4.2.1 Infrared spectroscopy

Although the infrared portion of the electromagnetic spectrum covers 14000–10 cm ^− 1, the majority of analytical applications use the mid-infrared range, approximately 4000–400 cm^− 1. Many different techniques are currently available and only those related to the scope of this chapter will be summarized below:

4.2.2 Raman spectroscopy

when a sample is irradiated with a monochromatic radiation of high intensity, such as solid lasers, a fraction of the radiation is scattered and the shifts produced with respect to the wavelength of the incident beam depend on the chemical structure of the molecules. Although Raman spectra and infrared spectra somewhat resemble each other (the peaks corresponding to vibrational modes active in both spectroscopies have the same energy shifts, that is, they appear at the same wavenumbers); the differences are also important, the relative intensity of the peaks differs and in some instances some peaks may not appear. Differences with infrared spectroscopy are not surprising because the mechanisms involved are also different, change in dipolar moment for FTIR-active groups and change in polarizability for Raman-active groups, and these differences make them complementary rather than competitive techniques. In particular, raman spectroscopy is very useful for samples containing water, because water does not cause interference in raman, as it strongly does in infrared spectroscopy. Another useful advantage is that glass or quartz cells can be used instead of salt crystals, which are environmentally unstable and fragile. As for FTIR spectroscopy, various methods are available in Raman spectroscopy whose selection will depend on the sample characteristics and the specific aims of the characterization.

4.3.1 Characterization of cellulosic materials

Lignocellulosic fibers consist of bundles of hollow cellulose fibrils. Their cell walls are reinforced with spirally oriented cellulose in a hemicellulose and lignin matrix. Therefore, the cell wall is a composite structure of lignocellulosic material reinforced by helical microfibrillar bands of cellulose. The composition of the external surface of the cell wall is a layer of lignaceous material and waxy substances that bond the cell to its adjacent neighbors (Li et al., 2000; Jacob et al., 2005). These components are the ones to be considered in the study of the interaction with polymer matrices. Thus, to discuss their spectra and modifications or their interactions with different polymers, one should begin by identifying their corresponding absorption bands. Table 4.1 summarizes some FTIR peak assignments for lignocellulosic materials (Marcovich et al., 1996a; Bessadok et al., 2009; Jayaramudu et al., 2009).

On the other hand, Fig. 4.2 is a good example of the information that can be obtained from different techniques and modes applied to a non-treated cellulose paper (Proniewicz et al., 2001). The ATR technique (Fig. 4.2a) is a non-destructive method that requires no sample preparation. The results are collected from a region between the surface and a depth of about 0.5–5 μm depending on the sample characteristics. Intimate contact between the surfaces of the sample and the crystal is required, thus information can vary with sample topology, because of the interfering signal of interstitial trapped air and owing to the beam penetration in the sample.

The older method of transmission FTIR can be used on a neat sample if it is transparent or thin enough to ensure a relatively large percentage of the beam goes through the sample. In Fig. 4.2 b and c the differences found by using pellets prepared from the neat sample and that diluted with KBr pellets are shown. Thus, when the sample is too concentrated or too thick (Fig. 4.2b), some high absorbent regions show essentially zero transmission. That is the case for the 2600–3500 cm^− 1 region, corresponding to the C–H and O–H stretching vibrations, and 1000–1300 cm^− 1 region, which comprises vibrations from the C–C stretching and the COH and CCH deformations. Usually, samples are diluted with KBr and pressed into a pellet to eliminate the problem of highly absorbent samples. However, this method is destructive and time consuming because the cellulose has to be in the form of a fine powder and well dispersed into the salt in order to avoid spectral artifacts. When sampling plant fibers, the pressing step can also have the added disadvantage of destroying the cell walls, exposing internal groups, in particular hydroxyl groups that were originally not available. Thus, a difference can be expected in the -OH bands if other not so invasive methods are utilized.

