Tian Tan,Xiaoning Tang,Heng Zhang,Xin Gao,*

1.Faculty of Chemical Engineering,Kunming University of Science and Technology,Kunming,Yunnan Province,650500,China

2.CAS Key Laboratory of Magnetic Materials and Devices,Ningbo Institute of Materials Technology and Engineering,Chinese Academy of Sciences,Ningbo,Zhejiang Province,315201,China

Abstract:Cellulose nanofibrils(CNFs)were obtained through one-step mechano-partial dissolution by ball milling in N,N-dimethyl acetamide with a low concentration of LiCl from agricultural waste bagasse pith(BP).Compared with fibrous cellulose,parenchyma cellulose(PC)is less uniform in diameter and less aligned,causing PC to dissociate into CNFs during this process without pretreatment.The results showed that the CNFs prepared from PC of BP had a diameter of 30-200 nm and a length of several micrometers.The as-obtained CNFs,along with dissolved cellulose,formed a wet-stable and highly transparent nanopaper in a sorbitol aqueous solution bath,which exhibited a high strain of 101%and a low Young's modulus of 4.3 MPa owing to the addition of the plasticizer sorbitol.This type of nanopaper with favorable transparency,high tensile property,and low Young's modulus has great potential for use as electronic skin and medical dressing material.

Keywords:cellulose nanofibrils; parenchyma cellulose;bagasse pith;partial dissolution;tensile strength;Young's modulus

1 Introduction

Cellulose is often considered one of the most important natural resources that can be obtained from different sources such as wood,agricultural biomass,marine animals(tunicates),algae,and fungi[1-2].With advances in nanoscience,researchers have focused on the production and application of nanocellulose(NC)on an industrial scale.Depending on the configurations and production conditions,NC can be divided into three main categories: cellulose nanocrystals (CNCs),cellulose nanofibrils(CNFs),and bacterial cellulose(BC)[3-5].Compared with BC and CNC,CNF is a natural nanomaterial that is extensively used because of its nanoscale lateral dimensions,high mechanical strength,good flexibility,and tendency to form strong entangled networks[6].These properties enable the application of CNFs in hydrogels, aerogels,biomedicine,and other fields[7].Conventionally,ultrasonic,grinding,chemical,enzymatic,and other physical methods have been used to destroy the hydrogen bonds among celluloses from wood and nonwood sources to obtain CNFs[8-10].In extensive production,fibrous cells of higher plants remain the main source of cellulose[11].However,the multilevel cell wall structure and tight cellulose chains make cellulosic fibers more resistant[12], resulting in excessive energy consumption and equipment damage when using physical treatment to obtain CNFs.Therefore,a series of pretreatments,such as acid prehydrolysis,alkaline treatment,oxidative treatment,and enzymatic pre-hydrolysis,is usually required before the physical process[13].In addition to fibrous cells,plant tissues contain several other cell types,including parenchyma cells[14-15].Parenchyma cells are an alternative source of cellulose that have been reported rarely and may be a potential raw material for CNFs.

In China,the annual output of bagasse pith(BP)is approximately 60 million tons[16].Being rich in nonfibrous cells,BP is considered an agricultural and industrial waste.In industrial production,BP is usually used as a supplementary fuel for boilers at cane mills as well as bagasse pulp mills[16].However,as an important biomass resource,BP has greater value in applications other than simple burning.Cellulose microfibrils in parenchyma cell walls are less uniform in diameter and less aligned than those in fibrous cells[17].Moreover,parenchyma cellulose(PC)has lower crystallinity and a larger surface area compared with fiber cellulose[14].Several studies have shown that PC has distinct advantages for the preparation of functional materials.For example,Lamaming et al[18]reported that CNCs isolated from oil palm trunk PC exhibited lower crystallinity than that of vascular bundle cellulose.Ren et al[12]further found that cellulose chains in bamboo parenchyma were looser than those of fiber cellulose.In our early research,PC from maize stalk pith(MSP)was found to be dissolved in N,N-dimethyl acetamide(DMAc)/LiCl more easily than fiber cellulose[14].Based on the above features and examples,we speculate that PC can be dissociated more easily than fibrous cellulose to obtain CNFs.

In this study,PC from BP(BPPC)was chosen to produce CNFs through one-step mechano-partial dissolution by ball milling in DMAc with a low concentration of LiCl.The resultant CNFs had diameters of 30-200 nm and lengths of several micrometers.CNFs were used to form wet-stable nanopapers during the regeneration process in a water bath with the addition of sorbitol as a plasticizer;these nanopapers showed high transparency,favorable tensile strength,and a low Young's modulus.

