Surface functionalization of nanofibrillated cellulose extracted from wheat straw: Effect of process parameters

Carbohydrate Polymers 150 (2016) 48–56

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Surface functionalization of nanofibrillated cellulose extracted from wheat straw: Effect of process parameters Mandeep Singh, Anupama Kaushik ∗ , Dheeraj Ahuja Dr. SSB University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India

a r t i c l e

i n f o

Article history: Received 28 November 2015 Received in revised form 22 April 2016 Accepted 23 April 2016 Available online 28 April 2016 Keywords: Nanofibrillated cellulose Surface modification SEM-EDX TEM Contact angle

a b s t r a c t Aggregates of microfibrillated cellulose isolated from wheat straw fibers were subjected to propionylation under different processing conditions of time, temperature and concentration. The treated fibers were then homogenized to obtain surface modified nanofibrillated cellulose. For varying parameters, progress of propionylation and its effects on various characteristics was investigated by FTIR, degree of substitution, elemental analysis, SEM, EDX, TEM, X-ray diffraction, static and dynamic contact angle measurements. Thermal stability of the nanofibrils was also investigated using thermogravimetric technique. FTIR analysis confirmed the propionylation of the hydroxyl groups of the cellulose fibers. The variations in reaction conditions such as time and temperature had shown considerable effect on degree of substitution (DS) and surface contact angle (CA). These characterization results represent the optimizing conditions under which cellulose nanofibrils with hydrophobic characteristics up to contact angle of 120◦ can be obtained. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Nanocellulose is one of most abundant and renewable natural material that has induced immense potential in global research for development of new green nanocomposites (Khalil, Bhat, & Yusra, 2012; Siró & Plackett, 2010). It is the main building constituent of wood and other lignocellulosic fibers and is responsible for providing them the required strength and stiffness. (Abe, Iwamoto, & Yano, 2007; Eichhorn et al., 2010; Habibi, Lucia, & Rojas, 2010; Siró & Plackett, 2010). In its native form, cellulose possesses crystalline fibrous structure, aligned parallel to each other, embedded in an amorphous matrix of lignin, pectin and hemicellulose. It is classified into (1) nanowhiskers and (2) nanofibrillated cellulose depending upon the morphology obtained after its isolation from the plant source (Belbekhouche et al., 2011; Khalil et al., 2014, 2012). Its super molecular structure along with strong and complex network of hydrogen bonds give it a crystalline structure which makes it insoluble in water and organic solvents under ambient temperatures. The nanostructure of cellulosic fibers provides a large specific surface area along with numerous free hydroxyl groups on its surface rendering it hydrophilic, which makes its dispersion difficult

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A. Kaushik). http://dx.doi.org/10.1016/j.carbpol.2016.04.109 0144-8617/© 2016 Elsevier Ltd. All rights reserved.

in hydrophobic polymers matrix (Gardner, Oporto, Mills, & Samir, 2008; Jackson et al., 2011; Volkert, Lehmann, & Hettrich, 2014). But the presence of free hydroxyl groups offer numerous opportunities for surface modifications of reactive sites (Gardner et al., 2008). Esterification is one of the important chemical modification techniques that have been extensively used to modify microstructures of cellulose (Bledzki & Gassan, 1999; Volkert et al., 2014). The technique adjusts the hydrophobic/hydrophilic balance in cellulose thus improving its solubility in numerous organic solvents, augmenting dispersion and interfacial adhesion in different polymeric matrices (Goussé, Chanzy, Cerrada, & Fleury, 2004; Johansson, Tammelin, Campbell, Setälä, & Österberg, 2011; Jonoobi, Harun, Mathew, Hussein, & Oksman, 2010; Missoum, Belgacem, & Bras, 2013; Missoum, Bras, & Belgacem, 2012; Pahimanolis et al., 2011). The acetylation of surface hydroxyl groups of nanocellulose is comparatively a new area of research. Many authors have adopted separate strategies and reported effects of surface acetylation on properties of isolated nanofibrillated cellulose (NFCs) (Bulota, Kreitsmann, Hughes, & Paltakari, 2012; Jonoobi, Harun, Mathew, & Oksman, 2010; Jonoobi, Mathew, Abdi, Makinejad, & Oksman, 2012; Rodionova, Lenes, Eriksen, & Gregersen, 2011; Tingaut, Zimmermann, & Lopez-Suevos, 2009). Modification of nanofibrils using propionic anhydride is a less studied area but it has been found that use of propionic anhydride instead of acetic anhydride results in better dimensional stability of resultant fibers and their composites (Papadopoulos & Gkaraveli, 2003).

