Electromechanical characterization of multilayer graphene-reinforced cellulose composite containing 1-ethyl-3-methylimidazolium diethylphosphonate ionic liquid

2.1 Materials

Cellulose (Cel), [EMIM]DEP, and N,N-dimethyle acetamid (DMAc) were provided from Sigma-Aldrich (Steinheim, Germany). Multilayer Gr, with an average particle diameter of 5–10 μm, was purchased from Grafen Kimya Sanayi A.Ş. (Ankara, Turkey). Gold leaf, which has a thickness of 10 μm, was obtained from L.A. Gold Leaf (Azuza, ABD).

2.2 Fabrication of cellulose-based IPMC actuators

Cel (0.39 g) was dissolved in 5.58 g of [EMIM]DEP at 80°C (in water bath). DMAc (3 ml) as a plasticizer was added to the solution. After the solution was mixed for an hour, the proper amount of Gr was added to the mixture to obtain 0.2, 0.4, and 0.6 wt.% of total mass and was well dispersed by ultrasonic homogenizer. Then, the mixture was cast on a glass plate. The wet film was dried at room temperature for 18 h, and the film, which has a measured thickness of 1.0 mm, was obtained. Both surfaces of the film were coated with gold leaves to fabricate IPMC actuators.

2.3 Characterization methods

FTIR spectra of the cellulose-based samples were analyzed by using Perkin Elmer Spectrum BX-II. The spectra were recorded at a resolution of 4 cm^-1 in the range 4000–400 cm^-1.

XRD analysis of the cellulose-based samples was done via a Philips X-Pert Diffractometer with Ni-filtered Cu Kα radiation (λ=1.54 Å) at 45 kV and 40 mA. The samples were scanned from 5° to 80° (2θ).

Thermal behavior of the cellulose-based samples was determined by using TGA (Shimadzu, TGA 50). TGA was performed at a heating rate of 10°C/min at a range from 30°C to 600°C under nitrogen atmosphere with a flow rate of 1.0 ml/min.

SEM analysis of Cel-PO₄ and Gr-loaded Cel-PO₄ films (Cel-PO₄-Gr) and the cross-sections of films were conducted by using Quanta FEG 250 SEM (at an accelerating voltage of 5 kV).

The tensile strength and tensile modulus of cellulose-based films and actuators were obtained by a Shimadzu universal testing machine with a 100-N load cell at a cross-head speed of 0.1 mm/min.

2.4 Electroactive properties

Electroactive behaviors of Cel-PO₄-based actuators were investigated under DC voltages of 1, 3, 5, and 7 V. As signal source, data acquisition hardware (NI-PXI 7854R) and a buffer circuit were utilized. The maximum tip displacements of actuators were measured by Keyence LK-51 Laser Displacement Sensor.

3.1 FTIR analysis

Figure 1 shows the FTIR spectra of Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6. In the FTIR spectra of cellulose, a broad band between 3600 and 3200 cm^-1 indicates the -OH stretching vibrations. The sharp peak at 3342 cm^-1 can be related with the -OH stretching vibrations due to the intramolecular hydrogen bonding [30], [31]. The absorption bands between 3000–2800 cm^-1 and 1500–1250 cm^-1 are attributed to the C-H and CH₂stretching vibration, respectively [31], [32], [33]. The absorption peak around 1630 cm^-1is caused by the presence of water [34]. The peak at around 1160 cm^-1 originated from C-O-C stretching vibration. The peak at 1042 cm^-1 is related to C-O stretching vibration [31], [35], [36].

Figure 1:

FTIR spectra of samples (A) Cel, (B) Cel-PO₄, (C) Cel-PO₄-Gr0.2, (D) Cel-PO₄-Gr0.4, and (E) Cel-PO₄-Gr0.6.

For [EMIM]DEP, the absorption band between 1505 and 1590 cm^-1 at around 1570 cm^-1 is due to stretching vibration of the phosphate group (PO₄)^3- [37]. The P-O stretching vibrations are observed between 1200 and 900 cm^-1 [38], [39]. The band between 1200 and 1250 cm^-1is assigned to asymmetric stretching vibration of P=O [39].

