Study of Structural Morphology of Hemp Fiber

Appl Compos Mater (2007) 14:89–103 DOI 10.1007/s10443-006-9032-9

Study of Structural Morphology of Hemp Fiber from the Micro to the Nanoscale Bei Wang & Mohini Sain & Kristiina Oksman

Received: 9 October 2006 / Accepted: 27 December 2006 / Published online: 30 January 2007 # Springer Science + Business Media B.V. 2007

Abstract The focus of this work has been to study how high pressure defibrillation and chemical purification affect the hemp fiber morphology from micro to nanoscale. Microscopy techniques, chemical analysis and X-ray diffraction were used to study the structure and properties of the prepared micro and nanofibers. Microscopy studies showed that the used individualization processes lead to a unique morphology of interconnected web-like structure of hemp fibers. The nanofibers are bundles of cellulose fibers of widths ranging between 30 and 100 nm and estimated lengths of several micrometers. The chemical analysis showed that selective chemical treatments increased the α-cellulose content of hemp nanofibers from 75 to 94%. Fourier transform infrared spectroscopy (FTIR) study showed that the pectins were partially removed during the individualization treatments. X-ray analysis showed that the relative crystallinity of the studied fibers increased after each stage of chemical and mechanical treatments. It was also observed that the hemp nanofibers had an increased crystallinity of 71 from 57% of untreated hemp fibers. Key words cellulose nanofibers . hemp . microfibrils . nanostructures . characterization

1 Introduction Lately, there has been considerable interests in the isolation and study of novel nanomaterials manufactured from renewable resources. An important class of nanomaterials has been nanofibers and fibrils from different cellulose sources and cellulose crystals B. Wang : M. Sain (*) : K. Oksman Centre for Biocomposites and Biomaterials Processing, Faculty of Forestry, University of Toronto Earth and Science Centre, 33 Willcocks Street, Toronto, ON M5S 3B3, Canada e-mail: [email protected] B. Wang e-mail: [email protected] K. Oksman Manufacturing and Design of Wood and Bionanocomposites, Luleå University of Technology, SE-93187 Skellefteå, Sweden

DO9032; No of Pages

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Appl Compos Mater (2007) 14:89–103

(whiskers) [1–4]. These novel nanofibers, fibrils and crystals have been shown to result in unique properties when incorporated in different polymers [1, 5–8]. The sources of these nanomaterials have been wheat straw, bacterial cellulose, kraft pulp, sugar beet pulp, potato, and swede root [2, 5, 7–10]. Since 1998, Canada has grown industrial hemp for seed and for fiber. Interest in hemp arises from the plant’s amazing versatility. The seeds are used to produce healthy food, nutraceuticals, and bodycare products and the stalk is starting to be processed into high performance fiber products such as paper, textiles, biocomposites and building materials [11]. The chemical constituents of the hemp plant’s cell wall consist not only of cellulose, but also of hemicellulose, pectin and lignin. The properties of each constituent contribute to the overall properties of the fiber [12]. The smallest building element of the cellulose skeleton is considered by some to be an elementary fibril. The fibril can be about 5–10 nm in diameter and its length varies from 100 nm to several micrometers depending of the source of cellulose [13]. The cellulose molecules are always biosynthesized in the form of nanosized fibrils; up to 100 glucan chains aggregate together to form cellulose nano-sized microfibrils or nanofibers [14–17]. The mechanical performance of cellulose nanofibers in terms of the tensile strength and Young’s modulus is comparable to other engineering materials such as glass fiber, carbon fiber, etc. Therefore, the cellulose nanofibers can be considered to be an important structural element of natural cellulose in a number of applications such as plastic reinforcement, gel forming and thickening agents [3, 18, 19]. Furthermore, a cellulose nanofiber has more than 200 times the surface area of isolated softwood cellulose [20] and possesses higher water holding capacity, higher crystallinity, higher tensile strength, and a finer web-like network. In combination with a suitable matrix polymer, cellulose nanofiber networks show considerable potential as an effective reinforcement for high quality specialty application of bio-based composites. Another type of nanoreinforcement that can be obtained from cellulose fibers are nanowhiskers. The elementary fibril is made up of amorphous and crystalline parts. The crystalline parts can be isolated by various treatments producing the cellulose nanowhiskers [4, 21]. Many studies have been done on extracting cellulose microfibrils from various sources and on using them as reinforcement in composite manufacturing [1, 2, 6, 7, 22–24]. These microfibrils can be extracted from the cell walls by three types of isolation processes: simple mechanical methods, a combination of chemical and mechanical methods, or an enzymatic approach. A purely mechanical process can produce refined, fine fibrils several micrometers long and between 20 to 90 nm in diameter [25]; however, this nano-scalar web-like structure of fibrils causes a reduction of strength [5]. In contrast, chemi-mechanical treatments can extract cellulose nanofibers from the primary and secondary cell walls without degrading the cellulose. A chemi-mechanical process can also achieve finer fibrils of cellulose, ranging between 5 and 60 nm diameter [1, 7]. Depending upon the raw materials and defibrillation techniques, the degree of polymerization, morphology and aspect ratio of the nanofibers will differ. The separation of nanoreinforcement from natural materials and the processing techniques have been limited to laboratory scale [13]. Therefore, it is important to develop new processing techniques which will be at use in large scale production. Removal of lignin left after chemical treatment of fibers is the goal of the bleaching process. Chlorine-based processes still dominate, but more environmentally friendly nonchlorine processes are becoming more prevalent. A bleaching treatment using a sodium chlorite solution was performed to remove phenolic compounds or molecules having chromophore groups, in order to whiten the fibers [1]. This is a popular technique at the laboratory scale to remove lignin from plants. Lignin is rapidly oxidized by chlorine and chlorites. Lignin oxidizing leads to lignin degradation and to dissolution in an alkaline