Similarly, Marcovich et al. (1996a) discussed the effect of sample concentration when using transmission and DRIFT modes for studying sawdust samples. They found that the opacity of the pellets limited the use of the transmission technique to weight concentrations equal or lower than 2% by weight in KBr. However, the DRIFT technique applied to neat samples gave good spectra with detailed band structure. Moreover, Fig. 4.3 illustrates that the frequency at which the bands appear is independent of the methods used, but the intensity depends strongly on the technique selected for collecting the spectra or preparing the sample. For example, the strong absorption band at 3000–3500 cm^− 1 (corresponding to –OH absorption) is highly dependent on the sample preparation, as well as the spectroscopic method selected: it appears with higher intensity in the transmission spectra, for which a KBr pellet was prepared by pressing. Although DRIFT requires also grinding of the sample (as in transmission), it is not necessary to apply pressure hence the band appears with much lower intensity. Similarly, the peak at about 1650 cm^− 1 corresponding to absorbed water appears clearly in transmission but not so much in DRIFT. Finally, the region between 900 and 1250 cm^− 1 looks similar for the diluted samples under the two FTIR methods, although the DRIFT trace of the neat sample shows distortion of the main absorption band (around 1100 cm^− 1).

The FT Raman technique displays a more direct correlation between band and bonds. However, it is not very sensitive to hydrogen bonds, as can be seen from the very low intensity of the band in the region of 3000 to 3500 cm^− 1 (Fig. 4.2d). Similarly the band at around 1625 cm^− 1, which appears in the infrared spectra and is assigned to the bending motions of water molecules, is almost missing in the Raman spectra.

4.3.2 Characterization of modified fibers (and nanofibers)

As already mentioned, the high-modulus, low-cost, lightweight characteristics associated with natural fibers make them attractive as polymer reinforcements. However, the performance of a composite depends not only on the properties of the reinforcement and matrix, but also on the bonding between them. A poor fiber–matrix interaction derived from polar fibers and, typically, non-polar polymers leads to composites with poor mechanical properties and low durability when exposed to aggressive environments. Thus, to improve interfacial adhesion between reinforcement and polymer, the fiber surface is frequently modified or coupling agents are included in the composite formulation. The hydrophilization of non-polar polymeric matrices is also possible, but most often treatments are performed on the reinforcements (Gauthier et al., 1998a; Nuñez et al., 2003). Lignocellulosic materials contain surface hydroxyl groups that can be bonded to molecules that have functional groups selected to improve polymer compatibility. The use of FTIr and raman spectroscopies in the analysis of the most frequently used chemical modifications is summarized below.

4.4.1 Alkaline treatment

The alkaline treatment, also called mercerization, involves the immersion and soaking of the fibers in an alkaline medium, typically a rather concentrated NaOH aqueous solution. This treatment removes practically all non-cellulose components except waxes (Idicula et al., 2006; Luz et al., 2008). Owing to the dissolution of lignin by alkali, some pores are formed on the fiber surface. The loss of the cementing material of the vegetable fibers produces some fibrillation because of the breakage and separation of the fiber bundles (Aranguren and Reboredo, 2007). These changes increase the contact area between the fiber and the matrix (Idicula et al., 2006).

