Exploring the effect of cellulose nanowhiskers isolated from oil palm biomass on polylactic acid properties

International Journal of Biological Macromolecules 85 (2016) 370–378

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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Exploring the effect of cellulose nanowhiskers isolated from oil palm biomass on polylactic acid properties M.K. Mohamad Haafiz a,∗ , Azman Hassan b,∗∗ , H.P.S. Abdul Khalil a , M.R. Nurul Fazita a , Md. Saiful Islam c , I.M. Inuwa b , M.M Marliana b , M. Hazwan Hussin d a

School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia Faculty of Chemical and Energy Engineering Universiti Teknologi, Malaysia c Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia d Lignocellulosic Research Group, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia b

a r t i c l e

i n f o

Article history: Received 5 November 2015 Received in revised form 17 December 2015 Accepted 1 January 2016 Available online 6 January 2016 Keywords: Cellulose nanowhiskers Microcrystalline cellulose Polylactic acid Tensile properties Thermal analysis

a b s t r a c t In this work, polylactic acid (PLA) reinforced cellulose nanowhiskers (CNW) were prepared through solution casting technique. The CNW was first isolated from oil palm empty fruit bunch microcrystalline cellulose (OPEFB-MCC) by using 64% H2 SO4 and was designated as CNW-S. The optical microscopy revealed that the large particle of OPEFB-MCC has been broken down by the hydrolysis treatment. The atomic force microscopy confirmed that the CNW-S obtained is in nanoscale dimension and appeared in individual rod-like character. The produced CNW-S was then incorporated with PLA at 1, 3, and 5 parts per hundred (phr) resins for the PLA-CNW-S nanocomposite production. The synthesized nanocomposites were then characterized by a mean of tensile properties and thermal stability. Interestingly to note that incorporating of 3 phr/CNW-S in PLA improved the tensile strength by 61%. Also, CNW-S loading showed a positive impact on the Young’s modulus of PLA. The elongation at break (Eb ) of nanocomposites, however, decreased with the addition of CNW-S. Field emission scanning electron microscopy and transmission electron microscopy revealed that the CNW-S dispersed well in PLA at lower filler loading before it started to agglomerate at higher CNW-S loading (5 phr). The DSC analysis of the nanocomposites obtained showed that Tg, Tcc and Tm values of PLA were improved with CNW-S loading. The TGA analysis however, revealed that incopreated CNW-S in PLA effect the thermal stability (T10, T50 and Tmax ) of nanocomposite, where it decrease linearly with CNW-S loading. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Recently, plethora studies have been done on the different approch in production and isolation of cellulose nanoparticle [1–5]. The use of this nanoparticles as a reinforcement phase in the composite production has attracted huge interest among researchers [1–7]. The principal reasons for the utilization of cellulosic materials are its high specific strength and modulus compared to other engineering materials, and its reinforcing potential [8]. Due to their availability, ease of chemical and mechanical modification as well as their biocompatibility, renewability and a high axis ratio (L/d), cellulose nanoparticles have attracted enormous attention as

∗ Corresponding author at: Division of Bioresources, Paper and Coating, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia. Fax: +60 4 657 3678. ∗∗ Corresponding author. E-mail addresses: mhaafi[email protected], mohamadhaafi[email protected] (M.K.M. Haafiz), [email protected] (A. Hassan). http://dx.doi.org/10.1016/j.ijbiomac.2016.01.004 0141-8130/© 2016 Elsevier B.V. All rights reserved.

an alternative to micro-sized reinforcements in composite materials [1,9]. It is well known that native cellulose can be readily hydrolyzed to micro or nanocrystalline. Hydrolysis of lignocelluloses has been reported to result in micro or nanocrystalline cellulose [1]. It was reported that these nanocrystalline cellulose or cellulose nanowhisker (CNW) are usually ∼100–300 nm in length and ∼3–10 nm in width [10,11]. The term whiskers are used to designate elongated crystalline rod-like nanoparticles, whereas the designation nanofibrils should be used to designate long flexible nanoparticles consisting of alternating crystalline and amorphous strings [3]. The use of CNW as reinforcement material will lead to a fully degradable and renewable biodegradable nanocomposite [12]. CNW have been obtained after the removal of the amorphous region that result in the formation of high-purity single crystals. This material has a mechanical strength equivalent to the binding forces of adjacent atoms [3]. The resultant highly ordered structure produces not only unusually high strength but also significant changes in electrical, optical, magnetic, dielectric,

