Preparation of CMSS hydrogel
The Effect of Percentage of CMSS
Fig. 1 shows the percentage of gel content and degree of swelling of various percentage of CMSS from 40 to 80% (w/v) in 1.0M HCl solution. The percentage of gel content increased gradually with the amount of CMSS until the optimum amount 60% indicating that the increasing amount of CMSS provide more crosslinking area thus increasing the gel content. However, higher than 60% w/v, the gel content started to decrease slightly. The formation of the crosslinked CMSS hydrogel with acid solution may be reaching its optimum at 60% (w/v) CMSS. When the percentage of CMSS was higher than 60% w/v, there was limited amount of H+ available in the mixture to promote crosslinking process as the same volume of HCl was used. Thus, resulting in less number of cross linkages between the CMSS chain although the percentage of CMSS increased. Meanwhile, the degree of swelling is inversely proportional to the percentage of gel content. Increased in gel content caused decreased in swelling capability. It may due to the degree of crosslinking which affected the water uptake [15]. Higher CMSS concentration produces more crosslinked point in polymeric chains thus increase crosslinking network which results in less swelling when it is brought in water. Increased crosslinking network, lead to formation of a tighter network. This caused difficulty for water to permeate and reduced the amount of water being trapped inside hydrogel network thus resulted in low degree of swelling [13, 15]. 60% (w/v) of CMSS gives highest gel content (62.05%), thus was chosen throughout the experiment to prepare hydrogel as a sorbent.
The Effect of HCl Concentration
The effect of different concentration (0.5 to 2.5 M) of HCl solution on gel content and degree of swelling of CMSS hydrogel is shown in Fig. 2. The percentage of gel content increased as the concentration of HCl increased. Similar result have been reported by Takigami et al., [11] stated that the number of crosslinks increases with the increase of acid concentration. However, the graph plotted shows a slight decrease of gel content at 2.5 M that might be due to acid hydrolysis of the linkage at high concentration of acid solution. From the data obtained, 2.0 M of HCl gave the highest gel content and has been chosen as a suitable concentration of HCl in preparing the hydrogel in the next parameter. The swelling capability of CMSS hydrogel as expected, decreased as the concentration of the HCl increased. With the increase of the gel fraction, water absorption decreased significantly depending on the acid concentration [11]. High swelling degree of hydrogel with 0.5M HCl indicates that it has high ability to absorb water. Pushpamalar et al., [16] reported in her study, hydrogels that are relatively weak and consist of a low number of intermolecular bonds are able to expand by absorbing water and holding it in their void. As the intermolecular bonds and the crosslink density increased, hydrogel becomes tightly packed, lessening the ability of the hydrogel to absorb water. This lead to the decrease of swelling degree of hydrogel with high concentration of HCl. Similar finding was reported by Sadeghi et al., [17] stated that higher crosslinker concentration diminished the free space between the polymer chain and subsequently resulting in highly crosslinked rigid structure that cannot be extended and hold a vast amount of water.
The Effect of Reaction Time
The third parameter investigated was the effect of reaction time (12 to 72 hours) for the crosslinking process to completely occur. Fig. 3 shows that there is no significant difference in gel content of CMSS hydrogel at 12, 24 and 36 hours, and it decreased slightly at 48 and 72 hours reaction time. A possible explanation may be the fact that the prolongation of the reaction time does not affect the gel content as the crosslinking process already achieved the maximum value in a short period of time. Although 36 hours reaction time gave 71.89% of gel content, 12 hours reaction time is preferable to avoid waste of time. Usually, swelling capability decreases as the gel content of the hydrogel increases. However, in this parameter, the swelling capability decreased with the increased of reaction time although the gel content did not show significant increment. As the reaction time increased, the structure of the hydrogel became stronger and denser. This could be due to a longer period of time increased the crosslink density and caused the decreasing the swelling capability of the hydrogel [18]. Kabiri et al., [19, 20] mentioned in their studies that there is a possibility of the additional linkage that increased the crosslink density, which resulted in reducing swelling capacity.
The Effect of Reaction Temperature
The effect of the reaction temperature of 60% CMSS in 2.0M HCl at 12 hours of reaction time was examined from room temperature (24ºC) to 70ºC. Fig. 4 shows slightly change in gel content as the temperature increase. This could be due to the degree of crosslinking of hydrogel decreased due to acid hydrolysis of CMSS at high temperature, thus reduced the number of long chain polymer that available for crosslinking resulting in decreased of gel content of the hydrogel. Similar result reported by Takigami et al., indicated that reaction at high temperature will cause starch hydrolysis and gives low gel formation [11]. The swelling capability in this study was different than the expected trend as it decreased with the decrease of the gel content. When hydrogel comes in contact with any solvent molecules, the solvent tries to attack the hydrogel surface and penetrate within the polymeric network structure [21]. However, in this experiment, it was observed that the appearance of the gel prepared at high reaction temperature was hard and rigid. This could prevent the hydrogel to swell as the water found difficulty to pass through the surface of the hydrogel thus decreased the swelling capability.
