Preparation and Dielectric Sensitivity of Polyurethane Composite Fiber Membrane Filled with BaTiO3

Polyurethane dielectric elastomer (PUDE) is considered a potential underwater flexible actuator material due to its excellent designability and environmental tolerance at the molecular level. Currently, the application of the polyurethane elastomer as an actuating material is constrained by such problems as the conflict between various properties such as dielectric properties and modulus and the low level of dielectric sensitivity. This is a common challenge facing polyurethane dielectric research related to the uneven distribution of dielectric fillers in the matrix. Besides, another challenge for the academic circles is the easy agglomeration of micro and nanofillers. Given the above-mentioned background of the application and technical problems, the coaxial electrospinning technology is proposed in this paper. The polyurethane fiber network is constructed with the preferred hydrolysis resistant polyether-Diphenylmethane diisocyanate (MDI) thermoplastic polyurethane elastomer as the matrix material. Dispersed by ultrasound, the micro nano dielectric filler is integrated into polyurethane fiber through the coaxial dual-channel design. Additionally, directional constraint molding is conducted to improve the agglomeration of small-scale particles induced by the loss of mechanical energy in traditional blending. After characterization, the distribution of BaTiO3 particles in the fiber bundle is relatively uniform. Compared to the polyurethane dielectric composites prepared by traditional blending (BaTiO3-Dielectric Elastomer, BaTiO3-DE), the dielectric sensitivity factor of the polyurethane composite fiber membrane (BaTiO3-Dielectric Elastomer Membrane, BaTiO3-DEM) is enhanced by over 25%; the electrostrictive strain of BaTiO3-DEM is boosted by least 10%.


Introduction
The concept of "electro active polymer" (EAP) was first proposed in 1880 by German scholar Roentgen who discovered that natural rubber film can deform under the action of an electric field [1]. After more than a century of constant development, there has been a considerable range of EAP materials developed and produced in large amounts for the research of new intelligent materials. Generally, EAP materials can be divided into ionic activated EAP and electronic activated EAP following the exact mechanism of deformation.
As a typical variety of electronic EAP materials, dielectric elastomer (DE) demonstrates advantages of significant electric deformation, excellent elasticity and flexibility, high electromechanical conversion efficiency and fast response. For this reason, it has the potential of widespread application in the research of intelligent materials, including bionic mechanical design, structural vibration and noise reduction [2][3][4][5].
Polyurethane dielectric elastomer (PUDE), as a novel variety of intelligent DE materials, presents high polarity, making it prone to orientation polarization under the electric field, high dielectric constant and large electro-strictive strain [6][7][8][9]. Meanwhile, this sort of material is characterized by a fast response (millisecond level), excellent mechanical properties and high environmental tolerance. The flexible actuator made of the material demonstrates structural simplicity, large stress-strain and the high efficiency of electromechanical conversion. Additionally, thermoplastic polyurethane (TPU) can lead to the occurrence of physical cross-linking owing to intermolecular hydrogen bonding. When heated, this material can transform into liquid before returning to an elastomer state because of cooling. This diversifies the method of TPU processing while endowing PUDE with the massive potential for development.
Concerning such intelligent actuating materials as PUDE, the barrier to research lies in how other properties of the material can be balanced with the improvement of its dielectric properties, which is significant to lowering the risk of excessively high actuating voltage [10]. Currently, the most effective solution to the dielectric modification of PUDE is to disperse the dielectric filler into PUDE; however, it remains difficult to prevent the agglomeration of filler in the matrix, as it can often result in an electrical breakdown [11,12]. Simultaneously, there is a sharp increase in the modulus and dielectric loss of the material induced by the presence of filler in large amounts, causing a significant deterioration in the comprehensive properties of the composite dielectric materials [13][14][15]. Hence, balancing the increase in material modulus with the improvement of dielectric properties possessed by PUDE is essential for enhancing the dielectric sensitivity of the material.
In this paper, the introduction of coaxial electrospinning technology is proposed [16]. First, the polyether MDI TPU was adopted as the matrix material intended for the construction of polyurethane dielectric fiber membrane. Then, BaTiO 3 dielectric filler [17,18] was filled into a polyurethane fiber bundle with the coaxial dual channel under ultrasonic dispersion. Additionally, the directional constraint molding method was employed to alleviate the agglomeration of small-scale particles caused by the loss of mechanical energy in traditional blending. In this way, the limited amount of BaTiO 3 can perform its dielectric function to the fullest, so as to improve the dielectric sensitivity of polyurethane materials. Table 1 shows the main reagents information used in this paper.  Table 2 shows the main devices information used in this paper.  Table 3 shows the design formula of PUDE in this paper.  Table 3 and place the mold release agent to the molds with a thickness of 0.5 mm in a 100 • C environment.