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Fig. 1. Optical micrograph images for (a) before hydrolysis (b) after hydrolysis.

conductive, and even superconductive properties. The tensile strength properties of whiskers allow the processing of the highest attainable composite strengths and high-volume content reinforcements [13,14]. A disadvantage of CNW is that the crystallites have to be isolated, and incorporation of the crystallites in a matrix usually involves problems in controlling the dispersion level [13,14]. However, according to Pandey et al. exposure to mechanical dispersion or ultrasonication would permit the dispersion of CNW aggregates and finally produce stable colloidal suspensions [15]. In addition proper treatment of MCC with sulfuric acid (H2 SO4 ) will not only generate isolated cellulose whiskers but also a negatively charged surface resulting from the esterification of hydroxyl groups by sulphate ions formed a stable colloid system [16]. These nanomaterials which show unique properties when incorporated in different polymers have been sourced from kraft pulp, sugar beet pulp, wheat straw, bacterial cellulose, and hemp fiber [17]. Malaysia is currently acclaimed as the largest producer and exporter of palm oil in the world. Earlier studies by Basiron [18], showed that the Palm Oil industry in Malaysia generates more than 18 million tonnes of palm oil annually, thus becoming a major economic pillar for the country. Consequently, enormous amounts of lignocelluloses residues from oil palm industry such as oil palm empty fruit bunches (OPEFB) were generated by the palm oil industry [19]. OPEFB, a non-woody fibrous residue, which remains after the liquid oil has been extracted, has not received much commercial utilization. This biomass is readily available at minimal cost. The development of a technique that can process these bio-residuals into high-value added product (i.e., CNW) is of

great interest. Therefore this study focused on the isolation of CNW from oil palm empty fruit bunches microcrystalline (OPEFB-MCC) by acid hydrolysis technique and designated as CNW-S. The nanosized cellulose particle obtained was then used as reinforcement phase in the polylactid acid (PLA) matrices for the development of green nanocomposites (PLA-CNW-S). PLA is biopolymers which can be derive from the fermentation of corn. It has good mechanical and biodegradable properties. However, due to low thermal stability, slow degradation rate, and medium gas barrier properties limited the use of this biopolymer [16]. Therefore incorporating PLA with the renewable reinforcement filler like CNW-S could be of great interest in order to enhance some of PLA limitation while maintaining their transparency and biodegradability properties. 2. Materials and methods 2.1. Material Polylactic acid (Nature WorkTM PLA 3001D) in pellet form was obtained from NatureWork® LLC, Minnetonka, MN USA. It has a specific gravity 1.24 g/cm3 and melt flow index (MFI) around 15 g/10 min (190 ◦ C/2.16 kg). All chemicals were used as received and were secured from Merck, Malaysia. 2.2. Preparation of PLA and PLA-CNW-S nanocomposites A CNW-S was produced from OPEFB-MCC and used as reinforcement filler. The production of OPEFB-MCC and CNW-S were described in detail in author’s early publication [6,20]. A 10 wt%

Fig. 2. The AFM images for (a) OPEFB-MCC and (b) CNW-S.

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Fig. 3. FESEM micrograph of fracture cross-section for (a) PLA, (b) PLA-CNW-S 3, (c) PLA-CNW-S 3 (detail view), (d) PLA-CNW-S 5 and (e) PLA-CNW-S 5 (detail view).

solution of PLA pellets in chloroform was prepared by stirring the solution in the water bath at 60 ◦ C for 2 h until the pellets were fully dissolved [21,22]. The PLA solution was immediately cast on the clean glass plates and left for the solvent to evaporate at ambient temperature for 48 h. The films obtained from the cast solution were approximately 100 ␮m and noted as pure PLA. To prepare the PLA-CNW-S nanocomposite, 10 wt% solution of PLA was mixed with different amounts of CNW-S (1, 3 and 5 phr) and the mixture was kept at 60 ◦ C with strong agitation until the PLA pellets were fully dissolved. The nano filler used in this stage was in suspension form. Therefore solvent exchange was done through centrifugation by using Universal 32 Hettich (Newport Pagnell, England). In this stage water was exchanged with acetone and acetone was exchanged with chloroform. The filler was then sonicated in a Branson 2510 bransonic bath for 5 min to make sure the CNW-S is dispersed homogenously inside the chloroform. The dispersed CNW-S was

