Sugar palm nanofibrillated cellulose (Arenga pinnata (Wurmb.) Merr): Effect of cycles on their yield, physic-chemical, morphological and thermal behavior

Accepted Manuscript Sugar palm nanofibrillated cellulose (Arenga pinnata (Wurmb.) Merr): Effect of cycles on their yield, physic-chemical, morphological and thermal behavior

R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin PII: DOI: Reference:

S0141-8130(18)33147-7 https://doi.org/10.1016/j.ijbiomac.2018.11.124 BIOMAC 11001

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

25 June 2018 7 October 2018 13 November 2018

Please cite this article as: R.A. Ilyas, S.M. Sapuan, M.R. Ishak, E.S. Zainudin , Sugar palm nanofibrillated cellulose (Arenga pinnata (Wurmb.) Merr): Effect of cycles on their yield, physic-chemical, morphological and thermal behavior. Biomac (2018), https://doi.org/ 10.1016/j.ijbiomac.2018.11.124

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ACCEPTED MANUSCRIPT Sugar palm nanofibrillated cellulose (Arenga pinnata (Wurmb.) Merr): Effect of cycles on their yield, physic-chemical, morphological and thermal behavior

Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products,

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R.A. Ilyas1, S.M. Sapuan1, 2*, M.R. Ishak3, E.S. Zainudin2

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Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400

Department of Aerospace Engineering, Universiti Putra Malaysia, 43400 UPM Serdang,

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UPM Serdang, Selangor, Malaysia

Selangor, Malaysia

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* Corresponding author. Tel.: +603-89471788; Fax: +603-86567122

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E-mail address: [email protected]

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ABSTARCT

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Nanofibrillated cellulose (NFCs) were extracted from sugar palm fibres (SPS) in two separate stages; delignification and mercerization to remove lignin and hemicellulose, respectively.

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Subsequently, the obtained cellulose fibres were then mechanically extracted into nanofibres using high pressurized homogenization (HPH). The diameter distribution sizes of the isolated nanofibres were dependent on the cycle number of HPH treatment. TEM micro-images displayed the decreasing trend of NFCs diameter, from 21.37 to 5.5 nm when the number of cycle HPH was increased from 5 to 15 cycles, meanwhile TGA and XRD analysis showed that the degradation temperature and crystallinity of the NFCs were slightly increased from 347 to 347.3°C and 75.38 to 81.19% respectively, when the number of cycles increased. Others analysis also were carried

ACCEPTED MANUSCRIPT on such as FT-IR, FESEM, AFM, physical properties, zeta potential and yield analysis. The isolated NFCs may be potentially applied in various application, such as tissue engineering scaffolds, bio-nanocomposites, filtration media, bio-packaging and etc. Keywords: Sugar palm nanofibrillated cellulose; thermal behavior; high pressurize

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homogenization (HPH).

1. Introduction

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One of the most important biopolymers is cellulose as it is being extensively used in our daily life.

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This is due to its sustainability, easily and abundantly availability, biological degradability, and biocompatibility. Cellulose fibres are made up of a bundle combination of nanofibres with

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diameter range from 5 to 70 nm, and a length of more range from 100 nm to several micrometers.

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These single nanofibres are usually named as nanowhiskers, nanocrystalline cellulose, whiskers, microfibrils, microfibrillated cellulose, nanofibres, or nanofibrillated cellulose [1,2], and these

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nanofibres can be isolated from numerous sources such as kenaf [3,4], softwood wood [5], cotton

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[6], wheat straw [7], rice straw [8], coir fibre [9], sugarcane bagasse, banana peels [10], pinecones (Jack pine: Pinus banksiana Lamb) [11], potato tuber cells [12], carrot [13], alfa (Alfa

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tenassissima), eucalyptus and pine fibres [14] , corn husk and oat hulls [15] and bamboo fibre [16,17]. Besides that, these nanofibres have attracted interest of researchers in recent decades due to their unique physical properties (large surface area, high aspect ratio, abundant surface of hydroxyl groups and high crystallinity), good mechanical properties (high specific strength, stiffness and modulus), thermal properties (high thermal resistance and low coefficient of thermal expansion), low cost of production and environmental friendly nanomaterial.

ACCEPTED MANUSCRIPT Currently, nanofibrillated cellulose (NFCs), have been used in various application including reinforcements for polymer composites [3,4], oxygen –barrier films [18], self-healing polymer film [19], flexible displays, optical devices, food packaging, automobile windows [20,21], edible coatings and packaging materials [22], continuous papermaking [23], optoelectronic and medical

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devices [24], light-weight and high performance materials for defense, infrastructure and energy

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as well as scarfolds for tissue regenerations [1,25].

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There are several methods for producing NFCs have been utilized by numerous researchers to

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isolate highly purified nanofibres from treated cellulose. Besides, different methods of disintegration would lead to different types and sizes of nanofibres materials. These processing

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methods include mechanical treatments (high pressurized homognizer (HPH), microfluidization,

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ultrafine grinding or refining, ball milling, aqueous counter collision, steam explotion, extrusion, cryocrushing in liquid nitrogen, high intensity ultrasonication (HIUS), and high speed blending),

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chemical treatment (acid hydrolysis, carboxylation, carboxymethylation, quaternization,

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sulfonation), biological treatments (enzymatic hydrolysis), synthetic and electrospinning treatment, TEMPO-mediated oxidation and consequent mechanical treatment, or, a combination

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of two or several of the stated treatments [1,26,27].

