SILPOT184/CAT184 (DuPont Toray Specialty Materials K.K.) was used as PDMS. It is an additional reaction type PDMS. As polar group-containing materials, cyanoethyl sucrose CR-U (Shin-Etsu Chemical Co., Ltd.), which possesses a cyanoethyl group (-CH₂CH₂CN) was used.

The base SILPOT184 and curing agent CAT184 were mixed in a weight ratio of 10:1 as a PDMS solution. CR-U is very viscous and difficult to handle as it is but is soluble in THF and ACT [12] [13] [14]. Thus, 50 wt% CR-U solutions of THF (CR-U (T)) and ACT (CR-U (A)) were prepared. CR-U (T) and CR-U (A) were mixed with PDMS solution so that the CR-U content to the base SILPOT184 and CAT184 was 42.9 wt%, at which the maximum bending deformation under an electric field was attained [10] [11]. THF/ACT weight ratio was from 0/100 to 100/0. They were stirred at 2000 rpm for 6 min using a revolution stirrer (ARE-310, THINKY Co., Ltd.) to prepare a “cast solution” of PDMS/CR-U/solvent. Table 1 shows the composition of PDMS/CR-U/solvent cast solution.

The cast solutions were poured onto a PET film and were cast with a four-sided film applicator (052-6, All Good Co., Ltd.). The solutions were kept for 30 min at room temperature, and then heated and cured in an oven at 150˚C for 30 min. The surface in contact with air and PET film was defined as “AIR-side” and “PET-side” respectively. PDMS composite films of PDMS/CR-U (Ax/Ty) were

prepared, where x and y were ACT and THF content (wt%) to the total amount of solvent, respectively. THF content was defined as the weight ratio against the total solvents.

The viscosities of cast solutions were measured with the tuning-fork vibrometers (SV-10A, A & D Instrument Ltd.) at 20˚C.

The cross-sectional images were observed with a laser microscope (OLYMPUS LEXT OLS4100, Olympus Corporation).

The tensile elastic modulus of the PDMS composites was measured using a tensilon-tensile measuring device (RTC-1250A, A & D Co., Ltd.). The PDMS composites were strained at 50 mm/min in a 10 N load cell. The tensile elastic modulus was calculated from the initial slope of the tensile stress curve at low tensile stress.

The dielectric constant was measured with an impedance analyzer (SI1260 and SII1296, Solartron Co., Ltd.), high voltage interface (HVI-100, TOYO Corporation), and bipolar amplifier (BOP-1000M, KEPCO, INC.). The dielectric constant (1 Hz) under a constant voltage was measured under 500 V because the dielectric actuator is driven by applying a voltage.

The surface resistivity was measured with a resistivity meter (Hiresta-UX MCP-HT800, probe type “URS”, Nittoseiko Analytech). The applied voltage was 500 V, a similar electric field for the bending deformation and space-charge distribution measurements.

Figure 1 shows the specimen dimensions and the experimental setup for measuring bending deformation. The PDMS composite film was cut to a width of 5 mm and a length of 20 mm, and two sheets of gold foils having a width of 4 mm, a length of 15 mm, and a thickness of 0.1 μm (KATANI SANGYO Co., Ltd.) were attached to both surfaces of the film to obtain a specimen. A maximum output of 850 V (square wave output) was used as a DC power source. One side of the specimen was fixed with the Kelvin clip (12 mm from the clip to the tip of the specimen was set in a cantilever state) and was settled perpendicularly to the floor surface. A 3 kV/mm DC voltage was applied between the gold foils of a specimen to observe the bending deformation as shown in Figure 1. First, the electric field was applied for 30 minutes so that the AIR-side (skin layer side) was the anode. Then, the polarity of the electric field was reversed. The bending deformation was observed and analyzed with a digital microscope.