Addition of [EMIM]DEP, Gr, and DMAc led to some changes in the FTIR spectrum of cellulose. The formation of new absorption bands was observed at 1570, 1216, 944, and 794 cm^-1. It was observed that the peaks at 1436, 1366, 1164, and 1042 cm^-1shifted to the peaks at around 1450, 1390, 1169, and 1056 cm^-1. In addition, it can be seen that there are some changes in the spectra of samples after the addition of Gr. The intensity of the absorption peaks at around 3400 and 1366 cm^-1 decreased depending on Gr concentration. The strong vibration peak at 1042 cm^-1, which is assigned to C-O stretching in cellulose, shifted to 1056, 1047, and 1052 cm^-1 after Gr loading of 0.2, 0.4, and 0.6 wt%, respectively.

3.2 XRD analysis

X-ray diffractograms of Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6 are shown in Figure 2. The major crystalline peak of the Cel-PO₄ film occurred at 2θ angle of 23.9°. Essentially, the main peak at 2θ angle of 22° for cellulose corresponds to the 002 crystallographic plane of the cellulose I lattice [40]. It is interesting to note that no peak at 2θ angle of 22° was observed for the Cel-PO₄ film.

Figure 2:

X-ray patterns of (A) cellulose, (B) Cel-PO₄, (C) Cel-PO₄-Gr0.2, (D) Cel-PO₄-Gr0.4, and (E) Cel-PO₄-Gr 0.6.

The XRD analysis shows that the main diffraction peaks of Cel-PO₄ films, which are loaded with a low amount of Gr, are similar to those of Cel-PO₄. According to [41], if the regular stacks of graphite or graphite oxide are destroyed by exfoliation, the diffraction peak becomes weak or even disappears. Because there is no diffraction from the Gr, it is suggested that Gr has been distributed uniformly within the cellulose matrix [42].

3.3 Thermogravimetric analysis

Figure 3 shows the mass loss curves of Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6. The related TGA data of samples are summarized in Table 1. The mass loss of the cellulose occurs in two stages. The first stage resulted from evaporation of trapped moisture in cellulose [43]. The second stage of the mass loss is due to the thermal decomposition of cellulose [44]. The first mass losses of the samples that consist of Cel, [EMIM]DEP, DMAc, and different amounts of Gr are 1.0%, 20.3%, 21.5%, 18.1%, and 18.8% for Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6, respectively. The increase in trapped moisture of the samples resulted from the hydrophilicity of the [EMIM]DEP IL [45].

Figure 3:

Thermograms of samples: (A) Cel-PO₄, (B) Cel-PO₄-Gr0.2, (C) Cel-PO₄-Gr0.4, and (D) Cel-PO₄-Gr0.6.

Cellulose began to decompose at 312°C, whereas the Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6 started to decompose at around 245°C. As can be seen in Table 1, thermal stability of the Cel-PO₄ and Gr-loaded samples (Cel-PO₄-Gr) (279°C–283°C) was lower than that of original cellulose (340°C) on the basis of maximum decomposition temperatures. The decrease in the thermal stability probably resulted from the partial destruction of the crystalline part and hydrolysis of the cellulose [26]. However, Gr loading into Cel-PO₄ has not led to significant variation in maximum decomposition temperature. Besides, Gr loading of 0.6 wt.% decreased the initial decomposition temperature of Cel-PO₄ by 6°C. The decrease in thermal stability of Gr-loaded samples can also be associated with the catalyzing effect of the Gr on the thermal decomposition of Cel-PO₄[46].

The mass losses in the temperature range 25°C–600°C for Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6 are 88.2%, 67.3%, 57.6%, 62.3%, and 59.6%, respectively. The total mass loss of cellulose is higher than those of Cel-PO₄ and Gr-loaded Cel-PO₄. The char residue of Cel, Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6 are 11.8%, 32.7%, 42.4%, 37.7%, and 40.4%, respectively. The loading of Gr into Cel-PO₄sligthly increased the pyrolysis residue [47]. Because the formation of the char layer is important for thermal insulation and flame-retardant properties of samples, it can be expected that Gr loading into Cel-PO₄ has led to better thermal insulation and flame-retardant properties [48].

3.4 SEM analysis

SEM micrographs of Cel-PO₄ and Gr-loaded Cel-PO₄ films are given in Figure 4. Film surfaces (Figure 4A, B, E, and G) appear to be smooth and fairly homogenous without any pores. In order to see the Gr particles, the cross-sections of films (Figure 4C, D, F, and H) were investigated by SEM analysis. A closer examination on the cross-section of films is given in Figure 4D, F, and H for Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6, respectively. Gr particles cannot be seen on the surface and cross-sections of films even at high magnifications.