M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

This work involves the surface modification of nanofibrillated cellulose isolated from wheat straw fibers using an indirect esterification method using propionic anhydride (Belgacem & Gandini, 2009). The indirect method limits the reaction material on the exposed surface of nanofibrils without causing any destruction to the fiber’s inner structure (Jonoobi, Harun, Mathew, Hussein et al., 2010). Microfibrillated aggregates of cellulose (MFCs) were treated under different processing conditions i.e. reaction time, temperature and concentration of reagent and were characterized using FTIR, SEM-EDX, CHNSO-elemental analysis and degree of substitution. Whereas, microfibrillar aggregates and their nanofibrils were characterized using powder WA-XRD and contact angle measurement techniques, TEM and TGA analysis was also conducted for NFC’s to investigate the effect of modification on their morphology and thermal characteristics.

2. Experimental 2.1. Materials Microfibrillated aggregates of cellulose (MFCs) isolated from wheat straw fibers using a physico-chemical method as described elsewhere (Kaushik & Singh, 2011) were subjected to surface modification. Propionic anhydride was procured from SRL Chem. India Pvt. Ltd. and was used as received. The other chemicals used were Pyridine, Toluene, H2 SO4 , ethanol were procured from Merck India Pvt. Ltd. and they were also used as received. 2.2. Propionylation of microfibrillated cellulose fibers (MFCs) A mixture of toluene (∼50 ml) and pyridine (∼2.5 ml) with H2 SO4 (0.1 ml) catalyst was taken as a medium for esterification. Toluene restricts the swelling of fibers and do not allow the reactants to enter into the bulk sites of fibers. A small amount of pyridine was used for increasing the number of accessible reactive hydroxyl sites and enhancing the rate of acetylation (Jonoobi, Harun, Mathew, Hussein et al., 2010). For the surface treatment, microfibrillated aggregates of cellulose (MFCs) extracted from wheat straw were oven dried at 38 ± 2 ◦ C overnight. A fixed weight i.e. 1 g (±0.01) of dried MFCs were then soaked in the prepared reaction medium. The mixture was taken in the stoppers glass tube and a fixed concentration of propionic anhydride in fiber to reagent ratio (w/v) was poured and shaken vigorously for 5–10 min. Finally, the glass tube was put in the oil bath under different temperatures and reaction time. After completion of treatment, samples were immediately cooled and centrifuged with toluene for 15–20 min to terminate the reaction. The treated fibers were then vacuum filtered with ethanol (50% aq.) and de-ionized water and finally dried at 50–60 ◦ C for 24 h. In order to optimize the results, two different fibers to anhydride concentration ratio (w/v) of 1:2 and 1:4 were taken and temperature was varied from 60, 80 and 100 ◦ C for 0.45, 1.30, 3 and 5 h of reaction time. Sample designation based processing conditions are given in the Table 1. 2.3. Isolation of unmodified & modified nanofibrillated cellulose (NFCs) The unmodified and modified MFCs were subjected to high shear action of IKA T-18 digital homogenizer (8500 ± 200 RPM) to obtain nanofibrillated cellulose. In this process, different samples were taken in distilled water and subjected to high shear homogenization for mechanical separation of nanofibers from MFCs. Each sample was homogenized for 20 min to obtain uniform nanosized