The space-charge distribution was measured with a Pulsed Electrostatic Acoustic Non-destructive Test System (PEANUT) manufactured by Five Lab Co., Ltd [10] [11] [15] [16] [17] [18]. The specimens were sandwiched between the upper (semiconductive rubber) and lower (aluminum) electrodes. A pulsed electrostatic force acts on a charge in the specimen, and an elastic wave, which has information of the charge distribution, is generated. This elastic wave is converted into an electric signal by a piezoelectric element, and then into the space-charge distribution. Bias conditions were 3 kV/mm, and pulse conditions were 200 V × 400 Hz.

The viscosities of the cast solutions of PDMS/CR-U composites are shown in Table 1. The viscosity of the cast solution decreased from 11.00 Pa∙S (PDMS/CR-U (A100/T0)) to 0.27 (PDMS/CR-U (A0/T100)) with the THF content.

The film thickness of PDMS/CR-U composites was 150 to 200 mm. The thickness depends on the viscosity of the PDMS/CR-U/solvent solution. The higher the viscosity was, the thicker the thickness was. As shown in Figure 2, the cross-sectional images of the PDMS/CR-U composites showed a phase-separated structure, in which CR-U spheres dispersed in the PDMS continuous matrix. PDMS/CR-U (A100/T0) showed a symmetric cross-sectional, in which CR-U small spheres dispersed uniformly. On the other hand, PDMS/CR-U (A0/T100) composites showed an asymmetric cross-sectional structure in which CR-U large spheres dispersed in the PDMS matrix with a skin layer on one surface side.

Figure 2 also shows THF content dependence of CR-U spherical diameter and the thin-skin layer thickness obtained by cross-sectional image analysis of PDMS composite films. The CR-U sphere diameter gradually increased with THF content, but the thin-skin layer thickness increased significantly at THF content 87.5 wt% or more. Thus, the morphology changed significantly from a symmetric to an asymmetric at THF content 87.5 wt% or more.

The tensile elastic modulus of PDMS/CR-U composites is shown in Table 1. The tensile elastic modulus of composite films was lower than that of PDMS prepared without solvent (1.8 MPa). The tensile elastic modulus increased slightly with THF content. PDMS/CR-U (A0/T100) showed almost 1 MPa and possessed sufficient mechanical strength for practical use.

The dielectric constant of PDMS/CR-U composites is shown in Table 1. The dielectric constants of pure PDMS and PDMS/CR-U (A0/T100) were 2.9 and 8.76, respectively. The dielectric constant increased slightly with THF content.

Figure 3 shows the AIR- and PET-side surface resistivity of PDMS/CR-U composites films. The surface resistivity of pure viscose CR-U and pure PDMS film (without CR-U) were also shown as a reference. The surface resistivity on both sides of the composite films at THF content of 75 wt% and less were higher than 10¹⁴ Ω and out of range as well as pure PDMS. On the other hand, the surface resistivity of PDMS/CR-U (A0/T100) and (A12.5/T87.5) were about 10¹³ Ω which was lower than that of PDMS but higher than that of CR-U. The surface resistivity of PDMS/CR-U (A0/T100) was higher than that of PDMS/CR-U (A12.5/T87.5), which suggests the increase in THF content allows the dispersion of CR-U in the PDMS phase.

Figure 4 shows the time dependence of bending deformation of PDMS/CR-U composites under a 3 kV/mm electric field. The electric field was applied at t = 0 so that the AIR-side electrode became anode and released at t = 30 min. Negative values of the deformation indicate that the film has bent toward the PET-side. The deformation increased immediately after the voltage turned on and gradually reached a steady state with time. The deformation after the voltage off decreased instantly and recovered a steady state remaining some hysteresis in 10 minutes. The bending deformation was defined as the difference between the two steady states.

In Figure 5, the bending deformation of composite films was plotted against THF content. The bending deformation of the composite films at THF content

75 wt% or less was remarkably small. On the other hand, the composite showed substantial bending deformation when the morphology changed to an asymmetric cross-sectional structure at THF content of 87.5 wt% or more.

