Title: Fabrication and actuation characterisation of a new UV curing acrylic dielectric elastomer
Abstract: IET NanodielectricsVolume 5, Issue 2 p. 104-111 ORIGINAL RESEARCHOpen Access Fabrication and actuation characterisation of a new UV curing acrylic dielectric elastomer Wen-Zhuo Dong, Wen-Zhuo Dong State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, ChinaSearch for more papers by this authorYu Zhao, Corresponding Author Yu Zhao [email protected] orcid.org/0000-0003-1596-9431 School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan, China Correspondence Zhi-Min Dang, State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] Yu Zhao, School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan 450001, China. Email: [email protected]Search for more papers by this authorLi-Juan Yin, Li-Juan Yin State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, ChinaSearch for more papers by this authorZhi-Min Dang, Corresponding Author Zhi-Min Dang [email protected] State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, China Correspondence Zhi-Min Dang, State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] Yu Zhao, School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan 450001, China. Email: [email protected]Search for more papers by this author Wen-Zhuo Dong, Wen-Zhuo Dong State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, ChinaSearch for more papers by this authorYu Zhao, Corresponding Author Yu Zhao [email protected] orcid.org/0000-0003-1596-9431 School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan, China Correspondence Zhi-Min Dang, State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] Yu Zhao, School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan 450001, China. Email: [email protected]Search for more papers by this authorLi-Juan Yin, Li-Juan Yin State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, ChinaSearch for more papers by this authorZhi-Min Dang, Corresponding Author Zhi-Min Dang [email protected] State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing, China Correspondence Zhi-Min Dang, State Key Laboratory of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China. Email: [email protected] Yu Zhao, School of Electrical Engineering, Zheng Zhou University, Zhengzhou, Henan 450001, China. Email: [email protected]Search for more papers by this author First published: 12 March 2022 https://doi.org/10.1049/nde2.12035Citations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Most commonly used dielectric elastomers (DEs) such as acrylic dielectric elastomers VHBTM 4910 need a high actuation voltage and pre-stretching to obtain a large actuation strain, and present high mechanical loss caused by viscoelasticity. In this work, we fabricated a new acrylic elastomer by UV curing based on CN9021 and lauryl acrylate. By manipulating crosslinker content, crosslink density changed and physical entanglements of the new material can be affected. Therefore, mechanical properties such as Young's Modulus and mechanical loss of the new material can be controlled, and little change of its glass transition temperature was induced. Results of the actuation test show that the new DE is capable of 9.0% actuation area strain under 11 kV/mm and a good performance under oscillating voltage with different waveforms and frequencies. 1 INTRODUCTION With the development of technology, human beings have been putting forward the demand for materials of intelligence, flexibility and versatility. As a type of functional material, smart materials can perceive external environment and perform certain functions according to the external environment, which are one of the materials for the future [1-3]. Among them, dielectric elastomer (DE) is a potential soft material which can realise mutual conversion between electrical and mechanical energy [4]. As an actuator, DE can convert electrical energy directly into mechanical energy in a simple way, and comparable to natural muscles [5-7]. Coating flexible electrodes such as conductive carbon grease on both sides of the DE film and applying voltage between the electrodes, the DE film can deform due to Maxwell stress, becoming thin in the thickness direction and expanding in the plane. This Maxwell stress can be equivalent to a uniaxial compressive stress p in the direction of film thickness [8]: p = ϵ 0 ϵ r E 2 $p={{\epsilon}}_{0}{{\epsilon}}_{r}{E}^{2}$ (1)where ϵ0 is the permittivity of the vacuum, ϵr is the relative permittivity of DE, and E is the applied electric field. Actuation strain in thickness sz can be represented as: s z = − p Y = − ε 0 ε r E 2 Y = − ε 0 ε r U z 2 Y ${s}_{z}=-\frac{p}{Y}=-\frac{{\varepsilon }_{0}{\varepsilon }_{r}{E}^{2}}{Y}=-\frac{{\varepsilon }_{0}{\varepsilon }_{r}{\left(\frac{U}{z}\right)}^{2}}{Y}$ (2)where Y is the Young's