Enhanced Aramid/Al2O3 interfacial properties by PDDA modification for the preparation of composite insulating paper

To further improve the electrical insulation properties of meta-aramid paper, polydimethyldiallyl ammonium chloride (PDDA) was used for the modification of nano-Al2O3 in this work. The modified products were characterized by means of transmission electron microscopy, Fourier transform infrared spectrometer and liquid Zeta potential. The original and PDDA-modified nano-Al2O3 was doped into the aramid fiber composite, and the effects of PDDA and filler mass fraction on several properties such as electrical conductivity, breakdown strength and surface charge dissipation rate of insulating paper were studied. Finally, the interfacial properties of the composites were investigated using molecular dynamics. The results showed that PDDA was successfully coated on the surface of nano-Al2O3, and its surface potential was changed from negative to positive, which could improve the interface characteristics between filler and matrix and enhance the dielectric strength of aramid paper. Besides, saturation effect of PDDA modification was observed, and when the amount of PDDA was 10% and the content of modified nano-Al2O3 was 3%, the breakdown strength of aramid paper was increased by 20%, and the volume conductivity was decreased by 68%. The simulation result revealed that PDDA could improve the insulation performance of aramid paper by reducing the interfacial binding energy between aramid and filler.


Introduction
Enhanced Aramid/Al 2 O 3 interfacial properties by PDDA… which could inhibit the injection and movement of space charges [24]. In another work, the surface etching of TiO 2 by silane coupling agent could further improve its crystallinity, and epoxy resins with better insulating properties was achieved [25]. Kan et al. proposed a polymer-impregnated surface modification method based on the asperities and porous surface structure of nanoparticle, which could improve the tensile properties of the composites [26]. Functional groups with different activities were introduced into the surface of nanoparticles according to appropriate ratio, which could reduce the aggregation and non-specific binding between particles, making it easier to combine with organic molecules [27].
Poly dimethyl diallylammonium chloride (PDDA) is a common polycationic surface modification reagent, which could improve the performance of nanomaterials by means of producing physicochemical interactions between multifunctional long chains and Nano-fillers. Li et al. coated PDDA on the surface of nano-Fe3O4 by chemical co-precipitation. The Zeta potential of the synthesized composite material became positive and the particle size became smaller [28]. Ratajski et al. found that PDDA could improve the retention and dispersion of silica in the matrix without damaging the original composite structure [29]. In addition, PDDA exhibits good dissociation ability in a wide range of pH in both water and ethanol [30]. It is foreseeable that PDDA has high applicability to the pulp environment during the papermaking process. However, to the best of our knowledge, only few reports were concerned about PDDA-modified aramid paper fillers.
In this paper, a series of modified nano-Al 2 O 3 with different mass fractions of PDDA was synthesized and used as filler in the preparation of Al 2 O 3 /PMIA composite insulating paper via wet papermaking. The effect and reaction mechanism of cationic polymer-grafted nanoparticles were studied by comparing the physical, chemical properties, microstructure and binding energy of nano-Al 2 O 3 before and after modification and the conductivity, breakdown strength and trap properties of the samples were tested.
The Zeta potential of nano-Al 2 O 3 before and after modification in aqueous solution under neutral environment was studied by liquid Zeta particle potential analyzer (DLS, Zetasizer Nano S90). The chemical compositions of Al 2 O 3 , PDDA and P-Al 2 O 3 composites were analyzed by Fourier transform infrared spectrometer (FTIR, Nicolet IS5) with wavenumbers ranging from 400 to 4000 cm −1 . Field emission transmission electron microscopy (TEM, Tecnai G2 F20) was used to observe the morphology of Al 2 O 3 before and after modification in water. The sample was obtained by dispersing the powder in deionized water to prepare a 0.15% dispersion, the carrier network was selected as an ultra-thin carbon film, and the image was a bright field image. The section of the aramid composite paper was characterized by field emission scanning electron microscope (SEM, Quanta 250 FEG). The section obtained by brittle fracture in liquid nitrogen and treated with gold spraying. Thermogravimetric analyzer (TG-DSC, TA-Q600) was used to test the thermal decomposition weight loss of aramid paper during the process of raising the temperature from 20 to 800 ℃ at a heating rate of 10 K/min under nitrogen atmosphere.

