2.1

Materials

Cotton pulp (MVE, DP-4580) was supplied by Buckeye Technologies Co. Ltd. (USA) and Poly vinyl alcohol (PVA = 89 000-98 000 g mole^-1, 99% hydrolyzed). Extra pure lithium chloride (LiCl) was carefully dried by using an oven at 110 °C, 2 h before use, N,N-dimethylacetamide (DMAc) were purchased from Sigma-Aldrich. The DI water was used throughout experiment.

2.2

Preparation of PVA-cellulose hydrogel

Cellulose/PVA (CP) hydrogels were prepared by blending method. Before blending cellulose and PVA solutions were prepared. Here a common solvent was used for both solution preparations. Preparation of the cellulose solution was described in our previous paper¹⁶ and here is brief explanation. The pure cotton pulp was dispersed in 8.5 % LiCl/DMAc solvent and stirred by using magnetic stirrer at room temperature over a period of 5 days after that completion of this process obtained 1.5 wt % of cellulose solution and also 1.5 wt% of PVA solution was obtained by above similar process at 60 °C. After getting the both solutions were mixed with different ratios 75/25, 50/50 and 25/75, stirred at room temperature until homogeneous mixture getting after that the solution was poured in Petridis and kept in oven at 30 °C for a period of 3 days. After 3 days transparent hydrogel is formed and the formed hydrogel was immersed in 500 mL beaker containing 250 mL of DI water. The water was repeatedly changed every 6 h up to 5 days to remove LiCl/DMAC and unreacted materials from the hydrogel.

2.3

Characterization of PVA- Cellulose hydrogels

The FTIR spectra were used to study the formation of CP hydrogel. Before, analysis of the FTIR spectra the samples were completely dried in the oven at 60 °C for 6 h and the samples were examined between 500-4000 cm^-1 on Bruker Optics, Billerica, MA using the KBr pellet method. Transparency analysis of the CP hydrogels was performed by using UV-visible spectroscopy (HP8452A, Agilent). The hydrogels were cut into a desire shape for transparency measurement and the spectra recorded from 200-800 nm. The thermal behavior of the Cellulose and CP hydrogels were studied by thermogravimetric analysis (TGA) (NETZSCH, STA 409 PC), at a heating rate of 10 °C/min under a constant nitrogen flow (20mL/min).

2.4

Actuation test

The actuation test was carried out by using a laser displacement sensor (Keyance LK-G85, Tokyo, Japan) a high voltage amplifier connected to a function generator, (33220A, Agilent), Lab view software on a personal computer. Before conducting the actuation test the desired shape of the hydrogels (10 × 10 × 0.4 mm) were equilibrated in DI water for 24 h after reaching of equilibration, the swollen hydrogels were kept in between two electrodes (polyimide tape attached on ITO glass) and an electric field was applied between two electrodes. Displacement of the hydrogel was measure by the laser displacement sensor along with a data acquisition system (PULSE, B&K) as shown in Fig.1.

Figure 1.

Schematic setup of actuation test.

3.1

Fourier transform Infrared Spectroscopy

The FTIR spectra of the PVA, cellulose and CP hydrogels are showed in Fig.3. From Fig.3, the CP hydrogels show the similar peaks of PVA, which demonstrates that the cellulose is uniformly dispersed in PVA. The characteristic peaks at 1098 cm^-1 and 2923 cm^-1 are related to stretching vibrations of the C-O and C-H. A broad absorption peak at 3438 cm^-1 corresponds to the –OH stretching vibration of PVA. While these peaks were slightly shifted to 1098 cm^-1 and 2923 to 1063cm^-1 and 2917 cm^-1 in CP hydrogel and also the -OH group was slightly shifted from 3438 cm^-1 to 3425 cm^-1 in CP hydrogel. The intensity of this peak was slightly decreased and appeared to be a bit broader than the –OH of the PVA. This is due to the strong intermolecular bond formation between the PVA and cellulose matrix.¹⁷ An interestingly absorption peak was absorbed at 899 cm^-1 this the b-glucan group of the cellulose and also the fingerprint region peaks intensity increased due to the crystalline structure of the cellulose.^18,19,20 The pure cellulose hydrogel shows the above characteristic peaks of the CP hydrogel peaks with slightly changes (3427 cm^-1, 2920 cm^-1, 1069 cm^-1 and 896 cm^-1).

3.2

UV-Vis transmittance

The prepared CP hydrogel transparency was measured by using UV-vis spectroscopy. The transparency depends on the concentration of PVA in the prepared hydrogels. Figure 4 shows the transparency of the CP hydrogel. The transparency was decreased with increasing the PVA concentration in the hydrogel and also it depends on the thickness of the hydrogel. As increasing the thickness of the hydrogel the transmittance decreases. However the overall transparency was followed in the sequence of CP75<CP50<CP25<Cellulose hydrogel at 500 nm wavelength.

Figure 4.

UV-vis spectra of Cellulose and CP hydrogels

3.3

Thermogravimetric analysis

The thermal stability of the PVA, cellulose and CP hydrogels were measured by using thermos gravimetric (TGA) analysis. Figure 5 shows the thermal stability of the PVA, cellulose and CP hydrogels. All hydrogel curves exhibited mainly two degradation stages. A minor weight loss was observed below 200 °C due to the water molecules present in the samples. The PVA hydrogel shows the initial and final degradations at 310 °C and 408 °C and cellulose hydrogels shows the initial and final degradation at 264 °C and 329 °C. whereas the CP shows at 274 °C and 346 °C.²¹ The overall CP hydrogel shows the highest thermal stability than the PVA and cellulose hydrogel.

Figure 5.

Thermogravimetric analysis of cellulose and CP hydrogels

3.4

Actuation test:

The CP hydrogels were prepared using physical blending method and the actuation test was performed. 4- 8 µm displacement output was achieved with 1000 V/cm at 0.1 Hz. Figure 6 shows the displacement of the physical hydrogels. Figure 6A shows the electric voltage dependent horizontal displacement, at 0.1 Hz. Here the displacement increased with increasing the voltage. Figure 6B shows the frequency dependent horizontal displacement at 1000 V. The displacement decreased with increasing the frequency.²²

Figure 6.

Activation test of fabricated hydrogels: A) electric voltage dependent displacement at 0.1 Hz, B) Displacement output with frequency change.

Note that neat cellulose hydrogel shows higher displacement than the CP hydrogel due to the ion migration. Cellulose has some remnant ions caused by DMAC/LiCl solvent. When the PVA concentration increased in CP hydrogels the displacement decreased since intermolecular bond formation is increased, which results in weak ion injection.^23,24