The Main Reagents
(2) Subsequently, pre-mix the PUP and BaTiO 3 at the temperature of 75 • C, then place them into a shear mixer and regulate at the rotational speed of 5000 r/min for deep mixing (when BaTiO 3 exists). Label the mixture as material A. Moreover, mix the measured PA, BDO and DABCO evenly at 42 • C, and label the mixture as material B.
(3) Mix A and B. Then, place the mixture in a vacuum oven to remove bubbles. Next, transfer the mixture to the mold and mature at 100 • C for 24 h. Finally, eject the samples out and place them at the ambient temperature for one week for further tests.

Preparation of Polyurethane Fiber Membrane Filled with BaTiO 3
(1) Technical description and recipe setting Figure 1 illustrates a schematic diagram of the coaxial spinning device used in this experiment, which consists of a coaxial spinning generator, fiber membrane collection device and electrospinning control center. The mechanism is described as follows. The spinning solution in the two channels exhibited at the left end of the schematic diagram gives rise to Taylor cone flow because of high voltage. Then, the continuous fiber bundle accumulates on the collection roller and ends up with the generation of fiber film. The adjustment can be made to the concentration of the spinning solution, the speed of glue pushing, the size of voltage, the size of pipe diameter, the scanning speed of the spinning head, the distance from pipe orifice to the roller, the direction of rotation for the roller and environmental factors such as temperature and humidity. Furthermore, the micromorphology of the fiber membrane can be adjusted to obtain an ideal polyurethane composite fiber membrane.    Table 4 presents the component settings of a shaft sleeve and axial spinning solution in the coaxial dual channel. Notably, the thickness and composition of the polyurethane fiber film filled with the BaTiO 3 prepared in this section (BaTiO 3 -Dielectric Elastomer Membrane, BaTiO 3 -DEM) are consistent with those of BaTiO 3 -DE; the thickness measurement of this part depends on a thickness gauge with an accuracy of 1 µm. In other words, the approach facilitates the comparison in overall performance with BaTiO 3 -DE. Table 4. Setting of a shaft sleeve and axial spinning solution. (2) Experimental process (1) Inject the prepared spinning solution into the shaft axis and shaft sleeve channel, adjust the initial position of the spinning head, fix the tin foil on the collecting roller and then close the safety door when the preparation is complete.

Component DE 1 -DMF (phr) BaTiO 3 -DMF (phr)
(2) Switch on the control center, and then adjust the distance between the spinning head and the roller, glue pushing speed, high voltage valve, roller speed, scanning starting point and distance successively. Next, the spinning parameters are determined through the shape of the Taylor cone flow of the spinning head.
(3) At the end of spinning, it is confirmed that all parameters of the control center are closed. Then, the safety door is opened. The tinfoil is carefully removed from the roller and transferred to the blast oven. The temperature is set to 30 • C for 12 h. Finally, the fiber film is removed from the tin foil to conduct the characterization test. Among them, BaTiO 3 -DEM 1 can be prepared using the traditional electrospinning method, which is not detailed in this paper.

Characterization Projects
The first step is to examine the micromorphology of the cross-section of the fibrous membrane using the scanning electron microscope to observe the dispersion of BaTiO 3 in the fiber bundle. The device information is listed in Table 5. Table 5. The device information.

Device Model Remarks
Scanning Electron Microscope Zeiss Merlin The signal sources were backscattered signal and secondary electron signal; the accelerating voltage was 20 kV.

Density and Hardness Test
The density and hardness of BaTiO 3 -DEM of the fully prepared network structure are supposed to be lower than those of BaTiO 3 -DE. Therefore, the density and hardness tests are performed separately.
According to the national standard GB/T4472-2011, the sample mass m is measured with an analytical balance first. Then, the sample volume V is measured with a scale. The density of the sample is obtained using the formula ρ = m/V.
According to the national standard GB/T531.1-2008, the hardness of the sample is examined using the TH-200 hardness tester.

Dielectric Sensitivity Factor Test
The second step is to perform a dielectric sensitivity factor on the polyurethane composite fiber membrane. Considering that the dielectric sensitivity factor β is expressed in Equation (1), the dielectric constant and elastic modulus are tested as follows.
It can be seen from this equation that for the dielectric material, the greater the dielectric constant and the lower the elastic modulus, the higher the dielectric sensitivity factor. Therefore, it is essential to balance the elastic modulus of materials while improving the dielectric properties of materials.
(1) Dielectric Constant Test The dielectric properties of the samples are measured using the dielectric constant tester of model 6632-1s from the Teng Skye company. The test frequency ranges from 10 Hz to 500 Hz, the test temperature is room temperature and the sample size is 10 mm.
(2) Elastic Modulus Test According to the national standard GB/T 528-1998, the sample is cut into multiple 2 mm × 5 mm × 2 mm dumbbell-shaped splines, with the tensile rate as 200 mm/min. The elastic modulus is calculated based on the slope of the initial part (deformation less than 5%) on the stress-strain curve. It is necessary to take the median value of five parallel test values.