Experiment
(1) Preparation of Al 2 O 3 coated with PDDA The Al 2 O 3 nanoparticles were hydroxylation with sodium hydroxide (NaOH) before coating it with PDDA. 1.5 g nano-Al 2 O 3 was submerged into 120 ml 20%(v/v) DI water/ethanol and stirred for 10 min, then 10 ml NaOH solution with a concentration of 0.5 M/L was added, and the mixture was ultrasonically dispersed for 10 min, then hydroxylation for 0.5 h at 60 °C. The hydroxylation Al 2 O 3 was then washed with NaCl aqueous solution several times to remove NaOH; then, it was soaked in DI water overnight at room temperature before coating with PDDA in the next step.
The hydroxylation Al 2 O 3 was coated layer with PDDA as follows: The PDDA solution was prepared by adding 0.1 g of 40% (w/w) PDDA aqueous solution to 10 ml DI water and by subsequent stirring at 65 °C for 4 h. Subsequently, the hydroxylation Al 2 O 3 was submerged in PDDA solution with different mass ratio, (PDDA/Al 2 O 3 = 0, 10, 20, 40, 80%,) and the mixture was ultrasonically dispersed for 5 min, then stirred for 6 h in a sealed water bath at 80 °C to coat it with a layer of PDDA. Finally, the Al 2 O 3 powder was filtered and washed three times with DI water and dried in a 100 °C oven for 8 h. The obtained modified Al 2 O 3 powder was denoted as P-Al 2 O 3 .
(2) Preparation of modified Al 2 O 3 /PMIA paper PEO and DI water were prepared into a PEO solution as a dispersant in a mass ratio of 1:2000, which was put into a plastic bucket for use. 2000 ml ultrapure water and 130 ml PEO aqueous solution were poured in a standard fiber disintegrating machine, and revolved 15,000 r at a speed of 3000 r/min. Then, P-Al 2 O 3 in different concentrations were added and continued to disperse for 30,000 revolutions. After that, 1.2 g chopped and 2.8 g precipitating fibers were added and decompressed for 75,000 revolutions. Then, a paper machine was used for suction filtration and sheet making, and the finished paper was flat on a heating 1 3 Enhanced Aramid/Al 2 O 3 interfacial properties by PDDA… plate. Finally, papers were dried in a vacuum environment at 110 °C for 20 min to remove most of the water, and then the flat vulcanizer and chrome-plated mold were used for 3 times of hot pressing with the parameters of 270 °C, 10 MPa, each time for 20 s. The diameter of the aramid composite papers prepared by this process was 200 ± 2 mm and the thickness was 0.16 ± 0.02 mm; the quality loss during the period was less than 5%. The specific reagent dosage for each sample is shown in Table 1 and the specific process is displayed in Fig. 1.

Characterization
(1) FTIR test The infrared spectra of PDDA, Al 2 O 3 and P-Al 2 O 3 are shown in Fig. 2a. The broad peak at 3357 cm −1 in the spectrum of PDDA belonged to the result of N-H stretching vibration. 2932 and 1470 cm −1 were the peaks of C-H stretching vibration from methylene and C-H asymmetric deformation vibration from N-CH3 [31]. The absorption peak at 1638 cm −1 was attributed to the unsaturated C=C stretching vibration in the PDDA molecular chain; 1099 cm −1 was the C-N in-plane bending vibration. The -OH stretching vibration peak of Al 2 O 3 near 3472 cm −1 before modification belonged to the hydroxyl groups adsorbed on the surface of Al 2 O 3 . 642 and 606 cm −1 were the bending vibration absorption peaks of Al-O. The absorption peak intensity corresponding to -OH decreased significantly in the PDDA-modified composite material, indicating the involvement of hydroxyl group in the reaction during the coating process. The absorption peak at 2922 cm −1 was attributed to the antisymmetric stretching vibrations of methyl and methylene on the PDDA polymer chain. The peak at 1622 cm −1 could be attributed to the vibration absorption peaks associated with C-C, C-N, and the presence of carbon nitrogen heterocycle, indicating the presence of PDDA in the composite [32]. In addition, the characteristic peaks of P-Al 2 O 3 shifted to low wavenumbers. According to the above analysis, it was proved that PDDA was successfully grafted on the surface of nano-Al 2 O 3 by intermolecular hydrogen bonding and chemical bonding.