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Lately, the high pressurized homogenizer method was found to be a selective technique to disintegrate nanofibrillated cellulose has been appropriately discussed [10,15,28–30]. High pressurized homogenization (HPH) process includes passing the cellulose slurry at high pressure into the vessel through very small nozzle purposely for isolating celulose microfibres mechanically into NFCs. HPH uses efficient high pressure energy to break cellulose present in fluids to the smallest possible size of the fibres from micro to nanoscale. When fluid is passed through small nozzle under high pressure condition, these microcellulose are broken into nanofibre and become

ACCEPTED MANUSCRIPT homogenized by the process of cavitation and high shear force. According to Khalil [28], HPH is described as one of the eco-friendly methods for refining of cellulosic fibres due to its simplicity, efficiency, and foremost does not need organic solvents to operate it. In the past decades, a wide range of raw materials have been used as extractive sources for the

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preparation of NFCs among which were listed earlier. In tropical countries, sugar palm fibres at present are byproducts from sugar palm cultivation. These fibres, which are lignocellulosic and

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multicellular are found to obtain high percentage of cellulose content (52.29%) [31] . Therefore,

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these sugar palms fibres can be used for the purpose of industrial, by which they convert waste to high value nanomaterial products. Nanofibrillated cellulose can be isolated from the fibres of the

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plant Arenga pinnata which is a bonafide member of the Palmae family by high pressurize

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homogenizer treatment [32].

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In the present work, the NFCs entrenched within sugar palm fibres are successfully isolated using high pressurize homogenization treatment. The structural and physicochemical properties of the

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sugar palm fibres were collected and analyzed through the atomic force microscopy (AFM), field

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emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, zeta potential nanoparticle sizer and X-ray

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diffraction (XRD). Whereas thermal analysis was done using thermogravimetric analysis (TGA) and derivative thermogravimetry analysis (DTG).

ACCEPTED MANUSCRIPT 2. Experimental 2.1. Materials Sugar palm fibres were collected from Bahau (Negeri Sembilan, Malaysia). The chemical reagents

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utilized include sodium chlorite, ethanoic acid and sodium hydroxide (purchased from Sigma–

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Aldrich, Malaysia).

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2.2. Extraction of cellulose

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Cellulose fibres have been extracted from sugar palm fibres (SPF) by delignification and mercerization processes [33–36]. This technique was applied in consonance with the guidance of

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ASTM D1104-56 (1978) to synthesize the holocellulose via bleaching process. This action is solely responsible for the delignification of the SPF. Tap water was used to wash 20g SPF to aid

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the removal of dust and other impurities. Afterwards, the cleaned SPF was immersed in a 1000 ml

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beaker, which contained 650 ml hot distilled water (95 oC). The beaker and its content were

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transferred to a water bath, while the temperature of the system adjusted to 70 oC. Ethanoic acid of 4ml volume and sodium chlorite of 8 g mass were simultaneously introduced into the beaker on

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an hourly basis for a period of seven (7) hours. The color transition of the sugar palm fibre from

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light brown to white is an indicator of the degree of delignification. The product was rinsed, washed and filtered with distilled water and is known as holocellulose. Applying, the ASTM D1103-60 (1977) method, the produced holocellulose was converted to α-cellulose. 500 ml of 5% w/v sodium hydroxide solution was utilized to soak the holocellulose for a time duration of 2 hours at a temperature of 23±2 ºC. The generated α-cellulose was thoroughly filtered and immersed in 500ml distilled water containing almost 7ml ethanoic acid to aid the neutralization of the alkaline cellulose. The mixture was then stirred for an estimated period of 30 seconds, then it was left for

ACCEPTED MANUSCRIPT 5 minutes. After that, the fibres were continuously washed with distilled water until the cellulose was confirmed to be free of the acid as indicated by the pH meter. Finally, the resultant fibres, also known as sugar palm cellulose (SPC), then was placed in an oven and allowed to dry at temperature of 103 ºC overnight.

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2.3. Disintegration nanofibres SPNFCs

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2.3.1. Mechanical pre-treatment

A refining treatment before HPH was required in order to enhance fibre accessibility and

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processing efficiency. Hence, the sugar palm cellulose (SPC) was refined for 20,000 revolutions

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in a PFI-mill according to ISO 5264-2:2002. The process of refining fibres resulted in improvement of both external and internal fibrillation. Moreover, this process had improved the

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flow of fibres and avoided clogging during fluidization.

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2.3.2 Mechanical high pressurize homogenization (HPH)

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Nanofibrillated cellulose (NFCs) from sugar palm fibre cellulose were isolated by the process of high pressurized homogenization (HPH). Typically, 1.8 % fibre suspension in water was processed

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in a high pressurized homogenizer (GEA Niro Soavi, Panda NS1001L, Parma, Italy). The samples

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were passed with 3 variables cycles which is 5, 10 and 15 times cycle, denoted as SPNFCs-5, SPNFCs-10, and SPNFCs-15, respectively, through an intensifier pump that increased the pump pressure, followed by the interaction chamber which defibrillated the fibres by shear forces and impacts against the channel walls and colliding streams. Through this process, the fibres were broken down from micro-sized structure to nano-sized structures forming slurries of nanofibrillated cellulose. The high pressurized homogenizer was maintain to operate at 500 bar. Besides that, the fibrillation process was conducted under neutral pH. The temperature was not

ACCEPTED MANUSCRIPT controlled but fluidization was temporarily stopped when the temperature of the stock reached approximately 90oC, to prevent pump cavitation. The process was then continued when the samples had cooled to approximately 45 oC. The NFCs suspensions were then collected and freezedried at -110 ºC in ethylene gas. SPNFCs then was kept at cool place before it was sent for sample

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testing [37].

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2.4. Characterization 2.4.1. FESEM microscopy

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With the aid of the FEI NOVA NanoSEM 230 machine (FEI, Brno-Černovice, Czech Republic)

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which possessed 3 kV accelerating voltage, visualization characterizations of nanofibres were observed. The microstructure and nanostructure topography of SPNFCs were visualized. Samples

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were coated with gold to avoid over-charging when visualized through FESEM. [38,39].

