The Role of Montmorillonite Loading on the Physicochemical Properties of Regenerated Cellulose Nanocomposite Films Obtained from Microcrystalline Cellulose

The objective of this study was to find the optimum amount of montmorillonite (MMT) to be incorporated with microcrystalline cellulose (MCC) to enhance physicochemical properties of oil palm empty fruit bunch (OPEFB)-based regenerated cellulose (RC). In this research, RC was synthesised by mixing 6.5 wt% of OPEFB-MCC with 1-butyl,3-methylimidazolium chloride [BMIM]Cl ionic liquid at 75°C by continuous stirring. RC nanocomposites were prepared by adding 1–5 wt% of montmorillonite (MMT) into RC preparation base. Finished solutions of both types were subjected to solution casting process to obtain thin films. Fourier transformation infrared (FTIR) spectroscopy, tensile test, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), water absorption and contact angle measurements were used as the characterisation tools. FTIR spectroscopy denotes the non-derivative behaviour of [BMIM]Cl ionic liquid. According to SEM images, moderate MMT loading is preferred since high MMT loading shows coarse morphology beyond 3 wt% of MMT. Tensile strength of RC 11 MPa was increased up to 30 MPa as maximum in 4 wt% of MMT loading. Compared with pure RC, TGA curves were shifted to right side with the increased MMT loading and that implies good thermal stability. The highest thermal stability can be seen when the MMT content is 4 wt% since its TGA curve was The Role of Montmorillonite Loading 86 the rightmost than others. Therefore, MMT can be identified as a good inhibitor for RC to avoid thermal degradation at elevated temperatures. Water absorption was reduced as 91%–37% in 2 h and 96%–41% in 24 h when MMT was increased from 0–5 wt%. Similarly, their contact angle was increased from 30.5o–88.5o for the MMT loading of 0–5 wt%. According to all these results, 3–4 wt% of MMT loading can be proposed as the optimum amount to be added into an RC matrix to utilise these films in packaging, photocatalytic and electrically conductive applications.


INTRODUCTON
Cellulose is considered the most abundant natural polymer polysaccharide on the earth. 1,2 It has an estimated annual production of 90 × 10 9 metric tons and represents the most obvious renewable resource for manufacturing of biocomposites. 3 Cellulose is commonly used in textiles, paper, artificial fibres, paint, food and medical applications. 4,5 This valuable polymer can be isolated from various raw materials such as post-consumer newspaper, rice husk, agriculture crops and oil palm empty fruit bunch (OPEFB). [6][7][8][9] OPEFB was chosen in order to extract the cellulose from palm oil biomass because of the higher percentage of lignocellulosic composition available in OPEFB. Also, OPEFB is abundantly available in Malaysia ( Figure 1) as a biomass waste. [10][11][12] Properties of OPEFB can be enhanced by transforming it into micro or nano-scaled materials. 6 According to Haafiz et al., microcrystalline cellulose (MCC) has been isolated from OPEFB using Battista method by dissolving amorphous regions of cellulose network with the aid of acid hydrolysis. [13][14][15] Cellulose is made of β-D-glucopyranose as the monomer. Cellulose network is formed by intramolecular and intermolecular hydrogen bonds in between layers. 16 Hence, cellulose has a partially crystalline structure and it brings the high resistance of cellulose to dissolve in many liquids. 17 Scientists have attempted several solvents to perform efficient dissolution of cellulose. Viscose, carbocell and xanthation are some old derivatising methods whereas lyocell process, cupromonium process, molten salts, aqueous and non-aqueous dissolution are nonderivatising regeneration methods. 18 In late 90s, ionic liquids (IL) were identified as a favoured technology for cellulose dissolution with numerous advantages such as good dissolving power, ability to recover solvent, and minimum environmental impacts as a result of being less volatile. 19 The dissolved solute (cellulose) can be precipitated using an antisolvent which is capable to reduce the solubility of solute in used solvent (i.e., IL) further. 20 Therefore, cellulose separation is facilitated by this process which is known as regeneration. 21 Compressed CO 2 , ethanol and water are popular antisolvents. 20 RC can be prepared through three-step process: dissolution, shaping and regeneration. 22 The shape of the regenerated material depends on the regeneration method and conditions. Thin films having a thickness ranging from 0.1 mm to 0.2 mm can be obtained by solution casting. Currently, RC is utilised in food packaging, membrane filtration, medical goods and electronic devices. 22 In this research, MCC was dissolved inside IL [BMIM]Cl at 75°C followed by shaping and coagulation with distilled water as the antisolvent to obtain RC thin films as the throughput.
Developing nanocomposites using nanomaterials is an ideal technique to enhance the performance of a basic material. Montmorillonite (MMT) is one of the most popular layered silicate materials widely used in nanocomposites because of its low cost, chemical and thermal stability, high aspect ratio (100-1500) and high surface to volume ratio (700-800 m 2 g -1 ). 23,24 It consists of sodium cations between interlayer spaces and therefore natural MMT is hydrophilic and interactive with hydrophilic polymers. 25 During the past decade, several researchers have attempted to study the utilisation of nanomaterials (halloysites; HNT, sepiolite and MMT) with commercial MCC to manufacture RC. 26-28 But incorporation of MMT with OPEFB based MCC has not been addressed yet.
Thus, this research aimed to add MMT different loadings (1-5 wt%) into an RC synthesising IL solution containing OPEFB based MCC (6.5 wt%). Physicochemical properties of prepared RC/MMT nanocomposite films were evaluated as explained in Section 2. It is expected to use these RC/MMT films in packaging, photocatalytic and electrically conductive thin film applications. Therefore, the objective of this research was to determine the most appropriate nanomaterial composition to be combined with OPEFB based RC matrix to enhance the performance of RC.