The effects of the alkaline treatment on fabrics of Polyalthia cerasoides tree, palm fibers, sisal fibers, hemp and kenaf fibers, henequen fibers and wood flour were addressed by Alawar et al. (2009), Jayaramudu (2009), Marcovich et al. (2005) and Rahman (2009), Rong et al. (2001), Sgriccia et al. (2008), among others. Using FTIR and chemical analysis, these studies confirmed the reduction of the hemicellulose and lignin contents after fiber mercerization. The major change in the FTIR spectra of the fibers owing to the alkaline treatment was the reduction (or sometimes the complete disappearance) of the peak centered at 1730–1740 cm^− 1, attributed to the carbonyl group (C==O) stretching. This disappearance is a consequence of the extraction of hemicellulose and lignins and/or the formation of ionic carboxylates in the incompletely extracted samples, in which instance the corresponding peak appears at lower frequencies (1590 cm^− 1). It was also established that the peak located at 1240 cm^− 1, assigned to the C–O bond of the acetyl group in xylan, separated into two smaller peaks at 1260 and 1230 cm^− 1. The first one corresponds to vibrations in the guaiacyl structure of the lignin and the second to the syringyl structure (Marcovich et al., 2005; Reddy et al., 1990; Roy et al., 1991). Since NaOH modification happens to increase the amount of accessible polar hydroxyl groups from the cellulosic fibers (Marcovich et al., 1998; 1999), the water sensitive regions of the FTIR spectrum become useless to detect other changes. In this sense, FT Raman spectroscopy results are very attractive because water does not interfere with the sample signal. This advantage was interestingly used by Jähn and coworkers (2002) to study the extent of the polymorphic transformation of cellulose I into cellulose II as a function of the alkali concentration. They detected the polymorphic transformation of the cellulosic fine structure of the flax fibers in vivo by analyzing the FT Raman spectra at frequencies below 1500 cm^− 1, as denoted in Fig. 4.4. The Raman lines characterizing the cellulose modification I are denoted by an asterisk whereas the typical Raman lines of modification II are denoted by a cross. Particularly, in the range of the methylene bending vibrations (1400–1475 cm^− 1) the spectra show the differences between the conformational arrangements of the side chains of the anhydroglucopyranose residues of cellulose. They indicate the simultaneous presence of two stereochemically non-equivalent –CH₂OH groups in cellulose I (1476 and 1455 cm^− 1). In cellulose II, only one type of –CH₂OH group is present (1460 cm^− 1) and the two scissoring vibrations of the methylene groups merge into one signal. on the other hand, the Raman lines at 1120 and 1098 cm^− 1 assigned to the skeletal vibrational modes v_s (C–O–C) and v_as (C–O–C) of the β(1 → 4) glycosidic linkages of the β-D-glucopyranosyl units of cellulose can serve as characteristic marker bands for multicomponent systems such as cellulosic plant tissues. In particular, the vibrational mode v_s (C–O–C) at 1120 cm^− 1 is superimposed on skeletal modes of non-cellulosic carbohydrates (Himmelsbach and Akin, 1998) and lignin components (Agarwal and Atalla, 1986; Agarwal et al., 1995). For that reason, the intensity and the band shape of this cellulosic Raman line, which is also coupled with the C–C stretching mode of the breathing vibration of the glucopyranose rings at 1153 cm^− 1, is strongly affected. In contrast, the Raman line at 1098 cm^− 1 assigned to the asymmetric stretching mode v_as (C–O–C), is not influenced by non-cellulosic carbohydrates. Taking into account the previous considerations, the changes in the C–H stretching region could be studied and related to the effect of the alkali treatment on the molecular structures of cellulosic plant fibers.

4.4.2 Esterification

The esterification of natural fibers using various anhydrides has been studied extensively. The type of lignocellulosic material used as a substrate in the esterification reaction is of vital importance, because its three main components, i.e. lignin, hemicelluloses and cellulose exhibit different reactivity towards the anhydrides. The acetylation reaction is one of the most frequently informed in literature (Adebajo and Frost, 2004; Bessadok et al., 2009; Liu et al., 1995; Luz et al., 2008; Rong et al., 2001; van Hazendonk et al., 1996; Zafeiropoulos et al., 2002). For example, Luz et al. (2008) reported the changes observed in the FTIR spectra for cellulose from sugarcane bagasse and sugarcane bagasse without hemicellulose after attaching acetyl groups to the OH groups of the fibers. The authors reported the appearance of a band at 1758 cm^− 1, corresponding to the C = O carbonyl peak from bonded acetyl groups. Furthermore, after fiber acetylation, there was a decrease in OH band intensity, which was shifted from 3400 to 3700 cm^− 1 owing to the reduction of hydroxyl groups available to form hydrogen bonds and the higher energy of the remaining free-OH groups. Similar effects were found by Zhang and coworkers (2008), who performed a mechanochemical acetylation of cellulose powder (solid-state at ambient temperature) using acetic anhydride. They demonstrated that the reaction occurred under those conditions and essentially on the cellulose surface.