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2.4.2. TEM microscopy

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Nanostructural images of the SPNFCs were viewed using TEM analysis. The Philips Tecnai 20 machine with 200 kV acceleration voltage and standard slanted sample holder was employed in

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the production of TEM micrographs. Initially, dried SPNFCs were dispersed in distilled water and

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sonicated for 10 minutes to generate the SPNFCs suspension. Afterwards, a drop of SPNFCs suspension was mounted on a carbon coated metallic copper grid. This setup was dried at room temperature. Enhancement of the TEM nanostructure micrographs was carried out to pave way for improved resolutions [40]. 2.4.3. AFM microscopy

ACCEPTED MANUSCRIPT The analysis of AFM was performed by utilizing Dimension Edge with High-Performance AFM tool (Bruker, Santa Barbara, CA, USA) and a software which is known as Bruker Nanoscope analysis (Version 1.7) operated using Peak/ Force tapping mode with a single controller (Nanoscope V from Bruker). Besides that, this equipment was used to estimate the diameter values

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of the SPNFCs. Initially, a drop of SPNFCs suspension was placed on the surface of an optical

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glass slide and was allowed to air dry. The SPNFCs suspension was scanned in the air at room

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temperature and controlled relative humidity in tapping mode of the machine with OMCLAC160TS standard Si probes (radius of tip less than 10 nm, spring constant of 2.98 N/m and a

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resonant frequency of ~310 kHz) under a 1 Hz scan rate.

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2.4.4. Yield

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Solid content about 0.2% was diluted with distilled water and centrifuged at 4500 rpm for 20 min

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to separate the nanofibrillated material (in supernatant fraction) from the non-fibrillated and the partially fibrillated ones, which sedimented down, before being dried to a constant weight at 90 ºC

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in a halogen desiccator. The yield was calculated by using Eq. (1): 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡 (

(1)

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Yield % = ( 1 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑖𝑙𝑢𝑡𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒 × %𝑆𝑐) × 100

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where % sc is solid content percentage. The result represent the average values of three replicates. 2.4.5. Density

Gas intrusion under helium (He) gas flow aided by an AccuPyc 1340 pycnometer (Micromeritics Instrument Corporation, Norcross, GA, USA) was used to identify the SPNFCs nanofibres density. The samples of SPFs, stage-based treated fibres and SPNFCs were oven-dried at a temperature of 105 ºC for 1 day to eliminate moisture within the fibres. Then, the oven-dried samples were then

ACCEPTED MANUSCRIPT placed inside the desiccator to prevent atmospheric moisture absorption prior to their insertion into the pycnometer. Five replicates of measurements were performed at 27°C and the mean value was evaluated [41].

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2.4.6. Moisture content

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The moisture content experiment was carried out using five (5) prepared samples. All the samples

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were kept in the oven at a temperature of 105 ºC for a period of 24 hours. The initial weight of samples before the oven-drying process, Mi (g) and the final weight after the process, Mf (g) were

𝑀𝑖 −𝑀𝑓 𝑀𝑖

× 100

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2.4.7. FT-IR spectroscopic analysis

(2)

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Moisture content (%) =

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the samples was realized with the aid of Eq. (2).

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measured so as to evaluate the moisture content [41]. The computation of the moisture content of

The detection of possible changes in the existing functional groups in sugar palm nanofibre was

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performed with the aid of FT-IR spectroscopic measurements (Nicolet 6700 AEM, Thermo Nicolet

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Corporation, Madison WI, USA) in the range 4000 - 500 cm−1. Potassium bromide was mixed with nanofibres, and the mixture was pressed into thin transparent films that were then subjected to FT-

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IR analysis [42,43].

2.4.8.. X-ray diffraction (XRD) The investigation of the X-ray diffraction patterns of the raw, all stages treated and HPH of sugar palm fibres were uncovered using Rigaku D/max 2500 X-ray powder diffractometer (Rigaku, Tokyo, Japan) equipped with CuKα radiation (λ = 0.1541 nm) in the 2θ range 10-40º. The

ACCEPTED MANUSCRIPT crystallinity index of each fibre sample Xc, as depicted in Eq. (4) can be deduced from the empirical method reported by Segal et al., [44]. 𝑋𝑐 =

𝐼002 −𝐼𝑎𝑚

× 100

𝐼002

(4)

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where 𝐼002 and 𝐼𝑎𝑚 are the peak intensities of crystalline and amorphous materials, respectively.

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2.4.9.. Zeta Potential

The determination of the approximate size of SPNFCs and characterization of the surface charge

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property of nanoparticles were carried out using Zetasizer Nano-ZS (Malvern Instruments, Worcestershire, UK) by measuring the zeta potential of the fibre samples. Each fibre sample was

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diluted ten folds in pure water to a total volume of 1 mL and then subjected to a particle size

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analysis at 25°C. The measurement employed the electrophoretic mobility (μm/s) of the particles

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which was converted to zeta potential by inbuilt software based on the Helmholtz–Smoluchowski equation [45].

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2.4.10. Thermogravimetric analysis (TGA)

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The TGA analyser was used to investigate the thermal stability of composites with respect to weight loss due to increase in temperature. TGA was performed with a Q series thermal analysis

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machine from TA Instruments (Mettler-Toledo AG, Schwerzenbach, Switzerland) to determine the thermal degradation potential of sugar palm fibres at varying stages of extraction. Usually, the operating conditions include a dynamic nitrogen atmosphere in the temperature range 25–600°C and a heating rate of 10°C /min, the analysis was conducted by depositing the fibre samples in aluminium pans [42,43].

