An in-depth study of molecular and supramolecular structures of bamboo cellulose upon heat treatment

In this study, two methods including a common method using high concentration of alkali solution and a mild extraction method at ambient conditions were used to extract cellulose from bamboo. The results showed that two methods affected the initial state of the cellulose.

Carbohydrate Polymers 241 (2020) 116412 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol An in-depth study of molecular and supramolecular structures of bamboo cellulose upon heat treatment T Qiuqin Lin, Yuxiang Huang*, Wenji Yu Research Institute of Wood Industry, Chinese Academy of Forestry, Haidian 100091, Beijing, China A R T I C LE I N FO A B S T R A C T Keywords: Bamboo Cellulose Heat treatment Molecular structure Supramolecular structure Hydrogen bonding system In this study, two methods including a common method using high concentration of alkali solution and a mild extraction method at ambient conditions were used to extract cellulose from bamboo The results showed that two methods affected the initial state of the cellulose Celluloses obtained by the former was a hybrid of cellulose I and II while the latter was pure cellulose I However, their heat treatment results indicated that the heat treatment (≤200 °C) would not change the aggregation structure of bamboo cellulose, but it will cause the change of intramolecular and intermolecular hydrogen bonds, and the break of glycosidic bonds in the amorphous region and part of the crystalline region of cellulose Accordingly, the crystallinity of bamboo cellulose will decrease slightly after heat treatment Finally, the macroscopic morphology change of bamboo cellulose caused by heat treatment was the thermal expansion in the width direction instead of distort Introduction Bamboo, as a biological eco-friendly material, is growing to a top interest due to the special natural functional gradient structure and superior mechanical properties In order to meet the various demands of bamboo products, heat treatment is extensively used to give bamboo a new darken color, improve its dimensional stability and durability (Cheng, Jiang, & Zhang, 2013), but at the same time reduce the strength properties (Zhang, Yu, & Yu, 2013) Many efforts have been put into the exploration of heat treatment process (Cheng et al., 2013) and its effect on the macroscopic performance, such as physical-mechanical properties (Boonstra, Van Acker, Tjeerdsma, & Kegel, 2007; Zhang et al., 2013), color traits (Meng, Yu, Zhang, Yu, & Gao, 2016) and chemical contents (Ma et al., 2014; Meng et al., 2016; Sharma et al., 2018) The crystallinity of thermal modified bamboo increased gradually in the temperature range of 170–210 °C but decreased above 210 °C (Maheswari, Reddy, Muzenda, Guduri, & Rajulu, 2012) The phenonmenon that the cleavage of cellulose chain started as the temperature exceeded 150 °C was observed in the hydrothermal treated bamboo (Ma et al., 2013) The process of heat treatment was accompanied by the alterations of chemical composition and supramolecular structure in lignified cell walls (Huang, Meng, Liu, Yu, & Yu, 2019; Mehrotra, Singh, & Kandpal, 2010) Most studies attributed the decrement of mechanical properties to the degradation of hemicellulose, but little focus has been put on the changes in molecular and supramolecular ⁎ structure of bamboo cellulose during the heat treatment Cellulose is the structure and skeleton material of bamboo cell wall, which directly affects the physical and mechanical properties of bamboo Whereas, since the experimental samples were usually raw bamboo, it was difficult to ignore the other chemical contents, such as hemicellulose and lignin, when studying the effects of the heat treatment on cellulose Therefore, it is necessary to study the response of native cellulose to heat treatment Researches have noted that lignocellulose underwent an enormous changes during chemical and physical treatment, especially heating (Cai et al., 2015; Weimer, Hackney, & French, 1995) Irreversible transformation tended to occur in native aspen wood cellulose while exposing to elevated temperatures (R S Atalla, Crowley, Himmel, & Atalla, 2014) Moreover, anisotropic thermal expansion was observed in tunicate cellulose Iβ while heating (Wada, 2002) The changing average crystallite size upon heating has been reported in previous literature (R S Atalla et al., 2014; Kuribayashi et al., 2016) In many studies, triclinic structure (Iα) has a tendency to convert into a more stable monoclinic structure (Iβ) during heat modification, as a result of the formation of new types of hydrogen bonds (Ma et al., 2013; M Wada, Kondo, & Okano, 2003; Yildiz & Gumuskaya, 