(2) Zeta potential test The Zeta potential of nano-Al 2 O 3 modified with different PDDA concentrations is shown in Fig. 2b. The surface potential of unmodified nano-Al 2 O 3 was − 21.9 mV and became positive after PDDA modification. The Zeta potential varied from 28.9 to 36.4 mV with the amount of PDDA being changed. The zeta potential increased overall when the amount was too high but the distribution uniformity decreased [33]. When the value of the Zeta potential was greater than 30 mV, the particles had better dispersion and stability [34]. This might indicate that the electrical properties of the modified surface were reversed. Considering that meta-aramid always exhibits negative zeta potential in aqueous solution, PDDA-modified nano-Al 2 O 3 particles would be expected to achieve classical connection with aramid fibers, thereby improving the dispersion and stability of fillers in composites.
(3) TEM The transmission electron microscope (TEM) images of P-0, P-10 and P-80 at different magnifications are shown in Fig. 3. It could be seen from the comparison that the black part in Fig. 3 was the united nano-Al 2 O 3 particles and the gray part was the PDDA shell. PDDA was successfully coated on the surface of the nano-Al 2 O 3 particles, forming a core/shell structure. In Fig. 3b, the composite filler had an irregular shape when the mass fraction of PDDA was 10%. After adding PDDA, it aggregated on the surface of individual particle to achieve the coating effect instead of capturing more particles at the same time. In Fig. 3c, PDDA fell off from the surface of Al 2 O 3 after the concentration continued to increase and the polymer chain self-polymerized, weakening the coating effect.
According to the above microscopic characterization results, the reaction process of PDDA-modified Al 2 O 3 is shown in Fig. 4. The alkaline environment promoted the adsorption of a large amount of OH-in the solution on the surface of nano-Al 2 O 3 , resulting in a negative surface potential. PDDA dissociated Cl-and positively charged polymer molecular chains in the ethanol/water solution. The hydrophilic amino groups provided the lone electron pairs, and the positively Fig. 3 The TEM images of a P-0, b P-10, c P-80 charged amino groups further combined with the hydroxyl groups on the surface of nano-Al 2 O 3 . New chemical bonds or hydrogen bonds were formed to reverse the surface potential of nanoparticles to positive, thus achieving the purpose of modification. In addition, other groups in the polymer chain might undergo addition or dehydration reactions with the hydroxyl groups, which could also be adsorbed on the surface of nanoparticles.
The surface potential of aramid fiber was negative in a neutral environment; therefore, the nano-Al 2 O 3 with positive surface could be organically bonded to the aramid matrix through electrostatic attraction, and the rest of the long chain had lipophilic groups which would weaken the difference between inorganic powders and organic polymers. In addition, the viscosity of the aramid fiber could increase the interfacial bonding force between the filler and the matrix, thereby improving the interfacial properties of the composite material.
