Effect Of Micro- and Nano- Size of Cellulose Particles Extracted From Rice Husk on The Properties of Hydrogels

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1 Wan Hafizi Wan Ishak, Ooi Shok Yin and Ishak Ahmad Effect Of Micro- and Nano- Size of Cellulose Particles Extracted From Rice Husk on The Properties of Hydrogels Wan Hafizi Wan Ishak 1, *, Ooi Shok Yin 1 and Ishak Ahmad 1,2 1 School of Chemical Sciences and Food Technology 2 Polymer Research Center (PORCE), Faculty of Science and Technology Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia Summary The effects of different size of cellulose particles extracted from rice husk on the properties of gelatin/cellulose hydrogels were studied. Micro size cellulose was extracted by alkali and bleaching treatments followed by acid hydrolysis to obtain cellulose nanocrystal (CNC). Both cellulose (micro) and CNC (nano) were used in hydrogels formulation with gelatin (in ratio 25% cellulose dispersion to 75% gelatin). Gelatin was then subjected to chemical cross-linking using glutaraldehyde as crosslinking agent with CNC entrapped in gelatin to produce hydrogels. SEM observation revealed that swollen micro cellulose has irregular and bigger pores compared to cellulose nanocrystal (CNC) hydrogel which has homogeneous and smaller pores arrangement thus affecting their swelling degree. The cellulose fibers exhibit enhanced thermal stability but contrary with the cellulose nanocrystals which showed lower thermal stability. Keywords: Cellulose, Cellulose nanocrystal, Rice husk, Gelatin, Hydrogel 1. Introduction Hydrogels are defined as hydrophilic three dimensional polymers network that have the ability to absorb and retain large amount of water in its swollen state 1,2. Chemical or physical crosslinking is required to avoid dissolution of hydrogels into the aqueous phase 3. Hydrogels have received special attention in pharmaceutical and biomedical applications including artificial tendons, drug delivery systems, drug delivery, contact lens materials and matrices for tissue engineering 4. Stimuli responsive hydrogels which response towards external changes in environmental condition, such as temperature, light, ph, electric signal, and ions possess a dramatic property change (like network structure, *Corresponding author: hafizi.wan29@yahoo.com Smithers Information Ltd., 2016 permeability, swelling behavior, or mechanical strength) 5,6. Polymeric hydrogels produced from natural materials such as polysaccharides, possess unique advantages such as non-toxicity, biodegradability, biocompatibility, biological functions and abundance which are promising for application in biomaterial field. Recently, the researchers have focus the fabrication of hydrogels on natural polymers such as cellulose, gelatin, starch and hyaluronic acid 7,8. Cellulose is one of the most natural polymer studied by researchers due to their outstanding properties such as high surface area, biocompatibility, biodegradability, hydrophilicity, and high mechanical property 4. Cellulose also known as the most renewable and abundant natural polymer in earth existed as one of the main constituent of plants and natural fibers 9,10. The abundance hydroxyl groups in cellulose fibers is a promising factors that allow easy preparation of hydrogels with fascinating structures and properties 1. In this research, cellulose fibers are extracted from rice husk. Rice husk can be easily found as an agricultural waste in northern part in Malaysia. In addition, a huge amount of rice husks produced during the paddy milling process from the fields as an agricultural waste. Normally, the rice millers dispose the rice husk waste by open burning that can leads to environmental problems 5. In this experiment, cellulose fibers were extracted from rice husk using alkali and bleaching treatment whereas acid hydrolysis was subsequently performed to obtain cellulose nanocrystal (CNC). Previously, hydrogels were synthesized using microcrystalline cellulose which differ in types and concentration 11. The aim of this study was to synthesize hydrogels based cellulose with Polymers & Polymer Composites, Vol. 24, No. 9,

