1 Introduction

Nanomaterials with at least one dimension having a size less than 100 nm are expected to revolutionize the field of materials science, considering that their chemical, physical, or biological properties are fundamentally different from bulk materials[1]. Nanocellulose is an organic nanomaterial that can be released from cellulosic resource, e.g.microcrystalline cellulose (MCC), cotton fiber,wood pulp, sisal, ramie, agricultural straw, tunicate cellulose[2]. Owing to its unique properties such as high aspect ratio, excellent strength, light weight, low coefficient of thermal expansion, biodegradability, and renewability, nanocellulose can be used to enhance the strength and barrier properties of composites,such as films, paper[3-4], as well as the rheological properties or stability of products, e.g. coatings, food products, cosmetics, and pharmaceutical products[5-6].It can also be used to manufacture recyclable, highly porous, flexible, transparent, or conductive materials for a number of applications[7-8]. Furthermore,nanocellulose has the potential for large-scale industrial manufacturing at a relatively lower cost compared to an inorganic nanomaterial, e.g. graphene[9], considering that the cellulosic biomass (raw material) is available in abundance and that the existing pulping/bleaching technologies can be used to produce nanocellulose to some extent; therefore, there has been a remarkable increase in the research activities, driven by the opportunities in potential applications of nanocellulose.

Nanocellulose derived from cellulosic fiber mainly has two nanostructured forms with different morphologies, i.e. cellulose nanofibril (CNF) and cellulose nanocrystal (CNC). These forms are obtained using different methods. The noodle-like CNF with a diameter of 2～100 nm and a length of a few micrometers can be produced using mechanical methods such as high pressure homogenization[3],microfluidization[10], grinding[11], ultrasonication[12],and enzymatic or chemical pretreatment (e.g. TEMPO-mediated oxidation) adopted prior to the physical nanofibrillation step, which can significantly lower the energy consumption[13]. In contrast, the rod-like CNC(referred to as crystals) has a diameter in the range of 2～30 nm and a length in the range of about 100～1000 nm[10], and it is typically prepared using hydrolysis by a strong inorganic acid.

It is well known that the properties of the nanocellulose products are highly dependent on fabrication methods and processing conditions[14]; for instance, increasing the acid concentration, reaction temperature, or time of hydrolysis can lower the size of the fabricated CNC. In our previous studies, we adopted formic acid (FA) hydrolysis for the purpose of developing a clean and sustainable approach for the preparation of nanocellulose products, considering that FA could be readily recovered and reused[15],and the esterification of the cellulose surface by FA could also take place simultaneously. Subsequently,we investigated this approach in detail including the tunable preparation of CNC by FeCl3-catalyzed FA hydrolysis[16], the tailored fabrication of CNF using FA pretreatment and homogenization[17], the simultaneous manufacture of hydrophobic CNC and CNF by FA treatment and homogenization in organic solvents[4], the detailed kinetic study of FA hydrolysis for the preparation of nanocellulose[18], as well as the controllable extraction of lignin-containing CNF (from tobacco stover) with a larger diameter compared to the CNF without lignin[7].

However, the impact of the starting materials on the properties of the nanocellulose products using FA hydrolysis has not been investigated in detail. In the present work, we measured and compared the characteristics of the nanocellulose products extracted from bleached softwood and hardwood pulps using FA hydrolysis and 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)-mediated oxidation. The conditions adopted for the fabrication of nanocellulose from two kinds of starting materials (aspen pulp and spruce pulp) were maintained identically. The fabricated samples of the two kinds of nanocellulose were compared with regard to morphology, surface property, and thermal stability to evaluate the impact of the starting materials.

