Fabrication of Glycidyl Methacrylate (GMA) Grafted Cellulose from Rice Straw

The purpose of this study was to prepare and characterize cellulose grafted with glycidyl methacrylate (GMA). Cellulose was isolated from rice straw by chemical method. The acid concentration used during the hydrolysis process affects the size of the cellulose produced, therefore in this study different concentrations of sulfuric acid (50%, 60%, and 70%) were used. Particle Size Analyzer (PSA) results showed the best concentration of sulfuric acid was 50%. Glycidyl methacrylate-grafted cellulose (GMA-g-C) was prepared by different concentration of GMA (15%, 20%, and 25%). GMA-g-C particles were characterized by FT-IR and XRD. FT-IR spectra confirmed the formation of GMA-g-C and XRD data showed the decrease in crystallinity of cellulose after grafting process. The optimum grafting conditions were obtained at 20% GMA with a grafting percentage (Pg) of 233.3% and grafting efficiency (GE) of 27.26%.


Introduction
Indonesia is an agricultural country with one of the largest commodities being rice.The majority of Indonesian people use rice as a staple food.One of the regions in Indonesia that produces rice is Aceh with a harvest area of 271.75 thousand hectares with a production of 1.51 million tons of GDG (Grinded Dried Grain), if converted into rice, the production reaches 0.87 million tons [1] .Based on this data, it can be seen that rice straw production in Aceh is very large.Currently, the use of rice straw in Aceh has not been carried out optimally.Its use is only in animal feed mixtures which are only used in small portions.
Rice straw is able to decompose naturally in the environment, but it takes a long time.To overcome this, finally the straw was burned so that the land could be replanted.Burning rice straw can have several negative impacts, such as causing the destruction of soil nutrients and producing gas emissions such as carbon dioxide (70%), methane (0.66%), carbon monoxide (7%), nitrous oxide (2.09%) and ash which causes environmental pollution and climate change.This has negative impacts on human and animal health [2].Rice straw has a composition of cellulose (38.3%), hemicellulose (31.6%), lignin (11.8%) and silica (18.3%) [3].Rice straw can be used as a source of cellulose, thereby increasing the use value of rice straw and being able to overcome environmental pollution.
Cellulose is the most abundant and renewable biopolymer available throughout the world.Physically, cellulose is a fibrous, hard, water-insoluble material which plays an important role in maintaining the composition of plant cell walls [2].Cellulose can be easily obtained from various types of plants, such as from rice straw [4][2] [5], jute [6], sugar cane bagasse [7], corn cobs [8], bamboo [9], empty palm oil bunches [10] and others.Cellulose can be used as an alternative material, such as making bioplastics, 1297 (2024) 012042 IOP Publishing doi:10.1088/1755-1315/1297/1/012042 2 adsorbents, and fillers as reinforcements in making polymer composites.Isolation of cellulose from rice straw can be done using chemical methods, namely the acid hydrolysis method.The acid used is a strong acid, one of which is sulfuric acid.Several studies have been conducted concerning the isolation of cellulose from rice straw using sulfuric acid, however there were differences in the acid concentration used, such as 64% [11], 45% and 55% [12].Therefore, in order to find the optimal acid concentration, this work was performed with various concentration of sulfuric acid.
Cellulose can also be used as a reinforcement in composites, because it has good thermal and mechanical properties [13].However, when compared with synthetic polymers, cellulose has a weakness, such as contains intramolecular and intermolecular hydrogen bonds, making it difficult to combine with other compounds in making composites [14].This can be overcome by modifying the cellulose, namely by graft copolymerization.Graft copolymerization is a technique for adding certain functional groups to cellulose chains.This process requires an initiator that will react to form species that can interact with the backbone and produce radicals that will initiate the polymerization reaction.The types of initiators that can be used are chemical, photochemical and radiation initiators [15].In this work, graft copolymerization was carried out with a chemical initiator.
Materials can be used to modify cellulose using the graft copolymer method include ethylenediamine [16], acrylic acid [17], N-isopropylacrylamide [18], glycidyl methacrylate [19], and other.Glycidyl Methacrylate (GMA) is the most popular monomer because it has very reactive vinyl groups and epoxy groups [20].So in this study GMA was used as the monomer and KPS (potassium persulfate) as the initiator, because KPS is an initiator that is easily soluble in water and the radicals formed are stable [21].This research aims to obtain (1) cellulose from rice straw (2) characteristics of graft copolymerization products which was carried out through functional group analysis using IR (infrared) spectroscopy techniques and analysis of the degree of crystallinity using XRD.

Materials
The raw material used in this work was rice straw taken after a few days of harvest obtained from Aceh, Indonesia.The chemicals used were analytical grade and purchased from Sigma-Aldrich.