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medium [1]. A further treatment is often required to fully bleach the suspension. Removal of a part of the flax fibers’ noncellulosic compounds by sodium chlorite was reflected in the mechanical and physical characteristics of the surface state [26]. Key issues are the size and the dispersion of the nano-sized reinforcements and the effect of this fine structure on fiber properties. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) aid the interpretation of structures from the nm to the μm size scale. Typical information obtained from conventional TEM is length, aspect ratio, shape and the aggregated or isolated state of fibers [27]. Thin evaporative carbon coatings were used for TEM sample preparation in this study. Recently, AFM has also been used to examine plant cell walls at a similar resolution to that of the TEM [28–30]. This type of microscopy has the important advantage of reducing the risks of introducing artefacts resulting from the preparative techniques. In order to investigate the potential of cellulose nanofiber as a reinforcement in polymer composites, this study was focused on the development of a new isolation technique to extract cellulose nanofibers from hemp by chemi-mechanical treatments. This research aims to clarify how the various levels of high pressure defibrillation affect the morphology from long hemp fiber towards nano-scale fibrillated cellulose and to compare the morphology of bleached and unbleached fibers at the different stage of the individualization of the nanofibers. The structural details were studied with SEM, TEM and AFM. The crystallinity was determined before and after different stages of the chemi-mechanical treatments of hemp fibers. The changes of the chemical composition of fibers after different treatments were studied. Infrared measurements were performed to identify the removal of the pectins. Further research work is required for incorporating these nanofibers into a polymer matrix to evaluate the mechanical properties of nanocomposites.

2 Experimental 2.1 Materials and Methods The raw material used in this study was hemp fibers (Cannabis sativa L.) from southwestern Ontario, Canada (Hempline, Ontario). These fibers have diameters of approximately 22–25 μm and lengths of 15–25 mm. Reagent grade chemicals were used for fiber surface modifications and bleaching, namely, sodium hydroxide, hydrochloric acid, sodium chlorite, chlorine dioxide, peroxide and sulfuric acid. 2.2 Individualization Process The individualization process of nanofibers is a multi-step process, shown in Fig. 1. Chemical and mechanical treatments together are applied onto the hemp fibers to individualize nanofibers. The chemical treatments include pre-treatment, acid hydrolysis, alkaline treatment and bleaching. The mechanical treatments include cryo-crushing by liquid nitrogen and high-pressure defibrillation [1]. 2.2.1 Chemical Treatments The main objective of the chemical treatments was to remove the starch, hemi-cellulose, lignin/pectins surrounding cellulose. Generally, the first step for all of the fiber surface