modulus of DE, U is the applied voltage and z refers to the current thickness of DE. As shown in Equation (2), to gain a large actuation strain, high ϵr, low Y and high E are demanded [9]. However, at present the commonly used DEs cannot reach an enough high permittivity. For example, the permittivity of VHBTM 4910 is about 4.5 and that of silicone rubber without modification is about 3 [10], due to the limitation of their molecular structures. At the same time, in order to have the ability to withstand mechanical forces, Young's modulus of DE cannot be too low and thickness cannot be controlled being too thin. These make the actuation voltage a high value up to thousand volts to obtain a large actuation strain [11]. High actuation voltage may cause electrical breakdown and electromechanical instability (EMI) of DEs [12, 13]. Besides, high viscoelasticity of DE leads to low energy conversion efficiency, which is apparent in DEs such as VHBTM 4910 [14]. These defects mentioned above significantly limit the application of DEs. A lot of work aimed at reducing actuation the voltage of DEs, preventing premature failure caused by electrical breakdown or EMI to improve the actuation strain of DEs at low actuation voltage, and at the same time focussing on decreasing the viscoelasticity of DEs. Based on the VHBTM acrylic elastomer produced by 3M, Pelrine et al. found that pre-stretching the original DE films can significantly improve the actuation performance, which makes them thinner, decreases actuation voltage, enhances actuation strain and also improves the breakdown field strength [5]. Koh et al. showed that the main reason for the pre-stretched VHBTM acrylic elastomer film's improving actuation strain is that pre-stretch modifies the stress-strain curve of DEs [15]. By pre-stretching or designing 'short chain' polymer network, a better voltage-stretch curve can be achieved to suppress EMI and a higher actuation electric field can be achieved to enhance the actuation strain. However, this theory still has limitations and cannot be promoted to all material systems [16]. Based on the significant effects of pre-stretching on actuation performance enhancement, a composite material with a second interpenetrating polymer network (IPN) can be formed to fabricate an internal supported DE film [17]. Therefore, without external pre-stretching devices, a good actuation performance was achieved, as well avoiding problems such as stress relaxation and fractures caused by stress concentration. Some other works were devoted to the regulations of mechanical and electrical properties by adding additives or by designing their molecular and network structures [18]. By doping dioctyl phthalate (DOP) and TiO2 into NBR rubber material, Nguyen et al. explored the influence of the contents of two additives on the material's Young's modulus, permittivity, breakdown strength and dielectric loss, etc [19]. Using styrene and butyl acrylate as raw materials, Ma et al. synthesised an ABA triblock copolymer SBAS (S stands for polystyrene, BA stands for polybutylene acrylate) [20]. Due to the difference of glass transition temperature Tg, phases A and B formed microphase separation. Phase A formed the crosslinking nanodomain and phase B formed the main body, and the mechanical and electrical properties of the elastomer can be regulated by adjusting the proportion of phase A and phase B. In this work, a new acrylic DE was prepared by UV curing. By adjusting the crosslinker content, crosslinking and physical entanglement degree were affected, hence realised an effective control of mechanical properties of the new material. Eventually the new material can realise large actuation strain under relatively low voltage without pre-stretching. At last, we tested the dynamic actuation performance of new DE under different waveform and different frequencies. The results will provide some guidance for its application. 2 EXPERIMENTAL Materials Laurel acrylate monomer and photo-initiator (2-hydroxy-2-methylphenylacetone) were purchased from Tokyo Chemical Industry (TCI) and Macklin, respectively, and CN9021 (bifunctional acrylic resin) from Sartomer Company. All the chemicals were used without further purification. Fabrication of dielectric elastomer films The fabrication process of the films is illustrated in Figure 1. The monomer, photo-initiator and CN9021 were dispersed by ultrasound and stirred to form a homogeneous solution. The prepared mixed solution was poured into a home-made mould [14, 21], and then the bubbles in the solution were removed by vacuuming for 15 min in the vacuum box. After being irradiated for 3 min by UV light, the mixed solution was cured into DE films of 1 mm. Finally, the DE films were placed in the oven at 50°C for 12 h to remove the remnant volatile small molecules. The