Isolation of Cellulose from Rice
Straw. 1 kg of rice straw was washed using clean water 1-4 times to remove various impurities.Then sun dried for 1-2 days (until dry).The dried rice straw was then cut into 3-5 mm pieces and crushed with a blender until it becomes rice straw powder [4].Cellulose isolation begins with a dewaxing process by mixing 50 g of rice straw powder with n-hexane and ethanol (2:1) for 24 hours to remove wax, oil and pigments, followed by a neutralization process and dried using an oven at 50 o C for 20 hours [3].The second stage was an alkalization process, the dried dewaxed product was treated with 12% NaOH (1g/10 mL) at 121 o C for 1 hour to remove lignin and hemicellulose, neutralized and dried in an oven at 50 o C. The third stage was hydrolysis, the alkalization product was mixed with H2SO4 in a ratio of 1: 20 (g/mL) with different concentration of sulfuric acid (50%, 60% and 70%) at 50 o C for 1 hour.The hydrolysed product was added to cold distilled water and centrifuged (10,000 rpm & 30 minutes) to remove excess sulfuric acid and washed until neutral pH was reached [2].The final stage was bleaching, where the neutral hydrolysed product was mixed with 10% NaClO at 75 o C for 60 minutes with a pH in the range of 3-5 (adjusted with glacial acetic acid), neutralized and dried in an oven at 50 o C [6].

Preparation of GMA Grafted
Cellulose.0.5 g of cellulose was added to a glass beaker containing 20 mL of KPS 0.0925 M (initiation stage) and stirred at a temperature of 60 o C for 1 hour with tightly closed conditions [18].Then added 20 mL of GMA (GMA was dissolved in a solvent containing methanol and water in a ratio of 4:6 and the GMA concentration variations were 15%, 20% and 25%).
The mixture was stirred for 4 hours and then rinsed using distilled water and continue with methanol to remove residual monomer, dried and weighed [16].Percentage of grafting (Pg) and grafting efficiency (% GE) were calculated using the equation: Where, W0 = cellulose weight, W1 = weight of monomer, dan W2 = weight of grafted cellulose

Fourier Transform Infrared (FT-IR).
FT-IR was used to identify functional groups present in cellulose and GMA grafted cellulose.The infrared area used was in the range 4000-400 cm -1 , with a resolution of 4 cm -1 and 50 scans.This analysis was carried out at the Chemical Laboratory of the Bandung Institute of Technology.

X-ray Diffraction (XRD).
XRD was used to determine the degree of crystallinity of cellulose and GMA grafted cellulose.The sample was measured at 2θ = 5 -90° with a Kα (Cu) 1.54 ray source.This analysis was carried out at the Lhokseumawe Polytechnic Chemistry Laboratory.The degree of crystallinity can be calculated using the equation: Dimana, Iam = minimum intensity (pada 2θ ≃18,3°) dan I(002) = main crystal peak (pada 2θ ≃ 22,5°).

Isolation of Cellulose from Rice Straw
Cellulose isolation from rice straw was conducted in 4 stages; dewaxing, alkalization, hydrolysis, and bleaching.The dewaxing process was performed using n-hexane as a nonpolar solvent and ethanol as a polar solvent.These two solvents were used to remove polar and nonpolar impurities [22].The dewaxed product is shown in Figure 1a.The alkalization process which is also known as delignification was conducted using 12% NaOH in order to dissolve the lignin and hemicellulose in rice straw and to purify the cellulose [2].This process produced smaller fibers that form aggregates (Figure 1b).Hydrolysis process was performed with the aim to remove the amorphous part of a cellulose chain so that the crystalline part of the cellulose can be isolated.The acid hydrolysis process involves breaking down the β 1,4 glycosidic bonds of glucose units in the cellulose material.In the acid hydrolysis process, the hydronium ions produced will penetrate the cellulose material and disrupt the amorphous part contained in the cellulose material by entering the hydrolysis agent.The amorphous part of cellulose has a lower density than the crystalline part of cellulose, this causes the amorphous part of cellulose to break apart more easily and release the crystalline part when contacted with acid [23].The products of hydrolysis process of rice straw with different concentration of sulfuric acid are shown in Figure 2.They appeared to be black in color.It was probably due to high concentration of sulfuric acid used not only disrupted the amorphous part of cellulose but also further led decomposition into carbon [24].The use of 50% sulfuric acid concentration produced a less black product than products using 60% (Figure 2b) and 70% (Figure 2c) sulfuric acid.Bleacing process was conducted to remove color from the fiber due to the remaining lignin resulting from hydrolysis because in the alkalization process, lignin cannot be 100% dissolved [11].NaOCl in water will produce hydroxyl ions and hypochlorous acid (HOCl) which were strong oxidizers and can break lignocellulose bonds and ether bonds in the lignin structure, so that the degree of whiteness of the fiber will increase.It was observed by the formation of yellowish white pulp.The yield percentage of cellulose produced at each variation in H2SO4 concentration are shown in Table 1.