then transferred into PLA with strong agitation approximately 2 h. The dissolved PLA containing CNW-S was then sonicated for another 5 min. The solution was then casted on a clean glass plate. The nanocomposite with approximately 100 ␮m in thickness was obtained by solvent evaporation at surrounding temperature for 48 h before analysis. The PLA nanocomposites were designated as PLA-CNW-S 1, PLA-CNW-S 3 and PLA-CNW-S 5. 3. Characterization 3.1. Optical microscopy In this study, an optical microscope (OM) Leica DM 3000 was used to observe microscopic changes on MCC, after hydrolysis treatment. One drop of diluted (0.1 mg/100 ml) suspension was dropped on the glass slide and observed with 10× magnified objective lens.

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Fig. 4. TEM micrograph for (a) PLA, (b) PLA-CNW-S 3 and (c) PLA-CNW-S 5.

3.2. Microscopy analysis Atomic force microscopy (AFM) measurements were performed using SPA-300HV atomic force microscope with an SPI 3800 controller. A dilute drop of CNW-S suspension (0.1 mg/100 ml) was dispersed on the mica surface and allowed to dry at room temperature prior to analysis. For the nanocomposites, the samples were observed by using a field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM was conducted using a FESEM-EDX Oxford INCA 400 model at an acceleration voltage of 10 kV. The samples were sputter-coated with gold to avoid charging. For TEM observation, the TEM model LEO LIBRA was used; specimens with the thickness about 60 nm were prepared using Leica Ultracut ultramicrotome with diamond knife. The samples were examined at an accelerating voltage of 120 kV. The TEM images were obtained by using soft imagine system software.

The sample was heated at 5 ◦ C/min in nitrogen flux from room temperature to 250 ◦ C. Approximately 10 mg of nanocomposites samples were transferred into hermetic aluminum pan and sealed. The sample was then placed in the analyzer with an empty hermetic aluminum pan as reference. The data of sample was recorded continuously over the temperature and time intervals. 3.5. Thermogravimetric analysis Thermogravimetric analyzer (TGA) Model 2050, (TA Instruments, New Castle, DE) was used to characterize the thermal stability of PLA and PLA-CNW-S. The specimens were scanned from 30 ◦ C to 600 ◦ C at the rate of 10 ◦ C/min and analysis was performed under a nitrogen gas flow [22]. 4. Results and discussion

3.3. Tensile test

4.1. Optical microscopy

Mechanical test was done using the Instron 4400 Universal Tester to measure the tensile strength at the point of breakage for each sample. Tensile tests were carried out at room temperature, according to the ASTM D882 type V. A fixed crosshead rate of 12.5 mm/min was utilized in all cases and the results were taken as an average of five tests [22].

Fig. 1 shows an optical micrograph of the OPEFB-MCC before and after acid hydrolysis treatment. From the figure it can be seen clearly that OPEFB-MCC (Fig. 1a) displayed large particles due to a strong tendency of agglomeration [20]. It is however noteworthy that after hydrolysis treatment the large aggregates had been broken down, whereby no particles could be observed (Fig. 1b). This indicates that they might be in the nanosize particles. The optical microscope resolution however is inadequate technique to observe or detect the dimension and physical characteristic of CNW-S particles [13]. Therefore to confirm the separation of individual whiskers after the hydrolysis treatment diluted suspension of CNW-S was

3.4. Differential scanning calorimetry Differential scanning calorimetry analysis was done by using a PerkinElmer Pyris 7 Thermal Analyzer under nitrogen purge.

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Fig. 5. Effect of CNW-S loading on PLA nanocomposites (a) tensile strength, (b) elongation at break, and (c) Young’s modulus.

Fig. 6. The DSC curve for PLA and PLA-CNW-S nanocomposites.