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Raw Material (hemp) Successive Bleaching Pretreatment (12% w/w NaOH, 2h) Chlorine Dioxide Stage Acid Hydrolysis (1M HCl, 80°C, 1.5h) Extraction Stage Alkaline Treatment (2 % w/w NaOH, 2h, 80°C) Acid Stage

Cryo-crushing in Liquid Nitrogen Peroxide Stage

High Pressure Defibrillation Fig. 1 Isolation of nanofibers

treatments is mercerization (pretreatment process) which will change the crystal structure of cellulose. The essence of mercerizing hemp is that in the swelling of cellulose fibers due to exposure to alkalis, the natural crystalline structure of the cellulose relaxes and under an appropriate tension, the dimensions can be set by the conditions [31]. Hemp fibers were soaked in a sodium hydroxide solution of 12% w/w at room temperature for 2 h, enabling chemical molecules to penetrate through the crystalline region of the cellulose. Acid hydrolysis with 1 M hydrochloric acid followed by alkaline treatment with 2% w/w sodium hydroxide was applied to remove the undesired components. After the successive chemical treatments, lignin was still remained within the fibers and removed by multi-stage bleaching. The bleaching was done in four different stages: 1) Chlorine dioxide stage (D), where the fiber consistency was adjusted to 3.5%. Sodium chlorite solution was applied based on the Kappa number of the fibers. Therefore the Kappa number was determined and then chlorine dioxide was calculated based on lignin content in the sample. The retention time was 1 h. 2) Extraction stage (E), where the consistency was adjusted to 10% using boiling water. Sodium hydroxide and peroxide was added to the fiber stock based on 1% OD fiber under mechanical stirring. 3) Acid stage (A), where the consistency was adjusted to 4% and sulfuric acid was added to fiber mixing well for 1 h. 4) Peroxide stage (P), where the consistency was adjusted to 10% and peroxide was added to fiber. After filtration, the fiber was washed and air dried.


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Process for chemical pulping was obtained from literature [32]. However, at this purification level, microfibrils are not individualized and further steps of mechanical treatments are needed. 2.2.2 Mechanical Treatments The hemp fibers were first cryo-crushed with liquid nitrogen to reduce the length and size. The objective of the cryo-crushing is to form ice crystals within the fiber cell wall. When high impact is applied on the frozen fiber, ice crystals exert pressure on the cell wall, causing a rupture and thereby liberating the microfibrils [33]. The chemically treated and cryo-crushed fibers were then diluted in water and dispersed evenly in a disintegrator (Cramer) for 10 min. The disintegrator is used to disperse the fibers uniformly in the water suspension before the high pressure defibrillation process. The water suspension with higher concentration of fibers (1–2%) was subsequently passed through the defibrillator (purchased from GEA-modified by the Centre for Biocomposites and Biomaterials Processing Laboratory, University of Toronto, Ontario). The pressure was above 500 bar and several passes were needed to crush the cell wall and fully release the nanofibers. The detailed method for mechanical treatment to produce the nanofibers is described in recent patents [33, 34]. 2.3 Microscopy Characterization Scanning electron microscope (JEOL JSM-840, Tokyo, Japan) was used as a routine for microstructural analysis of the fibers after various stages of chemical and mechanical treatments. All images were taken at an accelerating voltage of 15 kV. The sample surfaces were coated with a thin layer of gold on the surface using an Edwards S150B sputter coater (BOC Edwards, Wilmington, MA) to provide electrical conductivity. Transmission electron microscopy (TEM) observations were achieved with a Philips CM201 (Philips, Eindhoven, The Netherlands) operated at 80 kV. A drop of a dilute cellulose nanofiber suspension was deposited on carbon-coated grids and allowed to dry. Atomic force microscope (AFM) study was obtained using a digital instruments dimension 3100 AFM (Veeco Metrology Group, Santa Barbara, CA) with a nanoscope IIIa controller. The system was operated in tapping mode at room temperature with DI tapping mode tips having a resonant frequency of 280 kHz. A droplet of the aqueous nanofiber suspension was allowed to dry on a cleaved mica surface. 2.4 Chemical Characterization of Fibers Over the different stages of nanofiber development, untreated hemp fibers, acid/alkali treated fibers, bleached fibers and nanofibers were chemically analyzed for hemicellulose, lignin and cellulose contents. The procedure used here for cellulose determination was given by Zobel et al. [35]. Lignin content was determined based on Tappi T 222 om02.2002 (acid-insoluble lignin in wood and pulp) and Tappi useful method UM250 (raw material and pulp-determination of acid-soluble lignin). 2.5 Spectroscopy Fourier transform infrared spectroscopy (FTIR), Tensor™ 27, Bruker Optics, Billerica, MA) was used to identify the removal of pectins at different purification levels by measuring the transmitted radiation of various infrared light wavelengths of pectin