2007) However, there is a gap in knowledge about the understanding in molecular-level of bamboo cellulose when it exposing to elevated temperatures While exploring the effects of heat treatment on the supramolecular structure of bamboo cellulose in our previous study, it was found that Corresponding author E-mail addresses: yxhuang@caf.ac.cn, hyx333100@163.com (Y Huang) https://doi.org/10.1016/j.carbpol.2020.116412 Received 24 February 2020; Received in revised form 17 April 2020; Accepted 30 April 2020 Available online 11 May 2020 0144-8617/ © 2020 Elsevier Ltd All rights reserved Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al The reaction system was put on a magnetic stirrer at 300 rpm The same dosage of NaClO2 was added to the solution once a week and the pH was also adjusted to 4.0 every time During the delignified process, the solution was saturated with chlorine dioxide, causing the dispersion presented in the color of canary yellow The delignified procedure lasted for months at ambient conditions The residue was filtered thoroughly with DW to neutrality Then the delignified bamboo was further treated with wt% NaOH solution for 72 h at 300 rpm at ambient temperature After that, the cellulose was filtered and washed with DW to a neutrality and the final native cellulose was obtained after freeze drying the cellulose extracted by 17.5 % caustic soda from heat-treated (180 °C and 200 °C) bamboo was more prone to distort and shrink with the changes of aggregation structure (Huang et al., 2019) It was likely that efficient transformation of bamboo cellulose was due to the improving Na+ accessibility to crystalline lattice as the degradation of lignin and hemicellulose upon heating, promoting swelling actions within the internal molecular chain (Das & Chakraborty, 2006; Ma et al., 2013) Whereas, whether the heat treatment itself transforms the structure of cellulose or not is still uncertain Furthermore, the drastic structural changes of alkali-extracted cellulose from heat-treated bamboo have not excluded the effects of temperature history on supramolecular structure so far For further explaining the above two doubts, two types of cellulose were extracted from bamboo by two methods including high concentration of alkali solution and a mild extraction method at ambient conditions, and then they were heat treated at 180 °C and 200 °C by exposing in superheated steam environment The purposes of this study are as follows: 1) to explore structure transformation of alkali-extracted cellulose from heat-treated bamboo was ascribed to heat history or alkali treatment; 2) to deeply study the effects of heat treatment on the molecular and supramolecular structure of cellulose, especially the important hydrogen-bonding system 2.3 Heat treatment of cellulose Two types of extracted cellulose were placed in the glass container in an oven saturated with overheated steam at specified temperature (180 °C and 200 °C) for h The samples of alkali-extracted cellulose and the subsequent heat-treated cellulose were denoted as A-Cell-Co, ACell-180 °C and A-Cell-200 °C, respectively And the cellulose isolated at ambient conditions and the subsequent heat-treated samples were denoted as N-Cell-Co, N-Cell-180 °C and N-Cell-200 °C, respectively 2.4 Characterization Experimental The morphology of all the cellulose samples were imaged by SEM (Hitachi SU8020, Japan) The crystal structure and crystallinity of cellulose was determined by XRD (Bruker, D8 ADVANCE) The crystalline index (CrI) of alkali-extracted cellulose and the heat-treated samples were calculated using the formula CrI (%) = (I200 - Iam)/I200, where I200 and Iam are the intensity of the crystalline portion at about 2θ = 22.4° and the amorphous portion at about 2θ = 18°, respectively In the case of the cellulose samples isolated at ambient conditions, the degree of crystallinity was determined by amorphous subtraction method using a software named “maud” The surface chemical groups of samples were recorded by FT-IR (Nicolet IS10, USA) The spectra were detected in the range of 4000 cm−1 to 500 cm−1 2.1 Materials Moso bamboo plant (Phyllostachys edulis) obtained from a forest farm in Anji, Zhejiang, China was selected as the raw material for the study These bamboo culms were obtained with the diameters of 7–10 cm and the thickness of 7–10 mm After air-drying, the culms were machined into smaller strips upon peeling the outer and inner layer The moisture content of bamboo strips was dried in an oven at 85 °C for 24 h to reach about 10 % and then the strips were conditioned in a room at 20 °C and 50 % relative humidity Finally, the bamboo strips were processed into bamboo powder (40 mesh) using a grinder Benzene (C6H6, 99.5 %, AR), ethanol (C2H6O, 95 %, AR) and acetic acid (CH3COOH, 99.5 %, AR) were purchased from Beijing chemical Sodium chlorite (NaClO2, 80 %, AR), sodium hydroxide (NaOH, 96 %, AR) were purchased from Aladdin Results and discussion 3.1 Heat treatment of alkali extracted cellulose 3.1.1 Cellulose morphology by SEM Fig shows the changes of morphology of alkali-extracted bamboo cellulose before and after heat treatment Bamboo cellulose remains in the shape of fiber cells and parenchyma cells after removing the lignin and hemicellulose As seen from Fig 1, the morphology of cellulose after heat treatment basically remained unchanged, which presented particularly well ordered, parallel and rigid structure Meanwhile, the typical aggregation of microfibers on the surface of wrinkles after treatment with high concentration of alkali solution was observed (Chen et al., 2016; Das & Chakraborty, 2006) In our previous study, bamboo was heat treated at 180 °C and 200 °C first and then the cellulose was extracted from the heat-treated bamboo by 17.5 wt% NaOH solution The results showed that the cellulose from heat-treated bamboo at high temperature was prone to distort and shrink (Huang et al., 2019) As is known to all, mercerization phenomenon would appear during the process of extraction by alkali with above 15 % concentration, in which the morphology of fibers could change (Das & Chakraborty, 2006; Eronen, Osterberg, & Jaaskelainen, 2009) Because all the samples were treated with the same alkali extraction conditions, the mercerization was not the reason for the morphology change of bamboo cellulose At that time, two reasons were speculated The first was that the heat treatment of bamboo directly resulted in the morphological changes of cellulose, and the other was that the history of heat treatment would promote the degree of mercerization in the subsequent alkali extraction process In this study, the raw bamboo and 2.2 Extraction of cellulose 2.2.1 Alkali-extracted cellulose The cellulose was extracted from the natural moso bamboo according to GB/T 2677.10−1995 and GB/T 744−1989 And the details of the operation process and reagent dosage have been described in our previous paper (Huang et al., 2019) Benzene-ethanol extraction, lignin removal and hemicellulose removal have been carried out successively to obtain the finally alkali-extracted cellulose 2.0 g bamboo powders was extracted by benzene-ethanol (2:1, v: v) first Then, the residue was surrounded by 65 mL distilled water (DW) in a container in 75 °C water bath 0.5 mL CH3COOH and 0.6 g 80 % NaClO2 were added to the solution per hour until the powders become white After washing to neutrality, the mixture was further treated by 17.5 wt% NaOH in 25 °C water bath After washing to a neutral pH and drying in an oven at 105 ± °C, the cellulose was isolated from the bamboo All the above operations were carried out in a fume hood 2.2.2 Cellulose extracted at ambient conditions According to the reported study (R S Atalla et al., 2014), the native cellulose was isolated at ambient conditions 10 g bamboo powder was used to be delignified at ambient temperature with 18 g 80 % NaClO2 and 400 mL DW in a 400 mL beaker Notably, the pH of the solution should be adjusted to 4.0 with CH3COOH The beaker was capped with glass-surface vessel for the release of the redundant chlorine dioxide Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al Fig The morphology of alkali-extracted cellulose before (a1 overall; a2 cellulose in fiber; a3 cellulose in parenchyma cell) and after the heat treatment at 180 °C (b1 overall; b2 cellulose in fiber; b3 cellulose in parenchyma cell) and 200 °C (c1 overall; c2 cellulose in fiber; c3 cellulose in parenchyma cell) methods for extraction and heat treatment were the same as our previous study and the only difference was the order of processing The first step was to extract cellulose from bamboo, and then heat treated the cellulose The results showed that heat treatment alone did not change the morphology of cellulose The reason why heat-treated bamboo was more prone to deformation after cellulose extraction can only be attributed to the promoting effect of heat treatment history Although there were no obvious changes in the morphology of cellulose, elevated temperature could cause the thermal expansion of cellulose (Fig 1b2 and c2) and also exacerbate the surface aggregation of microfibers However, it seemed that heat treatment had little effect on the morphology of parenchyma cells (Fig 1a3, b3 and c3), which could be ascribed to the different cell wall structure and microfibril arrangement of fiber cells and parenchyma cells The average width of cellulose before heat treatment was 12.02 ± 2.2 μm while the diameter of