Impact of purification on iota carrageenan as solid polymer electrolyte

2.4.1 Impedance spectroscopy

The samples bulk resistance (Rb) measurement was carried out by using electrical impedance spectroscopy (EIS), high-frequency response analyzer (HFRA) model Solartron 1260 Schlumberger with a frequency range of between 1 Hz and 10 MHz with 10 mV amplitude. The samples were sandwiched between the blocking stainless steel with a surface contact area of 2.0 cm². The conductivity (σ) values of polymer electrolyte were calculated from Eq. (1)

2.4.2 Attenuated total reflectance Fourier transform infrared (ATR-FTIR) analysis

ATR-FTIR analysis was performed by using Perkin–Elmer Spectrum 2000 in the range of 4000–600 cm⁻¹ with a scanning resolution of 4 cm⁻¹. The analysis was carried out to observe powdered and polymer electrolytes characteristic bands changes and shifting on the spectra.

2.4.3 X-ray diffraction (XRD) analysis

XRD measurements were done for powdered and polymer electrolytes in order to determine whether they are crystalline, amorphous or both. The measurements were performed by using model D5000 Siemens. The data were collected from a range of diffraction angles at 2θ from 3° to 6° at 0.05° s⁻¹ rate.

3.1.1 Attenuated total reflectance Fourier transform infrared (ATR-FTIR)

ATR-FTIR spectra of ῑ-carrageenan and purified ῑ-carrageenan powder are shown in Fig. 1. There are nine basic chemical bonding in ῑ-carrageenan, which are C—O—C of 3,6-anhydro-D-galactose at 927 cm⁻¹, ester sulfate O⚌S⚌O symmetric vibration at 1214 cm⁻¹, —O—SO₃ stretching vibration at D-galactose-4-sulfate (G4S) and D-galactose-2-sulfate (DA2S) at 848 cm⁻¹ and 802 cm⁻¹, C—O bridge stretch at 1157 cm⁻¹, C—O stretch at 1066 cm⁻¹, C—H stretch at 2912 cm⁻¹, O—H stretch at 3369 cm⁻¹ and water deformation at 1637 cm⁻¹, respectively, for purified ῑ-carrageenan (Michel et al., 1997). Meanwhile, for purified ῑ-carrageenan the C—O—C of 3,6-anhydro-D-galactose was at 928 cm⁻¹, ester sulfate O⚌S⚌O symmetric vibration at 1220 cm⁻¹, —O—SO₃ stretching vibration at D-galactose-4-sulfate (G4S) and D-galactose-2-sulfate (DA2S) at 844 cm⁻¹ and 801 cm⁻¹, C—O bridge stretch at 1150 cm⁻¹, C—O stretch at 1065 cm⁻¹, C—H stretch at 2920 cm⁻¹, O—H stretch at 3376 cm⁻¹ and water deformation at 1638 cm⁻¹.

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Fig. 1 ATR-FTIR spectrum and XRD of (a) iota carrageenan and (b) purified iota carrageenan powder.

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It can be seen that the major observation from the spectrum is the intensity of the peaks, especially for O—H stretching, water deformation and ester sulfate O⚌S⚌O symmetric vibration. The O—H stretch of ῑ-carrageenan at 3376 cm⁻¹ was shifted to a lower wavenumber at 3369 cm⁻¹ after purification. The %T value calculated from the spectrum showed that the O—H stretch of purified ῑ-carrageenan powder (27%T) is more than the ῑ-carrageenan (10%T) itself. The H₂O deformation peak increased significantly from 8%T of ῑ-carrageenan up to 27%T for purified ῑ-carrageenan powder. Meanwhile, the ester sulfate peak also increased in value from 10%T up to 27%T from ῑ to purified ῑ-carrageenans. Based on a work report by Janaswamy and Chandrasekaran (Janaswamy and Chandrasekaran, 2001), there are strong interactions between the sulfate group of neighboring helices mediated by calcium ions and water molecules (Mobarak et al., 2015; Funami et al., 2008). The ῑ-carrageenan double helix was stabilized by interchain hydrogen bond from the O—H group. The water molecules enhanced the structural stability by connecting the sulfate groups to the neighboring hydroxymethyl group. Meanwhile, the sulfate groups were linked together by the calcium ions. The increase in intensity and shifting of the peak towards a lower wavenumber were caused by the breakdown of the interchain hydrogen bond in the double helix that nucleates the dissociation into single strand after purification in dilute solution (Mobarak et al., 2015; Funami et al., 2008).