2 Experimental

2.1 Materials

The samples of the bleached softwood (spruce) and hardwood (aspen) pulps were gifts from UPM (Finland).The spruce pulp contained 80.1 wt% cellulose, 17.6 wt% hemicelluloses, and 0.4 wt% lignin, while the aspen pulp had 75.4 wt%, 20.2 wt%, and 4.8 wt% of the respective constituents. FA (98 wt%) and NaClO solution (active chlorine 14 wt%) were purchased from VWR International LLC., while NaBr (extra pure) was from Merck KGaA. TEMPO (98 wt%) was obtained from Sigma-Aldrich (Finland). All chemicals were used as received.

2.2 FA hydrolysis of pulp

The XRD patterns of the samples of the pulp and the nanocellulose samples were recorded at ambient conditions on an X-ray diffractometer (Bruker Discover D8, Germany), whereby the freeze-dried samples were pressed into flattened sheets on a sample holder before the tests. The detailed test conditions were maintained as the same as those in our previous report[15]. The crystallinity index (CrI) of each sample was calculated using the empirical Equation (1), and the crystal size(Dhkl) was estimated using the Scherrer Equation (2)[20]shown as follows:

2.3 TEMPO-mediated oxidation of CNF

The CNF obtained by FA hydrolysis was further treated by TEMPO-mediated oxidation[19]. The desired amounts of TEMPO (0.1 mmol/g-fiber) and NaBr (1 mmol/g-fiber)were dissolved in a certain volume of water, and then added to the CNF suspensions (the final consistency of CNF was 1 wt%) followed by stirring at 700 r/min.Subsequently, NaClO solution (10 mmol/g-fiber) was added dropwise to the CNF suspension during the first 10 min of the total 1 h oxidation time. The reaction was conducted at a pH value of 9.5 by adding 0.5 mol/L NaOH dropwise at room temperature. Upon completion of the reaction, the mixture was washed by an equivalent volume of ethanol and then centrifuged at 3500 r/min for 10 min. Subsequently, the supernatant was decanted, and the obtained cellulose gel was washed by de-ionized water twice, using the same washing procedure as mentioned above. Finally, the resultant nanocellulose suspension was stored in a cold room (4℃) for further analyses. According to the TEM results, the morphology of the obtained nanocellulose was changed to rod-like CNC after the TEMPO-mediated oxidation, and the CNC products were named as Aspen T and Spruce T, respectively.

2.4 Preparation of CNC by sulfuric acid hydrolysis

The CNC prepared by sulfuric acid (64 wt% H2SO4,45℃ for 30 min) was used for comparison, and the CNC products obtained from the aspen and spruce pulps were named as Aspen S and Spruce S, respectively.

首先，本文变化替代弹性的取值，而其他参数设定为基准值。文献中国产大豆和进口大豆的短期替代弹性为1.72，长期替代弹性为4.5，因此在进行敏感性分析时，本文将替代弹性的取值区间设定为[1,5]，每隔0.5取一个值。但在政策效果模拟过程中，当替代弹性取值为3时，消费者福利变化已经由正变负，故在文中仅给出替代弹性从1变到3的政策模拟结果(表4)，以便节省篇幅。

2.5 Particle size distribution and Zeta potential

The particle size distribution and Zeta potential of the nanocellulose samples were measured using a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., UK) based on dynamic light scattering. All the sample suspensions were ultrasonically treated (VWR,45 kHz and 180 W) for 10 min before measurement,and the solid concentration was maintained at 0.3 wt%for all samples. Every sample was tested in triplicate,while 7～15 runs were performed for each test, and the average values were reported. In addition, the content of the carboxyl group in the nanocellulose samples was determined using conductometric titration method[19].