fibers became slightly larger after heat treatment at 180 °C (14.4 ± 1.5 μm) Previous study has shown that the thermal expansion behaviors of cellulose was ascribed to the intermolecular hydrogen bonding systems (Hori & Wada, 2005; Wada, 2002), which will be further analyzed in section 3.1.3 Fig XRD diffractograms of the natural bamboo, alkali-extracted cellulose and its heat-treated samples at 180 °C and 200 °C cannot change the structural type of cellulose but the process of heat treatment was an efficient manner for transforming the cellulose I to II during conventional alkali treatment, which was of great significance to the mercerization of cellulose (Huang et al., 2019) The empirical measurements of CrI were used to allow rapid comparison of the changing of crystallinity of cellulose samples upon heat treatment According to Table 1, the CrI of A-Cell-Co (69.70 %) 3.1.2 Crystal structure and crystallinity of cellulose by XRD In addition to morphology, we are most concerned about whether heat treatment will affect the supramolecular structure of bamboo cellulose or not Fig shows the X-ray diffractograms of natural bamboo, alkali-extracted cellulose and the cellulose with heat treatment at 180 °C and 200 °C The typical peaks of both cellulose I and II are observed, indicating that the alkali extraction process changed the cellulose crystal type For natural bamboo, three major diffraction planes of cellulose I named 200, 110 and 004 were presented at 2θ = 22.23°, 15.74° and 34.51° (Maheswari et al., 2012) For other cellulose samples, their X-ray diffractograms presented two additional diffraction planes of 1–10 (2θ = 12.23°) and 110 (2θ = 19.54°), which belonged to typical cellulose II structure This indicated that the alkali extraction process indeed changed the supramolecular structure of cellulose There was no new peak in the diffractogram of heat-treated cellulose, nor the dramatic changes from cellulose I to II seen in our previous study The results confirmed that heat treatment (≤200 °C) itself Table The CrI (%) of natural bamboo, alkali-extracted cellulose and its heat-treated samples at 180 °C and 200 °C Sample Iam I200 CrI (%) Bamboo A-Cell-Co A-Cell-180 °C A-Cell-200 °C 7547 6602 5940 4222 15,756 21,783 19,689 16,780 52.10 69.70 69.83 74.84 Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al Fig Hydrogen-bonding schemes between center chains in cellulose I and II (a), origin chains in cellulose II (b) and center chain and origin chain in cellulose II (c) schemes of 200 crystal plane, in which the scheme A was the typical form for cellulose I In the case of scheme A, the intramolecular hydrogen bonding (intra eOH) formed in intramolecular chain, involving O3 and O2 as donors and O5 and O6 as acceptors (O3H—O5 and O2H— O6), respectively Meanwhile, the intramolecular hydrogen bonding (inter eOH) of O6H—O3 was found between adjacent center chains, which was almost the main form of inter eOH in cellulose I However, some studies considered that there was a possible inter eOH in cellulose I, which involved O6 as donor and O2 as accepter (Oh, Yoo, Shin, & Seo, 2005) Even so, O6H—O2 was energetically less competitive, that is, little contribution can be made to the stabilization of the supramolecular organization due to the very weak interaction between two groups (Mazeau, 2005) As for cellulose II in the scheme B, only one intra eOH and one inter eOH were formed: (1) between the O3H group and the neighboring O5 atom (intra eOH); (2) between the O6H group and the O2 atom of the glucose ring of the adjacent chain (inter eOH) This was because the alteration of cellulose conformation happened during the mercerization process, causing more O6H-O2 generated or retained (Sturcova, His, Wess, Cameron, & Jarvis, 2003) In addition, another specific inter eOH (O2H—O2) was formed between the origin increased by 17.6 % with the removal of lignin and hemicelluloses, compared with that of natural bamboo (52.10 %) Heat treatment, within a certain range, can improve the crystallinity of cellulose (Weimer et al., 1995) The heat treatment at 200 °C improved the CrI of cellulose from 69.70 % to 74.84 % This was a consequence of the partial recrystallization of amorphous regions or the partial co-crystallization of crystalline zones in adjacent fibrils (Wan, Wang, & Xiao, 2010) From the FT-IR spectrum in Fig 4a, the bands at approximately 1736 cm−1 and 1512 cm−1 were assigned to the hemicellulose (C]O stretching in unconjugated ketones) and lignin (C]C stretching of the aromatic ring) After extraction, the peak of lignin disappeared while there was a residual peak at hemicellulose peak The residual hemicellulose, as the amorphous substance, degraded during the subsequent heat