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Experimental Result and Discussion

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3.1. SPNFCs yield and FT-IR spectroscopy physicochemical analysis

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Fig. 1. Yields of attained nanofibres after several cycles

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SPNFCs-10

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Transmittance (%)

SPNFCs-15

3500

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Wavelength (cm-1)

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4000

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SPNFCs-5

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Fig. 2. FT-IR spectra of SPNFCs-5, SPNFCs-10 and SPNFCs-15

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The isolated SPNFCs yields from sugar palm fibre for various treatment according to specific HPH cycle of 5, 10 and 15 cycles were summarized in Fig. 1. It can be observed that the

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lowest yield is dominated by SPNFCs-5 yielded 69.91 % and increase slightly by 8.21%, reaching a yield of 78.12% for SPNFCs-10. The tremendous increasing trend throughout the extended number of cycle from 10 cycles to 15 cycles can be determined by observing the increasing yield of SPNFCs-15 by 14.4%, with yield of 92.52 % compared to SPNFCs-10. Further passing the nanofibres at 10 cycles did not bring substantial increase on the yield, compared to 15 cycles. This is due to the difficulty to disintegrate the force of cohesion of the amassed fibres within the cell

ACCEPTED MANUSCRIPT wall to release the nanofibrils. The significant increasing trend of SPNFCs yield may be attributed by the high pressure homogenization which gradually defibrillate cellulose fibres via mechanical approach into NFCs [28]. During the process of homogenization, a cellulose suspension was inserted into the vessel through very small nozzle between the impact rings and homogenizing

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valve, these fibres were then isolated from micro-size fibre to nano-size fibres by subjected to the

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process cellulose fibrillation through the effect of cavitation, impact and shear forces. Therefore,

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as the cycles of homogenization increase, the yield of nanofibres also increased, due to the frequent expose of nanofibres to the cavitation, impact and shear forces which turn more microfiber to

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nanofibre.

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It is surprisingly to note that the yield obtained nanofibres through mechanical approach in this current experiment was significantly higher than those of two similar experiments [33,46]

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which used similar starting material (sugar palm fibre and sugar palm bunch, respectively) for

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isolation of nanocellulose, however, both of them applied chemical approach. The differences of the yield gained is depending on the source sample, pretreatment and condition of mechanical

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approach, i.e. homogenization pressure, number of cycles, and temperature used [28].

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The study on changes of SPNFCs chemical structure after several number of passes through homogenization treatment was carried out by FTIR analysis. Fig. 2 shows the physicochemical

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analysis of the FTIR spectra of SPNFCs-5, SPNFCs-10 and SPNCs-15. FTIR analysis is an important method to attain direct information on changes occurred in the chemical structure of nanofibres after passes through several cycles of HPH mechanical treatment [47]. FTIR analysis indicated that there are similarities of all spectra found in all samples which is a sign that all nanofibres have similar chemical composition. This result was in good agreement with those

ACCEPTED MANUSCRIPT reported in the literature by Besbes [14] when isolating nanofibrillated cellulose from alfa, eucalyptus and pine using altered number of HPH passes and pressure. The physiognomies of cellulose 1 was indicated in all nanofibres spectra at absorption peak that was located around 3400, 2900, 1370, 1316, 1160, 1030, and 897 cm-1 [33,47,48]. Besides

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that, wide absorption band situated within 3200-3500 cm-1 specify the existent of O-H groups, and

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this absorbance becomes more deep with the addition of number of HPH cycles, where it is an

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indication of the reflectance of hydrophilic properties because of the presence of hydroxyl group in nanofibre [49]. A peak located at 2900 cm-1 is associated to C-H group of cellulose [50,51]. The

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presence of absorption crest at around 1645 cm-1 in all nanofibres samples in revealing of

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absorption of water by cellulose, in which, this peak is also interrelated to the bending modes of water molecules because of strong adhesion between water and nanofibre. Moreover, the

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absorbance bands around 1424 cm-1 is linked with intermolecular hydrogen attraction at C6 group

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[47]. In the meantime, according to the report by Janoobi et al., [50] and Haafiz et al., [47] in the study on isolation of nanofibres from OPEFB and waste sugarcane bagasse, respectively, stated

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that the absorbance band that located within the spectra range of 1330—1370 cm-1 which present

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in all nanofibres samples, might be credited to the present of C-H and C-O2 group of aromatic ring in polysaccharide. The glyosidic ether band at 1105 cm-1 links to C-O-C and C-C ring breathing

1

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band was at asymmetric valence vibration was at 1163-1167 cm-1. The prominent band at 897 cmwas observed in all nanofibres samples, is associated with C-H rocking vibration of cellulose.

Besides that, the common water absorption related characteristic peak around 1637 cm-1 (H-O-H stretching vibration) was found increasing for its intensity from SPNFCs-5 to SPNFCs-15 [52]. Therefore, FTIR analysis showed that the number of cycles that was implemented to the attained nanofibres from sugar palm cellulose did not altered the chemical structure of nanofibres.

ACCEPTED MANUSCRIPT This phenomenon proved that there was no chemical reaction occurred and the chemical structure of the resultant nanofibres were totally stable, however, morphologies surface of the fibre treated may be affected.

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3.2. Physical Properties of Sugar Palm Nanofibrillated cellulose (SPNFCs) and particle size analysis

In this section, Image J software was used to investigate and analyze the morphological

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properties of the sugar palm nanofibrillated cellulose (SPNFCs), which was collected from the

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micro-image of FESEM, TEM and AFM.

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Fig. 3. Aqueous suspension (2 wt%) (a-c), TEM (d-f), FESEM (g-i), AFM (j-l) images of SPNFCs-5 (a, d, g, j), SPNFCs-10 (b, e, h, k) and SPNFCs-15 (c, f, i, l)

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Fig. 4. Transmission electron (TEM) micrograph of length and diameter histograms of SPNFCs-5

Table 1

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(a), SPNFCs-10 (b) and SPNFCs-15 (c).

Dimension size of SPNFCs-5, SPNFCs-10 and SPNFCs-15 analyzed by TEM, FESEM, and AFM microscopies.

Samples

TEM (diameter size range) (nm)

AFM (diameter size range) (nm)

Sugar Palm Cellulose (SPC)

-

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FESEM (diameter range) (nm) 11870

SPNFCs-5

21.37 ± 6.91

65 ± 6.19

90 ± 17.98

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11.54 ± 2.77

27.5 ± 7.35

50 ± 25.04

SPNFCs-15

5.5 ± 0.99

9 ± 1.89

35 ± 8.61

Numerous studies conducted in the topic have summarized a convergence between the effects of experimental conditions on the yielded SPNFCs physical properties. The studies showed

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that the number of cycles passes through high pressurize homogenization process is crucial in

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determining the morphological structure and dimensions of the yielded SPNFCs. Fig. 3 displayed

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the aqueous suspension (2 wt%), TEM, FESEM and AFM microscopies of SPNFCs-5, SPNFCs10 and SPNFCs-15.