Figure 6 shows the space-charge distribution of PDMS/CR-U composites. The horizontal axis represents the sample thickness (position). The electrode interfaces were defined as zero, and the vertical axis represents the space-charge density. The maximum value of the opposite charge accumulated in the film in the vicinity of the AIR-side electrode interface was defined as the hetero charge value.

In Figure 6(A), the interface of the anode/AIR-side (skin layer side) is defined as zero of the depth axes. The accumulation of a small and significantly large negative charge (shown by diagonal lines in the figure), which is the opposite to

the polarity of the anode, that is, the hetero charge was observed inside the PDMS/CR-U (A12.5/T87.5) and PDMS/CR-U (A0/T100), respectively, in the vicinity of the anode interface. The hetero (positive) charge accumulation was not almost observed inside the film near the cathode interface, thus, the asymmetry of the space-charge distribution was confirmed. On the other hand, the space-charge distribution of PDMS/CR-U (A100/T0) to (A25/T75) showed no hetero charge accumulation inside the film.

In Figure 6(B), the cathode electrode/AIR-side of the film interface is defined as zero of the depth axes. The hetero charge accumulation of PDMS/CR-U (A0/T100) and PDMS/CR-U (A12.5/T87.5) was confirmed in the vicinity of both electrodes, and the accumulation at the cathode side was more extensive than that at the anode side. As a result, an asymmetry of the space-charge distribution was confirmed. On the other hand, the space-charge distribution of PDMS/CR-U (A100/T0) to (A25/T75) showed no hetero charge accumulation inside the film.

PDMS/CR-U (A0/T100) to (A12.5/T87.5) indicated the significant hetero charge accumulation in the vicinity of the AIR-side/electrode interface regardless of the electric field polarity. The location of the hetero charge accumulation was nearly consistent with that of the thin-skin layer. Thus, the thin-skin layer suppressed the electric charge leakage, and the electric charge was accumulated in the vicinity of the thin-skin layer. The composite showed the asymmetric space-charge distribution when the morphology changed to an asymmetric cross-sectional structure at THF content of 87.5 wt% or more.

Figure 7 shows the relationship between the hetero charge value of the AIR-side (thin-skin layer side) estimated from Figure 6 and THF content. The hetero charge value was not observed for the composite films at THF content of 0 to 75 wt%, however, the charge accumulation was increased significantly at THF content 87.5 wt% or more regardless of the electric field polarity.

For PDMS/CR-U (A100/T0) to PDMS/CR-U (A25/T75), the charge injection did not occur when (even if) the electric field was applied because of their high surface resistivity. As a result, the composites showed almost no bending deformation. The significant increase in hetero charge was consistent with the bending deformation of the PDMS/CR-U composite. The charge accumulation is a primary factor for bending deformation. The bending deformation was due to the electrostatic repulsion among the charges that caused the thin-skin layer expansion induced the bending deformation.

The morphology of the composite film strongly depends on the (composition of the) casting solution. So, first, the viscosity of the solutions of PDMS/CR-U/solvent was discussed. The viscosity of polymer solutions is sometimes helpful to get insight into the polymer morphology in the solution. Figure 8 is a plot of the viscosity η against THF content. The η decreased sharply with the THF content and showed a constant low value above the THF content of 87.5 wt%.

In the high concentration polymer system, the viscosity is very sensitive to the polymer chain length. η increases with N (number of monomers (units) in a polymer chain), according to the Equation (1) [19]:

η ∼ N m η (1)

where m_η is scaled 3.3 - 3.5, which is still controversial. When the relation of Equation (1) can be applied to the authors’ system, the change of N, an apparent size of PDMS aggregates can be reflected in the change of η. No matter the value of m_η is, the larger the η, the bigger the N. Then, the following discussion will be available:

In solutions with a THF content of 87.5 wt% or more, η reached a certain small value. In other words, small PDMS chain aggregates were distributed in the CR-U containing solution. On the contrary, the aggregate size increased with ACT content. Furthermore, the PDMS/CR-U (A100/T0) solution became paste-like sol right after mixing, suggesting the CR-U solution is secluded in the large aggregates of PDMS chains by micellization.