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Fig. 7. TGA and DTG curves for PLA and PLA-CNW-S nanocomposites.

observed using atomic force microscopy as discussed in detail in next section. 4.2. Atomic force microscopy The atomic force microscopy (AFM) images of OPEFB-MCC are shown in Fig. 2a.The figure shows that OPEFB-MCC had regular spherical particles. It is interesting to note that, after the hydrolysis treatment, the morphological change on OPEFB-MCC was observed as shown in Fig. 2b. The AFM analysis confirms that the individual

rod-like CNW-S from OPEFB-MCC with good aspect ratio (L/d) was successfully produced after the treatment. Hence, this will provide the better reinforcement effect when CNW-S is incorporated with PLA, which are discussed in the mechanical properties section. 4.3. Field emission scanning electron microscopy Since the mechanical properties of composites or nanocomposite are leaning on the polymer/filler interaction [23], the field emission scanning electron microscopy (FESEM) was carried out to

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elucidate the interaction formed between CNW-S and PLA. FESEM images of fractured surfaces of PLA, and P-CNW-S nanocomposites are shown in Fig. 3. Fig. 3a shows a glossy and ordered morphology of PLA fractured surface as also reported by Haafiz et al. [21,22]. On the other hand, the P-CNW-S displays homogeneous surface structure, which suggested that CNW-S were uniformly distributed in the PLA. The good dispersion of CNW-S could be due to the strong interaction between CNW-S and PLA. Besides that, higher degree of crystalline of CNW-S as reported earlier could be the reason for a better dispersion of CNW-S in PLA [6]. This result is in good agreement with previous observations reported by Garcia and Lagaron [24]. and Roohani et al. [25]. Fig. 3c and e shows the detail view of P-CNW-S fractured surface at 3 and 5 phr. From the figures there is no clear evidence pertaining to the present of the individual CNW-S in the PLA. This could be attributed to the nano size dimensions of CNW-S. It however, can be suggested that the white dot that appeared on the fracture surface of PLA-CNW-S were assumed to be the perpendicular plane of the CNW-S inside PLA. Wang et al. as well as Roohani et al. observed the similar outcomes when CNW used as reinforcement in poly(vinyl alcohol) and in soy protein thermoplastic respectively [17,25]. However, at 5 phr loading the aggregation of CNW-S in PLA is readily available (Fig. 3d). The fracture surface shows a relatively rough structure suggesting that the interfacial adhesion between PLA and filler is rather low [26]. This could be the reason, why the tensile strength and elongation at break (Eb ) of PLA-CNW-S decreased gradually as discussed later in the mechanical properties section. 4.4. Transmission electron microscopy The enhancement in mechanical properties of composites and nanocomposite are depending on the absence of void, intact position of filler, interfacial bonding between filler and the absence of filler agglomerations [26,27]. In order to get a better understanding in the changes on mechanical performance of PLA with additional of CNW-S, transmission electron microscope (TEM) was used. TEM images of PLA and PLA-CNW-S at 3 and 5 phr loading are shown in Fig. 4(a–c). It can be clearly seen in Fig. 4a that the microtome section of PLA displays a smooth and clean surface feature. Meanwhile, Fig. 4b reveals that at 3 phr CNW-S loading, the filler are able to disperse homogenously inside the PLA matrices. The good dispersion of CNW-S in PLA could be the main factor in tensile strength improvement. However, at the highest filler loading (5 phr) the aggregated of CNW-S occurred as displayed in Fig. 4c. This phenomenon is similar to the observation by Liu et al. [28] could be the reason for reduction in mechanical properties of PLA-CNW-S 5. 4.5. Mechanical properties 4.5.1. Tensile strength The effects of CNW-S loading on the tensile properties of pure PLA and PLA-CNW-S nanocomposites are shown in Fig. 5(a–c). The incorporation of microcrystalline cellulose from oil palm empty fruit bunches (OPEFB-MCC) into the PLA matrices did not show any improvements in both tensile strength and elongation at break (Eb ) of the composites as compared to pure PLA [21]. However, the addition of CNW-S from OPEFB-MCC in PLA matrix indicated a different behavior in tensile strength properties. It is clear from Fig. 5a that the tensile strength of the nanocomposites was improved with the increasing in CNW-S loading up to 3 phr before decrease with higher CNW-S loading. This enhancement has been attributed to the good dispersion and stiffness between the filler and polymer matrix. Consequently, filler-matrix interaction becomes more pronounced and better interfacial adhesion was formed between the CNW-S and PLA. Therefore the stress transfers to the filler, which