Materials
In this study, IL [BMIM]Cl with purity of ≥ 98% was purchased from Sigma Aldrich. OPEFB was supplied by United Oil Palm, Nibong Tebal, Penang, Malaysia and its composition was displayed in Table 1. MMT was purchased from Nanocor, Arlington Heights, United States and its constituents were displayed in Table 2. Further, 25% NaOH, 32% HCl and NH 4 OH were supplied by Merck, Malaysia. As the cellulose stabiliser, tannic acid (TA) extra pure was purchased from HmbG Chemicals. It is a dark yellowish powder with a melting point of 218°C. MCC was isolated from OPEFB according to the method described by Haafiz et al. 13 Initially, OPEFB fibrous strands were pre-hydrolysed at 170°C for 60 min. Then, soda pulping was done inside 25% of NaOH. The mixture obtained after soda pulping is known as total chlorine free (TCF) pulp. It was put for oxygen, ozone and hydrogen peroxide treatments (bleaching). 13 The bleached pulp was off white. Afterward, the pulp was hydrolysed with 2.5N HCl (1:20 pulp to liquor ratio) at 105°C ± 2°C for 30 min with constant agitation using a magnetic stirrer. Then, the mixture was filtered at room temperature and the precipitate was washed by distilled water, 5% diluted NH 4 OH and again with distilled water until it was free from acid. The wet solid mass was dried under vacuum at 105°C ± 2°C until a constant weight was attained. At the end of this step, dry solid mass could be viewed in snowy-white colour. This was subjected to grinding process using rotatory ball mill to obtain MCC as a fine dust. Before use, the ground product had been kept in a storage bottle.