The morphology of the composite film is strongly dependent on the casting solvent. Figure 9 illustrates a schematic diagram of the morphology formation (after curing).

Figure 9(A) shows a schematic diagram of the PDMS/CR-U (A0/T100) to (A12.5/T87.5) solution and the composite.

Since CR-U is miscible in THF and ACT, CR-U dissolves in any composition of THF-ACT solvent. PDMS is also soluble in THF-rich solvents. However, when PDMS solution and CR-U solution are mixed, the mixture instantly forms an untransparent dispersion. The PDMS phase, which has a small size PDMS aggregate, is dispersed in a CR-U solution of THF-ACT. As shown in (a) of Figure 9(A), the PDMS (green sphere) with a smaller PDMS aggregates is highly mobile and less viscous.

As PDMS aggregates with the solvent evaporation and the progress of crosslinking, CR-U is gradually squeezed out from the PDMS aggregates and forms a droplet phase of CR-U, accompanying the sedimentation of the CR-U droplet,

because the density of CR-U (1.23 g/cm³) [14] is higher than that of PDMS (1.03 g/cm³) [20]. Therefore, CR-U droplet phases settle during the evaporation/curing process (film formation) in the middle of sedimentation. On the other hand, the sedimentation process accompanies the skin layer formation in the airside of the film. The phenomenon is observed in the films in the range of PDMS/CR-U (A0/T100) to (A12.5/T87.5). Thus, an asymmetrical PDMS/CR-U film with a PDMS skin layer is formed and some CR-U remains in the thin skin layer and the film matrix even after curing.

As shown in Table 1, the segregation and gradual curing process of PDMS in the film result in a slight increase in the tensile modulus due to the compaction of the PDMS phase and the dielectric constant due to the CR-U continuous channel remaining in the PDMS phase. In addition, the increased permittivity facilitates charge injection, and the dense skin layer prevents discharge, thus inducing an asymmetric space-charge distribution. Thus, the composite becomes electrically active. As a result, the film can bend when an electric field is applied and be used for dielectric actuators.

Figure 9(B) shows the case of a PDMS/CR-U (A50/T50) solution. It is an intermediate case of Figure 9(A) and Figure 9(C). Therefore, in Figure 9(B), PDMS aggregates are intermediate size. The morphology of the composite film also displays an intermediate structure. However, the PDMS phase contained almost no CR-U, and the composite material is an insulator to an electric field similar to Figure 9(C).

Figure 9(C) shows a schematic diagram of a PDMS/CR-U composite ranging the solvent composition from (A100/T0) to (A75/T25). PDMS is immiscible in ACT but is colloidally dispersed in ACT-rich THF solvent. Immediately after mixing a PDMS solution and a CR-U solution, the aggregates of PDMS molecules instantly seclude the CR-U solution to form paste-like aggregates, as shown in (d) of Figure 9(C). The CR-U solution is covered with PDMS aggregates. The micelles have large PDMS aggregates, and the aggregates grow by hydrophobic interactions. Therefore, the solution showed high viscosity immediately after mixing.

The composite film obtained by evaporating the solvent in the micelle and curing the PDMS has the finely dispersed CR-U domain coated with PDMS, and the CR-U phase remains isolated and finely dispersed as shown in (e) of Figure 9(C). CR-U is not included in the PDMS matrix, since PDMS forms a continuous phase right after mixing. Therefore, the composite has a symmetrical cross-sectional structure, a small and evenly dispersed CR-U phase, and exhibits high electrical insulation properties. This insulation characteristic effectively suppressed bending deformation.