2.6 TEM analysis

Surface chemical properties of the pulp and nanocellu-lose samples were characterized using FT-IR,and the corresponding spectra are shown in Fig.4. As can be seen,the wave number band between 3500 cm-1 and 3250 cm-1 was related to O—H stretching, and the band close to 2900 cm-1 was related to the C—H stretching vibration in CH2 groups[24]. The small shoulder peak at 1725 cm-1 in the spectrum of bleached aspen pulp was associated with the C=O stretching in acetyl groups of hemicelluloses[15],and the peak near 1635 cm-1 was ascribed to O—H bending vibration of the adsorbed water[24].In contrast, the sharp peak at 1725 cm-1 in the spectrum of Aspen F was probably due to the C=O stretching vibration in the adsorbed FA on the surface of cellulose. On one hand, FA could be adsorbed onto the surface of cellulose owing to the formation of hydrogen bonds[25].On the other hand, FA could swell the cellulose fibers and react with them, thus forming cellulose formate;therefore, the strong peak at 1725 cm-1 was also partially attributed to the C=O stretching in cellulose formate[26]. As measured, the carboxyl group content of Aspen F was (0.525±0.015) mmol/g (Table 1). As for the spectrum of Aspen T, the peak at 1725 cm-1 was related to the C=O stretching in COO-, and the peak at 1610 cm-1 was because of the antisymmetric stretching of COO- in carboxylate salts. The presence of these two peaks indicated that the carboxyl groups were introduced on Aspen T after the modification of the surface of Aspen F by TEMPO-mediated oxidation,and the corresponding carboxyl group content of Aspen T was (1.501±0.022) mmol/g, as listed in Table 1.Comparable results were also obtained for the spruce samples, as presented in Fig.4(b), but the corresponding carboxyl group content of Spruce T was only(1.292±0.001) mmol/g (Table 1).

2.7 Atomic force microscopy (AFM) analysis

AFM images of the nanocellulose samples were carried out with a Nanoscope V (MultimodeTM series,Bruker) AFM microscope in Peak Force Tapping TM mode at a peak force of 790～1150 pN, a scan speed of 0.68 s/μm, a peak force amplitude of 50 nm, and a peak force frequency of 2 kHz. The images were obtained at ambient conditions (33% RH, 25℃) using a SCANASYST-AIR cantilever (Bruker) with a spring constant of 0.406 N/m (thermal tune). The Scanning Probe Image Processor (SPIPTM, Image Metrology,Denmark) software was used for processing and analyzing the AFM images. Before the AFM analyses, one drop of nanocellulose dispersion (2 mg/mL) was applied on a freshly cleaned mica and air dried at room temperature.

从上述政策中，我们可以看出，我国越来越多地重视东部对西部高校的“对口支援”工作，制定了相当多的“对口支援”政策，有力地促进了西部民族地区的高等教育发展；同时，也加大了对少数民族高校的人力投入，通过政策的支持，为少数民族地区输入了大量的人才，有力地支持和促进了少数民族高等教育的发展。

2.8 Fourier transform infrared (FT-IR) analysis

（四）防治 对于生猪霉玉米黄曲霉素中毒到目前尚无特效解毒药。临诊中仅限于对症处理并加强肝脏解毒功能。首先是立即停喂霉烂变质饲料和可疑料，其次是根据临床症状采取一些解毒措施，如应用中药轻泻，促使病猪采食后的霉饲料尽快排出体外，同时注射肌苷、柴胡保肝，用维生素E和高渗葡萄糖输液。对部分出现心力衰竭的猪只用安钠咖注射液强心。并注意控制继发感染。全天用5%板蓝根浆饮水，连饮3～5 d。饲料中添加抑霉菌素3～5 d，增加蛋白质和脂肪含量可增强猪体对黄曲霉毒素的抵抗力并减少死亡率。也可发病时用三苯甲咪唑按20 mg/kg口服，日服三次，连用数天可治疗本病。

