Improved dielectric properties of PVDF nanocomposites with core–shell structured BaTiO 3 @polyurethane nanoparticles

: Polymer nanocomposites with improved dielectric permittivity and high breakdown strength are extremely desirable for the flexible electronic devices and power systems. The compatibility of fillers and polymer matrix is important in determining the dielectric and breakdown strength properties. The core–shell structure concept is useful to improve the compatibility of fillers with polymer matrix. Herein, an organic thermoplastic urethanes (TPU) polymer shell was successfully grafted on the surface of barium titanate (BaTiO 3 , BT) and such a TPU shell improved the permittivity and breakdown strength of TPU@BT/PVDF polymer nanocomposites greatly. The permittivity of TPU@BT/PVDF nanocomposites with 12 wt% fillers at 10 2 Hz was up to 13.5, which was 1.5 times higher than that of pure poly(vinylidene fluoride) (PVDF). The improvement of the dielectric properties could be attributed to the enhanced interfacial polarisation between BT nanoparticles and TPU shell. Besides, the compatibility of BT nanoparticles and PVDF matrix was improved after the introduction of TPU shell. Accordingly, a highest breakdown strength value about 373 MV/m was obtained for the TPU@BT/PVDF nanocomposites with 7 wt% fillers. The core–shell strategy could be extended to a variety of inorganic fillers to improve the dielectric and breakdown strength properties of polymer nanocomposites.


Introduction
High dielectric permittivity and high electrical breakdown strength materials are extremely desirable for the electric industry [1][2][3][4][5]. In the past few decades, great efforts have been made to develop flexible polymer nanocomposites with high dielectric permittivity and high electrical breakdown strength [6,7]. Polymers have good properties such as low dielectric loss, high breakdown strength, low cost and easy to process [8]. However, compared with inorganic fillers, the dielectric permittivity of polymers is relatively lower. More studies have been reported to fabricate inorganicorganic composites, which could integrate the advantages of inorganic and organic parts [9, 10]. Thus, the dielectric permittivity and breakdown strength could be simultaneously improved by the composites processing strategy.
The incorporation of inorganic fillers into polymer matrix to fabricate dielectric composites is one of the most effective way to increase the dielectric permittivity of polymer composites [11][12][13][14][15]. The dielectric permittivity of the composites can be effectively improved by adding inorganic fillers. Dang et al. [16] reported that the dielectric permittivity of poly(vinylidene fluoride) (PVDF) composites filled with barium titanate (BaTiO 3 , BT) particles increased significantly at low frequency. Meanwhile, the high breakdown strength of the composites can be maintained with the aid of polymer matrix. However, the obtained dielectric permittivity and breakdown strength of the composites cannot meet the requirements of the high-performance dielectric polymer composites. A high filler loading is usually needed to obtain a considerable dielectric permittivity, which would significantly deteriorate the flexibility and breakdown strength of polymer matrix [17]. The weak interaction between the inorganic fillers and polymer matrix hinders the improvement of the dielectric permittivity and breakdown strength at a small of filler loading [17,18]. Therefore, numerous efforts have been made to optimise the interface between the fillers and polymer matrix. For example, Zhou et al. [17] treated the surface of BT particles using hydrogen peroxide solution (H 2 O 2 ) and the obtained surface hydroxylated BT particles had strong interactions with PVDF matrix, thus resulting in the improvement of the dielectric properties. Dang et al. [16] modified the surface of BT particles with silane coupling agent KH 550. Zheng et al. coated the BT nanoparticles with ethylene propylene diene monomer (EPDM) and the polymer shell could be used to modify the compatibility between the polymer matrix and fillers, thus improving the dielectric properties [19]. In addition, the strategy of coating the metal particles with an organic polymer shell or inorganic shell has been widely adopted. Zhang et al. [20] coated the silver (Ag) particles with a polydopamine shell to optimise the dispersion of Ag particles in polymer matrix. Dang et al. [21] coated the Ag particles with a titanium dioxide (TiO 2 ) shell and the thickness of the inorganic shell could be controlled. The purpose of utilising metal particles in their work was trying to increase the dielectric permittivity of the composites and the purpose of utilising the TiO 2 shell was trying to improve the compatibility between Ag particles and polymer matrix.
The excellent compatibility of inorganic fillers and polymer matrix is extremely desirable for the design of composites [22][23][24]. The compatibility has a huge influence on the dielectric permittivity and breakdown strength of the composites. Our previous work demonstrated that the addition of TPU into PVDF showed excellent compatibility between the filler and matrix, and the breakdown strength of TPU/PVDF composite with proper loading of TPU was enhanced significantly due to the defect modification of TPU in the amorphous area of PVDF, which indicated a potential way to improve the properties of dielectrics [25]. Coating the inorganic fillers with an organic polymer shell is more effective to improve the compatibility between the fillers and polymer matrix, thus improving the dispersibility of inorganic fillers in polymer matrix [26]. In this work, we coated the BT nanoparticles with thermoplastic urethanes (TPU) shell and incorporated these core-shell structured TPU@BT fillers into PVDF matrix. The introduction of the insulating TPU shell on the surface of BT nanoparticles not only enhanced the compatibility between BT nanoparticles and PVDF matrix, but also improved the dielectric properties and breakdown strength of polymer composites.