2 Effect Of Micro- and Nano- Size of Cellulose Particles Extracted From Rice Husk on The Properties of Hydrogels different size of cellulose (micro and nanometers). The main reason to compare the properties of hydrogels with different size of cellulose is the changes in swelling behavior. The significant improve in the properties of hydrogels could broaden the function of the gelatin hydrogels in various biomedical applications. 2. Materials and Method 2.1 Preparation of Microcellulose Fibers Rice husk was grinded to obtain rice husk powder and then treated with 4 wt.% NaOH solution at 90 C for 2 h under mechanical stirring. The alkali treatment is repeated three times, in order to remove lignin and hemicellulose from the rice husk fibers. After each treatment, the fibers were then filtered and washed with distilled water to remove the alkalisoluble components. A subsequent bleaching treatment was conducted to remove the residual lignin. An equal part of solution used in this treatment were acetate buffer (solution of 2.7 g NaOH and 7.5 ml of glacial acetic acid dissolved in distilled water), 1.7% aqueous sodium chlorite NaClO 2 solution, and distilled water. Then, the mixture were stirred by mechanical stirring for 4 h at 80 C and repeated seven times. For each bleaching treatment, the bleached fibers were filtered, washed with distilled water and air dried. 2.2 Preparation of Nanocellulose Fibers Acid hydrolysis was performed on the cellulose with micro-sized at 50 C using 65 wt.% H 2 SO 4 (pre-heated) under continuous stirring for 40 min. The suspension was diluted with cold distilled water to stop the reaction before centrifuged for 10 min at 10,000 rpm. Centrifugation step was repeated a few times before a subsequent dialysis were performed against distilled water until a constant ph was attained. The suspensions were then freeze dried to produce nano cellulose powder. 2.3 Hydrogels Formulation Hydrogels formulation was prepared by mixing cellulose in distilled water and then homogenized to ensure a proper dispersion of cellulose. Gelatin was added into the cellulose suspension and stirred with magnetic stirrer at 55 C until a homogeny solution prepared. In this case, two formulations were prepared, the first one was using micro- cellulose (Hydrogels A) and the second one was using the nanocellulose (Hydrogels B). Next, glutaraldehyde (crosslinking agent) was added by dropping in about 3 wt.%. After 4 hours, the mixture was poured into petri dish and kept in the oven at 45 C until it dried. 2.4 FTIR Analysis FTIR analysis was conducted in order to elucidate functional group in hydrogels using FTIR spectra 2000 (Perkin Elmer). The samples were cut with dimension of 1 cm 1 cm and 2 mm thickness subsequently put on the ATR plate to be characterized. The samples were analyzed over the range of cm Swelling Degree The swelling degree of hydrogels against influence of solution was determined by evaluating its ability to swell in room temperature of the solution. Study on the influence of temperature was performed at 37 C. Degree of swelling of the sample in each medium were then specified using Eq. (1): (1) Weight of sample after swelling in medium for 48 h (G s ) and dried weight of extracted sample (G d ). 2.6 Morphological Studies Morphology of the hydrogels was observed using Variable Pressure Scanning Electron Microscopy (VP Leo 1450 SEM). Hydrogel samples were freeze-dried and quickly frozen with liquid nitrogen and images were taken under 500 magnifications at 15 kv. Transmission electron microscopy (TEM) was conducted in order to investigate the morphology of cellulose nanocrystal (CNC) by using a Philips CM 30 microscope. The contrast was enhanced by staining the nanocrystals with 2% of uranyl acetate solution (an organometallic complex) in deionized water for 1 min before dried at room temperature. 2.7 Thermogravimetric Analysis (TGA) TGA analysis was conducted in order to study thermal stability of the sample. Hydrogels were cut into small pieces to be analyzed. The analysis was performed under nitrogen gas flow at 10 ml/min and temperature (50 C- 900 C) range at the rate of 10 C/min. 3. Results and Discussion 3.1 FTIR Analysis The IR spectra of cellulose, gelatin, hydrogels A and hydrogels B are shown in Figure 1. FTIR spectrum of cellulose showed C-O stretching at cm -1, C-H stretching at 2897 cm -1 and O-H stretching at 3271 cm -1 due to intermolecular hydrogen bonding 12. Another peak at 898 cm -1 showed the present of cellulose structure 13. Gelatin spectra showed a distinctive peak at 3297 cm -1 and 3074 cm -1 represent stretching of N-H for amide I. Another peak at 1628 cm -1 corresponds to C=O stretch for amide II and the stretching of C-H for alkyl groups is showed at 2955 cm -1. The following peak of the spectra at 1544 cm -1 and 1336 cm -1 represent N-H for amide II and amide III. Similar results were also reported by Yang et al. 14. The analysis also shows that all peaks present in both hydrogels spectra 784 Polymers & Polymer Composites, Vol. 24, No. 9, 2016

3 Wan Hafizi Wan Ishak, Ooi Shok Yin and Ishak Ahmad are comparable, suggesting that the formation of hydrogels involved the same interaction between cellulose fibers and gelatin polymer chains 7. These spectra also proved that the chemical structure does not take part in modifying the size of cellulose and cellulose nanocrystal. However, the effect of size of cellulose may take part during the reaction. Hydrogels B with nano sized cellulose provides more surface area for the reaction as reported by Johari et al. 7. Figure 1. FTIR spectra of cellulose, gelatin, hydrogel A and hydrogel B 3.2 TEM Analysis The morphological observation using transmission electron microscopy (TEM) on cellulose nanocrystal (CNC) is to confirm the size of nanofibers. According to Azizi Samir et al. 15, the size of fiber will be reduced from micro scale into nano scale after acid hydrolysis. The amorphous region of the cellulosic microfibrils were expected to be cleaved during the acid hydrolysis keeping the crystalline rodlike or needle-like particle remained as showed in Figure 2. As can be seen from the micrograph, the diameter of the CNC were in the range of 6 to 10 nm with the length of 110 to 250 nm. Similar result were reported by Johar et al. 5. The CNC is expected to provide more surface area for the reaction lead to increase the swelling degree of hydrogels. Figure 2. The TEM micrograph of cellulose nanocrystals (CNC) extracted from rice husk fiber 3.3 Swelling Degree The swelling studies of swollen hydrogels were conducted in distilled water at room temperature and the hydrogels were let to swell for 8 hours. The swelling degree was calculated using Eq. (1). Figure 3 showed that, hydrogel B with nano cellulose has higher degree of swelling compared to hydrogel A with micro cellulose. The difference in particle size of hydrogel also affected water uptake and absorption speed, thus led to high or low swelling ratio of hydrogels. Hydrogel B exhibits faster water Polymers & Polymer Composites, Vol. 24, No. 9,