2.9 X-ray diffraction (XRD) measurement

The FA hydrolysis of the bleached pulp was carried out based on the previous method[15]. Bleached pulp(2 g) was added to 100 mL FA, and the mixture was vigorously agitated using a Mixstab TEFAL Swing (300 W,regular kitchen blender) for 1 min to obtain a pulp-like slurry. The reaction was then allowed to take place in a spherical flask (250 mL) placed in an oil bath at 95℃for 6 h with a magnetic stirring at 900 r/min. The flask was then immediately cooled to room temperature using tap water, and the reaction mixture was centrifuged(SORVALL R77 Plus) at 3500 r/min for 10 min. After separation of the spent liquor for recovery of FA, the cellulosic solid residue was washed by mixing with 200 mL of distilled water and then centrifuged at 3500 r/min for 10 min. The washing procedure was repeated three times. The fiber suspensions obtained were stored in a cold room (4℃) for further treatments and analyses. The yield of the isolated fibers was calculated based on the volume and solid content of the resultant fiber suspensions. Considering that the diameter and the length of the isolated fibers were 2～10 nm and several micrometers (based on transmission electron microscopy (TEM) analyses), respectively, the fiber samples isolated by FA hydrolysis were identified as CNF (named Aspen F and Spruce F, respectively).

Where, I200 is the maximum peak intensity at lattice diffraction (200), Iam is the minimum intensity between planar reflections (200) and (110), Dhkl is the crystal dimension perpendicular to the diffracting planes with Miller indices of hkl, λ is the wavelength of X-ray radiation (λ=1.5406 Å), and β1/2 is the full width at halfmaximum of the diffraction peaks.

2.10 Thermal gravimetric analysis (TGA)

Table 2 gives the crystallinity and crystal size values of pulp and nanocellulose products. The total crystallinity index (TCI), lateral order index (LOI), and hydrogen-bond intensity (HBI) were calculated on the basis of the FT-IR absorbance ratio of A1372/A2900,A1430/A897, and A3308/A1330, respectively[27]. The TCI, LOI, and HBI were sensitive to the degree of the intermolecular regularity of cellulose and the crystal system[28]. The crystallinity index (CrI) and crystal size(Dhkl) were calculated based on the XRD determination.

3 Results and discussion

3.1 Comparison of the size and morphology of nanocellulose products

In this work, the bleached aspen and spruce pulps were used as starting materials for the preparation of nanocellulose using the same method under identical conditions. In step I, FA hydrolysis was conducted, and the resultant nanocellulose was found to be CNF, i.e.Aspen F and Spruce F. In step II, a TEMPO-mediated oxidation of the CNF (obtained from step I) was carried out, and the resultant nanocellulose was found to be CNC, i.e. Aspen T and Spruce T. Table 1 lists the yield, mean size, Zeta potential, mobility, and carboxyl group content of the nanocellulose products. As can be seen, the yields of Spruce F and Spruce T were higher compared to Aspen F and Aspen T, respectively; this phenomenon was attributed to the higher values of the density, fiber length, and cellulose crystallinity (Table 2) of the spruce pulp in comparison with the aspen pulp[21-22]. Consequently, the mean sizes of Spruce F(5193 nm) and Spruce T (523 nm) were also larger than those of Aspen F (3280 nm) and Aspen T (276 nm),respectively, and the detailed distribution of the particle size is shown in Fig.1. In other words, the aspen pulp with shorter fiber was easier to be hydrolyzed compared to spruce pulp under the same treatment conditions.

图论问题在18世纪初期就已经开始有运用，由于现代计算机的出现，图论的应用则越来越广泛，包括结构化学、计算机网络以及数学竞赛等。使用图论可以提供给我们一个快速解决问题的方式和角度。数学题中应用题相对偏多，而应用题往往都是在实际背景下产生出来的题型，在数学竞赛中是一种相对比较重要且难度较大的题型，主要考查学生如何通过数学相关的知识来解决和分析实际问题的能力，而这个能力则是通过解决数学图论的能力来展现。

Table 1 Yield, size, Zeta potential, and carboxyl group content of nanocellulose

* The yields were on the basis of the dry weight of starting materials. F: CNF obtained by FA hydrolysis; T: CNC obtained by TEMPO-mediated oxidation of CNF.