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functional groups in the sample. The Tensor 27 standard FTIR spectroscopy was used to obtain spectra for the fibers after each chemical treatment. Fibers were ground and mixed with KBr (sample/KBr ratio, 1/99) to prepare pastilles. FTIR spectra were recorded in a spectral range of 4,000–400 cm−1 with a resolution of 4 cm−1. 2.6 X-Ray Analysis The crystallinity determination was made using a powder X-ray diffraction method (PXRD). X-ray crystallography was carried out to investigate the relative crystallinity after various stages of the chemi-mechanical treatment and of nanofibers obtained by air drying of the nanofiber suspension. A D8 advance Bruker AXS diffractometer (Bruker AXS, Madison, WI), Cu point focus source, graphite monochromator, and 2D-area detector GADDS system were used. Samples were analyzed in transmission mode.

3 Results and Discussion 3.1 Individualization of Hemp Nanofibers The individualization step of nano-sized fibers from the plant cell walls requires chemical and mechanical treatments. The properties of the nanomaterials cannot be physically measured without separating them from the plant cell wall. These treatments may result in significant chemical or mechanical damage on the fibers. Figure 2 shows how the fiber morphology is changed from the micro to the nanoscale during the individualization process. Figure 2a shows an untreated hemp fiber bundle where the individual fibers are bound together by lignin. The size of the bundle is around 100– 200 μm. In Fig. 2b, it is clearly visible that the chemical treatments are reducing the bundle size and the surface roughness compared to the fibers in Fig. 2a. Figure 2c shows how the morphology is affected by the cryo-crushing. This process imparted sufficient energy to break the bundles into single fibers which are around 20 μm in width. In Fig. 2d, the single fibers are defibrillated showing a web-like structure. No individual hemp fibers are visible after the defibrillation step and the size is reduced to the nanometer level. The fibrillar structure of individual fibers was revealed from the Fig. 2e after the fiber bleaching and may be due to the leaching out of waxes and pectic substances. In Fig. 2f, it was observed that the diameter of cryo-crushed fibril with bleaching is much smaller compared to cryocrushed fibril without bleaching. High pressure defibrillation provided high turbulence and shear that created an efficient mechanism of reduction in size. Figure 2g shows the structure after the high pressure defibrillation, showing nanoscale fibrils and microfibril bundles contributed a unique morphology of the interconnected web-like structure of fibrils. This combination of forces promoted a high degree of microfibrillation of cellulose fibers, resulting in cellulose nanofibers. Figure 3 demonstrates how the number of passes through a defibrillator is affecting the individualization of hemp nanofibers. Figure 3a shows the morphology after five passes which did not result in nanofiber structure. The fibers were still entangled with each other and the size was in the range of microns. In Fig. 3b, after 10 passes, the fibers were split apart into smaller bundles. Figure 3c and d show a large extent of defibrillation after 15 and 20 passes, these small bundles were additionally separated into thinner fibril bundles increasing the exposed surface area of the cellulose (Fig. 3d). High pressure and high energy were needed to defibrillate hemp fibers and achieve acceptable dispersion level. The