Tserkia and coworkers (2005) modified flax, hemp and wood fibers by acetylation and propionylation, and confirmed the esterification reaction by ATR-FTIR. The treatment with both anhydrides led to the appearance/increase of the absorbance in the regions 1735–1737 cm^− 1 (stretching vibration of the C = O group in ester bonds) and 1162–1229 cm^− 1 (C–O stretching) in all cases. A reduction of the strong absorption between 3400 and 3600 cm^− 1 (OH groups) was also noticed after fiber esterification.

Marcovich and coworkers (1996a, 1996b, 1997, 2005) showed that the DRIFT technique was effective in assessing surface modifications on wood flour resulting from esterification with maleic anhydride (MAN), even under mild conditions (room temperature without catalyst). The formation of new ester bonds owing to the reaction of the OH groups in the wood with the anhydride was confirmed by the increased intensity of the 1710–1740 cm^− 1 band (attributed to both the C = O groups in acid and ester groups attached to the filler) and the appearance of a new peak at 850 cm^− 1 related to the absorption of cis C = C conjugated to the carbonyl group in MAN (Bellamy, 1975). The similar intensity of the peaks corresponding to ester and acid carbonyls indicated that only one of the formed acid groups reacts with the filler surface and the other remains unreacted, in agreement with other studies (Ly et al., 2008; Kishi et al., 1988; Felix and Gatenholm, 1991; Maldas and Kokta, 1992). Samples esterified under stronger conditions (xylene reflux temperatures) showed an increase in the absorbance of the band at 1740 cm^− 1 that was also a function of the reaction time. This observation corresponds to the increase in the concentration of ester bonds.

Wood flour was also esterified with a commercial alkenyl succinic anhydride (ASA) (Acha et al., 2003; Marcovich et al., 2005, Fig. 4.6). The FTIR spectrum of the treated sample revealed the usual increase in the intensity of the carbonyl absorption at 1735 cm^− 1, owing to ester formation, and the absence of the anhydride absorption bands at 1600–1800 cm^− 1. An increase in the CH stretching bands (2900–3000 cm^− 1) was also observed owing to the CH groups of the attached ASA molecule. Additionally, a new peak at 1650 cm^− 1 appeared, which corresponds to the unsaturation of the attached ASA. As is the case with many unsaturated fatty acids, in which the double bond is located approximately in the middle of the molecule, the 1650 cm^− 1 absorption peak, characteristic of C = C absorption, did not appear in the FTIR spectrum of the neat ASA and had to be identified using Raman spectroscopy (strong peak). However, it clearly appeared in the spectra of the modified wood flour, because the reaction broke the symmetry of the molecule.

The attachment of longer chains on cellulose whiskers and flax strands is also well exemplified by Junior de Menezes et al. (2009) and Cañigueral et al. (2009), respectively. The cellulose was modified by grafting organic acid chlorides with aliphatic chains of different lengths, and thus, their spectra showed the expected peak at 1737 cm^− 1 and an increase in the signals at 2953, 2919 and 2850 cm^− 1 ascribed to the presence of grafted alkane chains (Fig. 4.6). The concomitant decrease of the magnitude of the broad band around 3300 cm^− 1 for modified whiskers compared with unmodified ones is attributed to the partial disappearance of OH groups, confirming the success of the grafting reaction with organic acid chlorides. On the other hand, the flax strands were modified with alkyl ketene dimer and the resulting spectrum showed the same signals discussed previously added to the peak at 1700 cm^− 1, which corresponds to the ketone group of the hydrolyzed reactive.