Samples Yield/% Mean size/nm Zeta potential/mV Mobility/(µm·cm·V-1·s-1) Carboxyl group content/(mmol·g-1)Aspen F 57.4±1.1 3280±171 -15.37±1.26 -1.21±0.10 0.525±0.015 Spruce F 61.8±2.1 5193±203 -16.67±1.08 -1.31±0.08 0.648±0.061 Aspen T 35.9±2.0 276±14 -61.92±3.54 -4.85±0.28 1.501±0.022 Spruce T 38.6±0.8 523±43 -59.57±4.45 -4.82±0.35 1.292±0.001

Table 1 also shows that both Aspen F and Spruce F had a relatively lower Zeta potential (absolute value).This was because no additional charge was introduced on the cellulose surface, and the hydrophobic ester groups were formed on the surface during FA hydrolysis[15]; thus, the obtained CNF (Aspen F and Spruce F) with hydrophobic surface could not be well dispersed in water, but they could be well dispersed in the solvents such as dimethylacetamide(DMAC), N,N-Dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)[17]. After the TEMPO-mediated oxidation,the Zeta potentials (absolute values)of the obtained CNC (Aspen T and Spruce T) were found to be increased, which was attributed to the significant increase of surface carboxyl groups (Table 1). The carboxyl group content of Aspen T(1.5 mmol/g) was obviously higher than that of Spruce T (1.3 mmol/g),which was also probably due to the relatively looser structure and lower crystallinity (Table 2) of the aspen pulp[21]; for the same reason, the aspen pulp was found to be easier to be reacted.

状态监测系统提供丰富直观的、可组态的多个监测画面，从不同的角度、分层次展现机组的状态信息。监测画面中将各种监测状态以图表、图形、数据等形式展示出来。

Fig.1 Particle size distribution of nanocellulose products

Fig.2 TEM images of nanocellulose products

Fig.3 exhibits the AFM topographical images of nanocellulose products. Both Aspen F and Spruce F fibers were found to be highly aggregated in line with the TEM images shown in Fig.2 but with a different appearance. In contrast, the individual fibers of Aspen T and Spruce T were distinctly identifiable, despite a few aggregations that were probably due to the high sample concentration. In addition, the calculated root mean square surface roughness (Sq values) for Aspen F,Aspen T, Spruce F, and Spruce T were 11.9, 5.1, 37.2,and 4.5 nm, respectively. These results indicated that smoother and thinner films could be formed with Aspen T and Spruce T after water evaporation compared to Aspen F and Spruce F. Similar results for birch pulp were also reported previously[15].

Fig.2 shows the TEM images of the nanocellulose products. Although both Aspen F and Spruce F fibers were found to be aggregated, the individual fibers were distinctly identifiable; the fiber diameter of Aspen F was approximately 2～4 nm,while it was approximately 6～9 nm in the case of Spruce F. After the TEMPO-mediated oxidation, the fibers of the obtained Aspen T and Spruce T were found to be well dispersed owing to the increased surface charge density[15], and the fiber length was clearly reduced as presented in Table 1, Fig.1, and Fig.2.Interestingly, the diameter of Aspen T continued to be in the range of 2～4 nm, whereas the diameter of Spruce T was decreased to 3～6 nm after the oxidation. These results also indicated that the aspen pulp with shorter fiber length, smaller fiber diameter, and lower cellulose crystallinity could be easier to be hydrolyzed and reacted compared with spruce pulp[23], thereby yielding the nanocellulose products with smaller size.

FT-IR analyses of pulp and nanocellulose samples were performed using a PerkinElmer FTIR-1000 spectrometer (USA), and the spectra were recorded in the wave number range of 400～4000 cm-1 with a resolution of 4 cm-1. Before the analysis, KBr pellets were prepared for each sample with a weight ratio (KBr to freeze-dried sample) of 100∶1.