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Fig. 2 Scanning electron micrographs of: a untreated fiber, b after acid and alkaline treatment, c after cryocrushing, d after defibrillation, e after bleaching, f bleaching followed by cryo-crushing, g bleaching followed by cryo-crushing and defibrillation

separated fiber bundles were shown to create small entanglements that were fibrillated into smaller entities as the number of passes through the defibrillator was increased [5]. Figure 4, compares the structure of unbleached and bleached nanofibers. Figure 4a shows that the unbleached nanofibers were either individual or bundles and coarser compared to the bleached nanofibers, shown in Fig. 4b. The Fig. 4b shows that bleached nanofibers were thinner and shorter than unbleached nanofibers in diameter and length. It is likely that harsh chemicals used for bleaching will reduce the chain length of the cellulose which can result in cutting the length and weakening cellulose nanofibers. Aspect ratio (fibril length to diameter ratio) is one of the most important parameters in determining reinforcing capability of the nanofibers. Aspect ratios of the extracted cellulose nanofibers were estimated from transmission electron micrographs. In some cases total fibril length was not visible, therefore only the visible portion was considered for the calculation in TEM graphs (provide statistical significance of this assumption). The aspect ratio of these bleached nanofibers (82) is comparable to unbleached nanofibers (88). We can expect a high reinforcing capability from both nanofibers. There is a direct relation between degree of polymerization and length of the nanofibers, as cellulose synthesizes in extended chain conformation. Degree of polymerization (DP) of nanofibers was calculated using intrinsic viscosity method (ASTM D1795–96). Unbleached nanofiber has a similar valve of DP (1,155) compared with that of bleached nanofibers (1,138). The nanofiber suspension obtained after the high-pressure defibrillation was also analyzed to determine the width using AFM, shown in Fig. 5. It is seen that the fibers are indeed nano-sized and the width is within the range of 30–100 nm. The length is estimated to be at micrometer level. The network of nanofibers can also be seen in force mode image

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Fig. 3 Scanning electron micrographs of: bleached hemp nanofibers after 5 (a), 10 (b), 15 (c) and 20 (d) passes during the defibrillation

Fig. 5a by interwoven microfibrils overlapping each other. As evident from the TEM and AFM images, high-pressure defibrillation leads to individualization of the cellulose nanofibers from the cell wall without degrading them. Figure 6 shows the width distribution of the nanofibers obtained through chemi-mechanical treatments. In Fig. 6a, it was observed that the widths of unbleached nanofibers were estimated between 50– 100 nm and most of them had a diameter range of 70 to 100 nm. Bleached nanofibers

Fig. 4 Transmission electron micrographs of hemp nanofibers (a) unbleached, (b) bleached under the same magnification (15,000×)


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Fig. 5 Atomic force micrographs of unbleached hemp nanofibers a force mode image, b height mode image

(a) Frequencyof Width

0.4 0.3 0.2 0.1 0 Width Range (nm)

(b) Frequencyof Width

Fig. 6 Size distribution of hemp nanofibers a unbleached, b bleached

<30

30-50

<30

30-50

50-70

70-100

>100

0.5 0.4 0.3 0.2 0.1 0 Width Range (nm) 50-70

70-100

>100

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Table 1 Chemical analysis of hemp fibers after selective chemical treatments

Untreated fibers Acid treated Acid and alkaline treated Bleached Nanohemp

Holocellulose (%)

α-cellulose (%)

Hemicellulose (%)

Insoluble lignin (%)

Soluble lignin (%)

Total lignin (%)