Breakdown Voltage and Electrostrictive Strain Test
The perfluoropolyether conductive electrodes are uniformly coated on both sides of the dielectric elastomer; then, a simple driving unit is obtained. Next, the variable voltage is applied by the high-voltage generator on the dielectric elastomer coated with the electrode. At this time, the area changes in the dielectric elastomer are recorded using the high-definition digital camera and scale. This phenomenon is called electrostrictive strain. Afterward, the recorded image is extracted and recognized, while the electric deformation of the dielectric elastomer is obtained under different loading voltages. The schematic diagram of electrostrictive strain and the electrical breakdown test are exhibited in Figure 2.  On this basis, the loading voltage is continuously boosted until electric breakdown occurs to the dielectric elastomer. Meanwhile, the voltage is recorded as breakdown voltage. On this basis, the loading voltage is continuously boosted until electric breakdown occurs to the dielectric elastomer. Meanwhile, the voltage is recorded as breakdown voltage.

Result Analysis
(1) Filling state of BaTiO 3 in fiber bundle With BaTiO 3 -DEM 2 as an example, the diagram of the electron microscope analysis and energy spectrum analysis for the distribution of BaTiO 3 in polyurethane fiber is presented in Figure 3. The red area in Figure 3a suggests that the white bright spots (BaTiO 3 ) present a regional directional arrangement in the fiber bundle. Figure 3b indicates the contents of elements Ti and Ba. Figure 3c shows the corresponding Figure 3a. According to the energy spectrum analysis of the medium red area, the diagram of Ba element distribution and that of Ti element distribution on the right are obtained, respectively, after element separation. It reveals that Ba elements and Ti elements are evenly distributed in the fiber membrane. The above results suggest that BaTiO 3 is oriented in the fiber bundle, and BaTiO 3 is basically uniformly distributed in the fiber membrane. According to past research, when BaTiO 3 is added to the polymer matrix by more than five parts, due to the strong dipole interactions between the polarized BaTiO 3 spheres, a more pronounced particle aggregation is observed [19], and the properties of composites will be greatly affected. Therefore, it is very meaningful to make the limited amount of BaTiO 3 uniformly filled in the polyurethane matrix.  (2) Density and hardness Table 6 shows the density and hardness data of the BaTiO3-DE series and BaTiO3-DEM series samples, respectively.  (2) Density and hardness Table 6 shows the density and hardness data of the BaTiO 3 -DE series and BaTiO 3 -DEM series samples, respectively. According to Table 6, the density and hardness of the BaTiO 3 -DEM series' materials have declined to varying degrees compared with the BaTiO 3 -DE series' materials produced through traditional mechanical blending. This is because the BaTiO 3 -DEM series' materials possess a fiber network structure, and the internal multi void structure plays a role in weakening their density and hardness. This structure contributes to a decrease in the modulus of materials, as listed in Table 7. (3) Dielectric sensitivity factor The above table shows the dielectric properties, modulus and dielectric sensitivity factor of the BaTiO 3 -DE series and BaTiO 3 -DEM series samples at 10 Hz and 100 Hz. The modulus results are reciprocally substantiated with the above contents. Figure 4 is presented according to the contents in the table to compare the differences in performance between the coaxial spinning fiber film filled with BaTiO 3 and the dielectric elastomer. (3) Dielectric sensitivity factor The above table shows the dielectric properties, modulus and dielectric sensitivity factor of the BaTiO3-DE series and BaTiO3-DEM series samples at 10 Hz and 100 Hz. The modulus results are reciprocally substantiated with the above contents. Figure 4 is presented according to the contents in the table to compare the differences in performance between the coaxial spinning fiber film filled with BaTiO3 and the dielectric elastomer. As observed in the above figure, the dielectric sensitivity factor of the BaTiO3-DEM series' materials prepared by coaxial spinning technology is improved by at least 25% compared to the BaTiO3-DE material formed by traditional blending. Specifically, the di- As observed in the above figure, the dielectric sensitivity factor of the BaTiO 3 -DEM series' materials prepared by coaxial spinning technology is improved by at least 25% compared to the BaTiO 3 -DE material formed by traditional blending. Specifically, the dielectric sensitivity factor is increased by more than 130% when the test frequency and the additional amount of BaTiO 3 reach 10 Hz and 0.5%. Following the data listed in Table 7, the above results are closely associated with a significant reduction in the modulus of BaTiO 3 -DEM materials produced by coaxial spinning.
(4) Breakdown voltage and electrostrictive strain Table 8 shows the maximum breakdown voltage and electrostrictive strain of BaTiO 3 -DE series and BaTiO 3 -DEM series samples at 5 kV, 10 kV and 30 kV, respectively. Figures 5 and 6 are presented based on the contents of the table to facilitate the comparison in performance between the coaxial spinning fiber film filled with BaTiO 3 and the traditional blend dielectric elastomer.   The figure suggests that the maximum breakdown voltage of BaTiO3-DE material obtained by the traditional blending method drops significantly with a progressive increase in the amount of the BaTiO3 addition. In contrast, the maximum breakdown voltage of the BaTiO3-DEM material produced by the coaxial spinning technology reveals barely any reduction. Generally, the breakdown strength is related to the distribution of dielectric fillers in the matrix. The higher the content of micro and nano dielectric fillers, the less difficult the agglomeration of fillers in the matrix, and the easier it is to break down. The results in the figure demonstrate that the application of coaxial spinning technology plays a certain role in improving the distribution of the micro-nano BaTiO3 dielectric filler in polyurethane materials.   The figure suggests that the maximum breakdown voltage of BaTiO3-DE material obtained by the traditional blending method drops significantly with a progressive increase in the amount of the BaTiO3 addition. In contrast, the maximum breakdown voltage of the BaTiO3-DEM material produced by the coaxial spinning technology reveals barely any reduction. Generally, the breakdown strength is related to the distribution of dielectric fillers in the matrix. The higher the content of micro and nano dielectric fillers, the less difficult the agglomeration of fillers in the matrix, and the easier it is to break down. The results in the figure demonstrate that the application of coaxial spinning technology plays a certain role in improving the distribution of the micro-nano BaTiO3 dielectric filler in polyurethane materials. The figure suggests that the maximum breakdown voltage of BaTiO 3 -DE material obtained by the traditional blending method drops significantly with a progressive increase in the amount of the BaTiO 3 addition. In contrast, the maximum breakdown voltage of the BaTiO 3 -DEM material produced by the coaxial spinning technology reveals barely any reduction. Generally, the breakdown strength is related to the distribution of dielectric fillers in the matrix. The higher the content of micro and nano dielectric fillers, the less difficult the agglomeration of fillers in the matrix, and the easier it is to break down. The results in the figure demonstrate that the application of coaxial spinning technology plays a certain role in improving the distribution of the micro-nano BaTiO 3 dielectric filler in polyurethane materials.
According to this figure, the electrostrain of BaTiO 3 -DEM material under three voltage loads is more satisfactory compared to the BaTiO 3 -DE material, which is consistent with the magnitude trend exhibited by the dielectric sensitivity factors of these two materials. The increase in the electrostrictive strain of the BaTiO 3 -DEM material is more significant than that of the BaTiO 3 -DE material with the increase in the BaTiO 3 addition. This verifies that the application of coaxial spinning technology is advantageous over the traditional physical blending method in improving the dielectric properties of comprehensive dielectric materials. The result is directly related to the fact that the dielectric sensitivity factor of BaTiO 3 -DEM is greater than that of BaTiO 3 -DE. In the subsequent research on the electrostrictive properties of a new intelligent material, improving the dielectric sensitivity factor of the composite can be regarded as a research idea which will greatly improve the efficiency of scientific research.