49

fibers with entangled network after disintegrating the MFCs. The final suspension of NFCs was then dried overnight at 50 ± 0.5 ◦ C. 2.4. Measurements Perkin Elmer RX−FTIR spectrophotometer was used to identify variations in the functional groups as result of surface treatment. Fiber samples were prepared using KBr disk methods and spectra were recorded in a spectral range of 4000–450 cm−1 with a resolution of 2 cm−1 . Degree of substitution (DS) of propionyl groups on the fiber surface was estimated using a titration method as described in literature (Kim, Nishiyama, & Kuga, 2002; Rodionova et al., 2011). The percentage of ester content and DS values for each sample were calculated by Eqs. (1) and (2) given below. Each test was performed thrice to ensure reproducibility. Ester Content (%) =

DS =

[[Blank (ml) − Sample (ml)] × Molarity of HCl] × 100 Sample Weight (g)

(1)

[162 × Ester Content (%)] [4300 − (42 − Ester Content (%))]

(2)

A Perkin-Elmer 240C CHNSO-Elemental Analytical Instrument (USA) was used to conduct elemental analysis of unmodified and modified MFCs. Surface elemental analysis was also conducted using low energy dispersive X-ray cartography (EDX) of INCAX-Act, Oxford Instruments coupled with Scanning Electron Microscope (SEM). The surface morphology of unmodified and modified MFC samples was also observed by using a separate scanning electron microscope model JSM JEOL-6490. The network and surface topography of unmodified and modified nanofibrillated cellulose was observed using Transmission Electron Microscope (TEM) model Hitachi-2100. All the images were taken at 80 kV accelerating voltage. The samples of unmodified and modified NFCs were analyzed in powdered form using a Philips X’Pert Pro X-ray diffractometer. Crystallinity index (CrI) of the nanofibrils was calculated using Eq. (3) derived from the Segal empirical method (Krassig, 1985; Segal, Creely, Martin, & Conrad, 1959). Crystallinity Index (CrI) = 100 ×

ITotal − IAm ITotal

(3)

where, ITotal is the maximum intensity of the (002) lattice diffraction and IAm is the intensity diffraction at 18◦ 2␪ diffraction angle. The crystallite size was also estimated using Scherrer’s equation (Eq. (4)) (Bodor, 1991). thkl =

 ˇhkl cos 

(4)

where, thkl is the thickness of crystallites at the (hkl) plane of diffraction, ␭ is X-ray wavelength (␭ = 0.1542 nm for CuK␣),  is the Bragg angle of the reflection, ␤hkl is the pure integral of width of the reflection at half maximum height, and K is the Scherrer constant. The surface properties of different modified MFCs and its NFCs were estimated for static & dynamic contact angles (DCA) measurements for a check time of 0.2 s. These measurements were done with a Dynamic Absorption Tester DAT 1100 at 23 ◦ C and 50% RH. Thermal characteristics of untreated and surface treated NFCs were determined using Perkin Elmer STA-6000 Thermal gravimetric Analyzer (TGA). Each test was performed in nitrogen atmosphere at a heating rate of 10 ◦ C/min−1 from room temperature to 500 ◦ C.

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M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

Table 1 Value of degree of substitution and elemental analysis depicting carbon, hydrogen and oxygen weight percentage for unmodified microfibril aggregates of cellulose* and its modified samples of MFCs at different concentration of reagent used (fiber to anhydride). Sample Name

Time (h)

T (◦ C)

Reagent ratio (w/v)

Reagent ratio (w/v)

