Preparation, optimization and swelling study of carboxymethyl sago starch (CMSS)–acid hydrogel
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In this study, sago starch was modified in order to enhance its physicochemical properties. Carboxymethylation was used to introduce a carboxymethyl group into a starch compound. The carboxymethyl sago starch (CMSS) was used to prepare smart hydrogel by adding acetic acid into the CMSS powder as the crosslinking agent. The degree of substitution of the CMSS obtained was 0.6410. The optimization was based on the gel content and degree of swelling of the hydrogel. In this research, four parameters were studied in order to optimize the formation of CMSS–acid hydrogel. The parameters were; CMSS concentration, acetic acid concentration, reaction time and reaction temperature. From the data analyzed, 76.69% of optimum gel content was obtained with 33.77 g/g of degree of swelling. Other than that, the swelling properties of CMSS–acid hydrogel in different media such as salt solution, different pH of phosphate buffer saline solution as well as acidic and alkaline solution were also investigated. The results showed that the CMSS–acid hydrogel swelled in both alkaline and salt solution, while in acidic or low pH solution, it tended to shrink and deswell. The production of the hydrogel as a smart material offers a lot of auspicious benefits in the future especially related to swelling behaviour and properties of the hydrogel in different types of media.
KeywordsCarboxymethyl sago starch Optimization Hydrogel Gel content Swelling in different media
Sustainable chemistry is a green approach in science and technology for environmental protection where this approach is hoped to overcome serious issues related to the ecosystem. Researches on carbohydrates polymer have been actively done due to their sustainability and biodegradability properties. These biodegradable polymers such as starch , chitosan  and carrageenan  can simply be modified via crosslinking, cationization, UV-irradiation, microwave and electron beam irradiation . In recent years, numerous types of technologies are used to improve the world’s climate which brings biodegradable technology as one of the examples that offers environmental solutions without harming the planet.
Starch is a natural polymer produced by green plants to store energy that can be easily found in leaves, stems, roots and seeds. Sago starch is isolated from the sago palm through the process of extraction and purification. Malaysia as world’s largest sago exporter has been exporting sago products in the volume of 44,000 tonne per year to Japan, Europe, America and Singapore . Sago palm is produced commercially in Sarawak, where the crop is mainly grown on peat soils. The most common sago species grown is Metroxylon sagu because this type of sago plant gives higher quality products . According to Flach , the advantages of the crops are; environmentally friendly, uniquely versatile and promote socially stable agroforestry systems. Plus, this crop is imperious to some minor natural disasters such as floods, drought, fire and strong winds because of its large fibrous root. Sago has been widely used around the world and the diversity has led to the use of sago in many areas.
The versatility of sago starch is due to its physicochemical properties that can easily be altered through chemical or physical treatment . The modification of sago starch is crucial to intensify its industrial properties and these modifications have been reported to improve its swelling, solubility and light transmittance . Modification by crosslinking can be established via chemical reaction that is initiated by the change in pH, radiation, heat or pressure . Crosslinking treatment is performed to increase chemical bonds at random locations in a granule to make it stable and strengthen the relatively tender starch. According to Haroon et al. , the treated starch via crosslinking may embellish the tensile strength and thermal stability. Modified sago starch such as carboxymethyl sago starch (CMSS) is proclaimed to improve physicochemical properties such as swelling ability in cold water, freeze–thaw stability and low retrogradation tendency .
Hydrogel is a polymeric three-dimensional (3D) network gel that is formed by polymer chains crosslinking, composed of hydrophilic groups such as hydroxyl and carboxyl to store water and biological fluid. To ensure that the hydrogel is equipped with hydrophilic character, carboxylic acid groups (R-COOH) is needed as the side groups of the hydrogel backbone. The absorption capacity and swelling properties of this sensitive hydrogel are very important in most of its applications. The hydrogel has a potential to swell in different media, highly associated with the network porosity and depends on; crosslinking density and hydrogel-media attraction . Hydrogel is an example of smart material because of its ability in changing structure due to certain responses. This smart hydrogel is able to change its volume in different environmental responses such as temperature, pH, ions and substances concentration . Hundreds of hydrogels from natural polymer have been fabricated using starch, alginate and chitosan because of their potential application in biomaterial field due to their safety, hydrophilicity, biocompatibility and biodegradability. Year by year, researchers are doing their best to modify and improve the hydrogel properties so that its usage can be expanded and not limited only to certain areas.
In this research, sago starch was chosen to be modified due to its abundancy and low cost. The aim of this research was to optimize the preparation of CMSS–acid hydrogel and to study its swelling properties in different media.