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The morphological structure of SPNFCs after mechanical defibrillation treatments was

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discovered through the FESEM at micron and submicron level. Upon closer observation of FESEM in Fig. 3 (g), (h), (i), it can be seen that the morphological structure of SPNFCs was altered

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as the number of HPH cycles increase. Besides that, it can also be observed that the SPNFCs-5

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display aggregated SPNFCs web-formation, and a larger morphological diameter of nanofibres, with apparently would give lower aspect ratio (length/ diameter). This phenomena might be

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attributed to the aggregated SPNFCs that are made up of strong cohesion of hydrogen bonding

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between thousands of individual nanofibres [53,54]. The tendency of fibre defibrillation can undoubtedly be seen after all treatments gave rise to intermittent fibrillar structure and further

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reduction in intra-fibrillar diameter. This result was in good agreement with earlier finding reported in the literature by Besbes et al., [14]. The diameter value of SPNFCs-5 was observed higher through the FESEM micro-image compared to TEM and AFM analysis, which is 21.37±6.91, 65±6.19, and 90±17.98 nm, respectively. Higher value of the SPNFCs nanofibre diameter noted from the FESEM analysis might be attributed from the coating layer of gold applied on the surface of the sample. This thin layer was applied by sputtering with gold in order to avoid

ACCEPTED MANUSCRIPT over-charging, to increase the surface electric conductivity, and enhanced the image resolution when visualized through FESEM. In order to obtain more precise idea about the dimension scale of the SPNFCs, TEM nanoscopic observations were carried out to observe the changes of surface morphological and to

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characterize nanofibrillated cellulose. From the Fig. 3 (d), (e), (f), it can be observed that the shape

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displayed by SPNFCs-5, SPNFCs-10 and SPNFCs-15 nanostructures produced from sugar palm-

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derived cellulose possess thread-shaped form. Besides, from the Fig. 3 (f) also it can be seen that SPNFCs-15 revealed well-distributed and uniform nanofibres morphology in relevant to good

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stability in colloidal suspension. The SPNFCs-5 possess diameter of approximately 21.37 ± 6.91

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nm, which presented insignificant different size of diameter as those of SPNFCs-10 approximately 11.54 ± 2.77 nm and SPNFCs-15 with an average value around 5.5 ± 0.99 (Fig. 4 and Table 1).

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Present discovery in this TEM analysis were supported by the previous literatures with the

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estimated diameter size of nanofibrils from banana (5 nm) [55], and flax fibre (5 nm) [56], and wider than prickly pear fruit (2-5 nm)[57], and smaller than raw cotton linter (Gossypium hirsutum)

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(12 nm) [45], alfa (Alfa tenassissima), pineapple leaf (30 nm) [58], pineapple leaf fibre (32.5 nm)

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[59], sugarcane bagasse (35 nm) [60], wheat straw (30-70 nm) [7], hemp fibre (10-60 nm) and rutabaga (80 nm) [56].

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Besides that, based on Table 1, the reduction diameter of the SPNFCs-5 compared to SPC was 99.82%. This might be attributed to the high shearing forced which break down the physical cohesion of the aggregated fibril, which subsequently caused the release of microfibrils into individualized nanofibres after passing through the high pressurize homogenizer. The process of the defibrillation of SPNFCs continued for the SPNFCs-10 and SPNFCs-15 for 10 and 15 cycles, indicated the changes in the diameter size of the SPNFCs which was reduce by 46% and 74.3%

ACCEPTED MANUSCRIPT compared to SPNFCs-5, respectively. This is because of the addition of the cycles assisted by high impact and shear force, which defibrillated microfiber into nanofibres. Besides that, this is also due to the external and internal fibrillation, where external fibrillation occurs by the harsh actions on the fibres surface, while internal fibrillation occurs as a result of breaking the hydrogen bond

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because of mechanical action [28]. The effect of fibrillation process could decrease the diameter

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size of nanofibre. In addition, Khalil et al., [28], reviewed that there is a relation between number

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of cycles to the length and dimensions of nanofibre. Increasing the number of cycles would resulted in reduction of length and dimension of nanofibre. However, the size, dimension and the shape of

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NFCs are depending on the variety of cellulosic fibre source and the conditions of the HPH process

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[50,61,62].

Moreover, it can be seen that the size length of SPNFCs was dominated by SPNFCs-5 with

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length of 2615.2 ± 740.81 nm and decrease slightly by 13.34%, reaching a length of 2266.4 ±

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120.73 nm for SPNFCs-10. The slight decreasing trend throughout the extended cycle from 10 cycle to 15 cycles can be determined by observing the decreasing length of SPNFCs-15 by 15 %,

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with an average value of 2223.0 ± 727.81 nm compared to SPNFCs-5. With the obtained values

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of both length and diameter, optimum aspect ratios (length to diameter) can be calculated. Thus, from the calculated value of the optimum aspect ratios of SPNFCs-5, SPNFCs-10 and SPNFCs-

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15, it can be seen that the value of aspect ratio were around 122.4, 196.4, and 404.12, respectively. The aspect ratios of SPNFCs-15 were not as much different from each other as their actual dimensions, however this value was higher compared to the value of those reported on NFCs from other cellulosic source, such as banana peel nanofibres (380.22) [63]. Whereas, from the AFM images, SPNFCs-5 (Fig. 3j) was displayed as long and wide fibrous form by AFM topographic image, corresponding to the bundle morphology as presented

ACCEPTED MANUSCRIPT via TEM analysis. The SPNFCs-5 and SPNFCs-10 (Fig. 3k) displayed more dense packed nanofibril than SPNFCs-15 (Fig. 3l), besides all samples showed thread-like particles which is in good agreement with the TEM observation. The calculated average values for SPNFCs-5, SPNFCs-10 and SPNFCs-15 from AFM analysis were within the diameter size range of FESEM

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microscopy, however were larger than that of TEM analysis (Table 1). This might be attributed to

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the tip convolution effect of AFM in overestimating the nanoparticles dimension [64,65].