1:2

1:4

DS

C

H

O

DS

C

H

O

SM60-1 SM80-1 SM100-1

0.45 0.45 0.45

60 80 100

0.34 ± 0.02 1.56 ± 0.08 1.61 ± 0.08

42.21 42.25 42.22

6.58 6.67 6.87

45.39 46.43 45.97

0.56 ± 0.03 1.47 ± 0.07 1.69 ± 0.08

41.58 42.78 42.69

6.42 6.7 6.82

44 46.21 46.32

SM60-2 SM80-2 SM100-2

1.30 1.30 1.30

60 80 100

0.79 ± 0.04 1.67 ± 0.08 1.83 ± 0.09

42.82 42.14 42.39

6.62 6.67 6.83

45.94 46.19 45.53

1.13 ± 0.05 2.14 ± 0.11 2.56 ± 0.13

42.19 41.97 42.2

6.71 6.67 6.59

44.69 45.08 45.44

SM60-3 SM80-3 SM100-3

3 3 3

60 80 100

1.57 ± 0.07 2.48 ± 0.12 2.17 ± 0.11

43.5 54.02 47.65

6.87 6.85 6.79

48.53 37.62 45.42

1.87 ± 0.09 2.07 ± 0.10 1.93 ± 0.09

43.5 44.26 42.63

6.8 6.87 6.83

48.63 47.73 44.78

SM60-4 SM80-4 SM100-4

5 5 5

60 80 100

2.13 ± 0.10 1.16 ± 0.06 0.89 ± 0.04

41.63 43.15 42.58

6.42 6.75 6.67

44.56 45.91 44.62

1.89 ± 0.09 0.70 ± 0.03 0.67 ± 0.03

41.93 43.49 42.51

6.74 6.85 6.55

44.74 45.67 44.53

*

For unmodified MFCs: weight percentage of carbon = 41.94%, hydrogen = 6.54% and oxygen = 43.49% as depicted by elemental analysis.

3. Results and discussion The reaction mechanism of surface modification of nanofibrils is a two stage indirect method that first involves esterification of the primary and secondary hydroxyl groups of cellulosic surface present in MFCs followed by isolation of surface modified NFCs using mechanical disintegration. The esterification reaction of anhydride groups and hydroxyl groups of MFC occurs under heterogeneous reaction conditions proceeding with formation of sulphate groups by reaction with sulphuric acid, which acts as the catalyst (Volkert et al., 2014). The whole mechanism of indirect esterification can be explained with the help of two hypothetical models on acetylation of cellulosic fibers as described by Sassi and Chanzy (1995) and Jonoobi, Harun, Mathew, Hussein et al. (2010). The first stage as described by Sassy and Chanzy model (Fig. 1) involves two step solid-liquid esterification involving dissolution of reacted chains of cellulose and subsequent reaction of these reacted chains with an anhydride group in liquid phase. The sulfation reaction substitute one hydroxyl group of the cellulose with one sulphate group first and then reaction proceeds further (Dubey et al., 2006; Volkert et al., 2014). The esterification reaction is relatively slower as compared to sulfation reaction and thus the rate limiting step (Bodirlau & Teaca, 2009). The second stage follows the Jonoobi et al. model that involves isolation of aggregates of esterified microfibrils (MFCs) into fine surface modified NFCs using high shear homogenization. The FTIR spectra of unmodified and modified MFCs in Fig. 2(i) and (ii) show significant differences due to propionylation treatment. Growth of new bands in the regions 1730–1745 cm−1 (carbonyl area), 1368 cm−1 (C H bonds in O(C O) CH3 group) and 1262–1129 cm−1 (C O stretching in ester group) confirms the formation of ester bonds with free hydroxyl groups present on the exposed surface of fibers (Adebajo & Frost, 2004; Bodirlau & Teaca, 2009; Ly, Thielemans, Dufresne, Chaussy, & Belgacem, 2008; Sun & Sun, 2002; Tserki, Zafeiropoulos, Simon, & Panayiotou, 2005). There is an absence of peaks at 1720 cm−1 (stretching of C O) and at 1760 cm−1 which confirms the absence of propionic acid and monomers/dimers in the treated samples (Tserki et al., 2005). Further, absence of peak near spectral region 1760–1800 cm−1 also confirms that treated fibers do not contain free anhydride groups (Sun & Sun, 2002). A strong band near 1642 cm−1 may be attributed to H O H bending of adsorbed water in the crystalline regions of cellulose or due to propionylation. As the samples were completely dried at 50 ◦ C overnight before analysis so the first possibility can