treatment, resulting in an increase in the relative proportion of crystalline zone 3.1.3 Hydrogen-bonding system of cellulose by FT-IR Figs and shows the three kinds of hydrogen-bonding patterns in both cellulose I and II, which are defined as scheme A, B and C, respectively The scheme A and B were the two hydrogen-bonding Fig The FT-IR absorbance spectra of bamboo and alkali-extracted cellulose and its heat-treated samples at 180 and 200 °C (4000 - 500 cm−1) (a), the peak separation of hydrogen bonding OH stretching of all the samples (3800 - 3000 cm−1) (b) and the content and wavenumber assignments of free eOH (peak 1), intra OeH (peak 2) and inter eOH (peak and 4) (c) Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al into a more swollen state with a rough surface when exposing at elevated temperatures (Hori & Wada, 2005, 2006; Wada, 2002) The hydrogen bond system in cellulose has a great correlation with the lateral size, which will be discussed in following Section 3.4 More importantly, the cellulose extracted by the two methods used in this paper did not distort and shrink after heat treatment, proving that heat treatment did not have a great impact on the morphology and structure of cellulose again chain and center chain in 110 crystal plane (Scheme C) Fig 4a displays the infrared spectrum of natural bamboo, alkaliextracted cellulose and heat-treated samples As discussed above, the lignin was almost completely removed, but some hemicellulose remained in the extracted cellulose Fig 4b shows the peak separation of hydrogen bonding OH stretching of samples ranging from 3800 to 3000 cm−1, and the corresponding parameters of different hydrogen bond types are shown in Fig 4c For the pure cellulose I, it was generally considered that inter eOH was stronger than intra eOH In other words, the former was more difficult to be broken during the heat treatment (El Oudiani, Msahli, & Sakli, 2017; Popescu, Popescu, & Vasile, 2011) Nevertheless, the mercerizing cellulose extracted by high concentration of alkali solution was the mixture of cellulose I and II Therefore, the contribution of hydrogen-bonding system of mercerized cellulose should be considered Moreover, it has generally been deemed that the length between donor and acceptor atom was inversely proportional to the bond energy Inter eOH had a higher average distance than that of intra eOH in mercerized cellulose, thus, part of inter eOH tended to be destroyed prior to intra eOH during the heating process Therefore, the above two conditions competed during the process Concerning these experimental results, the alkali extraction process made the situation from intra eOH dominating to inter eOH taking majority The sum number of inter-chains slightly decreased after 180 °C and 200 °C treatment (Fig 4c), which indicated that the heat treatment mainly influenced the inter eOH of alkali-extracted cellulose In the case of 200 °C treated cellulose, the contents of the intermolecular and intramolecular hydrogen bonds were approximately the same 3.2.2 Crystal structure and crystallinity of cellulose by XRD Fig shows the x-ray diffraction patterns of untreated cellulose by mild extraction method and heat-treated cellulose It can be seen from Fig 6a that the cellulose obtained by the mild extraction method had a typical cellulose I structure, and there was no diffraction peak of cellulose II This indicated that this extraction method would not destroy the original structure of cellulose Compared with the N-Cell-Co, there were no new peak and no large shift of peak in the diffraction curves after heat treatment, which suggested that heat-treated cellulose samples remained typical cellulose I structure The results indicated that the process of high temperature did not lead to the transformation of cellulose from parallel-chain structure to anti-parallel-chain of structure The degree of crystallinity was further obtained by amorphous subtraction method, that is, crystallinity was determined by subtracting the amorphous contribution from diffraction spectra via an amorphous standard The crystallinity of N-cell-Co was 79.68 % and then an approximately 12 % decrease of degree of crystallinity was occurred after the heat treatment at 180 °C (70.08 %) and 200 °C (69.06 %) It was worth noting that the crystallinity of cellulose extracted by the first method with high concentration of alkali solution increased after the heat treatment This difference may come from two aspects On the one hand, the cellulose obtained from the first extraction method was a hybrid structure of cellulose I and cellulose II, and the crystallinity calculated by using the empirical formula was not very accurate On the other hand, as mentioned above, the cellulose obtained