Introduction

Cellulose nanofibers have attracted considerable attention because of their potential reinforcing capabilities offered by their inherently high strength (Bras et al. 2011). They promise good prospects for various applications, such as polymer composites (Ben Mabrouk et al. 2011; Bras et al. 2010; Wang et al. 2010), flexible substrates for electronics (Nogi and Yano 2008), films (Azeredo et al. 2012), medicines and functional materials (Luo et al. 2012). Cellulose nanofibers that consist of monocrystalline domains with molecular chains parallel to the microfibril axis had a high Young’s modulus and did not deform appreciably under load (Xu et al. 2006). Cellulose is a renewable, abundant and naturally occurring polymer that is obtainable from numerous resources, and it is considered an attractive alternative reinforcement to produce environmentally friendly materials for future-oriented applications (Pasquini et al. 2010; Rosa et al. 2012). Recently, there has been a burgeoning interest in extracting cellulose nanofibers from various plants and organisms, for example, bamboo (Visakh et al. 2012), rice husks (Rosa et al. 2012), grass (Pandey et al. 2009) and mulberry (Li et al. 2009), etc. Among various sources, such as plants, algae, sea animals and bacteria, natural cotton fibers hold the highest percentage of cellulose (>95 %). In nature, cotton fibers with a diameter of 20–30 μm consist of numerous fibrils in nanometer scale; therefore, cellulose nanofibers can be obtained from cotton fibers by the hydrolysis method to remove amorphous phases between crystalline domains. There were a few reports at the forefront of exploration in extraction nanofibers from natural cotton fibers (Teixeira et al. 2010a, b, 2011). Suspensions from nanofibers were prepared by the acid hydrolysis of white and naturally colored cotton fibers, and the morphology was investigated by AFM and SEM (Teixeira et al. 2010a). A chemical-ultrasonic method was used to isolate cellulose nanofibers from four kinds of plant cellulose fibers, and the chemical composition, morphology, crystallinity, and thermal properties of the nanofibers and their intermediate products were characterized and compared (Chen et al. 2011). Chemical-physical properties of nanofibers isolated from rubber wood and shells of oil palm were also analyzed (Jonoobi et al. 2011). However, the morphological evolution of cotton fibers during hydrolysis has been rarely paid much attention, but it is very important to guiding the method for isolation of nanofibers from cotton fibers. Although both native cellulose and completely mercerized cellulose have been used extensively to produce cellulose nanofibers, partly mercerized cellulose, which may be the best starting material for conversion to nanofibers, has not been paid much attention.

In this work, cellulose nanofibers using partly mercerized cotton fibers as the starting materials were prepared by acid hydrolysis, under a joint effect of ultrasonic radiation and mechanical stirring. The structural evolution of the hydrolyzed cotton fibers was investigated, and the formation mechanism of cellulose nanofibers was discussed.

Experimental section

Materials and preparation

Chemical reagents and material: Sodium hydroxide (NaOH, 99 %), sulfuric acid (H2SO4, 98 %), hydrochloric acid (HCl, 37 %) and ethanol (C2H5OH, 98 %), purchased from the commercial market, were analytical grade reagents and were used without further purification. Cotton fibers used in this experiment were Xinjiang long-staple cotton (produced in Xinjiang, China).

Mercerizing process: Cotton fibers were completely cut into small pieces of 2-mm length, then mercerized in sodium hydroxide with a concentration of 5–25 % at 150 °C for 3 h to remove wax and grease. The mercerizing process made the cotton fibers swell and become more accessible to acid reagents. Finally, mercerized cotton fibers were washed using distilled water until the pH of the washing water reached 7.

Preparation of cellulose nanofibers: The above mercerized cotton fibers were hydrolyzed for 0–12 h at 80 °C in a mixture of 5 % H2SO4 and 5 % HCl, under a joint effect of ultrasonic and mechanical agitation, to form a suspension. The suspension was alternately washed and centrifugally separated at 10,000 r/min until the pH approached the range of 6–7. Finally, the products were washed using ethanol, then remained at room temperature first for 6 h followed by a drying process at 60 °C for 4 h.

Measurement and characterization

FTIR analysis: For Fourier transform infrared (FTIR) analysis, dried cellulose nanofibers were diluted with potassium bromide at a ratio of 1:100 and made into a KBr disk. This disk was measured using an IR Prestige-21 FTIR spectrometer in transmission mode.