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By comparing the microscopies among TEM, FESEM and AFM, it could be assumed that TEM gave the clearest insight of resultant SPNFCs-15 morphology with diameter size of 5.5 ±

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0.99 nm. Moreover, it is well known that AFM microcopy is usually carried out for accurately

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measuring height of nanoparticles. The well-distributed thickness values of SPNFCs measured by

be 7.703 nm, 5.781 nm and 4.655 nm.

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AFM. The peak nanofibres height of SPNFCs-5, SPNFCs-10 and SPNFCs-15 were calculated to

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The morphological structure analysis of the SPNFCs presented in Fig. 4 revealed that nanofibres with widths between 21.37 ± 6.91 nm and 5.5 ± 0.99 nm. Higher magnification of TEM

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analysis shows an aggregated structure of SPNFCs-5, which forming a highly entangled web-like

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layer compared to others. These nanofibrils were unconstrained and individualized after passing through the several numbers cycles of homogenizer, where high impact and shearing forces break

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down the physical cohesion of the fibrils.

3.3 Physical properties, particle size analysis and zeta potential Table 2 Physical properties and zeta potential of SPNFCs-5, SPNFCs-10 and SPNFCs-15. Samples SPNFCs-5

Z-average (nm) Zeta-potential (mV) Conductivity (mS/cm) Density (g/cm-3) 2615.2 ± 740.81 -39.5 0.01546 1.11 ± 0.0030

ACCEPTED MANUSCRIPT SPNFCs-10 2266.4 ± 120.73 SPNFCs-15 2223.0 ± 727.81

-36.6 -34.2

0.01442 0.00717

1.10 ± 0.1100 1.10 ± 0.0026

Density is one of the criteria that have to be considered in process of material selection due to it affect the performance of the products. Insignificant decreasing trend was observed throughout

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the extended of number of cycle in high pressurize homogenization process, in which the density value of SPNFCs-5, SPNFCs-10 and SPNFCs-15, were 1.11 ± 0.0030, 1.10 ± 0.1100 and 1.10 ±

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0.0026 g/cm-3, respectively. This declining trend might be attributed to the changes of the diameter

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size of the nanofibres, however, it showed insignificant changes as no strong chemical reaction was acted on the process of defibrillation of SPNFCs. This result was supported by the FTIR

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analysis results. Besides, the raise in fibre volume with loss in weight affected density value to

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reduce [66]. SPNFCs density values of all treatments showed lower value compared to

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conventional fibres likes aramid (1.4 g/cm3), carbon (1.7 g/cm3) and glass fibre (2.5 g/cm3) [67]. The zeta potential study is very well-known method for characterization of micro/nano-

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material surface charge from the perspective of classical principle of colloidal stability. In easier

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word, it measures surface activity in colloidal particles. It also represents the electrostatic charge density induced near the surface material by the action of polymers and ions in suspension [68].

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Zeta potential and conductivity values of SPNFCs that was analyzed using Zetasizer Nano-ZS (Malvern Instruments, Worcestershire) was demonstrated in Table 2. From this table, we can observe that SPNFCs-5, SPNFCs-5, SPNFCs-10 and SPNFCs-15 displayed high negativity charged value of zeta-potential (mv), with mean value of -39.5 mv, -36.6 mv and -34.2 mv. from this value, it can be concluded that SPNFCs have a good stability of dispersion in aqueous suspension as the absolute value were greater than -27 mV [69]. High value of zeta potential also could prevent the formation of accumulation of nanocellulose, giving more stable colloid

ACCEPTED MANUSCRIPT suspensions with individualized nanofibres, subsequently, stimulating a strong adhesion network of nanofibres within the polymer composite. Besides that, the value of conductivity also was found to increase as the value of zeta potential increase.

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3.4 X-ray diffraction measurements

250

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300

SPNFCs-45

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SPNFCs-30 200

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Intensity

SPNFCs-60

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150

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100

10

15

20

25

30

2 (degree)

35

40

45

50

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0 5

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50

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Fig. 5. X-ray diffraction patterns of (a) sugar palm fibres, (b) SPNFCs-5, (c) SPNFCs-10, and (d) SPNFCs-15. Table 3

XRD analysis data of SPNFCs-5, SPNFCs-10 and SPNFCs-15. Samples SPNFCs-5

2𝜃 (am) (◦)

2𝜃 (0 0 2) (◦)

Xc%

19.46

22.74

75.73

ACCEPTED MANUSCRIPT SPNFCs-10

18.52

22.54

75.38

SPNFCs-15

19.7

22.6

81.19

Table 3 shows the crystalline degree of the ensuing nanofibers for SPNFCs- 15 nanofibre was

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increased with the fibrillation action. The increased in crystallinity index was enhanced by the

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number of high pressurized homogenization (HPH) cycle. This table also shows all X-ray

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diffraction patterns of SPNFCs nanofibres present a sharp diffraction peak around 22.6o (2θ) which is known for the characteristic of typical polymorps cellulose 1 structure (typical peaks at 2θ=15º

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and 22.6º) [70]. However, the amorphous region was detected at low intensity peak around 18º

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(2θ) [44]. Fig. 5 displayed the pattern of crystallinity of nanofibres and the values were as shown in Table 3. Besides that, one can observe that the sharp intensity peaks for SPNFCs-5, SPNFCs-

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10 and SPNFCs-15 were located at around 22.74º, 22.54 º, and 22.6 º and minor intensity peaks of

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19.46 º, 18.52 º, and 19.7 º, respectively. The highest crystallinity index was belong to SPNFCs60, and the lowest crystallinity index was SPNFCs-30, with value of 81.19% and 75.38%,

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respectively. The obtained outcomes showed that the nanofibres crystallinity index was