composition of the prepared DE films is shown in Table 1. FIGURE 1Open in figure viewerPowerPoint Schematic illustration of the preparation process of the UV cured dielectric elastomer (DE) films TABLE 1. Formulations (Parts of Weight) of the UV cured dielectric elastomer (DE) films Name Laurel acrylate CN9021 Photo-initiator A-60 188 60 1.5 A-70 188 70 1.5 A-80 188 80 1.5 A-90 188 90 1.5 Characterisation 2.3.1 Swelling experiment test The crosslinking degree of the prepared DEs can be estimated qualitatively by the swelling experiment, considering the complex network structures of cured films. All the samples were cut into 20 mm × 20 mm squares and their original mass were measured. Then, they were immersed in the tetrahydrofuran solvent for 4 days. Their volumes after swelling were measured to obtain the swelling ratio Q, which is defined as the volume of the swollen elastomer to its initial volume. The gel fraction is defined as the ratio of the mass of the swollen elastomer after drying over its initial mass. The average molecular weight between crosslinks, Mc, is proportional to Q5/3 as the density of the samples is almost the same as measured in this work [22]. 2.3.2 Tensile test The tensile test was used to measure the mechanical loss under large strain and to estimate the Young's modulus of DE through uniaxial tensile test on INSTRON 3343. The 1 mm thick DE films were cut into 50 mm length by 10 mm width, and the ends of the length were 10 mm each for fixing. The tensile rate was fixed at 100 mm/min, the elongation ratio was 100% and the force (F) and stretch displacement (D) were read. Each sample was stretched for 5 cycles and three samples were tested for each formulation. The F-D curve was recorded in each tensile test. Integrate the curve in the stretching and retracting process in a tensile cycle to obtain the integral values (take the absolute value) S1 and S2: S 1 = | ∫ s t r e t c h i n g F d D | ${S}_{1}=\vert \underset{stretching\hspace*{.5em}}{\int }FdD\vert \hspace*{.5em}$ (3) S 2 = | ∫ r e t r a c t i n g F d D | ${S}_{2}=\vert \underset{retracting\hspace*{.5em}}{\int }FdD\vert $ (4)then the mechanical loss η in a cycle can be described as: η = S 1 − S 2 S 1 × 100 % $\eta =\frac{{S}_{1}-{S}_{2}}{{S}_{1}}\times 100\%$ (5)The strain(ε) and the nominal stress(σ) were calculated by Equations (6) and (7): ε = D l 0 $\varepsilon =\frac{D}{{l}_{0}}$ (6) σ = f A 0 $\sigma =\frac{f}{{A}_{0}}$ (7)where l0 (30 mm) is the original length stretched and A0 (10 mm2) is the original cross section area before stretching. Y can be calculated as shown in Equation (8): Y = σ ε $Y=\frac{\sigma }{\varepsilon }$ (8)where Y was estimated at small strain when D/l0 < 0.1. Values of η and Y for each sample were the averages of five tensile tests, and the final results for each formulation were the average of three samples. 2.3.3 Dynamic thermomechanical analysis (DMA) test The dynamic thermomechanical properties, including storage modulus and loss factor (tan δ) of the DE films were measured with a TA Q800 DMA. All tests were carried out at 1 Hz frequency, < 2% stain, and a temperature ramping rate of 7°C min−1. 2.3.4 Dielectric performance Permittivity and dissipation factor of the DE films were measured by a novocontrol GmbH in the frequency ranging from 10−1 Hz to 107 Hz. The diameter of the testing electrodes was 20 mm. 2.3.5 Actuation performance The method for the actuation performance test is shown in Figure 2. The conductive carbon grease was coated on both sides of the film as flexible electrodes. The film region with the coated electrode was a circular region of 20 mm diameter, which was the actuation area. Then DE film was fixed without stretching between two acrylic rings with an inner diameter of 20 mm. Copper wires wound around the rings wired from both electrodes to connect the voltage source could supply constant DC voltage and oscillating voltages of different waveform and frequencies from an oscilloscope EDUX1002 G and a High-Voltage AC/DC generator Trek, Inc.615-3. When the voltage was applied to both sides of the film, the film deformed out of the plane. Bulging height was measured with the laser displacement sensor and recorded with a LabView programme. Actuation area strain could be calculated through the data of the bulging height [23]. FIGURE 2Open in figure viewerPowerPoint Schematic illustration of actuation performance test 3 RESULTS AND DISCUSSION The result of the swelling experiment is shown in Figure 3, which can make a qualitative estimate of the crosslink density of the UV cured DE films. Gel fractions of all the samples remain between 70% and 85%, indicating an effective crosslink through photopolymerisation. From A-60 to A-70, the increase of the crosslinker content results in a significant increase in the gel fraction