Materials
The BT nanoparticles were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD. TPU and PVDF polymers were provided by Bayer Company and Arkema Corporation, respectively. 4, 4′-diphenylmethane diisocyanate (MDI) was purchased from Alfa Aesar Company. N, N-dimethylformamide (DMF) and methylbenzene were offered by Sinopharm Chemical Reagent Co., LTD.

Preparation of the TPU@BT/PVDF nanocomposites
Firstly, 3 g BT nanoparticles were dispersed into 300 ml solution with 30 wt% H 2 O 2 , and then the mixture was sonicated for 15 min with the aid of magnetic stirring. After that, the mixture was heated to reflux for 6 h at 106°C in a round-bottomed flask. Afterwards, the modified BT nanoparticles were washed with deionised water for three times through centrifugation treatment. The obtained powders were subsequently dried in the vacuum oven overnight. These powders were denoted as HO-BT.
Secondly, 6 g MDI was dissolved into 100 ml methylbenzene solution with the help of stirring. After completely dissolved, 3 g HO-BT nanoparticles were added into above solution and they were reacted at 60°C for 6 h. After the reaction, the obtained powders were washed with toluene for three times to remove the excess MDI. Then these powders were vacuum freeze-dried at −55°C for 8 h. These powders were denoted as MDI@BT.
Thirdly, 3 g TPU were dissolved in 20 ml DMF solution at 35°C, and then a certain number of MDI@BT nanoparticles were added into above TPU solution and they were reacted for 8 h at room temperature. Finally, the obtained powders were centrifuged and washed with DMF for several times. After the drying process, the TPU@BT nanoparticles were obtained.
Lastly, the prepared TPU@BT nanoparticles were introduced into PVDF matrix with the aid of solution blending method in DMF solvent. Then the solution was cast on the glass plates to fabricate TPU@BT/PVDF nanocomposites film. The films with various filler concentrations were prepared for the following characterisations.

Characterisation
Thermal gravimetric analyser (TGA, SDT Q600) was used to test the thermal stability property of the nanoparticles at a heating rate of 15°C/min from 30 to 800°C in the nitrogen atmosphere. Fourier transform infrared spectroscopy characterisation was carried out to check the typical absorption peaks of the functional groups. The dispersion of nanoparticles in polymer matrix was observed by scanning electron microscope (SEM, Hitachi, S-4800). The morphology of the TPU@BT nanoparticles was directly observed by transmission electron microscope (TEM, Hitachi HT7700). Impedance analyser (Agilent 4294A) was applied to measure the dielectric properties of the nanocomposites. DC breakdown strength of the nanocomposites was tested using voltage tester (Changsheng, CS2674A). Fig. 1 shows the preparing process of core-shell structured TPU@BT nanoparticles. Generally, the polymerisation of TPU is carried out by the reaction of the hydroxyl and isocyanate groups. In our work, the first step was the hydroxylation of BT nanoparticles. Then the HO-BT nanoparticles were reacted with the MDI monomer to fabricate MDI@BT. Lastly, the TPU was grafted onto the surface of BT nanoparticles and the obtained nanoparticles were denoted as TPU@BT. Fig. 2a displays the characteristic absorption peaks of the functional groups in BT and modified BT nanoparticles. For example, the broad peak at 3430 cm −1 demonstrated the successful hydroxylation of BT. In addition, the displayed peak at 2250 cm −1 can be attributed to the -N = C = O group, which indicates that MDI and TPU have been grafted onto the BT surface successfully. Fig. 2b exhibits the TGA curves of the BT and modified BT nanoparticles. Compared with the pristine BT nanoparticles, MDI-BT and TPU-BT exhibit the obvious weight loss with increasing the temperature. More importantly, the total weight loss of TPU-BT is higher than that of MDI-BT and the TGA curve of TPU-BT shows two decomposition stages, which indicates the successful grafting of TPU on the surface of BT nanoparticles.