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insignificantly decreased. According to Samir et al., [71] this phenomena occurred due to the broken of cellulose chain, subsequently, causing the crystal structure region between cellulose

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chains to collapse and damage during HPH process. However, after several number of passes from 10 to 15 passes, the crystallinity index increases gradually. This might be due that HPH not only breaks the amorphous regions, but also restructures and enriches the semi-crystalline cellulose regions. Therefore, it can be inferred that the higher crystallinity index of sugar palm nanofibrillated cellulose was contributed by the capability of strong defibrillation via process of high pressurize homogenization. This extended HPH process leads to the disintegration of cell walls into nanofibrils [72]. This result was in good agreement with earlier findings presented in

ACCEPTED MANUSCRIPT the work done by Besbes et al., [14], where this theory is reasonable for fibres that are considered to be composed of bundle cellulose nanofibrils that are aggregated into larger form of structure which embedded into hemicellulose with imperfect axial orientation and less ordered interlinking regions between the crystalline structure within the fibres. The process of defibrillation was

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estimated to begin with weakest region, which was less ordered interlinking zone of amorphous

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region. According to Jonoobi et al., [50], during the process of HPH, the crystalline region

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proportions were increased due to the alteration between cellulose fractions of disordered and ordered taking place. This mechanical process has capability to reorganize the constrained part of

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nanofibres, which were previously disordered, into crystals as a result of the increased mobility of

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the nanofibres in water [50].

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The crystallinity index calculated were similar to the nano-sized structures that were extracted from other agro-residue sources such as kenaf bast fibre (Hibiscus cannabinus v36) (81.5%) [73],

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and higher than pineapple leaves (54%) [59], eucalyptus Kraft pulp (60 nm)[74], bamboo (70.6%)

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[17], pinecone (Pinus banksiana Lamb) (70%) [11], pine (75%) [14] and pineapple leaf (75.38%) [58]. However, it was lower than cotton linter (90%) [45], eucalyptus (86%) [75] and alfa (Alfa

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tenassissima) (92%) [14].

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There are relations between the number of crystallinity degree region and the stiffness of cellulose, where increment in number of crystallinity region will raise the stiffness of fibres. Higher crystallinity in the chemical treated fibers is interrelated with higher tensile strength of the fibers. Therefore, the mechanical properties of the nanocomposite material can be improved by using these treated fibers as nanofiller (Bhatnagar, 2005; Rong et al., 2001).

3.5 Thermogravimetric analysis

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SPNFCs-15

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80

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T

100

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SPNFCs-5

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60

40

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Weight (%)

SPNFCs-10

200

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100

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0

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20

300

400

Temperature (oC)

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Fig. 6. TG curves for SPNFCs-5, SPNFCs-10 and SPNFCs-15

500

600

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0.0000

DTG (w%/min)

-0.0005

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T

-0.0010

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-0.0015

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-0.0020

-0.0025

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SPNFCs-15 SPNFCs-10 SPNFCs-5 100

200

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-0.0030

300

o

400

500

600

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Temperature ( C)

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Fig.7. DTG curves for SPNFCs-5, SPNFCs-10 and SPNFCs-15

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Table 4

Onset temperature (TOnset), degradation temperature on maximum weight-loss rate (TMax), weight

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loss (WL) and char yield for SPNFCs-5, SPNFCs-10 and SPNFCs-15 obtained from the TG and DTG curves. Samples SPNFCs-5 SPNFCs-10 SPNFCs-15

Water Evaporation TOnset (°C) TMax (°C) WL (%) 31.51 99.58 8.78 26.90 102.26 10.05 28.71 102.80 8.94

1st thermal degradation TOnset (°C) TMax (°C) WL (%) 203.31 347.02 74.07 203.63 346.22 73.01 192.71 347.30 72.44

Char yield W (%) 15.00 17.17 16.02

ACCEPTED MANUSCRIPT Nanofibres samples from sugar palm-derived cellulose were thermogravimetrically examined to compare the degradation characteristics at different number of passes. Fig. 6 and 7 display the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of SPNFCs nanofibres samples. Most of the SPNFCs nanofibres tend to lost their weights in the region 25 °C - 150 °C,

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is mostly due to the evaporation of water residue within the nanocellulose sample [77]. Moreover,

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all SPNFCs displayed continuing thermal degradation in the temperature region of 190 °C–350

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°C, basically connected to the α-cellulosic chain degradation [33]. The obtained thermogravimetric (TG) analysis data concluded in Table 4 displayed that SPNFCs-15 showed lowest onset thermal

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decomposition of 192.71 °C among the SPNFCs samples, this phenomena might be due to the

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possible owing to the difference in crystalline structures organization after defibrillation processes [78]. For instance, the well-aligned crystallites structure of SPNFCs-5 and rearranged crystallites

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structure of SPNFCs-10 can influence the thermal resistance of nanofibres toward initial

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temperature decomposition [79,80]. From first thermal decomposition, both SPNFCs-5 and SPNFCs-10 prolonged their higher thermal decomposition temperatures until 347.02 °C and

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346.22 °C, respectively when compared with SPNFCs-15. However, as the temperature continued

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in range of 350 °C, SPNFCs-15 showed as the most stable curve and highest thermal stability from 192.71 °C and 347.30 °C. This is because of the unaffected and strong native crystals order in

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crystalline structure of SPNFCs-15 [80,81]. Besides that, from the Table 4, insignificant shifting in major decomposition temperature for all samples of nanofibres was observed. SPNFCs-15 showed higher decomposition temperature than others nanofibres. This might be due to the fact that raw sugar palm-cellulose derived material is organized into smooth fibrils which are surrounded by a matrix of hemicellulose and lignin, although they are small in percentage. Hemicellulose and lignin which is in the form of amorphous

ACCEPTED MANUSCRIPT structure are softer than cellulose and act as glue between the cellulose fibres by virtue of the interaction or linkages between cellulose and the matrix. Hence, these impurities initiate more active sites and begin to accelerate earlier in the thermal decomposition process. Form the same table, the SPNFCs-15 shows the lowest carbon residue compared to others nanofibres at extended

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temperature of 600 °C. This phenomenon might be occurred due to the disintegration of amorphous

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region, which is hemicellulose and lignin, during prolonged of high pressurize homogenization

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process, and better accessibility formation of cellulose crystallinity in the nanofibres. The lower char yield of the nanofibres was because of the absence of nanocellulosic constituents in the system

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[75].