and a decrease in the swelling ratio Q, suggesting an increase of the crosslink density [24]. Gel fractions of A-70, A-80 and A-90 show a fluctuation when crosslinker content increases, while still greater than that of A-60, and Q value moves towards a balance after a drop from A-60 to A-70. We speculated that, although the crosslinker content increased, the reaction rate of crosslinking was affected because the UV irradiation time remained constant and the photo-initiator content was unchanged. Therefore, increase in the crosslinker content does not keep causing a significant decrease in the Q value. Taking into account the complex reaction process of photopolymerisation including photo initiation, propagation, transfer and termination, as well as the influence of various factors such as temperature and polymerisation depth, the formation of the gel network structure is complex, so the gel fraction does not show a monotonous trend with the increase of the crosslinker content. FIGURE 3Open in figure viewerPowerPoint Swelling ratio Q and gel fraction of UV cured dielectric elastomer (DE) films with different crosslinker content The F-D curves of UV cured DE films are demonstrated in Figure 4. Calculated mechanical loss and Young's modulus of the DE films are shown in Figure 5. With the increase in the crosslinker content, Young's modulus increases and mechanical loss decreases monotonically. The same as A-60 to A-70 when there is an obvious decrease in Q value and an increase in gel fraction, A-70, A-80 and A-90 also show an obvious monotonic change in the case of slight increase in Q value and obvious fluctuation in gel fraction as shown in Figure 3. We presumed that it was because more physical entanglements were introduced into the DE network with the increase of the long-chain crosslinker content [25, 26]. Even in the absence of a significant increase in chemical junctions, these physical entanglements as temporary junctions improve the toughness of the material and prevent slippages among the molecular chains which can induce internal frictions under tensile strain. Therefore, the mechanical loss decreases when the crosslinker content increases. FIGURE 4Open in figure viewerPowerPoint F-D curve of (a) A-60, (b) A-70, (c) A-80 and (d) A-90 for 5 cycles in the tensile test FIGURE 5Open in figure viewerPowerPoint Mechanical loss and Young' modulus of UV cured DE films with different crosslinker content As shown in Figure 6, the Tg of the samples are about 0°C where the storage modulus E′ drops sharply, and the values of Tg are with no significant difference among films with different crosslinker content. Above Tg, when the material is in elastic state, tan δ gradually decreases with the increase in the crosslinker content, which is in agreement with the tensile test result. When the temperature drops below Tg, the tan δ of the samples changes in an opposite pattern. The opposite change patterns before and after the glass transition temperature may be caused by the difference of the loss mechanism. In the elastic state above Tg, the relative slippages of free ends or free molecular chains in the crosslink network cause losses. The larger the crosslinker content, the more physical entanglements and relatively higher crosslink density, which decreases the slippages in the network. When molecular chains tend to be in frozen below Tg, it is not easy to slip among chains and the internal friction generated by slippages is no longer the main loss mechanism. Instead, more chemical crosslink and physical entanglements will cause increasing resistance when DEs deform and therefore increase the loss and tan δ values. FIGURE 6Open in figure viewerPowerPoint Storage modulus(top) and loss factor(bottom) of UV cured DE films with different crosslinker content Permittivity and dielectric loss of A-60, A-70, A-80 and A-90 were measured and the results are shown in Figure 7. The permittivity of the samples is stable in the range of 4.2–4.4 @103 Hz and VHBTM 4910 is 4.5 @103 Hz, and the frequency response characteristic of the dielectric loss does not change with the change of the crosslinker content. The possible reason is that compared with the monomer laurel acrylate, CN9021 as a kind of polyurethane acrylate does not increase much dipoles or polar side chain groups in new materials, which can significantly change the dielectric properties of the material. FIGURE 7Open in figure viewerPowerPoint Frequency response characteristics of DEs' dielectric properties According to the previous test results, we chose A-80 as the actuation DE film due to its relatively low mechanical loss and small Young's modulus in the condition of the maintained permittivity. The breakdown strength of A-80 was measured before actuation test and for comparison that of A-60, A-70 and A-90 