The cross sections of the aramid nanocomposite paper filled with P-Al 2 O 3 were characterized by SEM, and the SEM images of different samples are shown in Fig. 5. The fillers in P-0 exhibited agglomeration phenomenon and the particle size was larger before PDDA modification. The overall particle size of the filler shown in Fig. 5b became smaller, and the dispersion was more uniform, indicating that PDDA modification could effectively prevent the agglomeration of nanoparticles and improved its dispersion in the aramid matrix [35]. In Fig. 5c, PDDA acted as "flocculants" when the amount of modifier continued to increase, resulting in the fillers re-agglomeration and a decrease in the modification effect [36]. In Fig. 5d, when the amount of filler increases, the PDDA-modified layer fell off during the papermaking process, and agglomeration occurred again with the electrostatic repulsion between the fillers decreased.  The normalized TG and DTG curves are shown in Fig. 6(a)-(b), respectively. The decomposition rate of the sample was obviously accelerated at 400 ~ 600 ℃. The ash ratio was used to reflect the residues of filler and undecomposed PMIA. The ash residue of the samples after adding PDDA was slightly higher than that of the unmodified ones, indicating that the modification had a delaying effect on the decomposition of the matrix. The decomposition peaks of PDDA appeared at 345 ℃ and 455 ℃, and the corresponding decomposition peaks appeared in the vicinity of the modified composite paper, indicating that PDDA was successfully integrated into the composite system [37]. In addition, the composite paper decomposition peaks appeared at 436 ℃ and 516 ℃, while the corresponding peaks shifted to the right for the modified samples, indicating that PDDA could delay the matrix decomposition [38].

Conductivity properties
The conductivity was measured by Geely 6514 galvanometer and HPPS0619 high-power DC power. The experiment was tested by three-electrode measurement as shown in Fig. 7a. The measuring electrode was connected to the electrometer and the high-voltage electrode was connected to the DC power through the protection resistor. The whole platform was encapsulated in a stainless steel shielding cavity. The conductivity was tested under 1.8 kV DC voltage for 3 min to stabilize before reading the conductivity current from the electrometer. In order to reduce the influence of residual charge, electrostatic cloth was used to discharge the surface of the paper sample after each test, and then stand on the grounding table for 5 min.
The electrical conductivity of the aramid insulating paper calculated by formula (1) is shown as Fig. 7b. where L was the thickness of the sample, I V was the conductance current, r was the radius of the test electrode, U was the applied voltage and takes 1.8 kV. Each sample was tested 10 times and the average value was used for analysis.
The bulk conductivity of P-0 was 1.12 × 10 -16 S/m. With the increase in PDDA concentration, the bulk conductivity first decreased and then increased in the saturation state. When the PDDA mass fraction was 10%, the minimum volume conductivity was 3.62 × 10 -17 S/m, which decreased 68% relative to P-0, and the bulk conductivity of P series was lower than that of pure Al 2 O 3 /aramid composites. According to the "multi-core model" [39], PDDA could provide the polymer chains and functional groups required for the bonding layer, enhance the bonding between nanoparticles and the matrix, effectively inhibit the migration of carriers in the composite material and reduce its bulk conductivity. However, as a strong electrolyte, it would provide carriers and lead to an increase in conductivity when the amount of PDDA was too high. With the increase in the doped nano-Al 2 O 3 concentration, the conductivity of the insulating paper first decreased and then increased. It is the smallest at 3% doping concentration, which is 53% lower than that at 1% doping concentration. Since Al 2 O 3 had good insulating properties and its trace doping could destroy the carrier channels inside the PMIA matrix. The interface between the matrix and the Nano-filler had an "electric double layer" structure [40], and the conductivity of the electric double layer was higher than that of the polymer. The particle spacing in the polymer decreased as the amount of filler continued to increase, resulting in the electric double layers of adjacent particles would overlap, and the electrical conductivity of the overlapping region would increase significantly which in turn affected the charge transport mechanism of the entire polymer. Combined with the electrical conductivity characteristics introduced above, it was found that the breakdown strength of aramid paper was very consistent with the change trend of volume conductivity, and the two were negatively correlated. Therefore, it could be considered that the charge Enhanced Aramid/Al 2 O 3 interfacial properties by PDDA… transport ability of the polymer was an important factor affecting the breakdown performance.

Breakdown properties
The column-column electrode was used for testing breakdown strength, as shown in Fig. 8a. The upper and lower electrodes were brass, and the diameter and height of the column electrode were 25 mm. The chamfer radius was 3 mm. During the test, a positive AC voltage was applied to the sample, and the breakdown strength of the aramid insulating paper under air conditions was measured by the method of continuous and uniform voltage increased as 2 kV/s. Each sample was tested 10 times on different parts to obtain the breakdown voltage, the thickness of the paper sample at the corresponding position was measured, and the average breakdown strength was calculated.