Conclusions
It is a common and effective method to improve the dielectric sensitivity of the materials by blending the polyurethane dielectric elastomer and micro nano BaTiO 3 dielectric filler, so as to strengthen its electrostrictive strain for adaptation to the application in intelligent driving. In this paper, the polyurethane fiber membrane filled with BaTiO 3 is prepared by coaxial spinning technology. The conclusions of this study are drawn as follows.
(1) The dielectric sensitivity of the polyurethane fiber film filled with BaTiO 3 improves by more than 25% compared with BaTiO 3 -DE obtained by traditional physical blending. Additionally, the breakdown voltage of the former declines by nearly 50% with the increase of BaTiO 3 content, while that of the latter is merely 22.6%.
(2) Under the voltage loading of 5 kV, 10 kV and 30 kV, the electrostrictive strain of BaTiO 3 -DEM is 10% higher than that of BaTiO 3 -DE at minimum, with some reaching 50%.
Author Contributions: Writing-original draft preparation, formal analysis, G.L.; methodology, G.L. and C.S.; funding acquisition, C.S.; writing-review and editing, C.S. and X.Y.; investigation, resources, Y.L.; writing-review and editing, X.H. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.
Acknowledgments: The work is also strongly supported by the New Materials and New Technology Laboratory of the institute. Particular thanks to the editors and reviewers of the journal for the precious time in editing and reviewing.

Conflicts of Interest:
The authors declared that they have no conflict of interest in this work. We declare that we do not have any commercial or associated interest that represents a conflict of interest in connection with the work submitted.