was also measured. Breakdown data are analysed using Weibull distribution, and the result in Figure 8 shows when electric field is 31.3 kV/mm, the breakdown probability of A-80 is 63.2%. FIGURE 8Open in figure viewerPowerPoint Weibull distribution of A-60, A-70, A-80 and A-90's breakdown strength data Considering the viscoelasticity of the sample in the actuation test, the measured values of the bulging height under constant DC electric fields were read after the voltage was maintained for 1 min under each different electric field. The calculated results are shown in Figure 9. The measured data of VHBTM 4910 are drawn together in this figure. FIGURE 9Open in figure viewerPowerPoint Actuation area strain of A-80 without pre-stretching under constant voltage As shown in Figure 9, A-80 shows a larger actuation strain than VHBTM 4910 under the same electric field. A-80 exhibits 9% actuation area strain at 11 kV/mm while VHBTM 4910 with the same thickness exhibits less than 2% actuation area strain at 15 kV/mm, which is mainly due to the lower modulus of A-80 and the close permittivity. The Young's modulus of A-80 is 0.07 MPa which is much lower than 0.26 MPa of VHBTM 4910 measured under uniaxial tensile test in this work. Figure 10, Figure 11 and Figure 12 show the actuation performance of A-80 driven by voltages of the rectangular wave, sinusoidal wave and sawtooth wave at the frequency of 0.2 Hz, 0.5 Hz, 1 Hz and 2 Hz, respectively. The actuation voltage amplitude is fixed at 5 kV. The maximum bulging height actuated by the oscillating rectangular voltage at 0.2 Hz is only about 20% of the height under constant DC voltage and drops to about 10% at 2 Hz. Figures 10, 11 and 12 also show that the bugling height actuated by the rectangular wave at each frequency is slightly higher than that of the sinusoidal and sawtooth wave, possibly because the rectangular wave lasts longer under high voltage and thus can continue to generate a larger Maxwell force to actuate the DE film. Meanwhile, under the rectangular wave voltage, the DE film has a displacement in bulging height almost perpendicular to the time axis, which then slowly increases, exhibiting both elastic and viscous properties of the DE [27]. The elastic displacement of height is nearly perpendicular to the time axis, because it requires a very short deformation time, and it is nearly the constant value of 0.02 mm at each frequency tested under the rectangular wave and sawtooth wave voltage when the voltage rises or drops sharply, as shown in Figures 10 and 12. It further inspires us to pay much attention to the elastic deformation ability of the DE under oscillating voltage of certain frequencies. Charging current measured in the actuation process under rectangular wave voltage was found only as a peak current along the rising edge of the voltage. Therefore, the influence of charging and discharging time on the actuation performance is not considered in the low-frequency test condition in this work. In Figure 11, an obvious phase lag can be seen and in Figure 12 when the voltage gradually rises from zero, the height displacement is still in the descending stage at each frequency, which are both due to the material's viscous property. FIGURE 10Open in figure viewerPowerPoint The bulging height of A-80 film actuated by rectangular wave voltages respectively at (a) 0.2 Hz (b) 0.5 Hz (c) 1 Hz and (d) 2 Hz FIGURE 11Open in figure viewerPowerPoint The bulging height of A-80 film actuated by sinusoidal wave voltages respectively at (a) 0.2 Hz (b) 0.5 Hz (c) 1 Hz and (d) 2 Hz FIGURE 12Open in figure viewerPowerPoint The bulging height of A-80 film actuated by sawtooth wave voltages respectively at (a) 0.2 Hz (b) 0.5 Hz (c) 1 Hz and (d) 2 Hz 4 CONCLUSION In this work, a new type of acrylic dielectric elastomer material was synthesised by UV curing. The results showed that by increasing the crosslinker content, mechanical loss reduced significantly, and though Young's modulus increased at the same time, it still maintained a small value so as to achieve a considerable actuation strain. Actuation performance test showed that under the actuating electric field of 11 kV/mm, A-80 could reach an area actuation strain of 9.0% without pre-stretching, which is much higher than that of VHBTM 4910. Meanwhile, the results showed that the deformation of creep caused by the viscosity of DE gradually decreased as the frequency of the actuation voltage gradually increased, resulting in the main change of deformation amount, while the amount of elastic deformation basically remained at a constant level. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51921005 and 51907177). CONFLICT OF INTEREST The authors declared that they have no conflicts of interest to this work. 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