-pentakis(hexyloxy) triphenylen-2-yl)oxy)ethyl methacrylate) (PMHT), poly(methyl methacrylate) (PMMA) and poly((methyl methacrylate)-co-(2-3,6,7,10,11-pentakis((hexyloxy) triphenylen-2-yl)oxy)ethyl methacrylate)) (P(MMA-co-MHT)).Preparation of block copolymer: Scheme 1d displays the synthesis route of block copolymers prepared by reversible addition-fragmentation chain transfer polymerization (RAFT).In the first step, PMMA-based macromolecular chain transfer was designed and prepared based on the method of literature, [10] and the specific process is as below: The material was added in the tube according to the molar ratio of Preparation of PMMA, P(MMA-co-MHT), PMMA-b-PMHT and PMMA/ PMHT blending films: PMMA-based copolymers thin films were fabricated by solution casting, as shown in Scheme 2. To prepare the 10 wt% homogeneous polymer solution, the calculated polymers were added to the THF solvent and the solution was stirred at 70 °C for 12 h.Subsequently, the polymer solution was cast on a clean glass plate and dried in the vacuum at 80 °C for 12 h.Finally, the uniform and high-quality polymer films were peeled off directly from the glass substrates after annealing at 140 °C (see Scheme 2).In addition, the copolymers films can be crimped and folded without breakage, suggesting the good flexibility of the films.Taking a similar preparation process, the blend films with the different molar ratios of PMMA and PMHT (99/1, 97/3, 95/5, 93/7) were obtained.
Characterization and DFT calculation: The test conditions for all samples and DFT calculation are described in detail in the Appendix S1.
c) T d (5% weight loss of the polymers) results were measured by thermogravimetric analysis.d) PMMA-based macromolecule chain transfer agent.Energy Environ.Mater.2024, 7, e12577 ), indicating that the above films possess high quality.As displayed in Figure 7g-j, except for PMMA/PMHT (99/1) film, nanoparticles and holes are discovered in other PMMA/PMHT composite films.Furthermore, this phenomenon becomes more and more obvious with the increase of PMHT filler, which is detrimental for the quality and the breakdown of polymer films.As the paramount parameters of dielectric materials, the values of permittivity (ε r ) and dielectric loss (tanδ) for all samples at frequencies from 40 Hz to 10 7 Hz and at room temperature were evaluated by impedance analyzer, as shown in Figure8a,b.As can be seen from Figure8a, the ε r value of all polymer films is greatly affected by frequency.

Figure 3 .
Figure 3.The TGA curves of the polymers.
parameter and E b is the breakdown strength when the cumulative failure probability [P(E)] is 63.2%.The E b and β values were obtained through Weibull curve analysis, and relationship diagram of the Weibull parameter with mole content of MHT units was made, as shown in Figure 9b.It can be seen from Figure 9b that the β values of PMMA-based copolymers exceed 10, reflecting the high reliability of the experimental E b .On the contrary, PMMA-based composites have low β values, especially the PMMA/PMHT blends (93/7), showing that the polymer composites are rather heterogeneous and exhibit a number of defects.As shown in Figure 9b, in all polymer films, the maximum E b of 870 MV m −1 was achieved in the random polymer P(MMA-co-MHT)1 with 1 mol% MHT structure units.

Figure 8 .
Figure 8. a) Permittivity and b) dielectric loss of all polymer films as a function of frequency.c) The permittivity of polymer films varies with the mole content of the MHT unit at 1 kHz.d) The relationship between frequency and conductivity of polymer films.

Figure 9 .
Figure 9. a) Weibull curve analysis of polymer films.b) Relationship between Weibull E b (Shape parameter (β)) and MHT unit molar fraction in polymer films.

Figure 11 .
Figure 11.a) The load-displacement curve of the samples.b) Young's modulus of the samples.

Figure 12 .
Figure 12.The a) band structure and b) state density of PMMA and copolymer P(MMA-co-MHT) were simulated by DFT theoretical method.Electrostatic potential distribution and area percent of c) PMMA and d) P(MMA-co-MHT) in each electrostatic potential range.

Figure 13 .
Figure 13.a) Polarization-electric loops of all sample films.b) Discharge energy density (U e ) and c) efficiency (η) of all sample films.d) The relationship between maximum U e and the content of MHT structure units.
Figure 14.a) Comparison of maximum U e and η between this work and previous work.b) Cyclic stability of P(MMA-co-MHT)1 films from 1 to 10 6 cycles at 200 MV m −1 .
H NMR and the related results are summarized in Table 1 (for the calculation Energy Environ.Mater.2024, 7, e12577 2 of 12 method, see Figure