be ruled out and chances of propionylation is confirmed (Ly et al., 2008; Tserki et al., 2005). Shifting of carbonyl peak in the region 1730–1745 cm−1 and change in the intensity of the peak occurred with the variation in process parameters i.e. concentration, reaction time and temperature. The shifting confirms the formation of ester bonds, the influence of process parameters on extent of propionylation, and also rejects the possibility of blend formation between propionic anhydride and cellulose (Tserki et al., 2005). Fig. 2(i) shows that increase in concentration i.e. fiber to anhydride ratio resulted in shifting of carbonyl peak in the region 1730–1745 cm−1 toward higher wave numbers confirming the formation of strong ester bonds on the fiber surface (Adebajo & Frost, 2004; Ly et al., 2008; Sun & Sun, 2002). Similar effects were seen with varying reaction time and temperature while maintaining lower fiber to anhydride ratio (1:2 w/v). Degree of substitution of propylated fibers varied from 0.34 to 0.48 while varying the reaction conditions (Table 1). MFCs treated at 80 ◦ C with 1:2 (w/v) fibers to anhydride ratio exhibit increase in DS up to 2.48 for SM80-3 followed by a steep decrease to a DS value of 1.16 for SM80-4. The reported decrease in DS for SM80-4 indicates the influence of longer reaction time which possibly caused reformation of cellulose hydroxyl groups in MFCs. The propionylation of cellulose is a reversible reaction in which reconstruction of cellulose hydroxyl groups may take place under suitable reaction conditions. This process is often called as de-esterification or de-propionylation (Adebajo & Frost, 2004; Freire, Silvestre, Neto, Belgacem, & Gandini, 2006; Sun & Sun, 2002). On increasing fibers to anhydride concentration to 1:4 (w/v), the MFCs of SM80-1 and SM80-2 exhibited sharp increase in DS which is probably due to increase in reaction rate at higher concentration. On the contrary, MFCs of SM80-3 and SM80-4 reported decrease in the DS value. The decrease in DS for 3 h and 5 h of reaction time by only increasing reagent concentration is possibly because of the competition between the esterification and hydrolyzed ester groups produced during the reaction (Freire et al., 2006; Freire, Silvestre, Pascoal Neto, & Rocha, 2005; Uschanov, Johansson, Maunu, & Laine, 2011). At higher temperatures, the increase in DS was limited by reaction time. The study revealed that longer reaction time i.e. 3 h and 5 h resulted in decline in DS at higher temperatures i.e. 80 ◦ C and 100 ◦ C. This trend is also confirmed by FTIR spectra for samples SM80-4, SM100-3 and SM100-4 that show gradual decrease in ester peak intensity in regions 1730–1745 cm−1 , 1368 cm−1 and 1234 cm−1 (Jandura, Kokta, & Riedl, 2000; Ly et al., 2008;

M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

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Fig. 1. Schematic of reaction mechanism of propionylation of NFCs.

Fig. 2. (i) FTIR spectra of microfibril aggregates of cellulose (MFCs) chemically treated fibers with propionic anhydride concentration 1:2 and 1:4 (w/v) and at 80 ◦ C under different time conditions with concentration 1:2 (w/v). Fig. 2(ii) FTIR spectra of microfibril aggregates of cellulose (MFCs) chemically treated for 3 h and 5 h time at different temperatures with fibers to propionic anhydride concentration 1:2 (w/v).

Pasquini, Belgacem, Gandini, & Curvelo, 2006). Initially, an increase in reaction time resulted in improved DS which is probably due to increased diffusion rate of reactants on the fiber surface. But as the reaction time is increased further, higher temperatures promotes the de-esterification or de-propionylation process also. The gradual decrease in ester peak intensity in regions 1730–1745 cm−1 , 1368 cm−1 and 1234 cm−1 directly indicates re-formation of cellulosic hydroxyl groups due to de-propionylation effect (Adebajo & Frost, 2004; Ly et al., 2008; Sun & Sun, 2002; Tserki et al., 2005). Table 1 also shows the results of elemental analysis of the MFCs. It indicates that except for some of the samples i.e. SM803 and SM100-3 in which DS ≥ 2.0, the rest of the samples show marginal variations in the carbon, hydrogen and oxygen concentrations which may be possibly due to absorbed moisture content or truncation of certain error during elemental analysis (Pasquini et al., 2006; Uschanov et al., 2011). As this technique is meant for elemental analysis in the bulk and the variations arising at the