Regarding commercial coupling agents, the use of maleated polypropylene (MAPP) or maleated polyethylene (MAPE) copolymers to improve the interface between cellulosic fibers and PP or PE matrices is a common practice and has been frequently reported in the scientific literature (Acha et al., 2006; Bledzki et al., 2005; Felix and Gatenholm, 1991; Gassan and Bledzki, 1997; Hristov et al., 2004; Ichazo et al., 2001; Joly et al., 1996; Lu et al., 2005; Miguez Suarez et al., 2003). Just to include an example, Lu et al. (2005) demonstrated by FTIR spectroscopy and ESCA (electron spectroscopy for chemical analysis) the chemical linkage formed between the wood fibers and several MAPE coupling agents through esterification and proposed that succinic and half succinic esters were the two primary covalent bonding products of the reaction. Their results are discussed further in 4.4.5.

4.4.3 Silanization

Researchers at Grenoble have been involved in a series of studies (Abdelmouleh et al., 2002, 2004, 2005 and 2007) aiming at understanding the silane–cellulose system. The main advantages of using silane coupling chemicals is that at one end, they bear alkoxysilane groups capable of reacting with the OHrich surface of the fibers, whereas the other end can be tailored to suit a specific matrix. One of the studies (Abdelmouleh et al., 2007) illustrates the grafting of the coupling agent through initial physical adsorption of the hydrolyzed silane followed by a curing process at 120 °C under an inert atmosphere. Figure 4.7 presents the DRIFT spectra corresponding to the cellulose samples treated with γ-methacryloxypropyltrimethoxy silane (MPS), before and after heat treatment. In both cases, the spectrum corresponding to untreated cellulose was subtracted to investigate the band issuing from the silane moiety. The two spectra show different bands at 1713 and 1636 cm^− 1, which are associated with the stretching vibrations of the C = O and C = C groups in the acrylic moieties, respectively. The broad shape of the C = O peak suggests that this group is interacting by hydrogen bonding with the surface hydroxyl groups. The broad intense bands around 1200 and 1135 cm^− 1 were assigned to the stretching of the –Si–O–cellulose and –Si–O–Si– bonds, respectively (Felix and Gatenholm, 1991; Navoroj et al., 1984). The strong increase in the intensity of these bands after the heat treatment suggested that the grafting of silane onto cellulose as well as the intermolecular condensation between adjacent adsorbed –Si–OH groups were substantially enhanced. The peaks near 1100 and 1080 cm^− 1 are related to residual unhydrolyzed Si–OCH₃ groups and their small intensity indicates that most of the silane adsorbed under these conditions was hydrolyzed. The large band around 1015 cm^− 1, present in the spectrum of the uncured sample, was attributed to –Si–OH groups. This band disappeared after the heat treatment and was replaced by a wide band around 1040 cm^− 1, characteristic of –Si–O–Si– moieties. These peak assignments are in agreement with those reported in other studies dealing with glass surfaces treated with the same coupling agents (Navoroj et al., 1984). However, FTIR spectroscopy applied to the study of silanized natural fibers has not always been reported as a successful analysis technique. Sgriccia et al. (2008) treated hemp and kenaf fibers with silane using gentler conditions than the Grenoble group and were unable to identify the characteristics of silane moieties on the surface of the fibers by FTIR spectroscopy.

Other studies (Herrera-Franco and Aguilar-Vega, 1997; Valdez-Gonzalez et al., 1999a, 1999b) have also used FTIR spectroscopy and ESCA to confirm the reaction of a silane coupling agent with henequen fibers and found that the fiber–matrix interaction became stronger when the fibers were previously treated with alkali. The pretreatment eliminated cementitious components and lead to a rougher topology and larger accessible surface for reaction.