Results and discussions
As shown in Fig. 3, an obvious core-shell structure of TPU@BT is observed as compared with the pristine BT nanoparticles and the thickness of TPU shell is about 5 nm.
The SEM images of the fracture surfaces of TPU@BT/PVDF nanocomposites with different filler loadings are shown in Fig. 4. As shown in Figs. 4a-c, the core-shell structured TPU@BT nanoparticles disperse well in the PVDF matrix as the filler concentration is below 10 wt%. However, the aggregations of TPU@BT nanoparticles are easily observed with increasing the filler concentration to 12 wt%. The severe aggregation of TPU@BT nanoparticles will lead to the deterioration of breakdown strength.
As shown in Fig. 5, the dielectric permittivities of TPU@BT/ PVDF nanocomposites increase with increasing the loading of TPU@BT nanoparticles at the frequency range of 10 2 -10 6 Hz. The permittivity of nanocomposites with 12 wt% TPU@BT loading reaches 13.5 at 10 2 Hz, which is 1.5 times higher than that of pure PVDF. The increased permittivity could be attributed to the intrinsic high permittivity of BT nanoparticles and the introduced interfacial polarisation between BT nanoparticles and TPU shell. While the barely changed dielectric loss with increasing the filler concentration from 0 to 10 wt%, probably because the insulating TPU shells on BT nanoparticles hinder the direct contact between them and decrease the leakage current. While the increased dielectric loss of TPU@BT/PVDF with 12 wt% fillers was possibly due to the nanoparticle agglomeration as shown in Fig. 4d  [1, 3]. Fig. 6 shows the Weibull distribution for the breakdown strength of TPU@BT/PVDF nanocomposites and the relationship between the breakdown strength and filler concentrations. The introduction of polymer shell on the surface of ceramic fillers could usually enhance the breakdown strength of polymer   composites. As shown in Fig. 6b, the introduction of a small number of TPU@BT nanoparticles into PVDF matrix can enhance the breakdown strength. The breakdown strength increases firstly from 350 MV/m for pure PVDF to 373 MV/m for the nanocomposites loaded with 7 wt% TPU@BT. However, with further increasing the filler concentration to 12 wt%, the breakdown strength decreases to 269 MV/m. The decrease in breakdown strength is resulted from the increase of defects and holes in the nanocomposites when the filler concentration is above 7 wt%. According to the dispersion of TPU@BT nanoparticles in PVDF matrix discussed above, the severe aggregation of TPU@BT nanoparticles was observed for the nanocomposites with 12 wt%, which leads to the increase of defects and holes. Therefore, the breakdown strength of PVDF nanocomposites decreases from 373 to 269 MV/m. The improvement of breakdown strength could be explained by the microstructure and molecular chain conformation of TPU shell on BT nanoparticles. The long TPU molecular chain contains -N = C = O groups and these groups may form hydrogen bond with PVDF molecules chain, which improves the compatibility of nanoparticles and PVDF matrix. Accordingly, the number of defects in the nanocomposites will be suppressed, thus enhancing the breakdown strength of PVDF nanocomposites. Meanwhile, the soft segments in TPU molecular chains can adjust their conformations to make them interact with PVDF molecular chain well. Therefore, the number of defects in the PVDF nanocomposites will be decreased and the breakdown strength will be improved subsequently.
To exhibit the improvements of TPU@BT/PVDF nanocomposites, some of our previous works were listed in Table 1. For example, comparing with thermal treated BaTiO 3 / PVDF, Al 2 O 3 @BaTiO 3 /PVDF-HFP and Al 2 O 3 @BaTiO 3 /PVDF [27][28][29], this work showed much higher breakdown strength due to the successful surface modification. In addition, the dielectric permittivity of TPU@BT/PVDF nanocomposites was about 1.9 times higher than that of EPDM@BT/PP [19].

Conclusions
The dielectric permittivity and breakdown strength were simultaneously improved by grafting TPU polymer chains on the surface of BT nanoparticles. The introduced interfacial polarisation between BT nanoparticles and TPU shell leads to the increase of dielectric permittivity. The introduction of TPU polymer shell could improve the dispersibility of BT nanoparticles in PVDF matrix. Therefore, the compatibility of BT nanoparticles and PVDF matrix was improved. The interaction of -N = C = O groups in TPU molecular chains and PVDF chains as well as the adjustment of the soft segments in TPU molecular chains were helpful to decrease the defects and holes in the nanocomposites, which would lead to the improvement of the breakdown strength. The core-shell strategy employed here provides an effective method to enhance the dielectric and breakdown strength properties of polymer nanocomposites.