Figure 4:

SEM images of samples: (A) Cel-PO₄, (B) Cel-PO₄-Gr 0.2, (C) Cel-PO₄-Gr0.2 cross-section, (D) Cel-PO₄-Gr0.2 cross-section, (E) Cel-PO₄-Gr0.4, (F) Cel-PO₄-Gr0.4 cross-section, (G) Cel-PO₄-0.6Gr, (H) Cel-PO₄-0.6Gr cross-section.

3.5 Mechanical properties

Figures 5 and 6 show the tensile strength and modulus with increasing Gr content in Cel-PO₄ films. As can be noticed from Figure 5, the filler content affects the tensile strength of Cel-PO₄ films. The tensile strength and modulus of Cel-PO₄ film were determined to be 12.57 MPa and 0.46 GPa, respectively.

Figure 5:

The effect of Gr loading on tensile strength of Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6.

Figure 6:

The effect of Gr loading on Young’s modulus of Cel-PO₄, Cel-PO₄-Gr0.2, Cel-PO₄-Gr0.4, and Cel-PO₄-Gr0.6.

After Gr of 0.2 and 0.4 wt.% were loaded into the Cel-PO₄ film, the tensile strength of the Cel-PO₄ film increased to 17.61 and 20.15 MPa, respectively. Thus, the tensile modulus of the Cel-PO₄film increased by about 50% and 61%, respectively, when Gr of 0.2 and 0.4 wt.% were loaded. These increments may be explained by well dispersion of Gr into the mixture of Cel-PO₄. However, when Gr of 0.6 wt.% was added into Cel-PO₄, the tensile strength and the modulus of Cel-PO₄-Gr0.6 film decreased by about 17% and 12%, respectively, compared with those of Cel-PO₄-Gr0.4. The detrimental mechanical properties may be described on the basis of aggregation theory [49].

3.6 Electroactive properties

Electromechanical behavior of the gold-coated films with different Gr loadings (0.2, 0.4, and 0.6 wt.%) was investigated under DC excitation voltages of 1, 3, 5, and 7 V. The tip displacements of the actuators were recorded as their time responses via laser displacement sensor. The observed time response curves are in exponential forms, and any back relaxation did not occur during the experiments. Therefore, the final value of the tip displacements corresponds to the maximum tip displacement values of the actuators. These maximum values, given in Figure 7, are used to evaluate the electromechanical behavior of the actuator.

Figure 7:

Maximum tip displacement of gold coated actuators with different Gr loading (1 mm in thickness).

The rows of Figure 7 show the effect of increasing excitation voltage on each actuator performance. It is seen that the maximum tip displacements of unloaded actuator and 0.6 wt.% Gr-loaded actuator increased slightly as excitation voltage increases from 3 to 5 V. On the other hand, the maximum tip displacement of 0.2 and 0.4 wt.% Gr-loaded actuator samples increased when excitation voltage increased from 1 to 3 V and then decreased when excitation voltage increased to 5 V.

The effects of Gr loading on the maximum tip displacements of the actuators are given in Figure 7. It is seen that Gr loadings of 0.2 and 0.4 wt.% increased the actuator performance for excitation voltages of 1 and 3 V. When 5 V experiments are considered, it is seen that the maximum tip displacement of the actuator with 0.2 wt.% Gr loading did not change, and further increase in Gr loading reduced the maximum tip displacement. The highest increase in performance is observed for 0.2 wt.% Gr-loaded actuators. It is seen that 0.2 wt.% Gr loading causes 400% increase in maximum tip displacement when the actuator is excited with 3 V. Lower Gr loading leads to considerable improvement in actuation behavior. Similar results for Nafion-based IPMC actuators have been obtained by Jung et al. [28]. It was emphasized that Gr at extremely low concentrations is advantageous and improves the actuation behavior of Nafion-based IPMC actuators [28]. The actuation performance was decreased clearly by greater Gr loading due to the agglomeration resulting from van der Waals forces and the π–π interactions between the Gr layers. This agglomeration may have restricted the diffusion of ions leading to actuation. Thus, probably, a decrement in maximum tip displacement came into existence.