X-ray diffraction analysis: X-ray diffraction patterns of all samples were obtained using a DX 2000 X-ray diffractometer with graphite-filtered Cu Kα (λ = 1.54 Å) radiation and analyzed using automatic powder diffraction (Jade 5.0) software. The diffracted intensities were recorded at 1.2°/min in the 2θ range of 5°–30°. The minimum intensity in the region of the recommended locations for the intensity of the amorphous fraction (18° 2θ for cellulose I and 16° 2θ for cellulose II) could not be measured correctly because of the severe overlap of the wider diffraction peaks (French and Santiago Cintron 2013). Therefore, the Segal crystallinity index was not used here to determine the crystallinity of the samples composed of cellulose I and II. Herein, the crystalline peaks of cellulose I, cellulose II and amorphous were separated from the fitted observed pattern using the Gaussian fitting function in Origin software. The amount of cellulose II was calculated on the basis of the separated area under the peaks of cellulose I and II. The degree of crystallinity (CI) was determined by comparing the areas under the crystalline peaks and the amorphous curve based on Borysiak’s method (Borysiak and Garbarczyk 2003). The CI formula is expressed as follows:

$${\text{CI}} = \left[ {\frac{{A_{c} }}{{A_{c} + A_{a} }}} \right]\text{ } \times \text{ }100$$
(1)

where A c (arbitrary units, a.u.) represents the area under the fitting crystalline peaks of cellulose I and cellulose II, and A a (arbitrary units, a.u.) represents the area under the amorphous backgrounds curve, respectively (see Fig. 1).

Fig. 1
figure 1

Diffractogram illustration of the crystallinity index determination of a mixture of cellulose I and cellulose II, obtained by partial mercerization in 15 % NaOH at 150 °C for 3 h

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FESEM observation: Morphology of the prepared samples was investigated by a 1530VP model field emission scanning electron microscopy (FESEM). The samples were attached to metal stubs using double-side adhesive tape and scanned at 5 kV. The surfaces of the samples were sputter coated with Au.

Results and discussion

XRD analysis

Figure 2 shows the XRD patterns of the mercerized cotton cellulose in NaOH solution with different concentrations. Three peaks at 2θ = 14.86° (1-10), 16.44° (110) and 22.75° (200) confirmed that only cellulose I was present in the native cotton cellulose. As the concentration of alkali increased, a significantly unique diffraction pattern assigned as cellulose II was observed. Cellulose I peaks were gradually replaced by peaks at 2θ = 12.18° (1-10), 2θ = 20.10° (110) and 2θ = 21.89° (020) (Fig. 2), indicating formation of cellulose II. This indicates that the native cellulose (cellulose I) was completely transformed into cellulose II through mercerizing with 25 % NaOH at 150 °C for 3 h. The pattern represents a transitional state of the partly mercerized cellulose, as observed in Fig. 2c. This shows that cellulose I was partially transformed into cellulose II after a mercerizing process with 15 % NaOH at 150° for 3 h. Figure 3 shows that the crystallinity of the partly mercerized sample is lowest between all mercerized samples. This implies that more amorphous materials occurred between cellulose nanocrystals during mercerizing. This characteristic structure of the half-mercerized cellulose (containing cellulose I and II) can be easily isolated into nanofibers by hydrolysis of amorphous materials between crystalline cellulose. In this experiment, partly mercerized cellulose from mercerizing with 15 % NaOH at 150° for 3 h was used for further acid hydrolysis.

Fig. 2
figure 2

XRD patterns of native cotton cellulose (a) and the mercerized cotton fibers in NaOH solution with concentrations of 10 % (b), 15 % (c), 20 % (d) and 25 % (e) for 3 h

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Fig. 3
figure 3

Relationship between the crystallinity of the mercerized cotton fibers and the NaOH concentration used for mercerizing

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Figure 4 shows that the intensities of all diffraction peaks of acid-hydrolyzed cotton fibers noticeably increased with increasing hydrolysis time until hydrolysis for 1.5 h and then stayed unchanging. Figure 5 shows a significant increase (about 15 %) in the crystallinity of the acid-hydrolyzed cellulose with increasing hydrolysis time (0–1.5 h) during the initial stage of the hydrolysis process, but it stayed almost unchanging in the latter stage. Although the amount of cellulose II in the hydrolyzed cotton fibers increased with hydrolysis time, it was not distinct. The amorphous parts were easily hydrolyzed and dissolved during hydrolysis compared to the crystalline phases. Comparing the crystalline phases, cellulose I could be hydrolyzed more easily than cellulose II.