Swelling Properties
Hydrogel is known with its ability to absorb and hold huge amount of solvent in its network structure. This swelling property is a very crucial factor for its further application. The swelling studies would suggest a good prediction on the ability of the hydrogel to interact with the surrounding media, its smart behavior could be used in many application especially in control release of drug delivery [7]. Fig. 5 shows the swelling degree for the CMSS hydrogel at different immersion media. PBS 7 gave high swelling degree indicating that this CMSS hydrogel swells well in PBS 7 solution. In acidic media (PBS 4), more –COOH groups are formed, which induce the formation of the hydrogen bond of –COOH with CMSS hydrogel, leading to a more compact network in CMSS hydrogel and caused a decreased in the swelling degree [7, 22]. Similarly, when CMSS hydrogel was immersed in HCl solution, the presence of H+ ions in the surrounding induced the hydrogen bond formation and resulting in low swelling degree. In salt solution (NaCl), the swelling ability of CMSS hydrogel significantly decreased compared to the swelling in deionized water. Gupta et al., [23] stated that the phenomenon can be explained on the basis of osmotic pressure developed due to the unequal distribution of ions in the medium and the polymer network. When the polymer immersed in NaCl solution, the development of osmotic pressure is much lower since the external solution contains Na+ and Cl-, the presence of Na+ ions in the outer solution causes a decrease in the osmotic swelling pressure which operates due to the difference of counter ions in a gel phase and solution phase, therefore, reducing the swelling ability. Sadeghi et al., [17] also reported that this behavior was often attributed to the charge screening effect and ionic crosslinking of monovalent cations. In alkaline solution (NaOH and PBS 10), it was observed that there was no swelling degree recorded as the CMSS hydrogel completely dissolved in the alkaline solution after 48 hours immersion time. This might due to the degradation of the CMSS hydrogel crosslinked network at high concentration of alkaline solution present in the surrounding resulting in reformation of –COONa thus dissolved in the media. These results indicated that CMSS hydrogel prepared in this study exhibited pH-sensitive characteristic and affected by its surrounding environment
Characterization of CMSS hydrogel
Fourier Transform Infrared analysis
FTIR spectroscopy is an effective technique to analyze the functional group of the materials. Fig. 6 shows the FTIR spectra of sago starch, CMSS, and CMSS hydrogel. The spectrum of sago starch shows the absorption band at 3273 cm-1, which is due to O-H stretching vibration, as well as intramolecular and intermolecular hydrogen bonds in glycosidic bond in the sago starch molecule [7]. The absorption band at 2910 cm-1 shows C-H stretching and 1643 cm-1 shows tightly bound water presence in the starch molecule. The appearance of the absorption band at 1350 cm-1 belongs to -CH2 symmetrical band, while the broad band at the range of 1100-990 cm-1 indicates the C-O stretching from C-O-C and C-O-H in glycosidic ring of starch molecule.
The FTIR spectrum of CMSS shows the shifting of O-H stretching band to lower wavenumber at 3175 cm-1. This probably due to the low intermolecular hydrogen bonds between O-H group left in the glycosidic rings of CMSS prior to the substitution reaction take place during carboxymethylation process [13]. Additional absorption band at 1597 cm-1 indicates substitution of CH2COO-Na+ group on the starch molecular chain during carboxymethylation.
CMSS hydrogel spectrum shows a new absorption band at 1723 cm-1 indicating that the Na in CMSS being exchanged to H from hydrochloric acid solution. The concentration of hydrochloric acid solution used was 2.0 M, thus a sharp absorption band can be observed indicating that most of the -COONa converted to COOH. The intensity of the absorption band depends on the concentration of the acid used, low concentration of acid shows a weak absorption band of conversion of -COONa to COOH [24]. A similar result was reported previously where the replacement of Na in carboxymethyl group with hydrogen from acid occurred depending on the type of acid and concentration [11]. The absorption band at 1421 cm-1 and 1238 cm-1 are respectively due to -CH2 scissoring and -OH bending vibration, while OCH-O-CH2 stretching represented by the absorption band at 1000 cm-1.
Thermogravimetric analysis
TGA analysis is a convenient analysis to study the thermal decomposition and thermal stability of polymer. It is a quantitative measurement of the weight change of a sample against the temperature applied. TGA thermograms of native sago starch, CMSS, and CMSS hydrogel are shown in Fig. 7. Native sago starch, CMSS, and CMSS hydrogel have two steps of decomposition. The first step below 150 ºC correspond to the removal of moisture from the surface and entrapped water in the sample. The second step occurred around 200 ºC to 500 ºC is due to the decomposition of starch backbone and leaving a residue. Zhang et al., [25] stated that water was the main product decomposition at temperature below 300 ºC and further heating up above 500 ºC resulted in carbonization and ash formation.
For CMSS, the maximum weight loss occurred at 295.33 ºC which is lower than native sago starch before carboxymethylation (309.57 ºC) with 49.71% of total CMSS decomposed and 42.37% residue due to the presence of ash and sodium salt in CMSS. It had been found that the carboxymethylation process decreased the thermal stability of starch materials [25]. The maximum thermal decomposition of CMSS hydrogel was 330.22 ºC with 60.22% major weight loss, slightly increased compared to native sago starch and CMSS. Increase in the maximum thermal decomposition of CMSS hydrogel could be due to the presence of cross-linkages in CMSS hydrogel [24]. High cross-linked structure and compact network improve the degree of stability of hydrogel resulted in higher temperature needed to decomposed them [7].
SEM analysis
Scanning Electron Microscopy (SEM) in an excellent technique in examining the surface morphology of a sample. Fig. 8(a) shows a smooth surface and oval-shaped granules sago starch with a diameter in the range of 20-40 µm similar to the previous study reported by Ahmad et al,. [26]. Fig. 8(b) shows massive distorted and irregular shape of carboxymethyl sago starch. In comparison with native starch, the surface of CMSS is rough and groove indicates that the breakage of chemical bond by a strong alkaline environment during carboxymethylation process. Thus, the carboxymethylation is not only the reaction on surface of starch granules but also from within [24]. The micrograph also shows that the CMSS remain intact and agglomerate after preparation with aqueous-alcoholic medium studied. Fig. 8(c), shows CMSS hydrogel has spongy surface with empty space called pores in structure and connected to each other to form networks as a consequence of the crosslinking process [27].