Preparation of RC/MMT Nanocomposite Films
Initially, 18.7 g of [BMIM]Cl was weighed into a 100 ml beaker and it was kept on a temperature controlled magnetic stirrer (at 75°C) to obtain liquid [BMIM] Cl. Once it appeared as liquefied IL, 10 ml of it was taken out to allow for MMT dispersion. In each MMT loading (0-5 wt%), the weighed amount of MMT was dispersed in [BMIM]Cl using a sonicator (Branson 1800) operated for 30 min and subsequently by magnetic stirring (500 rpm) for 4 h at 75°C. The remaining IL was mixed at 75°C with 2 phc TA by magnetic stirring (400 rpm) until a clear liquid was observed. Then, 1.3 g of MCC was applied into the beaker and next MMT dispersion was added. Mixing of MCC and MMT was conducted with continuous stirring at 75°C until a clear solution was observed. Then, the solution mixture was cast on a glass plate with the aid of a blade and quickly immersed inside a distilled water bath for coagulation up to 24 h. Once coagulation was completed, films were transferred into a polyethylene sheet. The films were allowed to dry at room temperature and conditioned at 23°C ± 2°C and 50% relative humidity before characterisation. All films with average thickness of 0.3 mm were prepared in triplicate and those were coded as MMT-0, MMT-1, MMT-2, MMT-3, MMT-4 and MMT-5 with respect to MMT loading.

FTIR Spectroscopy
Attenuated total reflectance Fourier transform infrared spectrometer (ATR-FTIR) IR Prestige-21, Shimadzu, Japan was used to characterise the chemical structure of prepared thin films. The films were clamped between the horizontal crystals and robust pressure clamp. The infrared spectrums were obtained within the wave number range of 600-4000 cm -1 . The significant transmittance peak at a particular wave number was measured with the aid of Find Peak tool provided by Nicolet OMNIC 5.01 software.

Tensile Test
ISO1924-2 method was selected to evaluate the mechanical behaviour of RC films. The RC films were uniformly cut into strips (140 mm × 15 mm × 0.03 mm) before clamping them on tensile testing machine (F81.50200, Frank-PTI, Birkenau, Germany) with respect to the standard ISO1924-2.
Initially, the thickness of each sample was measured with the aid of micrometre screw gauge on three distinct positions along the length and the mean value was multiplied by a 15 mm width of the sample to calculate the effective cross-sectional area. The initial grip separation and load cell value were 100 mm and 500 N, respectively. For every film, average values of five different measurements were taken to calculate the mean tensile strength (TS) (σ = F/A) and elongation at break (EAB) (Strain € = e/L). Young's modulus (E) was calculated by using Equation 1 based on Hook's law: where F is the load (N), A is the effective cross-sectional area (mm 2 ), e is tensile displacement (mm) and L is initial length (140 mm).

SEM
Scanning electron microscopy (SEM) was used to examine the molecular arrangement of the RC nanocomposite films. Leo Supra 50 VP, Carl Ziess, 5MT Field Emission SEM (Radeberg, Germany) was used for this experiment. Since the films are non-conductive, gold sputtering was made prior to analysis of crosssectional view of films that were subjected to tensile test.

Thermogravimetric Analysis
Thermal analysis was performed using Perkin Elmer-YGA7 (Radebergy, Germany) instrument. Approximately 10 mg of each film was cut into fine pieces and the particle size was reduced using motor and pestle. Then, those were placed on the sample holder and subjected for heating from room temperature to 600°C with a scan rate of 10°C min -1 under nitrogen atmosphere.

Water Absorption Measurement
ASTM D570 standard was followed to perform water absorption test in this research. The films were arranged into a square shape of 20 × 20 mm 2 and those were dried at 40°C for 24 h until a constant weight was attained prior to analysis. Then the films were immersed in distilled water at 25°C for 2 h and 24 h. Wet weight of the films were recorded by measuring the weight after completion of immersion time. The percentage of water absorption of each sample for every formulation was recorded using Equation 2: where W w is the wet weight after immersion and W i is the constant weight attained by long drying before immersion.

Contact Angle Measurement
The contact angle of RC/MMT films was measured by Attension KSV CAM 101 supplied from KSV Instruments Ltd., Finland as per sessile drop method. Two films of 20 × 20 mm 2 size were placed on a rectangular slide for each formulation. The test liquid deionised water was applied on the surface of the films by a micro syringe. Subsequently, the images were recorded at a speed of 40 frames s -1 . The contact angle value of a particular film was taken as an average of five different measurements in different parts of two samples prepared per each formulation.