2 Experimental

2.1 Materials and chemicals

BP was collected from Yunnan Xinping Nan'en Sugar and Paper Co.,Ltd.(Xinping,Yunnan Province,China),air-dried and sieved using 40 and 60 mesh stainless steel screens.

Sodium chlorite,acetic acid,potassium hydroxide,and DMAc were purchased from Aladdin(Shanghai,China)and used without further purification.Sorbitol was supplied by Shanghai Macklin Biochemical Co.,Ltd.,China.All reagents were of analytical grade,and deionized(DI)water was used in all experiments.

2.2 Preparation of CNFs from BP

PC was obtained by removing lignin and hemicellulose from BP using sodium chlorite/acetic acid and potassium hydroxide,respectively,as described in detail in our previous study[19].The entire procedure for obtaining CNFs is illustrated in Fig.1.Dried BPPC(1 g)was added to a 1.5 L nylon container and diluted to 2 wt%with DMAc.Subsequently,pre-lyophilized LiCl(3:100,w/V,compared to DMAc)and 0.5-1.5 mm zirconia balls were added to the nylon container.The milling process was performed at 400 r/min for 24 h using a ball mill(DECO-PM-1.5L,Changsha Deco Equipment Co.,Ltd.,China)to obtain CNFs.

2.3 CNF yield measurement

The CNF yields were measured using gravimetric analysis.The obtained CNFs were washed with DI water and freeze-dried to remove moisture;the final dried CNFs were designated asM2.The weight of the initially air-dried BP was determined asM1.The yield(Y)was calculated from the average of three parallel runs.The CNF yield was calculated as follows[20].

2.4 Formation of nanopaper

The partially dissolved CNFs were spread on a PTFE plate and placed in a BPZ-6123 vacuum drying oven(Shanghai Yiheng Scientific Instruments,Shanghai,China)at 25℃under vacuum(0.7 MPa for 1 h)to remove air bubbles.Subsequently,the PTFE was immersed in a 5 wt%sorbitol aqueous solution bath to regenerate the cellulose (Fig.1).The resulting nanopaper was removed from the PTFE plate and washed with DI water for subsequent experiments.

Fig.1 Schematic for the preparation of CNFs and nanopaper

2.5 Characterization

Wide-angle X-ray diffractograms (XRD) of BP,holocellulose,BPPC,and the nanopaper were recorded using an X'Pert 3 power diffractometer(PANalytical Co.,Ltd.,The Netherlands).The samples were then placed on a glass sample holder.Radial scans of intensity were recorded under ambient conditions over scattering 2θangles ranging from 5°to 60°(step size=0.01313°,scanning rate=13.77 s/step)using Cu Kαradiation(λ=1.5406Å),an operation voltage of 40 kV,and a filament current of 40 mA.The crystallinity index (CrI)was calculated based on the peak deconvolution results[21].

whereAtotalandAamrepresent the total area of the diffractogram and of the amorphous peak,respectively.The total intensity of the(200)peak of cellulose I was measured near 2θ=22.3°,and that of the(020)peak for cellulose Ⅱ was measured near 2θ=20.4°[19].Amorphous intensity was measured near 2θ=18°for cellulose I and 2θ=16°for cellulose II.

The room was only 13 square meters with two nails on the wall next to the door, one for her to hang her bag and the other for hanging umbrellas. At that time, she threw all her stuff such as her bag and umbrella on the floor once she entered the room, staring at the whole mess and feeling terribly upset.

Fourier transform infrared (FT-IR)spectroscopy measurements of BP,holocellulose,BPPC,and CNFs were obtained using a Vertex 70 FT-IR spectrophotometer(Bruker,Billerica,MA,USA).All samples were analyzed using the KBr disk method,and all spectra were recorded in the wavenumber range of 500-4000 cm-1at a spectral resolution of 4 cm-1.The nanopaper was analyzed with the attenuated total reflection(ATR)method using a Nicolet 6700 FT-IR spectrophotometer(Thermo Fisher,USA).The spectra were recorded in the reflection mode in the wavenumber range of 500-4000 cm-1.

The morphologies of BPPC,CNFs,and nanopaper were observed using a Nova Nanosem 450 scanning electron microscope(SEM,FEI,Hillsboro,OR,USA)at an accelerating voltage of 5.0 kV.Transmission electron microscopy(TEM)image of CNFs was recorded on a Tecnai G2 F30 S-Twin(FEI,Hillsboro,OR,USA)operated in the high-contrast mode at 100 kV.Atomic force microscopy(AFM)image of CNFs was observed using a Dimension Icon(Bruker,USA).

The optical transmittance of the nanopaper was measured using a TU-1950 ultraviolet-visible spectrophotometer(UV-vis,PERSEE,China)at a wavelength range of 200-800 nm.