Table 1 Glass transaction temperature (Tg ), cold crystalline temperature (Tcc ) and melting temperature (Tm ) for PLA and PLA-CNW-S nanocomposites. Formulation

Tg (◦ C)

Tcc (◦ C)

Tm1 (◦ C)

Tm2 (◦ C)

PLA P-CNW-S 1 P-CNW-S 3 P-CNW-S 5

48.1 48.9 49.1 49.47

104.1 103.34 101.1 95.64

146.90 148.10 147.10 146.11

150.05 151.03 151.22 151.60

the load bearing entity becomes efficient which then improved the tensile strength [26]. For the specimens with more than 3 phr by the tensile strength of PLA-CNW-S nanocomposite is dropped by 22%. This drop in tensile strength is attributed to the agglomeration of CNW-S in PLA (Figs. 3 e and 4 c). This aggregation could act as stress-centralized point and reduced the surface area interaction between CNW-S and PLA which leads to decrease the tensile strength of PLA-CNW-S 5 nanocomposites. On the other hand, elongations at break (Eb ) of nanocomposites decrease dramatically with the additional of CNW-S in PLA matrix as shown in Fig. 5b. The Eb decreased gradually as the CNWS concentration increased which rendered PLA more brittle. This observation may be attributed to the stiffening action of the CNW-S by restricting the segmental chain movement of PLA during tensile testing. Similar result was reported by Bulota et al. [29] when they studied the mechanical behavior of PLA reinforced TEMPOOxidized cellulose. Beside that the Eb is also affected by the volume fraction of the added reinforcement, the dispersion and interaction between the reinforcement and the matrix as reported by Pei et al. [30]. when cellulose nanocrystal from cotton cellulose used as bio-based nucleation agents in poly(l-lactide) (PLLA). The Young’s modulus of PLA-CNW-S nanocomposites increased with the addition of CNW-S as shown in Fig. 5c. This revealed that the additions of CNW-S give the positive impact to the PLA-CNWS nanocomposites. According to Cheng et al. [31] the increase in Young’s modulus with increasing filler loading can be explained by increased in stiffening effect from the filler which is a typical characteristics filler/polymer composite. Beside that the high crystallinity index of filler also is the reason for increasing of Young’s modulus [22,23,28]. 4.6. Differential scanning calorimetric analysis The differential scanning calorimetric (DSC) was carried out to observe the effect on thermal properties of PLA after incorporated with CNW-S. In order to minimize the influences of any residual moisture presented, thermal history as well as other side effect such as solvent traces in the samples, the DSC data presented are taken from the second heating cycle [32]. DSC thermogram of PLA and PLA-CNW-S are shown in Fig. 6 and the resultant data of glass transition temperature (Tg ), cold crystalline temperature (Tcc ) and melting temperature (Tm ) are summarized in Table 1. Fig. 6 indicates the Tg , Tcc and Tm of the PLA-CNW-S nanocomposites. The Tg of PLA-CNW-S is slightly higher than that of the PLA (Table 1). This indicates that the addition of CNW-S lead to the reduction in PLA chain flexibility as Tg value is mainly related to the flexibility of polymeric segments. This observation is in a good agreement Table 2 Thermal properties of PLA and PLA-CNW-S nanocomposites. Formulation

