Enhanced electromechanical performance through chemistry graft copper phthalocyanine to siloxane‐modified polyurethane and interpenetrate with siloxane silicon rubber as composite actuator material

National Natural Science Foundation of China, Grant/Award Numbers: 51973080, 51903100 Abstract Researchers are devoted to developing dielectric elastomers (DEs) with excellent electromechanical properties as an artificial muscle material. The authors report a new class of semi‐interpenetrating network (semi‐IPN) composites that contains siloxane‐modified linear polyurethane (PU) and silicone rubber through reasonable design of polymer structure. The organic‐filler copper phthalocyanine (CuPc) is chemically grafted into the semi‐interpenetrating network as a cross‐linking point and exhibits excellent dispersibility in the matrix. The various properties of the obtained composite films are also evaluated. The dielectric constant (8.65 at 1 kHz) and maximum actuation strain at 30 MV m (5.32%) are significantly higher than those of semi‐IPN composites.


| INTRODUCTION
Nature has endowed muscles with many ideal characteristics, and so researchers have been working on developing materials that mimic muscle-like behaviour [1,2]. Since the 1990s, electroactive polymers (EAPs) have attracted increasing attention by virtue of their attractive properties that exhibit substantial deformation in response to applied voltage, resulting in them being considered as the main material of ̒ artificial muscle' [3]. Based on the actuation mechanisms, EAPs can be divided into several kinds, including ionic and field-activated EAPs [1,3,4]. Dielectric elastomers (DEs) as a branch of field-activated EAPs appear to provide the best combination of properties such as light weight, high actuation strain, fast responses and scalability for true muscle-like actuation [5,6]. When stimulated by an electrical field, the membrane of DEs reduces the thickness and expands the area, known as actuation strain. The actuation performance is always determined by the electromechanical sensitivity β = ε r /Y according to the electrostatic equation S z = −ε r ε 0 E 2 /Y (where ε 0 and ε r denote the vacuum permittivity [8.85 � 10 −12 F/m] and the relative permittivity, E is the applied electric field, and Y is elastic modulus of the DEs) [7][8][9]. Therefore, great efforts have been focussed on developing new DE materials with a large ε r and a low Y over the past decade [9][10][11][12].
Silicone possess high elasticity, fast response, broad response temperature range and long-term reliability, making it an interesting candidate for dielectric materials [13,14]. However, single-component silicone rubber materials cannot meet all the requirements in practical applications (low Young's modulus, high dielectric permittivity etc.). Combining two different networks to create interpenetrating polymer networks (IPNs) is an effective strategy to improve the performance of silicone DE devices [8,9,15,16]. Polyurethanes (PUs) could be introduced as the second polymer network because it has an intrinsically high dielectric constant with easy processability of formulations. Cazacu et al. used PU as a partner with PDMS and showed increased dielectric permittivity and electrically induced actuation compared with selected commercial dielectric elastomers [17]. Although great achievements have been made, due to the inherently low dielectric permittivity and other properties, the practical applications of polymers, especially in the biological and medical fields, are limited by their high driving electric field (>100 MV m −1 ).
In recent years, the dielectric permittivity of DEs has been easily modified using nanofillers [2,18], such as conductive carbon-based filler [19] and ceramic powders [20][21][22] to achieve high driving strain under a lower driving electric field. However, the introduction of inorganic nanoparticles will sharply increase the elastic modulus and reduce the flexibility of elasticity. Copper phthalocyanine (CuPc) oligomer processes a giant conjugated structure, and can increase the dielectric permittivity significantly while retaining the flexibility of the matrix as an organic semiconductor filler [23][24][25]. However, the excessive agglomeration of filler due to the molecular stacking of CuPc and incompatibility with the polymer matrix results in a large electrical conductivity and increases the dielectric loss. In the investigations carried out by Zhang et al., chemically grafting the filler to the dielectric elastomer was shown to remarkably reduce the aggregation of CuPc particles in matrix [26].
A silicone rubber using polydimethylsiloxane (PDMS) and tetraethyl orthosilicate (TEOS) was prepared, and a linear PU used as the second component of the semi-IPN. Through the reasonable design of the structure, the authors introduced silicon-oxygen bonds (-Si-O-Si-) as the soft segment of the PU, which can increase the compatibility of the two polymer networks and maintain excellent chain flexibility. By changing the ratio of interpenetrating components, the mechanical, dielectric characteristics and electromechanical behaviour of different formulations were evaluated. Furthermore, an amino-functionalised copper phthalocyanine was prepared as the cross-link points of PU to form a double interpenetrating polymer network. The covalent bonding increased the dispersibility of CuPc and improved the dielectric permittivity of the matrix, but the elastic modulus remained at a lower level. The authors provide a method for preparing a new type of all-organic composite actuator material with low modulus, high dielectric constant and large strains.