4 Effect Of Micro- and Nano- Size of Cellulose Particles Extracted From Rice Husk on The Properties of Hydrogels uptake compared to hydrogel A. The final hour of the experiment (level of plateau swelling in Figure 3) also showed that the hydrogels with smaller particle (hydrogel B) obtained higher water uptake. Similar behavior was also reported by Razmjou et al. 17 which could be due to greater surface area and interstitial volume of the smaller particle. Another reason proposed by Omidian et al. 18 that the rate of absorption increased for the smaller Figure 3. Comparison of swelling degree on Hydrogel A and Hydrogel B Figure 4. SEM micrographs for swollen hydrogels of (a) hydrogel A (microcellulose/gelatin) and (b) hydrogel B (CNC/gelatin) particle size attributed to an increase in surface area of absorbent accessible to the water. 3.4 SEM Analysis SEM images revealed the changes in morphology of swollen hydrogels cellulose/gelatin with different size of cellulose (micro and nano). The hydrogels were freeze dried and then quickly frozen with liquid nitrogen in order to preserve a better quality of morphology images. Similar observation were also reported by Dash et al. 4 and Liao et al. 16. Based on the observation, hydrogel A showed a bigger and irregular pores compared to hydrogel B with smaller and regular pores. The distribution of pores in hydrogel B which is more frequent in quantity and uniform promotes accessible of water into the hydrogels that lead to higher water uptake with faster responses in time. This observation proved that the swelling behavior in Figure 3, where hydrogel B has higher swelling degree than hydrogel A. Figure 5. TGA curves of hydrogel A (microcellulose/gelatin) and hydrogel B (CNC/ gelatin) 3.5 TGA Analysis Thermogravimetric analysis (TGA) is used to obtain information about the thermal stability of both hydrogels cellulose/gelatin with different size (micro- and nano- size). According to Mu et al. 19, the increase of temperature decomposition refers to the degradation of material which has better thermo stability. In this experiment, the first degradation of hydrogels occurs at around C which refers to water and other molecules which can easily evaporated. This behavior also observed by Inamura et al. 20. Figure 5 also showed the degradation of hydrogels occurs at around C which represented the rapid decrease of about 70% of cellulose fibers. At this stage, the molecular chains of cellulose fractured and gradually transformed into CO 2 and volatile hydrocarbons 21. Based on the observations, hydrogel B (CNC) started 786 Polymers & Polymer Composites, Vol. 24, No. 9, 2016

5 Wan Hafizi Wan Ishak, Ooi Shok Yin and Ishak Ahmad to degrade at 290 C whereas hydrogel A started to degrade at 310 C. It shows that hydrogel B (CNC) degrades at lower temperature compared to hydrogel A. This distinction is mainly due to the difference in the particle size between cellulose nanocrystal (CNC) and microcellulose. The reduction in the thermal stability of hydrogel B (CNC) is due to the increase of highly specific surface area and free end chains of the nanocrystals resulting from the high amount of short cellulose chains. These end chains started to decompose at low temperature Conclusions This study clearly demonstrates the potential of modification of cellulose by reducing its size from microcrystalline cellulose into cellulose nanocrystal (CNC). The hydrogels formation from both type of cellulose shows significant difference in their performance including swelling and thermal stability. Swelling capacity of hydrogels was improved by using crystal cellulose nanocrystal due to the proper formation and distribution of pores. In addition, CNC reduces the thermal stability of hydrogels due to high amount of surface area in a certain region. The study verifying that the properties of the hydrogels can be induced by modifying the cellulose to obtain nanocrystalline cellulose. These findings will be very beneficial in improving biomedical applications such as drug delivery, wound dressing, tissue engineering, and artificial tendon and so on. Acknowledgements The authors acknowledge the financial support from the Ministry of Higher Education (MOHE) and Ministry of Science, Technology and Innovation (MOSTI) under FRGS grant FRGS/2/2013/SG06/UKM/02/2 and Sciencefund grant SF1106. References 1. Chang, C. and Zhang, L Cellulose-based hydrogels: Present status and application prospects. Carbohydrate Polymers 84(1): Buwalda, S. J., Boere, K. W. M., Dijkstra, P. 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