Chemical Behaviour of RC/MMT Nanocomposite Films
Evaluation of chemical stability of the system is a mandatory requirement in this kind of study. Figure 2

Mechanical Behaviour and Morphology
The variation of TS and EAB with respect to MMT content is shown in Figure 3. However, the TS of the films shows a reduction from 30 MPa to 21 MPa when the MMT loading is increased from 4 wt% to 5 wt%. According to Mahmoudian et al., the TS is decreased when the MMT loading is further increased because of the aggregation of MMT particles. Aggregation of MMT particles hinders its basic function and the matrix becomes as a low loading MMT. Therefore, the TS tends to reduce. 28 EAB has showed a decreasing trend with the presence of MMT. When the MMT content was increased from 0 wt% to 1 wt%, EAB has decreased from 0.02% to 0.01% and remained constant in every MMT loading. According to Hanid et al., the presence of MMT restrains the slippage movement of RC chains during deformation and it affects the reduction of EAB with MMT incorporation. 26 But further MMT loading in this range has not affected EAB.

Thermal Behaviour of RC/MMT Nanocomposite Films
Thermogravimetric analysis (TGA) was conducted to determine the effect of MMT on thermal stability of RC. TGA curves of the prepared nanocomposites films with different percentages of weight are shown in Figure 5(I). Further the derivative thermographic (DTG) curves were plotted in Figure 5(II). According to the plot, curves were shifted right side when the MMT loading was increased beyond 2 wt%. Figure 6 depicts T 20 , the temperature at which 20% of weight loss had occurred, T max , the temperature at which maximum weight loss can be observed and residue content. According to these data, in high MMT loading, thermal degradation temperature is increased compared with pure RC. Further residue content after the test is higher when MMT is introduced into RC which implies that degradation temperature can be increased by incorporating MMT nanoparticles on RC matrix.   According to Figure 6, residue content at 3 wt% of MMT is less than that in 2 wt% of MMT. According to Mahmoudian et al., residue content is responsible for the amount of char formation in thermal degradation. The reduction of residue content may be due to the decomposition of octadecylamine which is added into MMT 1.30TC galleries during its modification. Again residue content is increased with MMT loading beyond 3 wt% of MMT since MMT platelets (impermeable) hinder thermal diffusion through the film. 28

Water Resistance of RC/MMT Nanocomposite Films
Determination of water absorption is a mandatory requirement for films used for packaging, photocatalytic and electrically conductive applications. Figure 7

Surface Wetting Behaviour of RC/MMT Nanocomposite Films
Contact angle measurements of prepared films were considered to evaluate the hydrophobicity of thin films. Figure 8 demonstrates the contact angle variation of the prepared films in 10 s with MMT loading. The contact angle value that is 30.5°C in pure RC has increased up to 88.5°C when MMT loading is 5 wt%. A material is hydrophilic if its water contact angle is less than 90°C. 34

CONCLUSION
RC nanocomposite films were successfully prepared by incorporating MMT in the MCC dissolution system inside [BMIM]Cl and subsequent solution casting, coagulation and drying. FTIR spectrums confirm that MMT does not interfere with the basic chemical structure of cellulose as well as the spectrums further prove the non-derivatising behaviour of [BMIM]Cl. Increased amount of MMT provides improved TS and maximum TS was obtained at 4 wt% of MMT. According to SEM, smooth morphology is associated when the MMT loading is 3 wt%. TGA results show that the thermal degradation temperature is raised with the addition of MMT into RC. The higher the MMT content, the lower the water absorption of RC nanocomposite films will be. Contact angle measurements denote that addition of MMT brings RC less hydrophilic. Regarding water absorption and contact angle, the most preferred amount of MMT is 5 wt% since both of these properties were enhanced with MMT content. But when the MMT content is 5 wt%, the morphology becomes a coarse structure as a result of MMT agglomeration at high loadings (Section 3.2) while decreasing mechanical strength. Hence, concerning the overall figure, 34 wt% of MMT can be recommended to synthesise OPEFBbased nanocomposites for numerous RC-based applications such as packaging, photocatalytic and electrically conductive thin films.