| Preparation of PU/Si and PU-CuPc/Si nanocomposites
Polydimethylsiloxane-α,ω-diol (PDMS, M n = 372.8 kDa, PDI = 1.67) was prepared according to an already published procedure [28] by ring-opening polymerisation of octamethylcyclotetrasiloxane. Linear PU was synthesised by the reaction between the hydroxyl of KF-6002 and the isocyanate of MDI. After that, different proportions of silicone rubber prepolymer were added to prepare semi-IPN dielectric elastomer. Briefly, KF-6002 and MDI (molar ratio = 1:1) were added into 15 ml toluene, and the reaction solution was purged with nitrogen and added to 10 μl of DBTDL. After reacting at 60°C for 3 h, a set amount of pre-prepared PDMS was added to the reactor and stirred for 3 h at room temperature until completely dissolved. The TEOS (20 wt% relative to the PDMS) be introduced as the cross-linker. After dissolution for 30 min, 10 μl DBTDL was dripped into the reactor and stirred for another 10 min. The solution was drop cast as thin films on Teflon substrate moulds at room temperature for 12 h, and heated at 60°C for 24 h. PU films with 15, 20, 25 and 30 wt% of silicon rubber relative to the mass of PU were prepared, denoted as PU-Si x (where x is the weight content of silicon rubber in the semi-IPN) throughout the manuscript. In the preparation of linear PU, CuPc-NH 2 was added to react with MDI to prepare double interpenetrating network nanocomposite elastomer film, denoted as PU-y% CuPc-Si x (where y is the weight content of CuPc in the PU). Figure 1 presents the synthetic route of nanocomposites.

| Characterisation
The molecular masses of PDMS were determined through gel permeation chromatography (GPC) on a Malvern instrument, equipped with a Waters 1515 instrument with a guard column MIXED 7.5 � 50 mm PL column and two MIXED-C 7.5 � 300 columns and a Waters 2414 differential refractive index detector using chloroform (HPLC grade) as the eluent at 35°C with a flow rate of 1 ml min −1 . Infrared spectra were recorded on a Nicolet Impact 410 FT-IR spectrophotometer over the range of 4000-500 cm −1 at room temperature. Scanning electron microscopy images (SEM) and energydispersive spectrometer (EDS) mapping were obtained from a FEI Nova Nano 450 field emission to detect the morphologies of the cross-section of multilayer films. The sample was prepared by fracturing in liquid nitrogen and then the fractured surface was sputtered with gold. Dielectric properties of the samples were performed by 4294A Precision Impedance Analyzer (Agilent Technologies Co. Ltd.) in the frequency ranges from 10 3 to 10 6 Hz at room temperature. Stress-strain measurements were performed on a strip-shaped cut from thin films on an electronic universal material testing machine (AG-I 20 KN, SHIMADZU, Japan). Measurements were run at an extension rate of 50 mm min −1 , at room temperature. All samples were measured three times and the averages were obtained. The electromechanical actuation of the films was HUANG ET AL. evaluated by a laser distance-detecting instrument (LK-G150, Keyence) and high-voltage amplifier. All the above tests were performed without any pre-stretching.