86.22 91.42 92.82

75.56 85.66 89.78

10.66 5.76 3.04

4.89 4.49 4.40

1.72 0.66 0.53

6.61 5.15 4.93

95.72 96.12

93.87 94.53

1.85 1.59

2.83 2.53

0.35 0.18

3.18 2.71

produced smaller widths (30–100 nm) compared with that of unbleached nanofibers as shown in Fig. 6b. Most of bleached nanofibers had a diameter range of 30–50 nm. By the fiber image statistical analysis, the average width of unbleached nanofiber was 87.5 nm and of bleached nanofiber was 54 nm based on averaging 50 individual fibers. It is expected that better reinforcing effect will be obtained by using bleached nanofibers. It is clearly shown from The TEM picture (Fig. 4) and the fiber size distribution chart (Fig. 6) that the size is less in bleached nanofibers, but more surface areas will be contributed from these fibrils. 3.2 Chemical Characterization of Individualized Nanofibers The fibers obtained after the chemical treatment contained mainly alpha-cellulose with some hemi-cellulose and lignin. As shown in Table 1, the α-cellulose content in the chemically treated fibers was 94% as compared to the original 75% and the hemicellulose content was reduced to 1.6%. Chemical analysis of these fibers after each stage of the purification showed a drastic increase in cellulose content and a decline in hemicellulose and lignin content. Lignocellulosic fibers contain a bit amount of hemicellulose, which is a hetero polysaccharide consisting mainly of pentoses and hexoses. The treatment of cellulosic, starch, or hemicellulosic materials using acid solution to break down the polysaccharides to simple sugars allows the solubilization of both pectins and hemicelluloses. Dilute sodium hydroxide treatment of lignocellulosic fibers causes separation of structural linkages between lignin and carbohydrate and disruption of lignin structure [32]. Fibers samples from hemp appeared brownish in color even after carrying out the acid and alkali treatments. Chemically treated hemp fibers were bleached before proceeding to the mechanical treatment to ensure that most of the lignin was removed from the fibers. After bleaching treatment, it was found that nanofibers contain both soluble and insoluble lignin. The lignin content of hemp significantly decreased from 6.6 to 3.18%. According to the result of successive bleaching extractions, the nanofibers lost most of their non-cellulosic constituents. Cellulose can be also partially degraded during these bleaching. Chemical treatments lead to almost pure cellulose fibers, which ensure the high stiffness and strength. The mechanical behavior of nanohemp reinforced composites changes as a function of hemp cellulose microfibrils purity level [5]. Although cellulose possesses excellent strength and good stability, it can be partially degraded due to the harsh chemical and mechanical treatment. The alkali extraction is expected to hydrolyze pectin by a βelimination process and solubilize it [1, 7]. Figure 7 shows the transmission electron micrographs of hemp nanofibers under two controlled concentrations of NaOH solution for the fiber pretreatment. 17.5% of sodium hydroxide induced undesirable reactions to cut down the cellulose chains, therefore reducing the aspect ratio of nanofibers. Too harsh


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Fig. 7 Transmission electron micrographs of hemp nanofibers a under 17.5% alkali extraction, b under 12% alkali extraction (15,000×)

Transmittance (%)

treatments led to the loss of the microfibrillar morphology as seen in Fig. 7a. In Fig. 7b, it was shown that the hemp fibers released their nanofibers either individually or in bundles at a lower alkali content leading to the formation of a strong network of microfibrils. Pectin is a heterogeneous grouping of acidic structural polysaccharides. The untreated, chemically treated and bleached hemp fibers were characterized by FTIR, shown in Fig. 8. By this technique, it was possible to follow the removal of pectins due to the vanishing of

1739 1259 895 1631

400

600

800

1000

1200

1400

1600

1800

2000

2200

Wavenumber cm-1 Untreated

Chemically treated

Bleached

Fig. 8 FTIR spectra of hemp fibers a untreated, b after acid and alkaline treatment and c after bleaching

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the characteristic bands for the carboxylate groups at 1,740 cm−1, acetyl and methyl ester group at 1,590 and 1,240 cm−1 [7]. In the transmittance spectra, the absorption band assigned to the pectin carboxylic groups was observed at 1,739 cm−1 in untreated fibers, but disappeared upon chemical treatments and the successive bleaching. This is because the carboxylic groups were partially removed by alkali treatment through a process called deesterification. During de-esterification, the ester groups on the pectin can be removed as well. The alkali treatment allows the ionization of pectin carboxylic groups (–COOH) and the formation of the corresponding sodium carboxylate (COONa), which decreases the ability of hydrogen-type intermolecular bonds to form [7] and the solubility of the pectins. Reduction in the peak intensity found at around 1,631–1,633 cm−1 in chemically treated and bleached fibers indicates the partial reaction of the C=O bonds of hemicelluloses. The intensity of the 1,259 cm−1 peak is sharply weakened after the bleaching treatment, due to the removal of hemicellulose materials. The peak observed at 895 cm−1 in both untreated and chemically treated fibers indicates the presence of the glycosidic linkages between the monosaccharides and disappears in the spectrum for bleached fibers. 3.3 X-Ray Diffraction

Intensity (a.u.)