Materials and methods
Sago starch powder was purchased from Song Ngeh Sago Sdn Bhd, Sarawak, Malaysia. Sodium monochloroacetate (SMCA, Sigma-Aldrich), sodium hydroxide (NaOH, ChemAR®) pellets, isopropanol (IPA), methanol, ethanol, acetic acid, phosphate buffer saline (PBS) solution pH 2.0, 7.4 and 10.0 were purchased from the R&M Chemicals. All chemicals used in the study were of analytical grade. Deionized and distilled water were used throughout the experiment.
Preparation of CMSS
Determination of degree of substitution (DS)
The degree of substitution (DS) of carboxylic group in CMSS is defined by the average number of the hydroxyl group in the starch structure which was substituted by carboxymethyl groups. The DS of the sample was determined by the standard ASTM D1439 .
Preparation of CMSS hydrogel
Gel content and degree of swelling
Swelling test in different media
1.0 g of optimized CMSS–acid hydrogel was weighed into a teabag and immersed in a beaker of 150.0 mL of different medium for 72 h at room temperature. The media studied were; (1) 0.2 M of NaCl solution; (2) 0.5 M of NaCl solution; (3) 1.0 M of NaCl solution; (4) 1.0 M of NaOH solution; (5) 1.0 M of HCl solution; (6) PBS solution pH 2.0; (7) PBS solution pH 7.4 and (8) PBS solution pH 10.0. After the immersion, CMSS–acid hydrogel was weighed again and then, the degree of swelling was calculated using Eq. 2.
Fourier transform-infrared spectroscopy (FT-IR)
FT-IR spectroscopy is a technique used to determine the functional groups of the sample by measuring the infrared absorption spectrum. FT-IR spectra were recorded on FT-IR spectrometer (Spectrum 100 Perkin Elmer) with a wavenumber range between 400 and 4000 cm−1. The sampling technique used was attenuated total reflection (ATR) in conjunction with infrared spectroscopy, which enables the samples to be examined directly in the solid state.
X-ray diffraction (XRD)
X-ray diffraction is a technique used to reveal the information on the structure of a sample. This XRD characterization was carried out using Shimadzu XRD-6000 diffractometer with Cu Kα (λ = 1.5418 Å) radiation at room temperature operated at 30 kV and 30 mA. A sample was placed in an aluminium sample holder and a diffraction pattern plots intensity against the angle of the detector, 2θ and the scanning range 2° to 60° with rate of 2°/min with continuous scan mode.
Scanning electron microscopy (SEM)
Scanning electron microscopy is a technique used to study the surface morphology of a material and it basically focuses on the surface of the material and its composition. The samples were freeze-dried first and then gold sputter-coated to make the samples become conductive before the scanning process is done. The prepared samples were examined under scanning electron microscope (JEOL, Tokyo Japan) at a voltage of 15.0 kV and recorded at the range of magnification between 50 and 1000×.
Results and discussion
Degree of substitution (DS)
The degree of substitution (DS) of CMSS attributes to the average number of carboxymethyl groups per anhydroglucose unit (AGU) and theoretically, the maximum number of DS is 3.0 . In this study, the sago starch was modified via chemical modification using SMCA and the DS was found to be 0.6410.
Effect of CMSS concentration
The degree of swelling of CMSS–acid hydrogel decreased with increase of gel content. At the highest percentage of gel content, the degree of swelling shows the lowest value which was 38.51 g/g only. The previous study also recorded that higher degree of crosslinking can reduce the swelling power . When the degree of crosslinking is higher, there are more tendencies of crosslinkages to occur in the CMSS–acid hydrogel. Hence, it makes the water molecules more difficult to diffuse into the CMSS–acid hydrogel.
Effect of acetic acid concentration
The degree of swelling of the CMSS–acid hydrogel is inversely proportional to the percentage of gel content. The swelling decreases from 1.0 to 4.0 M but starts to rise up to 49.63 g/g from 4.0 to 5.0 M. At high acid concentration, the acid hydrolysis onto CMSS instead of crosslinking reaction may take place to break the bond and intermolecular forces between the CMSS molecules. The breakage has interrupted the firm structure of the hydrogel itself leaving some voids to the structure and the hydrogel to absorb more water.
Effect of time of reaction
Effect of reaction temperature
A similar trend as observed in the previous section was found for the degree of swelling of CMSS–acid hydrogel produced at a different temperature. The degree of swelling of CMSS–acid hydrogel decreases with the increase of gel content. At higher temperature, higher possibility of formation of the hydrogen bonding that caused tighter crosslinked structure which leaves fewer voids for water absorption and thus reduces the swelling ability of the CMSS–acid hydrogel to swell in water.