| RESULTS AND DISCUSSION
The interpenetration of the PU with different proportions of silicone rubber (PU, PU-Si 15 , PU-Si 20 , PU-Si 25 and PU-Si 30 ) was prepared to explore the influence of the amount of silicone rubber on the electromechanical drive performance. A crosslinked silicone rubber film was also prepared as a reference sample. The infrared spectra of semi-IPN were characterised, which labelled the characteristic peaks of PU and silicone rubber shown in Figure 2a. The characteristic peak at ∼1528 cm −1 (C-N) in the spectrum indicates that the NCO group was reacted with the hydroxyl group to form a carbamate bond. The Si-O-Si asymmetric stretch-specific absorption bands present at 1095-1014 cm −1 . The peak at ∼1258 cm −1 corresponded with the C-H symmetric rocking absorption peak in side group Si-CH 3 , 1600 cm −1 and 1415 cm −1 , 1715-1735 cm −1 represent the specific absorption bands of N-H and C=O, respectively [8,12,29].
Stress-strain curves of the obtained films are shown in Figure 2b, which evaluated the effects of the silicone rubber incorporation within a PU matrix. From the elastic modulus of each component as presented in Table 1, it is evident that the flexible siloxane segments in linear PU could significantly reduce the elastic modulus (1.38 kPa), which is much lower than previously reported. By contrast, silicone rubber exhibits the largest modulus (2.08 kPa) due to its cross-linked structure. With the blending of silicone rubber and PU in the resultant hybrids, all resulting materials have values below the corresponding pure silicone rubber, which indicates that the linear PU chains are well interspersed in the cross-linked silicone rubber. Pure PU exhibits the lowest elongation at break (399%), which can be attributed to the linear PU that has almost no cross-linking point and the molecular chain slips when stretched, so it is destroyed by a small force. Siloxane rubber has an elongation of 529%. The elongations of the prepared semi-IPN materials increase as expected compared with pure PU, with the increasing amount of siloxane rubber content, breaking occurs in the range of 450%-600% elongation. This indicates that the siloxane rubber network improves the PU network extension.
The aim is to increase the dielectric permittivity (ɛ r ) through the interpenetration of PU and silicone rubber, thereby affecting the electromechanical properties of the material. The ɛ r of different series of PU/Si semi-IPN was measured in the frequency range of 1 kHz to 1 MHz at room temperature as presented in Figure 3a, ɛ r values of PU (6.08 at 1 kHz, refer to Table 1) are significantly higher than those of pure silicone rubber (1.82), due to the polarity of the polyurethane fragments [13]. As the silicone rubber loading in PU to form the yield materials, an obviously increase of ɛ r (7.21, 7.40, 7.47 and 7.62 for PU-Si 15 , PU-Si 20 , PU-Si 25 and PU-Si 30 , resp.) might be appreciated, this being an expected effect due to a strong interface coupling between PU and silicone rubber [30]. According to Figure 3b, as the degree of semi-interpenetration increases, all samples had a similar dielectric loss factor (tan δ), lower than 0.02, indicating that the synthesised semi-IPN is a non-conductor, with excellent energy transformation ability and suitable as flexible dielectric components.
The simplest form of dielectric elastomer actuator was prepared by coating compliant electrodes on both sides of the elastomer. The electro-driven deformation was detected by a F I G U R E 1 Schematic representation and deforming mechanism of the PU/Si semi-IPN and PU-CuPc/Si nanocomposites 40laser displacement sensor, which conformed to the standards established by the EuroEAP society [31]. The thickness strain of the hybrid films was measured under an electric field from 0 to 30 MV m −1 (with a gradient of 5), and the strain field curves are shown in Figure 4a. With the increase of the electric field, all samples showed a higher response strain, and reached the maximum values at 30 MV m −1 ; the specific values are shown in Table 1. It can be clearly observed that the silicone rubber shows poor actuation displacement (1.03%), because it presents significantly higher elastic modulus and lower dielectric constant. The liner PU film has the higher strain at about 4.33%. With the increase in the silicone content on PU/Si semi-IPNs, the thickness strain value shows a trend of initial increase and subsequent decrease, and reaches a peak (4.82%) at the silicon content of 15 wt%, which is synergistically influenced by the dielectric constant and elastic modulus according to the formula of electromechanical sensitivity, β = ε r / Y. The value of β of each component, calculated by the  corresponding ε r and Y, is shown in Figure 4b. It is clear that a minimum β value of 0.87 is obtained for siloxane rubber under 30 MV m −1 . With an increase in the siloxane rubber content, the β of semi-IPN materials increased from 4.41 to 4.62 (PU-Si 15 ), and then began to decrease. The phenomenon can be attributed to the following factors. First, the introduction of silicone can improve the dielectric constant values of the hybrids, as indicated in Figure 3a. However, a high silicone content component (>15%) significantly increased the modulus of the semi-IPN (Figure 2b), thus leading to a decrease in the β value. The result is consistent with the observed electric-driven deformation behaviour.
To further improve the electromechanical response performances of elastomers, copper phthalocyanine with four amino groups (CuPc-NH 2 ) was blended into the PU/Si semi-IPN as a cross-linking agent for the linear PU component.
According to the previously discussed electromechanical drive results, the sample films PU-Si 15 with the CuPc-NH 2 content (10 wt%) were recorded as PU-10%CuPc-Si 15 and compared with PU-Si 15 as a reference. The fracture surface morphologies of the obtained films were observed by SEM, as shown in Figure 5a. The fracture surface can be observed to be smooth, indicating that, as an organic filler, CuPc-NH 2 can blend well with the polymer matrix. The authors also characterised the distribution of Cu in the PU-10%CuPc-Si 15 by EDS mapping. Figure 5b shows that CuPc-NH 2 was well dispersed into the matrix without obvious aggregation, indicating that chemical grafting effectively improves the dispersibility of CuPc. Figure 6 and Table 1