Preparation of GMA Grafted Cellulose (GMA-g-C)
The grafting process began with the initiation stage.In the first stage, KPS was thermally decomposed to form sulphate radical anions (Figure 4).These radicals were then used as initiators of the polymerization process.The second stage in the grafting process was the propagation stage.This stage began when the GMA monomer was added.Before being added, the GMA was dissolved using solvents, namely methanol and water (4:6).These two solvents were used because they match the polarity of GMA, so they can help the interaction process of GMA with the active groups on cellulose.The vinyl groups contained in GMA bonded with the active groups on cellulose and the epoxy groups were used to bond with other polymers.This grafted cellulose was called GMA-g-Cellulose.
The cellulose grafting process is influenced by several factors, one of which is the concentration of the grafting agent, which in this case was GMA.In this study, GMA concentration was varied and proven to influence grafting efficiency (GE) and percent grafting (Pg).The GMA concentrations in this study were 15%, 20%, and 25%.The pictures of GMA-g-C produced with different concentration of GMA are shown in Figure 3 and the Pg and GE obtained are shown in Table 2.  Based on Table 2, the largest Pg and GE were found at 20% GMA concentration, that was 233.3% and 27.26%, respectively.As the GMA concentration increases, Pg and GE also increase, because more GMA molecules are bound to cellulose [16].However, at a GMA concentration of 25%, Pg and GE decreased.This was because higher GMA concentrations allow homopolymerization to occur so that the amount of GMA interacting with cellulose will decrease.In addition, the viscosity of the solution increases so that the diffusion of GMA in cellulose becomes less [16] [18].Characterization of cellulose and GMA-g-C 3.3.1.Particle Size Analyzer (PSA).Figure 5 shows PSA results of cellulose isolated from rice straw with different concentrations of sulfuric acid.The average particle sizes of cellulose obtained using sulfuric acid concentrations of 50%, 60% and 70% were 417 nm, 420 nm, and 986 nm, respectively.As the acid concentration increases, the particle size increases.Supposedly, with increasing concentration, the cellulose particle size becomes smaller.However, it was found oppositely in this work.It was probably due to high concentration of sulfuric acid used not only disrupted the amorphous part of cellulose but also disrupted crystalline part of cellulose and became oligomers and formed aggregates.It was confirmed by the reduction of peak intensity at 2θ = 22.56° in XRD patterns of cellulose isolated using sulfuric acid concentrations of 60% (7c) and 70% (Figure 7d), where this peak was observed with high intensity in XRD pattern of cellulose isolated using sulfuric acid concentration of 50% (Figure 7b).Therefore, for grafting process, cellulose isolated using 50% sulfuric acid was used.

Fourier Transform Infrared (FT-IR).
Figure 6a shows absorption peaks at wave number 3410 cm -1 and 1639 cm -1 which indicate the stretching and bending vibrations of -OH and the absorption of water molecules in rice straw [26].These peaks shift to 3427 cm -1 and 1641 cm -1 in cellulose isolated using sulfuric acid concentration of 50% FT-IR spectra (Figure 6b) and shift to 3429 cm -1 and 1631 cm - 1 in cellulose isolated using sulfuric acid concentration of 60% and 70% FT-IR spectra.Figure 6e shows a decrease in intensity at wave number 3412 cm -1 which indicates the existence of intermolecular interactions in the cellulose grafting process [18].Peak appears at wave number 2918 cm -1 (Figure 6a and 6b), 2920 cm -1 (Figure 6c and 6d) and 2999 cm -1 indicates C-H stretching vibrations.Figure 6a shows an absorption band at wave number 1512 cm -1 which corresponds to a typical aromatic C=C absorption band representing the structure of lignin.This peak is not observed in Figure 6a-6e.It confirmed that the lignin was successfully removed [4].Peak at wave number 1161 cm -1 , (Figure 6a), 1163 cm -1 (Figure 6b) and 1153 cm -1 (Figure 6e) indicates the presence of asymmetric C-O-C stretching vibrations [4].However, this peak appears at 1095 cm -1 in Figure 6c and 6d, indicating that a different structure has been formed.These results are supported by XRD data (Figure 7c and 7d).
Figure 6e shows a sharper peak at wave number 1730 cm -1 which is the stretching vibration of the C=O group and peaks at 993 cm -1 and 864 cm -1 indicate the stretching vibration of the epoxy group in GMA monomer [25].Peak at wave number 758 cm -1 is also observed which is the stretching vibration of the oxirane ring in GMA [18].The presence of this new peak shows that GMA has been successfully grafted onto cellulose.

X-Ray Diffarction (XRD).
Figure 7a shows peaks appearing at 2θ = 14.98 o , 19.81 o , and 22.87 o with crystallinity 38%.In Figure 7b, peaks appear at 2θ = 15 o and 22.56 o which indicate the characteristics of cellulose [26].The of crystallinity obtained was 40%.The crystallinity of sample increases after isolation process using sulfuric acid concentration of 50%.However, the crystallinity decreases after isolation process using sulfuric acid concentration of 60% and 70%, where the crystallinity is 29.84% and 30.92%, respectively.The higher the acid concentration used in the hydrolysis process in this work reduce the crystallinity due to the deformation of cellulose crystal structure.Grafting process affects cellulose structure, where the grafting process reduces the crystallinity of cellulose and the XRD pattern of GMA-g-C shows an amorphous material.

Figure 1 .
Rice straw after dewaxing process (a) and (b) after alkalization process

Table 1 .
The yield percentage of cellulose produced at each variation in H2SO4 concentration.

Table 2 .
Pg and GE of GMA-g-Cellulose produced with different GMA concentration.