Fig.3 AFM images of nanocellulose products

3.2 Comparison of surface chemical and crystalline structure of nanocellulose

The TEM images of the nanocellulose samples were captured using a JEM-1400 Plus TEM microscope(JEOL Ltd., Japan) with an accelerating voltage of 120 kV.Prior to imaging, the samples were prepared based on our previous report[15].

供应商分级标准制定时应用正态分布的“钟形”曲线分布规律，根据均值μ和标准差σ制定分级界限，并结合2/8原则，期望80%左右的供应商落在曲线的中部，将正态分布概率密度函数的μ-1.28σ和μ+1.28σ值作为对供应商进行分级的分级界限。即当供应商综合评价结果大于μ+1.28σ时为A级供应商，供应商数综合评价结果小于μ-1.28σ时为D级供应商，分别占总数的10%，供应商综合评价结果落在分级界限区间内的为B级和C级供应商，占总数的80%；再根据样本均值划分B类和C类供应商。

Fig.4 FT-IR spectra of pulp and nanocellulose products

TGA of the pulp and nanocellulose samples was carried out on a thermogravimetric analyzer (Netzsch STA 449 F1 Jupiter, Germany). The temperature was raised to 600℃ from room temperature at a heating rate of 10℃/min under N2 (25 mL/min). Both TGA and differential thermogravimetric (DTG) curves were recorded.

As can be seen from Table 2, for the aspen samples,the TCI of Aspen F (1.44) increased compared to aspen pulp (1.22), which was attributed to the degradation of amorphous area of the cellulose during FA hydrolysis[7,15]; however, the TCI of Aspen T was lower than that of Aspen F, indicating that a part of the crystalline cellulose molecules in Aspen F developed a disorderly structure during the process of TEMPO-mediated oxidation. This was in agreement with the corresponding CrI values (Table 2). Similar results were also reported earlier[13]. Compared to the pulp sample, the successively increasing LOI of Aspen F and Aspen T indicated the orderly structure of the obtained nanocellulose products[28]. Similarly, the successively increasing HBI values of Aspen F and Aspen T indicated that more hydroxyl groups were exposed on the surface of CNF/CNC, thereby increasing the hydrogen bonding[15]. Furthermore, the crystal sizes of Aspen F and Aspen T were smaller than that of aspen pulp. The changes in crystallinity for spruce pulp had a similar trend, but the CrI values of Spruce F (74.3%)and Spruce T (66.9%) were higher in comparison with the corresponding Aspen F (72.8%) and Aspen T(60.4%). Also, the intensity peaks of XRD for spruce samples were stronger compared to the corresponding aspen samples, as shown in Fig.5. These results were probably due to the fact that the CrI of spruce pulp was higher than that of aspen pulp (Table 2). Thus, the CrI of the resultant nanocellulose was dependent on the original CrI of the starting material under identical preparation method and process conditions.

3.3 Comparison of thermal stability of nanocellulose

Fig.6 shows the TGA and DTG curves of pulp and nanocellulose samples. The corresponding thermal degradation onset temperature (Ton) and maximum decomposition temperature (Tmax) for all the samples are given in Table 3. Compared with aspen pulp, the thermal stability of Aspen F was clearly improved as evident from the increased value of Ton, and the increase was attributed to the removal of hemicellulose and part of amorphous regions of cellulose during FA hydrolysis[13,15]; however, the Ton and Tmax values of Aspen T and Aspen S were obviously lower compared with aspen pulp (Table 3) mainly due to the fact that the carboxyl and sulfate groups introduced on the surface of CNC products reduced the activation energies of the degradation of cellulose[14]; thus, the thermal stability of pulp and nanocellulose products could be sorted in an ascending order: Aspen S＜Aspen T＜aspen pulp＜aspen F.