Fig. 4
figure 4

XRD patterns of the partly mercerized cotton fibers through a certain hydrolysis time (0–6 h)

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Fig. 5
figure 5

Crystallinity and amount of cellulose II versus hydrolysis time, respectively

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FTIR results

Figure 6 shows the FTIR spectra of the partly mercerized cotton fibers through acid hydrolysis times of 50 min, 1.5, 3.5, 6 and 12 h, respectively. The strong hydrogen-bonded OH stretching vibration within the region of 3,000–3,600 cm−1 is shown in Fig. 6a. Intermolecular hydrogen bonding of O2–H–O6 for native cellulose (cellulose I) and O2–H–O6, O6–H–O6 and O2–H–O2 for mercerized cellulose (cellulose II) was shown at the 3,489, 3,445, 3,339, and 3,272 cm−1 positions, respectively. The OH stretching vibration was seen in the range of 3,000–3,600 cm−1, obviously decreasing with increasing hydrolysis time, but the intensity of other peaks in this range gradually increased. Decreased IR index values indicated that acid hydrolysis decreased the –OH groups on the crystal surface. Peaks in the range of 1,372–1,326 cm−1 could be attributed to >CO stretching of the acetyl ring. The peaks at 1,032 and 1,060 cm−1 could be attributed to >CO/C=C stretching vibration (Das and Chakraborty 2006). The absorbance of CH stretching at 2,901 and 2,890 cm−1 was assigned for native cellulose I and mercerized cellulose II, respectively. The absorbance at 1,429 and 1,370 cm−1 was assigned as CH2 bending and CH stretching. Peaks at 1,162 and 897 cm−1 were assigned to cellulose I and cellulose II, respectively. It could be seen that the band at 996 cm−1 was also attributed to cellulose II (Yue et al. 2012). There were C–O–C, C–C–O and C=C–H deformation modes and stretching vibrations in which the motions of the C-5 and C-6 atoms were at 894.37 cm−1. Comparing these spectra, no obvious difference could be found, except in the 3,000–3,600 cm−1 range. This indicates that the chemical composition of the remaining cellulose I and cellulose II nanofibers had no noticeable change after acid hydrolysis, and no preferential hydrolysis occurred for the samples composed of cellulose I and II.

Fig. 6
figure 6

FTIR spectra of the partly mercerized cotton fibers through acid hydrolysis for a different times, in the range of 3,600–2,700 cm−1 (a) and 1,450–850 cm−1 (b)

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FESEM observation

Figure 7 shows FESEM photographs of the as-hydrolyzed cotton fibers. The dimensions of the cotton fibers acid hydrolyzed for 50 min were almost identical to those of the partly mercerized cotton fibers used in the experiment. The circled area (partial region of cotton fibers) in Fig. 7a shows net-like or porous structures, implying a low crystallinity in these regions with an amorphous or semicrystalline structure. The polymer chains in these regions had a low degree of order and tended to be hydrolyzed easily by acid reagents at the beginning of hydrolysis. This also proves that the crystallinity and order degree of cellulose polymer were not consistent throughout the cotton fibers. The amplified image of the net-like region relating to Fig. 7a is shown in Fig. 7b. The net-like structure is composed of numerous microfibrils in an intertwining state, which have a length of approximately hundreds of micrometers and a width of 100–300 nm. Along the arrow direction in Fig. 7b, a microfibril was cut apart into several segments by narrow, black-line-like regions. In fact, the black-line-like regions were the amorphous or semicrystalline domains with a low resistance to acid reagents, also called a dislocation. Figure 7c, d shows that the cotton fibers hydrolyzed for 1.5 h were composed of a number of short cotton fibers and more net-like or porous structures built with numerous microfibrils with a length of several micrometers and a width of 100 nm. However, dislocation could not be observed in Fig. 7d. This implies that microfibrils had already been isolated apart along the dislocation regions, easily accessible to acid reagents. Traditionally, the cellulose superstructure has been described as crystalline, amorphous or froms between these two extremes based on the level of the orderliness of the cellulose fibrils. It is assumed that all microfibrils in a dislocation have similar orientations. This makes dislocations disconnected from the fiber cell wall sections rich in bordered pits, which also represent weak spots in fibers, in which cellulose microfibrils have no strong common orientation. A dislocation band is typically discernible orthogonally to the microfibril angle of the surrounding cell wall. The material properties of the cell wall dictate that a brittle crack-type failure is typically not seen when cell walls are exposed to excess mechanical stress in the absence of hydrolysis activity. Instead, fiber tearing and fibrillation due to the hierarchical and anisotropic structure take place, even when the failure is initiated within a dislocation. That is, the failure line typically does not follow a dislocation all the way across a fiber. Dislocations have long been known to be susceptible to acids. The hydrolysis rate is faster when the number of dislocations per unit length of fiber is higher. After acid hydrolysis for 12 h, cotton fibers were transformed into small pieces with dimensions of approximately several to dozens of micrometers in length (as shown in Fig. 7e). The hydrolyzed pieces were mainly composed of net-like structures. Figure 7f shows that the net-like structure is the twisting structure of cellulose nanofibers with a dimension of hundreds of nm in length and 70 nm in width.