X-ray crystallography was used to investigate the crystallinity of the sample after different treatments. X-ray powder diffraction photographs from untreated, acid and alkali treated fibers and nanofibers are shown in Fig. 9. The percentage crystallinity of these samples was calculated based on X-ray analysis by Eq. 1 and they are given in Fig. 9 as well. The main diffraction intensity was at about 2θ=21° for each sample. The peak observed close to 2θ= 22.4° is from cellulose. Untreated fiber exhibited very low crystallinity (57.4%) and a

10

15

20

25

30

35

Diffraction angle 2θ (degree) Untreated (57.4%) Acid & Alkaline treated (69.7%)

Acid treated (61.9%) Nanofiber (71.2%)

Fig. 9 X-ray diffractometry and crystallinity estimation after each stage of chemo-mechanical treatment for unbleached hemp fiber


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single peak at about 2θ=22.75° and a broad hump showing amorphous nature. It can be seen that acid treated and acid/alkali treated fibers show peaks at 2θ=22.8° and 2θ=21.4° respectively. In the case of acid treated fibers, an additional peak was seen at 2θ=20.4° with a small hump. It is possible that acid treated fiber contains some residual lignin and hemicellulose that contribute to the slightly lower crystallinity (61.9%). Further the hump has disappeared showing that amorphous chains have been rearranged to crystalline regions. Acid/alkali treated fiber shows only one prominent peak and exhibits more crystalline nature (69.7%) than acid treated fiber. Nanofibers showed a high crystalline feature with a narrow strong peak at 2θ=21.6°. This peak can be attributed to cellulose crystals. It was observed that the hemp nanofibers had an increased crystallinity from 57.4% of untreated hemp fibers to 71.2% of nanofibers. As shown in Fig. 9, the relative crystallinity of the samples increased after each stage of chemical treatments. Consecutive chemical treatments and processing of cellulose fibers give different X-ray patterns. According to X-ray testing on cellulose, cellulose is not made up of single perfect crystals. Disordered cellulose molecules as well as hemicelluloses and lignin are located in the spaces between the microfibrils. The hemicelluloses are considered to be amorphous although they apparently are oriented in the same direction as the cellulose microfibrils. Lignin is both amorphous and isotropic. It is believed that these crystallites are connected to each other by disoriented amorphous zones [36]. The crystalline nature of the cellulose nanofibers is not only influenced by the chain conformation but also by the packing of adjacent chains. Nanofibers are pure cellulose chains having different arrangements of the glucose chains as in the native cellulose of untreated fibers. The hemp nanofiber, after successive chemical treatments, only possessed 4.3% lignin and hemicellulose combined. The X-ray powder diffraction pattern showed a narrow peak which was more prominent and sharp for nanofiber, indicating the crystalline nature of this reinforcement and higher relative crystallinity. C; % ¼

Icrystalline 100 Icrystalline þ Iamorphous

ð1Þ

4 Conclusions This study has been concerned how the degree of individualization affects the hemp fiber morphology from the micro to the nanoscale. The chemi-mechanical process resulted in hemp nanofibers having a width in the range of 30–100 nm. The used chemical treatments resulted in the individualized hemp microfibers and further mechanical treatment formed a network structure of hemp nanofibers. The high pressure defibrillation contributed a unique morphology of the interconnected web-like structure of nanofibers. Chemical analysis of the cellulose fiber after each stage of purification showed an increase in cellulose content and a decrease in lignin and hemicellulose content. Successive bleaching helped with the cellulose purification. FTIR graph indicated the partial removal of the pectins during the fiber extraction. It was also seen that the relative crystallinity of the hemp fibers increased after each stage of chemical treatments. Acknowledgment We gratefully acknowledge financial support of this study given by NSERC (Natural Sciences and Engineering Research Council of Canada).

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Study of Structural Morphology of Hemp Fiber

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