3.1.2 X-ray diffraction (XRD)

X-ray diffraction of ῑ-carrageenan and purified ῑ-carrageenan powder are shown in Fig. 1. The ῑ-carrageenan intense peak at 25.5° 2θ was reduced and the peak at 23° 2θ has slightly shifted to 22° 2θ after purification. It was supported by the degree of crystallinity value before and after powder purification. The degree of crystallinity for ῑ-carrageenan powder was 53.12% and purified ῑ-carrageenan powder was 35.21%; hence, the crystallinity region was reduced after purification.

3.2.1 Attenuated total reflectance Fourier transform infrared (ATR-FTIR)

The purified ῑ-carrageenan powder was utilized to form a polymer electrolyte film. ATR-FTIR analysis are shown in Fig. 2. Acetic acid was used as a solvent in this polymer electrolyte system. The acetic acid dissociated into H⁺ and CH₃COO⁻ which is entrapped in the electrolyte system (Michel et al., 1997); and thus, they play a role in helping the ionic conduction mechanism with the polymer. The coordination happened between the H⁺ ion and several functional groups, as reported by Mobarak et al. (Jumaah et al., 2015). They are O—H stretch, O⚌S⚌O symmetric vibration, C⚌O stretching and C—O stretching. The H⁺ ions coordination is favorable towards the functional groups due to the oxygen atoms that are rich in electron (Jumaah et al., 2015; Mobarak et al., 2012).

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Fig. 2 ATR-FTIR spectrum of purified iota carrageenan (a) powder and (b) film and XRD of iota carrageenan and purified iota carrageenan films.

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The interaction occurred between the polymer hydroxyl group with the ions (H⁺ and CH3COO⁻) was indicated by the shifting of the O—H and C—O—H bands to a higher wavenumber; 3369–3383 cm⁻¹ and 1066–1071 cm⁻¹, respectively (Michel et al., 1997). The interaction of acetate ions with ῑ-carrageenan polymer was confirmed through the formation of a new peak around 1556 cm⁻¹ which belongs to COO⁻ stretch of acetic acid (Michel et al., 1997; Funami et al., 2008). The coordination of H⁺ occurred only on C—O—H (1066 cm⁻¹) but not with C—O—C (1157 cm⁻¹) because C—O—H group inhibit the coordination by blocking the interaction of H⁺ with C—O—C group. As a result, there are no observable changes of the C—O—C peak in the film spectrum. Ester sulfate group, O⚌S⚌O does not play any role in ionic conduction for this system since there is no shifting in the wavenumber, indicating the occurrence of the interaction. The ionic conduction of the purified ῑ-carrageenan based polymer electrolyte followed the same trend as proposed by Jumaah et al. (Jumaah et al., 2015).

3.2.2 X-ray diffraction (XRD) analysis

Fig. 2 shows the XRD pattern of ῑ-carrageenan and purified ῑ-carrageenan films. Both powders were used to form the film. As can be seen from the figure, a ῑ-carrageenan film, without purification, has many crystalline peaks arising from the contribution of extra salts and impurities. It possess the highest degree of crystallization, that is 53.8% at 29° 2θ. Other observable peaks were at 21°, 32°, 34° and 45° 2θ. After purification, the peaks have disappeared, forming a more amorphous region and the degree of crystallinity was reduced to 32%; thus, the ionic conductivity value of the purified ῑ-carrageenan film is expected to obtain a higher value of conductivity as compared to non-purified ῑ-carrageenan film.