Fig. 7
figure 7

FESEM photos of the cotton fibers hydrolyzed for 50 min (a, b), 1.5 h (c, d) and 12 h (e, f) under an acid concentration of 5 % H2SO4 and 5 % HCl and ultrasonic radiation

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The above results indicate that the crystallinity of the acid-hydrolyzed cotton fibers during hydrolysis from 1.5 to 12 h did not change, but the micromorphology and microstructure changed significantly. This implies that the amorphous region of cotton fibers was completely hydrolyzed after 1.5 h, and the structural change of the acid hydrolyzed cotton fiber was caused by hydrolysis of the crystalline region with more defects. Although the amorphous and disordered regions existing in cotton fibers or between the cotton microfibrils were completely hydrolyzed to isolate cellulose nanofibers, individually separated nanofibers could not be found in the 12-h hydrolysis samples; the net-like structure was still kept through the end of the polymer chain twisting between nanofibers. This means that the crystalline fibrils with more defects had a relatively low crystallinity compared to the bulk of nanofibers and could not be completely hydrolyzed at low acid concentrations. Figure 8 shows the morphologies of the acid-hydrolyzed cotton fibers hydrolyzed at a high acid concentration (in a mixed solution of 50 % sulfuric acid and 5 % hydrochloric acid) for 12 h. A number of individual cellulose nanofibers in a dispersive state are shown in Fig. 8, which have dimensions of 300–500-nm length and 70-nm width. The nanofibers have the same dimensions as the nanofibrils observed in Fig. 7f, which proves that the linking between the nanofibers was broken at a high acid concentration.

Fig. 8
figure 8

SEM images of the cotton fibers hydrolyzed at a high acid concentration (in a mixed solution of 50 % sulfuric acid and 5 % hydrochloric acid) for 12 h

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Based on the above analysis, the formation mechanism of cellulose nanofibers from cotton fibers was proposed as follows (refers Fig. 9). At the initial stage, amorphous or disordered regions existing between cotton microfibrils are quickly hydrolyzed under hydrogen cation attack and ultrasonic radiation; therefore, cotton fibers are broken into long microfibrils. At the second stage, acid reagents penetrate into the inner, dislocation regions or regions with more defects are steadily hydrolyzed. Microfibrils are broken into short microfibrils. Finally, the fine regions between fibrils with a relatively low crystallinity are further hydrolyzed. Therefore, a number of cellulose nanofibers remain. Ultrasonic radiation plays an important role in catalyzing the hydrolysis of hierarchical reactants during the whole process mechanically and chemically (Fig. 9).

Fig. 9
figure 9

Formation mechanism of cellulose nanofibers from cotton fibers by hydrolysis

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Conclusion

Cellulose nanofibers can be successfully fabricated from partly mercerized cotton fibers by acid hydrolysis. XRD results show that cellulose I was partially transformed into cellulose II by treatment with 15 % NaOH at 150° for 3 h. The crystallinity of this partially mercerized sample was lower than in the samples that were completely converted to cellulose II by higher concentrations of NaOH. The intensities of all of the diffraction peaks noticeably increased with increased hydrolysis time. FTIR results revealed that the chemical composition of the remaining nanofibers of cellulose I and II had no observable change after acidic hydrolysis, and there was no difference between the hydrolysis rates for cellulose I and II. The formation of cellulose nanofibers involves three stages: net-like microfibril formation, then short microfibrils and finally nanofibers.