PLA PLA-CNW-SO4 1 PLA-CNW-SO4 3 PLA-CNW-SO4 5

Degradation temperature (◦ C) Ton

T10

T50

Tmax

295.93 294.61 291.50 290.13

331.99 325.01 322.24 318.11

359.57 349.90 348.17 346.08

363.62 354.02 353.37 349.63

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with the results reported Baheti et al. [26], and Bulota et al. [29], in which they were using jute nanofiber and tempo-oxidized cellulose from birch pulp respectively as reinforcement in PLA. Besides that this phenomenon could be related to the hydrogen bonding interaction between OH group from PLA and CNW-S that induced a restricted mobility of polymer chain. Therefore the decrease in the chain mobility associated with Tg will increase the energy required for Tg to occur [28]. On the other hand the Tcc values of PLA-CNWS are shifted to lower temperature as compared to the PLA. The low Tcc observed during heating is indicated by faster crystallization induced by CNW-S which acts as nucleating agent for PLA. Therefore decreased the free energy barrier and fastens the crystallization [26,28,33]. From Fig. 6, it is noteworthy that the melting peak of PLA and PLA-CNW-S showed double melting peak (Tm1 and Tm2 ). In this study the highest temperature endotherms were taken as the Tm . According to Baheti et al. the highest temperature (Tm2 ) is a perfect crystalline structure of PLA rather than lower temperature (Tm1 ). From the observation the Tm value of PLA was enhanced with the addition of CNW-S [26]. The improvement in Tm value with addition of CNW-S could be due to nucleating ability of CNW-S to develop more heterogeneous crystalline morphology in PLA, consequences increase the Tm value. Similar conclusion was arrived from Baheti et al. [26], when wet mild jute nanofiber was used as reinforcement in PLA, and by Frone et al. [33] in characterized the thermal properties of PLA reinforced cellulose nanofiber obtained from commercial available MCC. 4.7. Thermogravimetric analysis In order to investigate the thermal performance of the PLA and PLA-CNW-S nanocomposites, thermogravimetric analysis (TGA) and derivative thermograms analysis (DTG) were performed. As reported earlier by Haafiz et al. [22], incorporated PLA with CNW obtained from OPEFB-MCC by swelling treatment improved the thermal properties of PLA nanocomposites. On contrary as can be seen from Fig. 7 and Table 2, the thermal stability of PLA-CNW-S nanocomposites shows different behavior where, the thermal performance of nanocomposites is slightly lower as compared to the pure PLA. The decreasing in thermal stability of PLA-CNW-S nanocomposites is readily apparent as the CNW-S loading increased (1–5 phr). The result was in agreement with earlier reported by Bras et al. [10]. During incorporated baggase cellulose whiskers as reinforcement in natural rubber nanocomposites. Beside that the lower decomposition temperature for PLA-CNW-S could be due the lowest onset, T10 , T50 , and Tmax temperature of CNW-S as reported earlier by the present author’s [6]. On the other hand the used of H2 SO4 in isolation of cellulose nanowhiskers probably cause the catalytic dehydration due to the present of sulfate group on the PLA surface which decreased the thermal stability of PLA-CNW-S [10]. However according to Wang et al [17]. This phenomenon could be overcome by neutralized the acid sulphate group in CNW-S with NaOH. When CNW-S was neutralized or H2 SO4 group was eliminated the degradation temperature of CNW-S will probably shifted to higher temperature, consequently will improve the thermal stability of PLA-CNW-S nanocomposite. 5. Conclusion Cellulose nanowhiskers (CNW-S) have been successfully isolated from OPEFB. Atomic force microscopy revealed that the CNW-S obtained had rod-like shape particles, with estimated aspect ratio (L/d) more than 30. The TEM and FESEM indicated that at 3 phr CNW-S loading, the fillers are homogeneously dis-