surface go undetected, the above marginal variations in the concentration of various elements make it evident that esterification is limited to the surface of fibers and did not enter in the bulk sites of MFCs. Another important conclusion of CHNSO elemental analysis is that all the samples possessed skeleton of carbon, oxygen and hydrogen only and any elemental impurity associated to the chemicals used in surface treatments such as sulphur was not detected. The SEM analysis evidenced the effect of propionylation on morphology of modified MFCs (Fig. 3). It can be seen that steam exploded and hydrolyzed MFCs of wheat straw possess cleaner and smoother surface whereas propionylated MFCs of SM80-3 possess rough surface. The roughness can be attributed to propynated groups on the surface. This roughness is more obvious for SM80-3 due to increased reactant diffusion rate and high rate of reaction on the fiber surface at higher temperatures i.e. 80 ◦ C for longer time (Hill, Khalil, & Hale, 1998).

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M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

Fig. 3. SEM image of (a) surface of wheat straw, (b1–b3) SEM-EDX spectra of pristine MFCs of steam exploded wheat straw, (c1 and c2) SEM-EDX spectra of MFCs of SM80-3 treated with fibers to anhydride regent ratio 1:2.

The EDX cartography of all MFCs reported in Fig. 3 shows higher percentage of carbon than that of oxygen typically synonymous to cellulose as also observed in elemental analysis. The oxygen to carbon weight percentage ratio (O/C ratio) for various unmodified and modified MFCs reflects the effect of propionylation. The decrease in O/C ratio was observed for the treated samples indicating the presence of reactive bonds on the fiber surface. It can be exhibited for the sample SM80-3, for which O/C ratio was as low as 0.88 while it showed the maximum value DS of 2.48. For pristine MFCs the O/C ratio was 1.12. High shear action of homogenizer for a fixed time interval resulted in disintegration of MFCs into fine NFCs, which is apparent from TEM images in Fig. 4. The images show that 99.99% of the nanofibrils for all samples possessed diameter below 100 nm while propionylated nanofibrils possess significantly different size and morphology as compared to pristine NFCs. The average diameter of nanofibrils in all the samples was calculated from the electron micrographs using digital image analysis software, UTHSCSA. Fig. 4 depicts the effect of surface modification on size distribution profile of nanofibrils as obtained from above analysis. The average diameter for SM80-2 sample was found to be between 0–20 nm with 95% of the NFCs below 30 nm, while the untreated NFCs show an average diameter between 10–30 nm with 95% of the nanofibrils below 50 nm. The NFCs of SM80-3 sample with highest degree of substitution of 2.48 depicts wide range of size distribution from 30 to ≥80 nm. A tendency of agglomeration is comparatively low for treated samples indicating that surface modification imparts good hydrophobicity. The SM80-2 possessed superior fiber network and uniform sized particles in comparison to SM80-3. This may be because surface modification for a longer time causes minor

destruction of the native fiber structure increasing the agglomeration tendency of nanofibrils thus effecting their isolation during homogenization. The contact angle measurements for different MFC and NFC samples at single fiber to anhydride concentration ratio (1:2, w/v) were measured and are summarized in Fig. 5 that shows the dynamic contact angle profile of SM80-1, SM80-2, SM80-3 and SM100-3 samples. There was a quintessential improvement in hydrophobic nature of the fiber surface as result of propionylation using indirect method. The contact angle of a water drop increased up to 121◦ and 119◦ for MFCs and NFCs of SM80-3 sample. Further increase in reaction time and temperature resulted in reduction of the contact angle down to 65◦ that can be seen in SM80-4 and SM100-4 samples. The dynamic contact angle results also evidenced reduced penetration of water in NFCs of SM80-2 and SM80-3 initially for 0.2 s signifying that the fibers have gained a stable and considerable level of hydrophobicity. The wide angle powder X-ray diffraction analysis of unmodified and modified NFCs treated by taking 1:2 (w/v) fiber to anhydride ratio have been shown in Fig. 6((i) and (ii)). All the samples displayed a typical diffraction profile of the native cellulose polymorph I with peaks at 2␪ value 34.4, 22.5, 16 and 15.1◦ related to diffraction planes at sites 040, 002, 101, 10¯ı, revealing that the native crystalline structure of the cellulose is preserved for all the samples (Hill et al., 1998; Krassig, 1985). This corroborates the fact that the reaction mostly involved the surface hydroxyl groups of cellulose. XRD spectra of SM60-3 and SM80-3 NFCs revealed that these fibers have maintained the crystalline pattern of untreated NFCs. The gradual increase of temperature to 80 ◦ C, significantly promoted the grafting of reactive constituents on the surface sites while a further increase in temperature to 100 ◦ C resulted in decrease in

