TURHAN ET AL .
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1240 cm 1 is typically related to C O and C O present in the hemicellulose component. 29,31 As the wood fibers undergo oxidation with NO 2 , these linkages are likely affected, leading to the cleavage of aryl ether bonds in hemicellulose. The decrease in intensity of this peak reflects the removal of the hemicellulose. Even if the intensities decrease, the peak positions might change because of the introduction of oxygen-containing groups or structural alterations. This could explain the shift observed in FTIR data (in Figure 2A,B). The peaks around 1106 and 1317 cm 1 corresponding to C O and CH 2 vibrations in crystalline and amorphous cellulose, respectively, were found to increase with increasing oxi- dation time, 36 due to a free fraction of cellulose after the modification with nitric acid vapor. Particularly, breaking the hydroxyl groups of lignin and hemicellulose could lead to increased cellulose fraction and the peak intensi- ties in the bands between 1317 and 1106 cm 1 in Figure 2B. As the reaction time increases, the structure becomes more hydrophilic as evidenced by a higher intensity of OH stretching at 3300 cm 1 , resulting from the breaking of intramolecular hydrogen bonds during the reaction. The removal of hemicellulose, lignin, and extractive sub- stances increased relative to the modification time, lead- ing to a relative increase in cellulose content and, thus, crystallinity. The XRD patterns in Figure 2C show the peaks at 2 θ = 16 , 22.2 , and 34.4 (corresponding to cel- lulose I structure) becoming more pronounced, resulting in a monotonic increase in the degree of crystallinity with modification time (34.4%, 38.4%, 43.0%, and 52.0% for raw, 15, 30, and 60 min CA'ed fiber, respectively). The modification of the wood fibers was determined by visual assessment using the UV – vis spectra in Figure 2D. The color of the wood gets lighter as the treat- ment time increases. This can be explained by the pres- ence of chromophores in the lignin structure. The strength of certain chemical groups ( OH, called auxo- chromes) in the lignin structure, which are responsible for certain color-related properties, decreases during oxidation with nitric acid steam. This reduction in auxo- chromes affects another set of groups (chromophores, characterized by C O double bonds) that absorb ultravi- olet (UV) radiation. 37 As a result of this steam treatment, the lignin structure undergoes degradation, leading to a lightening of color and altering how wood fibers reflect light in the visible spectrum. Thus, oxidation induces changes in the chemical composition of lignin, weakening its ability to absorb UV radiation. This process, in turn, affects the overall color of the wood fibers by diminishing the intensity of specific chemical groups responsible for coloration. The consequence is a lightening of the wood fibers, making them appear less colored. Additionally, the
modified lignin structure influences how these fibers inter- act with light in the visible range, altering their reflectance properties. This can be advantageous with a more stable color response of wood fibers to light, especially in the visi- ble spectrum. This enhanced light stability is valuable for maintaining the intended color of wood panels over time, preventing undesired fading or discoloration due to expo- sure to environmental factors. The structure of the modified wood fibers was exam- ined through SEM microscopy images. Figure 3A shows the surface of the unmodified wood, highlighting the open pit structures present. These pits play an important role in facilitating the water flow radially between neigh- boring fibers through diffusion. It is important to note that these pits have a semi-permeable, membrane-like surface that allows water to pass through wood inner structure. These pores are particularly important in the production of composite materials as the binding agents can effectively diffuse into the fibers through these pits and effectively fill the hollow fibers. This mimics the nat- ural transport of water in plants, highlighting the intri- cate contribution of these structural features to the functionality of wood fibers in composite material appli- cations. 38,39 In this study, after a 15-min treatment (Figure 3B), we observe the pits on the tracheid walls (in some cases partially) closed. This closure acts like a barrier, preventing the diffusion of excessive adhesive inside of the fibers. Thus, the closed pits act as gateways
FIGURE 3 (A) Unmodified wood fibers with open pits, (B) 15 min modified wood fibers with closed pits and irregulated wood fibers, (C) 30 min modified wood fiber with damaged pits and surface, and (D) 60 min modified wood fibers with fully damaged and open pits.
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