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persed in the PLA matrices before the agglomeration took place at 5 phr CNW-S loading. The tensile strength of the nanocomposites increased with addition of CNW-S up to 3 phr before decreasing at further fillers loading due to the agglomeration. The Young’s modulus of the obtained nanocomposite showed the positive impact to the filler loading. The elongation at break of PLA-CNW-S however, decreased linearly with the CNW-S loading due to polymer chain movement restriction. It interesting to note that, Tg, Tcc and Tm of PLA-CNW-S improved with the CNW-S loading as revealed by DCS analysis. The TGA and DTG of the nanocomposite however, slightly decrease as compared to the PLA when the CNW-S loading increase. The fabricated PLA-CNW-S nanocomposite had shown the potential to be used in coatings, membranes, and food agro based packaging as well as in the automotive applications where the high thermal stability is not a main requirement. Acknowledgment Short Term Research Grant 304/PTEKIND/ 6313194 and Fundamental Research Grant Scheme (FRGS) 203.PTEKIND.6711500. References [1] R. Li, J. Fei, Y. Cai, Y. Li, J. Feng, J. Yao, Cellulose whiskers extracted from mulberry: a novel biomass production, Carbohydr. Polym. 76 (2009) 94–99. [2] A.N. Fernandes, L.H. Thomas, C.M. Altaner, P. Callow, V.T. Forsyth, D.C. Apperley, C.J. Kennedy, M.C. Jarvis, Nanostructure of cellulose microfibrils in spruce wood, Proc. Natl. Acad. Sci. U. S. A. 108 (2011) 1195–1203. [3] S.J. Eichhorn, Cellulose nanowhiskers: promising materials for advanced applications, Soft Matter 7 (2010) 303–315. [4] F. Fahma, S. Iwamoto, N. Hori, T. Iwata, A. Takemura, Isolation, preparation, and characterization of nanofibers from oil palm empty-fruit-bunch (OPEFB), Cellulose 17 (2010) 977–985. [5] K. Oksman, A.P. Mathew, D. Bondeson, I. Kvien, Manufacturing process of cellulose whiskers/polylactic acid nanocomposites, Compos. Sci. Technol. 66 (2006) 2776–2784. [6] M.K.M. Haafiz, A. Hassan, Z. Zakaria, I.M. Inuwa, Isolation and characterization of cellulose nanowhiskers from oil palm biomass microcrystalline cellulose, Carbohyr. Polym. 103 (2014) 119–125. [7] M.K.M. Haafiz, A. Hassan, Z. Zakaria, I.M. Inuwa, M.S. Islam, Physicochemical characterization of cellulose nanowhiskers extracted from oil palm biomass microcrystalline cellulose, Mater. Lett. 113 (2013) 87–89. [8] S. Panthapulakkal, M. Sain, Preparation and characterization of cellulose nanofibril films from wood fibre and their thermoplastic polycarbonate composites, Int. J. Polym. Sci. (2011) 1–6. [9] M. Jonoobi, J. Harun, A.P. Mathew, K. Oksman, Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion, Compos. Sci. Technol. 70 (2010) 1742–1747. [10] J. Bras, M.L. Hassan, C. Bruzesse, E.A. Hassan, N.A. El-Wakil, A. Dufresne, Mechanical, barrier, and biodegradability properties of bagasse cellulose whiskers reinforced natural rubber nanocomposites, Ind. Crops Prod. 36 (2010) 627–633. [11] A.J. De Menezes, G. Siqueira, A.A.S. Curvelo, A. Dufresne, Extrusion and characterization of functionalized cellulose whiskers reinforced polyethylene nanocomposites, Polymer 50 (2009) 4552–4563. [12] L. Goetz, A. Mathew, K. Oksman, P. Gatenholm, A.J. Ragauskas, A novel nanocomposite film prepared from crosslinked cellulosic whiskers, Carbohydr. Polym. 75 (2009) 85–89. [13] D. Bondeson, A. Mathew, K. Oksman, Optimization of the isolation of nanocrystals from microcrystalline cellulose by acid hydrolysis, Cellulose 13 (2) (2006) 171–180. [14] M.A.S. Azizi Samir, F. Alloin, A. Dufresne, Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field, Biomacromolecules 6 (2005) 612–626. [15] J.K. Pandey, W.S. Chu, C.S. Kim, C.S. Lee, S.H. Ahn, Bio-nano reinforcement of environmentally degradable polymer matrix by cellulose whiskers from grass, Compos. Part B: Eng. 40 (2009) 676–680. [16] L. Petersson, I. Kvien, K. Oksman, Structure and thermal properties of poly(lactic acid)/cellulose whiskers nanocomposite materials, Compos. Sci. Technol. 67 (2007) 2535–2544. [17] N. Wang, E. Ding, R. Cheng, Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups, Polymer 48 (2007) 3486–3493. [18] Y. Basiron, Palm oil production through sustainable plantations, Eur. J. Lipid Sci. Technol. 109 (2007) 289–295. [19] W.D. Wanrosli, M.K.M. Haafiz, S. Azman, Cellulose phosphate from oil palm biomass as potential biomaterials, BioResources 6 (2) (2011) 1719–1740. [20] M.K.M. Haafiz, S.J. Eichhorn, A. Hassan, M. Jawaid, Isolation and characterization of microcrystalline cellulose from oil palm biomass residue, Carbohydr. Polym. 93 (2013) 628–634.

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