4.4.4 Urethane bonds

The reactivity of lignocellulosic fibers towards isocyanates has been frequently used as a compatibilization procedure in polymeric based composites (Kokta et al., 1990; Ly et al., 2008; Maldas et al., 1989a,b). As an example, Joly and coworkers (1996) demonstrated the success of the chemical modification by showing the presence of two new bands at 3343 and 1705 cm^− 1, attributed to the formation of urethane groups and associated with their NH and CO vibrations, respectively, after modifying Avicel cellulose, flax and ramie fibers with aliphatic isocyanates. More recently, Ly and coworkers (2008) modified cellulose fibers and kraft softwood pulps with both 1,4-phenylene diisocyanate (PDI) and methylene-bis-diphenyl diisocyanate (MDI). The FTIR spectra of the Soxhlet extracted modified fibers revealed a new band at 2275 cm^− 1, corresponding to the isocyanate function and a peak at 831 cm^− 1, associated with the presence of disubstituted aromatic ring. The small size and rigidity of the isocyanate molecules selected exclude the possibility of bridging two different fibers, which explained the presence of isocyanate functions. However, the expected urethane linkage signal was masked by a large broad signal from absorbed water (~1650 cm^− 1). Gandini et al. (2001) used TDI (toluene di-isocyanate) to treat cellulose and also confirmed the reaction by FTIR spectroscopy.

In a comprehensive experiment, Marcovich et al. (2006) treated cellulose nanocrystals (prepared from acid hydrolysis of microcystalline cellulose Avicel) with a polymeric MDI, followed by Soxhlet extraction with toluene to remove unreacted isocyanate. Because of the large area available for reaction in the cellulose nanocrystals, the modifications were distinctively observed in the FTIR spectra. The spectrum of the treated (unwashed) crystals showed new peaks corresponding to the absorption of isocyanate (2270 cm^− 1) and urethane (1720 cm^− 1). The spectrum of the extracted crystals showed that there was no change in the 1720 cm^− 1 peak, because the urethane bonds were formed by reaction with the OH in the surface of the cellulose crystals. On the other hand, a large, but incomplete reduction of the isocyanate band was reported, which results from the removal of non-attached MDI, and the presence of unreacted isocyanate in the attached moieties.

4.4.5 Acrylate derivatives

Kalia and coworkers (2008) reported the preparation of graft copolymers of flax fibers with methyl acrylate (MA) using a redox system. FTIR spectrum of original flax showed a broad peak at 3422.8 cm^− 1 owing to bonded –OH groups and at 2918.8, 1653.5 and 1058.7 cm^− 1 arising from –CH₂, C–C and C–O stretching, respectively. However, for the graft copolymer, an additional peak at 1731.2 cm^− 1 owing to the > C = O group of MA was observed, suggesting MA graft copolymerization onto flax fiber through covalent linkages.

Bessadok et al. (2009) treated agave (Americana L.) fibers with acrylic acid (AA), among other reactives. Owing to the introduction of new CH and CH₂ groups of the acrylic acid, the spectra of the treated agave fibers presents stronger bands at 2960, 2900 and 2855 cm^− 1 than that of the untreated fibers. At the same time the new ester groups resulting from the acidic AA treatment are highlighted by the band at 1730 cm^− 1 characteristic of the carbonyl (C = O) group. Analagously, Sangthong et al. (2009) treated sisal by admicellar polymerization with a poly(methyl methacrylate) film coating (Fig. 4.8). Figure 4.9 shows the FTIR spectra of pure PMMA, untreated sisal fiber, and admicellar-treated sisal fiber. The admicellar-treated sisal fiber spectrum, shows the peaks at 1734 cm^− 1 (C = O) and 1457 cm^− 1 (–O–CH₃), which are the characteristic peaks of pure PMMA whereas the spectrum of bare sisal fiber, shows the characteristic peaks of cellulose at 3421 cm^− 1 (–OH) and 1430 cm^− 1 (–CH₂OH). `These results confirm that the sisal fiber surface was successfully coated with PMMA by admicellar polymerization.