M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

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Fig. 4. TEM of (a) pristine NFCs, (b and c) NFCs of SM80-2 and SM 80-3 treated with reagent ratio 1:2 (w/v), (d) Fiber size (diameter) analysis of the given TEM images using UTHSCSA image tool software.

intensity of diffraction peak and thus crystallinity of the nanofibrils. This is probably due to thermal agitation inflicted in the sample as a result of temperature rise causing the reaction to proceed more aggressively (Tomé et al., 2011). Table 2 shows the value of Bragg’s angle, crystallite size (t) and crystallinity index (CI) for wheat straw, neat MFCs and NFCs of unmodified and modified samples. A decrease in crystallinity from 89% (for SM80-3) to (78% for SM100-3) indicates that de-crystallization in the nanofibrils occurred as the reaction temperature increased from 80 ◦ C to 100 ◦ C. The progress of esterification reaction on cellulose surface depends upon two parameters rate of diffusion of reactants and rate of reaction. As the reactants cannot diffuse into the crystalline region of cellulose, it being compact, reaction takes place only in the amorphous domains of the fibers. As the hydroxyl sites on the exposed surface of the cellulose get esterified, they become soluble and are removed. It results in degradation of exposed crystalline structure and formation of new amorphous domains with diffusion of reactants into the newly formed domains (Ifuku et al., 2007; Tserki et al., 2005; Zafeiropoulos, Williams, Baillie, & Matthews, 2002). This effect is often called de-crystallization in which two concurrent processes occurs i.e. reaction on the surface of cellulose

and simultaneously generation of more amorphous regions in the crystalline domains. In SM100-3 sample, this effect might be the reason for fall in crystallinity. The gradual decrease in DS and contact angle for this sample also suggests that reformation (or de-esterification) of surface hydroxyl groups and penetration of reactants into crystalline regions of intrinsic structure of cellulose have occurred simultaneously. The diffraction profile of samples SM80-2, SM80-3 and SM80-4 indicate that reaction at 80 ◦ C was initiated at the surface, penetrated into the microfibrillated bundles and then finally entered into the core structure of the nanofibers. The increased crystallinity of SM80-3 as compared to SM80-2 is possibly due to increased removal of the amorphous regions during the treatment (Ifuku et al., 2007). It seems that reaction is completed within 3 h thereby keeping the native crystalline structure, crystallinity as well as crystalline size, intact. However, further increase in reaction time to 5 h i.e. SM80-4 resulted in decrease in crystallinity. The decrease in DS as well as contact angle values for this samples also reaffirm that the reaction proceeded by simultaneous reformation of surface hydroxyl group and penetration of reaction species into the core structure of the fibers thereby promoting de-crystallization in the cellulose.

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Fig. 5. Dynamic contact angle plots of (a) pristine NFCs, (b–d) NFCs of SM80-2, SM80-3 & SM100-3. All the samples treated with fibers to PA regent ratio 1:2 (w/v).

Fig. 6. (i) XRD profiles of unmodified and modified NFCs at 60, 80 and 100 ◦ C for 3 h of reaction time with fiber to anhydride concentration of 1:2 (w/v). Fig. 6(ii) XRD profiles of unmodified and modified NFCs at 80 ◦ C for reaction time 1.30, 3 and 5 h with fiber to anhydride concentration of 1:2 (w/v).