The Weibull distribution was widely utilized to describe the phenomenon of overall failure of insulation properties. In order to make the results more reasonable, two-parameter Weibull model was used for analyzing. The cumulative probability function was shown as follows: where, V was the breakdown voltage. Pv was the breakdown failure. α and β were the paper scale and shape parameter and indicated the breakdown voltage at the probability of 63.2% and the stability of the sample, respectively.
The AC breakdown strength of the aramid paper is shown in Fig. 8b. The value α with a breakdown probability of 63.2% was extracted for comparison. It could be seen that the α of P-0 was 32.7 kV/mm. With the increase in PDDA concentration, the breakdown field strength of the paper showed a trend of increasing, then decreasing, and then increasing. When the mass fraction of PDDA was 10%, the highest breakdown strength was 39.3 kV/mm, which was 20% higher than P-0. Continuing to increase the concentration of PDDA, it was still higher than without modification. The average breakdown field strength of P-10 series first increased and then decreased with increasing doping concentration in a "V" pattern. The α of P-10/1 was 26.5 kV/mm, which was 48% lower than P-10/3. The breakdown field strengths of other doping concentration paper were lower than that of P-0, and the modification effect was poor, indicating that the concentration of Nano-filler was higher than that of the modifier. The impact on the breakdown strength of the aramid paper was more significant.
Nano-powder is easy to agglomerate in aqueous solution due to the strong cohesion, and the particle size became larger, which is unable to give full play to the excellent properties of nanomaterials. Adsorption of positively charged PDDA on the surface of nano-Al 2 O 3 by electrostatic attraction could increase the absolute value of surface Zeta potential, which could increase the electrostatic repulsion between Al 2 O 3 particles and prevent their aggregation, leading to the stably suspension and uniformly dispersion in the aramid matrix. In addition, PDDA has good dissociation ability in a wide pH range. In ethanol/water mixed solution, Cl − and positively charged polymer chain could be dissociated, and the polymer chain with NH 3 functional group could form N-containing hydrogen bond with the aramid fiber matrix, enhancing the bonding of Al 2 O 3 to the matrix interface. PDDA could form spherical micelles when its amount was slightly increased, and the particle size of nanoparticles was significantly reduced based on the theory of "adsorption and electric neutralization" of the polymer [41]. At the same time, the increased PDDA would provide more N elements to generate hydrogen bonds with the matrix. When the concentration was too high, the modified layer would Enhanced Aramid/Al 2 O 3 interfacial properties by PDDA… fall off, resulting in less repulsive than attractive force between the fillers, and the interface properties between the filler and the matrix were destroyed.

Trap properties
A shielding cavity was used to measure the surface charge dissipation of aramid paper. The devices are shown in Fig. 9a. The equipment includes the following: Trek 347 electrometer from TREK company in the United States, with a measuring range of 0 ~ ± 3 kV; LAS60300P high-voltage power supply produced by Boer High Voltage Power Co., Ltd., which output voltage was adjustable from 0 to 100 kV. In the test, the surface of the paper sample was first treated with electrical discharge with a conductive cloth, and the surface potential of the grounding plate was measured to determine the initial potential of 0. The sample was placed on the stage to preheat for 5 min after heating the grounded stage to 60 °C. The needle electrode was used to apply 5 kV positive DC voltage at 5 mm above the paper sample for charging for 2 min. After charging, the paper sample was moved to 2 mm below the capacitance probe to monitor the surface potential, and the potential decay signal was continuously collected within 10 min through the data acquisition card (NI USB-8452).