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Conclusions

Nanofibrillated cellulose with diameter of 21.37 ± 6.91 nm, 11.54 ± 2.77 nm, and 5.5 ± 0.99 nm

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were defibrillated from three different cycles/passes, named, SPNFCs-5, SPNFCs-10 and

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SPNFCs-15. A beating pretreatment was carried out on the sugar palm cellulose in order to prevent the high pressurize homogenizer from clogging problem due to its very small orifice size. Thus, it

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is necessary to reduce the fibre size before continuing with HPH process. The yield of

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nanofibrillated cellulose is the lowest for SPNFCs-5 compared to SPNFCs due to the some of the fibres that were not defibrillated into nanosize. The FTIR analysis shows that the chemical structure of nanofibres remains unchanged after several number of HPH passes. The SPNFCs-15 also displayed a high degree of crystallinity of 81.19%, which proved that it meets the requirement for load-bearing material application in composite structure. TGA analysis of SPNFCs-15 was found to display good thermal stability property, where it can withstand high temperature during

ACCEPTED MANUSCRIPT the composite fabrication. The SPNFCs established in this current work is aimed to be utilized in SPNFCs/Starch based nanocomposites for potential packaging and medical application. Acknowledgments

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The authors would like to thank Universiti Putra Malaysia for the financial support through the

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Graduate Research Fellowship (GRF) scholarship, Universiti Putra Malaysia Grant scheme Hi-

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COE (6369107) and FRGS/1/2017/TK05/UPM/01/1 (5540048). The authors are grateful to Dr. Muhammed Lamin Sanyang for guidance throughout the experiment. The authors also thank Dr.

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Rushdan Ibrahim for their advice and fruitful discussions.

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Fig. 1. Yields of attained nanofibres after several cycles Fig. 2. FT-IR spectra of SPNFCs-5, SPNFCs-10 and SPNFCs-15 Fig. 3. Aqueous suspension (2 wt%) (a-c), TEM (d-f), FESEM (g-i), AFM (j-l) images of SPNFCs5 (a, d, g, j), SPNFCs-10 (b, e, h, k) and SPNFCs-15 (c, f, i, l) Fig. 4. Transmission electron (TEM) micrograph of length and diameter histograms of SPNFCs-5

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(a), SPNFCs-10 (b) and SPNFCs-15 (c).

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Fig. 5. X-ray diffraction patterns of (a) sugar palm fibres, (b) SPNFCs-5, (c) SPNFCs-10, and (d)

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Fig. 6. TG curves for SPNFCs-5, SPNFCs-10 and SPNFCs-15

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Fig. 7. DTG curves for SPNFCs-5, SPNFCs-10 and SPNFCs-15

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Table 1 Dimension size of SPNFCs-5, SPNFCs-10 and SPNFCs-15 analyzed by TEM, FESEM, and AFM microscopies. Fibres

TEM (diameter size range) (nm)

AFM (diameter size range) (nm)

FESEM (diameter range) (nm)

Sugar Palm Cellulose (SPC)

-

-

11870

21.37 ± 6.91

65 ± 6.19

90 ± 17.98

11.54 ± 2.77

27.5 ± 7.35

50 ± 25.04

SPNFCs-15

5.5 ± 0.99

9 ± 1.89

35 ± 8.61

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SPNFCs-5 SPNFCs-10

Z-average (nm)

Zeta-potential (mV)

SPNFCs-5 SPNFCs-10 SPNFCs-15

2615.2 ± 740.81 2266.4 ± 120.73 2223.0 ± 727.81

-39.5 -36.6 -34.2

Conductivity (mS/cm) 0.01546 0.01442 0.00717

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Table 2 Physical properties and zeta potential of SPNFCs-5, SPNFCs-10 and SPNFCs-15 Density (g/cm-3) 1.11 ± 0.0030 1.10 ± 0.1100 1.10 ± 0.0026

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Table 3 XRD analysis data of SPNFCs-5, SPNFCs-10 and SPNFCs-15. 2𝜃 (am) (◦) 19.46 18.52 19.7

Samples SPNFCs-5 SPNFCs-10 SPNFCs-15

2𝜃 (0 0 2) (◦) 22.74 22.54 22.6

Xc% 75.73 75.38 81.19

Water Evaporation

1st thermal degradation WL (%)

TOnset (°C)

TMax (°C)

WL (%)

W (%)

SPNFCs-5

31.51

99.58

8.78

203.31

347.02

74.07

15.00

SPNFCs-10

26.90

102.26

10.05

203.63

346.22

73.01

17.17

SPNFCs-15

28.71

102.80

8.94

347.30

72.44

16.02

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Successful isolation of sugar palm nanofibrillated cellulose (SPNFCs) from sugar palm fibre (SPF) by a chemo-mechanical method. High pressurize homogenization (HPH) technique with 3 variables cycles of 5, 10 and 15 times cycle were used to obtain SPNFCs. TEM, FESEM and AFM micrograph and zeta sizer analysis showed the thread-shape and nano-size of SPNFCs. The SPNFCs was of ranging 5.5 – 21.37 nm in diameter, highly crystalline in nature and high thermal stability. TGA and XRD analysis showed improvement in thermal and crystallinity of SPNFCs.

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TMax (°C)

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Char yield

TOnset (°C)

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Table 4 Onset temperature (TOnset), degradation temperature on maximum weight-loss rate (TMax), weight loss (WL) and char yield for SPNFCs-5, SPNFCs-10 and SPNFCs-15 obtained from the TG and DTG curves.

Figure 1

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Figure 7