Table 2 Nanofiber diameter (thickness) and crystallinity index (CI) as estimate from WA-XRD analysis and decomposition characteristics as obtained from the DTG/TGA analysis of unmodified and surface modified NFCs samples with 1:2 fibers to anhydride reagent ratio (w/v) were taken for the analysis. Fiber Type

Thickness (nm) (Eq. (4))

CI (%) (Eq. (3))

To (◦ C)

Tp (◦ C)

Tm (◦ C)

Weight loss at To (MTo ) (%)

Weight loss at Tp (MTp ) (%)

Weight loss at Tm (MTm ) (%)

NFCs SM60-3 SM80-2 SM80-3 SM80-4 SM100-3

4.18 4.53 4.54 4.47 4.39 4.33

89.02 88.27 88.31 88.90 86.11 78.91

283.2 252.9 254.2 257.1 249.8 254.6

337.5 322.4 340.4 346.7 317.2 322.8

360.3 372.2 377.8 376.3 370.1 368.9

6.87 2.95 4.25 2.09 4.04 4.32

23.49 52.78 51.65 58.84 46.15 54.41

74.12 87.77 83.39 81.42 73.26 77.17

Thermal characteristics of nanofibrillated cellulose has important role to play in green nanocomposite synthesis especially where processing temperature goes beyond 200 ◦ C. Fig. 7 shows TGA and DTG plots of unmodified and modified NFCs of SM60-3, SM802, SM80-3, SM80-4 and SM100-3. All the samples show a single

weight loss step with maximum degradation at offset temperature region i.e. 350–380 ◦ C. Samples show 2–7% weight loss before the onset degradation temperature that lies between 268 and 305 ◦ C. The thermal characteristics are also influenced by the improved hydrophobicity of the nanofibrils. After modification decrease in

M. Singh et al. / Carbohydrate Polymers 150 (2016) 48–56

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Fig. 7. TGA/DTG plots of pristine and surface modified NFCs.

the percentage weight loss was observed at 100 ◦ C which is associated with the evaporation of adsorbed water. Table 2 depicts the extrapolated values of onset (To ), peak (Tp ) and offset (Tm ) temperatures and their corresponding percentage weight loss (MTo , MTp and MTm ) as obtained from the plots. TGA and DTG plots show that the modified nanofibrils are less stable at higher temperature. This behavior may be due to introduction of ester bonds on the cellulose structure or due to slight decrease in crystallinity of the samples obtained after the esterification (Jandura, Riedl, & Kokta, 2000; Tomé et al., 2011; Zafeiropoulos et al., 2002). De-esterification or reformation of cellulose hydroxyl groups is also indicated in Table 2 where net weight loss percentage (MTm − MTo ) has improved for SM100-3 and SM80-4 NFCs.

4. Conclusion The cellulose fibers isolated from wheat straw were treated under different reaction conditions of reaction time, temperature and concentration of reagent to get varying degree of hydrophobicity at the surface of NFCs. Effect of surface modification on other properties of nanofibers such as surface morphology, network and size distribution and thermal characteristics of the nanofibrils was also explored using different techniques. FTIR and DS analysis showed that the extent of modification is strongly dependent upon the processing conditions. The adopted reaction mechanism i.e. the indirect method for surface modification of nanofibrils has significantly controlled the DS of the fibers especially when lower fiber to reagent concentration ratio of 1:2, w/v was used. The fibers treated at 80 ◦ C showed better results while reactions carried out at 100 ◦ C were unstable due to increased diffusion rate of reactants and greater possibility of occurrence of de-esterification even in short time reactions. The contact angle measurements showed quintessential decrease in hydrophilic characteristics of nanofibers ranging from 75 to 120◦ were obtained as an effect of chemical treatment under specified reaction conditions which is a landmark achievement of this research.

Acknowledgements We gratefully acknowledge Council for Scientific Instrumentation & Research (CSIR), India for SRF-Direct fellowship to one of its author Mr. Mandeep Singh; All India Council of Technical Education (AICTE), India; University Grants Commission (UGC), India and Technical Education Quality Improvement Program (TEQIP-II)

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