There are two main ways of charge dissipation charge on the surface of the insulating paper: transfer into the body and dissipate along the surface of the paper [42]. The surface potential dissipation curves of the aramid paper are shown in Fig. 9b,c. It could be seen from the figures that the dissipation trend of all samples was first fast and then slow. The initial potential of the composite material increased first, then decreased, and then increased again with the increase in PDDA concentration, and first increase and then decrease with the increase in Al 2 O 3 doping concentration, the change trend of the decay rate was opposite to that of the initial surface potential, showing a good synergistic effect. The dissipation rate would increase with the concentration of PDDA increased because its excellent dissociation ability would provide more charge and conduction channels. Al 2 O 3 possesses excellent insulating properties and high dielectric constant. The greater the dielectric barrier height, the more difficult it was to inject charges into the dielectric body and the greater the accumulation of charge on the dielectric surface, the slower the dissipation rate. The micro-doped Al 2 O 3 could give full play to its good insulating properties and effectively hinder the transfer of charge to the interior of the matrix. The rate of dissipation began to increase again when a large number of conductive channels were formed between the excess fillers and the charges could migrate. It could be seen from the figure that the overall dissipation rate of P-10 series was higher than that of P series, indicating that PDDA and Al 2 O 3 would provide different conductive channels. The impact on composite materials was a process of trade-offs, and they must cooperate with each other for a better overall effect.
Since aramid paper was made of fibers with different structures bonded together through physical processes, there were many large physical pores and chemical impurities randomly distributed within its interior due to the randomness and inhomogeneity nature of the dispersion. The defects introduced localized or local energy state into the energy gap of the polymer, and these trapping energy levels could trap carriers, reducing their mean free path and then the accumulated energy. These structures present in polymers capable of trapping charges were defined as "traps" [43]. The trapped charges couldn't be neutralized from each other through recombination, and the internal electric field formed by them would affect the breakdown, aging and flashover characteristics of the dielectric under high field strength [44]. According to the isothermal surface potential decay method (ISPD), [45] Trap properties could be described by the trap density I(T) and the trap depth E T . I(T) and E T were obtained by Eqs. (2) and (3).
where ε 0 was the vacuum permittivity, ε r was the relative permittivity, e was the elementary charge, L was the thickness of the paper sample, dU/dT was the decay rate of the surface potential, k B was the Boltzmann constant, T was the temperature, v was the charge escape frequency, here the low frequency field was 4.17 × 1013 s −1 .
The trap distribution curves of the composites insulating material calculated by formula (2) and (3) are shown in Fig. 9d, e. The center of swallow and deep trap was listed in Table 2. It could be seen that P-0 had both deep and shallow traps. The energy center of shallow trap and deep trap was 0.932 and 1.015 eV, respectively. The depth and density of the deep traps were improved, and the shallow traps of P-10 disappeared After PDDA modification, and the depth of the deep traps was 1.040 eV at most. It could be obtained that the interface properties between the modified Al 2 O 3 and the aramid matrix were improved. The depth and density of the deep traps showed a saturated decreasing trend and the shallow traps reappeared with the continuous increase in PDDA, indicating that excessive use of PDDA would introduce new physical defects and chemical impurities and change the structure of deep traps or introduce shallow traps. The depth and density of traps first increased and then decreased with the increase in Al 2 O 3 content, and both were dominated by deep traps, confirming that the combination effect of aramid fiber and filler was better when the mass fraction of PDDA was 10%. Shallow traps reappeared when the filler content was 7%, and the density of deep traps decreased. The reason might be that many composite Nano-fillers could cause secondary agglomeration, resulting in an increase in particle size and specific surface area of the filler. Therefore, Al 2 O 3

3
Enhanced Aramid/Al 2 O 3 interfacial properties by PDDA… couldn't be closely combined with the matrix, and introduce more defects and interface problems, resulting in deep traps shallower. The breakdown strength of aramid nanocomposites had a very similar trend to the depth and density of deep traps, so it could be considered that deep trap was another important factor affecting the breakdown properties (Table 2).