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low level (<0.02), indicating that the covalent connection prevented the aggregation of CuPc, the grafted insulating polymer around the CuPc reduced the long-range intermolecular hopping of electrons and blocked the space charge conduction [23]. On the other hand, the PU-10%CuPc-Si 15 shows the lowest strain at break, at around 319%. This decrease can be attributed to the CuPc rigidity and its cross-linking effect on PU. Despite this, the elastic modulus ratio does not increase much due to the organic structure of CuPc-NH 2 , and the resulting composite exhibits almost the same elastic modulus as the polymer matrix. Thus, the sample of PU-10%CuPc-Si 15 has a significantly increased thickness strain value of 5.32% compared with the value found in PU-Si 15 . The thickness deformation value increased by 10.4% after the addition of CuPc-NH 2 , while the theoretical value β only increased by 4.11%, this may be ascribed to the change in the aggregation state of CuPc-NH 2 , which as the cross-linking point under the action of the electric field, results in the change of chain segment arrangement in the matrix, making the modulus lower than the value obtained by the extension experiment.

| CONCLUSIONS
In summary, we have successfully prepared a series of semi-IPN dielectric elastomers containing a linear PU and used in different percentages as a partner with silicone rubber. The influence of the interpenetration of two components on the elastic modulus and dielectric constant, which indirectly affected the electromechanical properties, has been discussed. Besides, the amino-modified phthalocyanine was used as a cross-linking agent in the PU phase and can effectively increase the dielectric constant at a filler content of 10% without causing a significant increase in modulus, showing the voltageinduced maximum strain of 5.32% at the electric field of 30 MV m −1 . This kind of dielectric elastomer also has excellent editability, which can be realised by the adjustment of the components and can guide the synthesis of dielectric elastomers for different purposes.