Table 2 Crystallinity and crystal size of pulp and nanocellulose products

Samples TCI LOI HBI CrI/% Dhkl/nm Aspen pulp 1.22±0.03 1.24±0.03 1.95±0.04 74.6±0.2 6.53±0.30 Aspen F 1.44±0.01 1.66±0.02 1.98±0.03 72.8±1.0 5.94±0.29 Aspen T 1.26±0.04 2.29±0.05 2.15±0.04 60.4±1.4 4.21±0.18 Spruce pulp 1.24±0.01 1.30±0.02 1.99±0.08 75.9±0.3 6.98±0.45 Spruce F 1.38±0.01 1.64±0.02 2.11±0.02 74.3±0.7 5.51±0.47 Spruce T 1.21±0.05 1.87±0.04 2.01±0.03 66.9±1.9 5.35±0.46

Fig.5 XRD patterns of the pulp and the nanocellulose products

Fig.6 TGA and DTG curves of pulp and nanocellulose samples products

The DTG curve of Aspen T shows two peaks in the vicinity of 224℃ and 282℃ related to the degradation of sodium anhydroglucuronate units and crystalline cellulose chains,respectively. The decreasing thermal stability of the crystalline cellulose chains in Aspen T was attributed to the presence of the thermally unstable anhydroglucuronate units (i.e.carboxyl groups)[29]. On the other hand, the smaller size of CNC could lead to exposure of a higher specific surface area to heat,thereby disrupting the crystal structure of the cellulose, and its lower crystallinity affected the thermal stability of cellulose adversely[13] . Fig.6 also exhibits the total weight losses of Aspen S, Aspen T, aspen pulp, and Aspen F as 72.4%, 73.6%,88.8%, and 92.8%, respectively. The higher char yields of Aspen S and Aspen T at 600℃ (27.6% and 26.4%,respectively) compared with the aspen pulp and Aspen F were attributed to the presence of sulfate and carboxyl groups that facilitated the char formation[14,29]. In addition, Fig.6 and Table 3 indicated a relatively higher thermal stability of the spruce samples compared to the corresponding aspen samples, which was attributed to the higher crystallinity (Table 2) and larger particle size (Table 1) of the spruce samples. For instance,the Ton values of spruce pulp, Spruce F, Spruce T, and Spruce S were 314, 329, 213, and 153℃, respectively,while the corresponding values of aspen pulp, Aspen F,Aspen T, and Aspen S were 307, 326, 212, and 146℃,respectively.

Table 3 The decomposition temperature of pulp and nanocellulose products ℃

Samples Ton Tmax Aspen pulp 307 345 Aspen F 326 347 Aspen T 212 224 Aspen S 146 203 Spruce pulp 314 347 Spruce F 329 348 Spruce T 213 293 Spruce S 153 198

4 Conclusions

This study investigated the properties of the nanocellulose products fabricated from the bleached aspen and spruce pulps under the same conditions by comparing their characteristics. It was found that the aspen pulp underwent hydrolysis and reaction more easily than the spruce pulp, and the resultant nanocellulose products derived from the aspen pulp exhibited a relatively smaller particle size, lower crystallinity, and lower thermal stability compared to the corresponding products obtained from the spruce pulp. This was attributed mainly to the relatively longer fiber length,higher crystallinity, and higher thermal stability of the spruce pulp compared to the aspen pulp; thus, the study helped establish that the starting materials influenced the properties of the resultant nanocellulose products under identical process conditions.

Acknowledgments

测量的同时对现场的车流量进行统计，得到表11。由表11发现，下行方向的车流量大于上行方向；且根据现场观察得知，上下行的车型无显著差别，以轿车和大中型卡车为主，但下行方向出现过车辆自行加高挡板的超载情况，即下行方向的行车荷载大于上行方向。

Financial support was from the Johan Gadolin Scholarship Programme at the Johan Gadolin Process Chemistry Centre at Åbo Akademi University (Finland),and the National Natural Science Foundation of China(No. 31470609). The authors would also like to thank Dr. Markus Peurla at Lab. of Electron Microscopy,University of Turku (Finland) for helping on the TEM measurements.

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