Molecular dynamics simulation
Aramid paper is composed of chopped fibers and precipitated fibers with the same structural formula. They are zigzag long molecular chains obtained by the polymerization of isophthalic acid and m-phenylenediamine as monomers with the chemical formula of (C 14 H 10 N 2 ) n . Aramid fibers exist in two states in the paper: crystalline and amorphous region. The molecules in the crystalline region are compact and ordered, while the molecules in the amorphous region are irregularly distributed and have larger air strikes and smaller intramolecular and intermolecular forces. The crystallinity of PMIA is about only 20 ~ 30%. Therefore, this paper only considered the construction of aramid amorphous structure, and all the simulations were carried out in the non-unit cell model in Accelrys Materials Studio (MS) software. Firstly, a PMIA amorphous box with a density of 1.3 g/cm 3 , a force field of COMPASS II, and a single molecular chain length of 10 was built using the Amorphous Cell module [46]. The alumina supercell was imported from the "Building" module database to form nanostructures, and the (012) section of the unit cell with a cutting thickness of 25 Å was selected for the study because this section exposed all types of O and Al elements. The PDDA molecular chain was built in building model. PMIA, Al 2 O 3 and PDDA models are shown in Fig. 10a, b, and c, respectively. The interface model between aramid fiber and aluminum dioxide using the Build layer is shown in Fig. 10c. Since the two materials were periodically distributed in the simulation setting, the thickness of the vacuum layer was set to 50 Å to prevent the influence of adjacent materials. Based on the above model, PDDA was introduced into the interface with a trans structure and DP of 1, as shown in Fig. 10d.
After the model was built, structural optimization was needed to make it more reasonable. The optimization step was 200,000 to ensure that the most stable and approximate structure of the real material was achieved [47]. The stable working temperature of aramid paper is about 453 K. Hence, 298 ~ 448 K was chosen as the simulation temperature range, and 4 certain temperatures with 50 K as the step for NVT dynamics simulation, adopting Andersen temperature control method. The total time step was 500 ps, each step was 1 fs, and the dynamic characteristics of each atom in the system were collected every 5000 ps for subsequent analysis. The initial velocity followed the Boltzmann distribution, the external pressure was standard atmospheric pressure, and the Berendsen method was used for control. Electrostatic interaction and van der Waals function were adjusted by Ewald and Atombased method, respectively, and the Dreiding force field was used for both structural optimization and dynamic simulation.
In nanocomposites, the interface accounts for a high-volume fraction, and the interface properties have a very important influence on the physical and chemical properties of the composites. The interfacial binding energy is often used to indicate the strength of intermolecular interactions in polymers. The larger the absolute value, the more energy required to break the interface and the more stable the structure. The definition is as follows: where E interaction was the interface binding energy, E total was the overall energy, E 1 was the energy possessed by PMIA, E 2 was the energy possessed by the alumina crystal plane. The interfacial binding energies of the model before and after modification at different temperatures are shown in Fig. 10e. With the increase in the simulation temperature, the models before and after modification showed a saturated increasing trend, and the increase rate was fast at first and then slow. The main reason was that when the temperature was low, a large number of hydrogen bonds could be formed between the interfaces, while high temperature would hinder the generation of hydrogen bonds. It could be clearly found that the interface binding energy of the modified model was significantly improved at different temperatures than before the modification, which was because PDDA could introduce N atoms to form NH…O hydrogen bonds and enhance the interaction between the components, thereby improving the dielectric properties of the composite material.

Conclusion
In this paper, PDDA was successfully coated on the surface of nano-Al 2 O 3 by electrostatic interaction and the modified nano-Al 2 O 3 exhibited great dispersion in the aramid fiber matrix. The effect of PDDA fraction and filler content was carefully studied and optimized formula was achieved. Satisfactory improvement in insulation performance and breakdown strength was observed for Al 2 O 3 /PMIA composite paper. After co-doping of 10 wt% PDDA and 3wt% Al 2 O 3 , the paper with conductivity of 3.62 × 10 -17 S/m which decreased by 53%, and breakdown strength of 39.3 kV/mm which increased by 20%. Moreover, the effect mechanism of PDDA on the electrical properties of Al 2 O 3 /PMIA composite insulating paper was studied. PDDA can increase the electrostatic repulsion between nano-Al 2 O 3 particles, which is conducive to the dispersion of fillers in the composite matrix; in addition, the introduction of PDDA reduced the interfacial binding energy of alumina and aramid by 39%, and improved the interfacial binding force between fillers and matrix. In conclusion, we believe this research can provide both theoretical support and practical guidance for the development of high-performance insulation composites.