by the first method still contained residual hemicellulose, thus affecting the results Therefore, the crystallinity obtained by the second mild extraction method was more reliable That is, the crystallinity of pure cellulose would decrease after the heat treatment at 180 and 200 °C 3.2 Heat treatment of cellulose extracted at ambient conditions The above experiments have proved that a single heat treatment process will not result in the transformation of the crystal structure of cellulose However, due to the influence of alkali concentration (17.5 %) and temperature (75 °C) in the extraction method, the obtained cellulose was a hybrid structure of cellulose I and cellulose II and had a heating history, which brought complexity to the subsequent structural analysis Therefore, another mild extraction method was adopted, which was carried out at room temperature and would not change the structure of cellulose 3.2.3 Hydrogen-bonding system of cellulose by FT-IR Fig shows the FTIR spectra of bamboo, untreated cellulose by the mild extraction method, and heat-treated cellulose Compared with the spectrum of bamboo, in the case of N-Cell-Co, the functional groups at 1738 cm−1 and 1505 cm−1 assigned to hemicellulose and lignin (Popescu et al., 2011), respectively, were totally disappeared This illustrated that pure cellulose can be obtained by this mild extraction method However, the band at 1738 cm−1 appeared again after the heat treatment at 180 °C and a more prominent band at 200 °C The reappearance of the C]O stretching peak indicated treatment at high temperature lead to the cleavage of cellulose and a rise of soluble species The active hydroxyl groups on the glucose ring in cellulose’s macromolecular chain were oxidized to aldehyde, ketone and carboxyl groups at high temperature The cleavage of cellulose chains had occurred when the temperature reached 150 °C (Ma et al., 2013) This may be the reason for the decrease of crystallinity of cellulose after the heat treatment The peaks at 1425 cm−1 and 897 cm−1 are generally called “crystallinity band” and “amorphous band” in cellulose, respectively, and the absorbance ratio A1425/A897 is considered as CrI (El Oudiani et al., 2017; Oh et al., 2005) Compared with the N-Cell-Co, the CrI of N-Cell-180 °C have decreased by 39 % The results again proved that high temperatures (≥180 °C) destroy not only the amorphous regions, but also the crystalline regions of cellulose The large band from 3600 to 3200 cm−1 was ascribed to the different types of hydrogen bonds Fig 8a shows the 2nd derivative of this region that can improve the resolution considerably and Table 3.2.1 Cellulose morphology by SEM Fig shows the micrographs of untreated and heat-treated bamboo cellulose derived from bamboo by mild extraction method Since the effects of stirring force caused by the magnetic stirrer, it can be seen that the morphology of fibers and parenchyma cells was damaged to some extent by mechanical effect, resulting cellulose clustered together (Fig 5a1) However, the presence of cellulose in the form of a single fiber (Fig 5a2) and a single parenchyma cell (Fig 5a3) was still observed at high magnification After the heat treatment at 180 and 200 °C, the morphology of cellulose remained essentially unchanged (Fig 5b1−3 and c1−3) Notably, a lot of pores with diameter about 40 nm–110 nm are observed in the N-Cell-200 °C sample (Fig 5c2) Cellulose, as a natural biopolymer, is composed with glucose units connected by β-1,4-glycosidic bonds (Osullivan, 1997) The active hydroxyl groups on the glucose ring tended to be oxidized at high temperatures, causing the degradation of cellulose (Ma et al., 2013; Yousefifar, Baroutian, Farid, Gapes, & Young, 2017) Therefore, these pores may be derived from the degradation of cellulose in the amorphous region under high temperature treatment This unique porous structure provided new possibilities for studying more potential applications of bamboo fibers, such as energy storage, drug or cell delivery, catalysis, separation, etc By repeated measurements of the diameter of the single fiber, the average diameters of N-Cell-Co, N-Cell-180 °C and N-Cell-200 °C was 6.01 μm, 9.20 μm and 9.87 μm, respectively The cellulose was changed Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al Fig The morphology of cellulose extracted by mild extraction method before (a1 overall; a2 cellulose in fiber; a3 cellulose in parenchyma cell) and after the heat treatment at 180 °C (b1 overall; b2 cellulose in fiber; b3 cellulose in parenchyma cell) and 200 °C (c1 overall; c2 cellulose in fiber; c3 cellulose in parenchyma cell) Fig XRD diffractograms of cellulose extracted by mild extraction method and its heat-treated samples at 180 and 