Swelling behavior in different media
From the optimized CMSS–acid hydrogel, the degree of swelling of CMSS–acid hydrogel in deionized water is 33.77 g/g. The first medium studied is sodium chloride (NaCl) aqueous solution. For this medium, 3 different concentrations of NaCl aqueous solution have been studied which were 0.2, 0.5 and 1.0 M. The swelling trend in NaCl aqueous solutions shows that the degree of swelling of the CMSS–acid hydrogel increases by decreasing salt concentration. The degree of swelling for 0.2, 0.5 and 1.0 M were 9.36, 7.09 and 5.33 g/g, respectively. The existence of the salt solution in the swelling medium may lead to the screening effect caused by cation (Na+) that leads to the osmotic pressure decrement between the CMSS–acid hydrogel and the external solution . The presence of the electrolyte salt solution also causing the CMSS–acid hydrogel to not swell well due to the exo-osmosis as it tends to shrink dramatically . The higher the concentration of the electrolyte salt solution, the higher the chances of hydrogel to collapse. The degree of swelling in 1.0 M of sodium hydroxide (NaOH) solution was the highest compared to other medium which is 23.64 g/g. The CMSS–acid hydrogel is an anionic hydrogel and as reported by Gupta et al. , the anionic hydrogel will swell in alkaline (high pH) solution. The pendant group of the anionic hydrogel, carboxyl groups, COO− are ionized in higher pH level and may lead to the electrostatic repulsion and causing the swelling of the hydrogel. The next medium used was 1.0 M of hydrochloric acid (HCl) solution which gives 5.12 g/g of the degree of swelling. The negatively charged carboxyl group of CMSS–acid hydrogel react with the strong acid which cause the hydrogel to shrink, deswell and inhibit the insertion of water molecules to the hydrogel network in the acidic environment.
Meanwhile, for the phosphate buffer saline (PBS) solution, three different pH values were used to study the degree for swelling of CMSS hydrogel. The pH values are: 2.0, 7.4 and 10.0. At the lowest pH value, which is pH 2.0, the degree of swelling for the hydrogel is only 4.07 g/g. For this pH value, there is no significant difference of swelling behaviour between pH 2.0 of PBS solution and 1.0 M HCl. This is because both solutions have low pH value. Hence, it can be said that there is some interactions between the hydrogen bonding of carboxyl group of CMSS hydrogel and the PBS solution that make the hydrogel to shrink. Plus, the excess cations, H+ may cause the “screening effect” and the protonation of carboxymethyl group which leads to shrinkage of the CMSS–acid hydrogel [26, 27].
As the pH values of PBS increased from pH 2.0 to pH 7.4, the degree of swelling has also increased. This could be due to the transformation of COOH to COO− and thus, breaking the hydrogen bonding. The breaking of hydrogen bonding led to swelling of the hydrogel. From Fig. 7, there was no obvious difference in the degree of swelling value for both PBS solutions at pH 7.4 and 10.0. The degree of swelling of CMSS–acid hydrogel slightly decreased at pH of 10.0 and similar finding was reported by Pushpamalar et al. .
Fourier transform-infrared spectroscopy (FT-IR)
X-ray diffraction (XRD)
The diffractograms of CMSS and CMSS–acid hydrogel, showed only broad patterns which attributed to the amorphous phase. This has confirmed that both samples lose their crystallinity, which may be due to the replacement of hydroxyl groups in the samples  and the breakage of starch due to heat with presence of water .
The loss of crystalline phase in sago starch can be seen after the modification to both CMSS and CMSS–acid hydrogel. This was due to the presence of strong alkaline, NaOH during carboxymethylation that transformed the hydroxyl groups of starch molecules (St-OH) into alkoxide group (St-O−). Effects from the repulsion of both negative charges caused a tension on neighbouring crystallites of starch molecule which pointed to the dissociation of double-helical regions and the disintegration of the crystalline structure .
Scanning electron microscopy (SEM)
In this study, CMSS was modified to obtain the CMSS–acid hydrogel. The preparation of the CMSS–acid hydrogel was successfully optimized with all parameters studied. Swelling in a different media of the hydrogel shows that the CMSS–acid hydrogel is a smart hydrogel that change its behavior depending on the surrounding behavior. The CMSS–acid hydrogel swells in both alkaline and salt solution but will shrink in acidic solution. Due to these smart properties of the CMSS–acid hydrogel, it can be used in various industrial applications.
NFATM designed the study, interpreted the results and wrote the manuscript. NZ developed the methodology for fabrication of the hydrogels as well as supervised the whole research. MA improved the methodology for hydrogels fabrication. SWT contributed to the discussion of results. All authors commented on the manuscript. All authors read and approved the final manuscript.
This research was financially supported by Ministry of Higher Education (MOHE), Malaysia (Trans-disciplinary grant scheme TRGS/2/2014/STG/UPM: VOT number: 5535401). Malaysian Nuclear Agency, Institute of Bioscience and Chemistry Department, UPM are gratefully acknowledged.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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