4.4.6 Enzymatic treatment

In some instances, the outermost surface of a lignocellulosic reinforcement is covered with hydrophobic noncellulosic components, i.e. lipophilic extractives and silica. This is true for wheat straws (WSs), stalks, husks, grasses, cereals and others. These materials impede uniform and efficient fiber–matrix adhesion and are responsible for the nonwetting behavior of these kinds of fillers when they are used for the production of particleboards (Han et al., 2001). Jiang and co-workers (2009) demonstrated that lipases from Candida cylindracea could effectively remove the hydrophobic lipophilic extractives and silica from the outer surface of untreated wheat straw, and increased the hydroxyl group content in the outer surface. They found, in the FTIR spectrum of the wheat straw outer surfaces, two strong and sharp peaks at 2916 and 2850 cm^− 1 assigned to the asymmetric and symmetric stretching, respectively, of the CH₂ group comprising the majority of the aliphatic fractions of waxes. Meanwhile, the small sharp band at 718 cm^− 1 being characteristic of the –(CH₂)_n – (n ≥ 6) in plane deformations rocking (Bio-Rad, 2005) was observed. After enzymatic treatment, the sharp peaks at 2916 and 2850 cm^− 1 significantly decreased and they turned into a weak rounded band, now associated with the contribution of the CH₂– group in lignin and polysaccharides (Fang et al., 2002; Moran et al., 2008). The peak at 718 cm^− 1, meantime, was reduced to a very weak shoulder. The two peaks at 790 and 970 cm^− 1, attributed to the Si–C stretching vibration and Si–O vibration, respectively (Das et al., 2007; Frost and Mendelovici, 2006), were reduced to two corresponding weak shoulders after the lipase treatment indicating that the silicon-containing components were also removed concurrently during the modification. Additionally, the intensity of several typical peaks assigned to lignin and polysaccharides (cellulose and hemicellulose) increased significantly after treatment: the band at 896 cm^− 1, characteristic of β-glucosidic linkages between the sugar units (Gupta et al., 1987), the infrared bands at 1422, 1600, 1502 cm^− 1, assigned to methoxyl group in lignin, the aromatic skeletal vibrations and the aromatic skeletal vibrations coupled with C–H in plane deformations, and the peaks at 1245 cm^− 1 (C–O stretching of acetyl group present in the lignin moiety as well as in hemicellulose, Subramanian et al., 2005) and 1160 cm^− 1 (C–O–C antisymmetric bridge in hemicelluloses and cellulose and to aromatic C–H deformation of the syringyl and guaiacyl units in lignin) were revealed. Moreover, the broad band associated with hydrogen bonded hydroxyl groups at 3320 cm^− 1 and the band at 1648 cm^− 1, attributable to the bending of adsorbed water (Brandrup and Immergut, 1989; Lojewska et al., 2005), increased significantly, illustrating that the lipase treatment increased the amount of accessible hydroxyl groups and thus, of water absorption owing to the higher hydrophilicity of the WS outer surface.

4.4.7 Irradiation

Kato et al. (1999) treated different types of cellulose fibers with vacuum ultraviolet (VUV) irradiation in order to ascertain the efficiency of this technique in terms of oxidation capacity and compared their results with other common oxidation techniques. They monitored the evolution of the surface chemical composition by FTIR spectroscopy through the intensity variations of the band at 1720–1740 cm^− 1 (stretching vibration of carbonyl) and showed that VUV irradiation induced surface oxidation as efficiently as chromic and nitric acids, with the additional advantage of neither causing losses in bulk mechanical properties nor coloring the cellulose fibers. Strlic et al. (2003) studied the effect of neodymium-doped yttrium aluminium garnet (Nd:YAG) laser cleaning at 1064 nm on the surface of Whatman cellulose, rag paper, cotton, linen, silk and wool. They used FTIR diffuse reflectance spectroscopy in order to establish the chemical changes occurring at the surface of these materials. The peaks affected by laser treatment were the vibration at 1063 cm^− 1, associated with C–O stretching, the band at 1111 cm^− 1 related with skeletal ring asymmetric stretching and that at 1413 cm^− 1 associated with scissoring, whereas the bands at 1492 and 875 cm^− 1 are new and cannot be associated with any of the usual bands appearing in FTIR spectra of pure cellulose. They concluded that chemical modifications of the surface were typical of the thermal degradation of cellulosic materials.

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