200 °C (a) and the amorphous subtraction analysis of N-Cell-Co (b), N-Cell-180 °C (c) and N-Cell-200 °C (d) Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al Fig The FT-IR absorbance spectra of bamboo, the cellulose extracted by mild extraction method and its heat-treated samples at 180 and 200 °C (4000–500 cm−1) (a), the peak separation of hydrogen bonding OH stretching of all the samples (3800–3000 cm−1) (b) and the crystallinity of all the samples, content and wavenumber assignments of free eOH (peak 1), intra OeH (peak 2) and inter eOH (peak 3) (c) inter eOH (O6H‑‑‑O3) After the extraction, the contents of intra eOH and inter eOH in NCell-Co were similar However, inter eOH gradually dominated in cellulose after the thermal treatment When the temperature reached 200 °C, the contents of inter eOH, intra eOH and free eOH were 67.11 %, 28.32 % and 4.57 %, respectively In cellulosic structure, the hydrogen bonds with high energy tended to form between cellulose molecules and chains (intermolecular) rather than in the same molecule (intramolecular) (El Oudiani et al., 2017; Popescu et al., 2011) Therefore, intra eOH was more easily to be damaged during heat treatment, bringing about the decreasing of its relative content From the changes of the content ratio of OH bands and CrI after the heat treatment, it can be inferred that there was no correlation between the crystallization degree of cellulose and the content ratio of the inter-H bonds and the intra eOH From the Fig 8a, the blue shifts were observed in all the absorption frequencies of the free eOH, intra eOH and inter eOH, which indicated an increase in bond energy and a decrease in bond length It could be considered that the alkali treatment had positive influence on the hydrogen bond energy of cellulose Nevertheless, the effect of temperature summarized the IR assignments of main functional groups for OH bond regions It was worth noting that the peak of cellulose Iβ (at 3273 cm−1) had a blue shift after 200 °C heat treatment, indicating an increase in bond energy and the formation of more stable groups Meanwhile, the absorbance at 3234 cm−1 assigned to cellulose Iα almost disappeared The crystalline dimorphism of native cellulose (R H Atalla & Vanderhart, 1984; Atalla et al., 2014), cellulose Iα and Iβ were considered to have different hydrogen bonds rather than the conformation (Janardhnan & Sain, 2011; Michell, 1993; Sugiyama, Persson, & Chanzy, 1991) Cellulose Iα (one-chain triclinic structure) can be irreversibly converted into cellulose Iβ (two-chain monoclinic structure) upon heating (Sun, Sun, Fowler, & Baird, 2004) According to Wada (Wada et al., 2003), the high temperature would induce the rearrangement of hydrogen bonds in cellulose, causing the transition from Iα to Iβ Hydroxyl degradation in cellulose mainly begins from the amorphous region and follows to the semi-crystalline and crystalline region (Mitsui, Inagaki, & Tsuchikawa, 2008) In order to explore the changes of hydrogen-bonding system in cellulose, Fig 8c displays the curvefitted OH bands of free eOH, intra eOH (O2H‑‑‑O6 and O3H‑‑‑O5) and Fig The 2nd derivative of FT-IR spectra (3800–3200 cm−1) for bamboo, cellulose extracted by mild extraction method and its heat-treated samples at 180 and 200 °C (a) and the hydrogen-bonding scheme in cellulose I (b) Carbohydrate Polymers 241 (2020) 116412 Q Lin, et al Table IR assignments of eOH bond region in the 2nd derivative of FT-IR spectrum (3800–3200 cm−1) Wave number (cm−1) Band assignment Reference 3582, 3539 3558 3458, 3411 3337 3273(3234) Free-OH Absorbed water weakly bound O2H‑‑‑O6 intramolecular hydrogen bonding in cellulose O3H‑‑‑O5 intramolecular hydrogen bonding in cellulose O6H‑‑‑O3 intermolecular hydrogen bonding in cellulose Iβ(Iα) (El Oudiani (Popescu et (El Oudiani (El Oudiani (El Oudiani on the bond energy of three types of hydrogen bonds was different The peak at 3576 cm−1 in cellulose shifted to 3565 cm−1 after 200 °C treatment, representing the decrement of bond energy of free eOH Meanwhile, the blue shifts from 3455 cm−1 and 3264 cm−1 to 3462 cm−1 and 3316 cm−1, respectively, indicated the both shortening of the intra eOH and inter eOH length and the increasing bond energy Studies has shown that the lateral behavior of thermal expansion was ascribed to the inter-molecular hydrogen bonding system in cellulose Plenty of inter eOH existed along the parallel direction to the glucose ring (b-axis) but little existed along the perpendicular direction (a-axis) (Hori & Wada, 2005; Mazeau, 2005; Wada, 2002; Wada et al., 2003) Thus, due to the decrement of the hydrogen bonds after the heat treatment, there was also a lack of inter eOH 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