The nanopaper was cut into rectangular strips of 15 mm×50 mm for tensile testing.An ETM 103A electromechanical universal testing machine(Shenzhen Wance Testing Machine Co.,Ltd.,China)was used to determine the Young's modulus and tensile strength of the nanopaper at a constant extension speed of 1 mm/min.At least five specimens of each sample were tested,and average values were reported.

3 Results and discussion

The chemical structures of BP,holocellulose,BPPC,and CNFs are compared in Fig.2(a).The absorption peaks at 3400 and 2900 cm-1were attributed to O—H stretching and C—H stretching in all spectra[20].For the BP sample,the peaks at 1730 cm-1represent the acetyl and uronic ester groups or the ester linkage of the carboxyl groups of ferulic andp-coumaric acids of lignin and hemicellulose[16].The adsorption peak at 1633 cm-1was assigned to the H—O—H stretching of the absorbed water in the sample[19].The intensity of the peak at 1514 cm-1in BP was attributed to the aromatic ring vibration and C—H deformation vibration of lignin[16].The peak at 1246 cm-1was attributable to ester,ether,or phenol compounds[22].The bands at 1160,1056,and 898 cm-1were associated with C—O antisymmetric bridge stretching,C—O—C pyranose ring skeletal vibration,andβ-glycosidic linkages between the glucose units in cellulose,respectively[9,16].After delignification,the peaks at 1514 and 1246 cm-1disappeared and weakened,respectively,in holocellulose,indicating the removal of lignin and partial removal of hemicellulose.For the BPPC sample,the absorption peak at 1730 cm-1disappeared,and the peak at 1246 cm-1weakened,indicating the removal of hemicellulose and lignin.Further,the FT-IR spectrum peak of the CNFs was consistent with that of cellulose.Fig.2(b)shows the ATR-FT-IR spectrum of the nanopaper.The peaks at 1085 and 1052 cm-1were associated with the stretching of C—OH and C—C—OH bonds for the secondary and primary alcohols of sorbitol,respectively[23].In the formation of the nanopaper,partially dissolved CNFs were regenerated in a sorbitol aqueous solution bath.The hydroxyl groups of cellulose and sorbitol were formed through hydrogen bonding in this process,and a wet-stable nanopaper was formed.

Fig.2 (a)FT-IR spectra of BP,holocellulose,BPPC,and CNFs;(b)ATR-FT-IR spectrum of nanopaper

To dissect the influence of representative samples on cellulose crystallinity,the XRD spectra of BP,holocellulose,BPPC,and nanopaper were analyzed,and the results are shown in Fig.3.There were three typical peaks of BP,holocellulose,and BPPC located at 2θ=15.8°,22.3°,and 34.5°,corresponding to the diffraction of(110),(200),and(400)lattice planes of cellulose I,respectively[24].This indicates that the crystal structures of these samples did not change during delignification and hemicellulose removal.However,theirCrIvalues were different.According to Eq.(2),theCrIvalues of BP,holocellulose,and BPPC were 50.4%,56.5%,and 60.6%,respectively.Owing to the removal of hemicellulose and lignin in the amorphous regions,which led to the realignment of cellulose chains,the crystallinity of the three samples was gradually improved[16].These results indicate that the crystallinity of cellulose from BP parenchyma cells was lower than that of cellulose from fibers,according to our previous studies[14,25].This may explain why BPPC is more easily dissociated into CNFs than cellulose from fibers.Further,the diffraction intensity around 2θ=12°of the nanopaper was enhanced,which corresponds to the(11ˉ0)lattice planes of cellulose II[20,26],suggesting that part of cellulose I was transformed into cellulose II.Additionally,dissolution and regeneration greatly influenced theCrIvalue of cellulose.Based on Eq.(1),theCrIvalues of cellulose I and cellulose II were 58.9%and 6.16%,respectively.These results demonstrated the coexistence of cellulose I and cellulose II in the nanopaper.

Fig.3 XRD patterns of BP,holocellulose,BPPC,and nanopaper

In this study,CNFs were obtained through one-step mechano-partial dissolution by ball milling.The CNF yield was calculated using Eq.(1).Owing to the removal of amorphous regions (lignin and hemicellulose),the CNF yield was 35.5%.To observe the change in cellulose morphology,SEM images of BPPC and CNFs are compared in Fig.4.Fig.4(a)and Fig.4(b)shows the SEM images of BPPC;the PC isolated from BP showed a translucent flaky shape,and apparent fibrillation on parenchyma cells,resulting in its fuzzy and rough surface.After milling in the DMAc/LiCl system,cellulose was partly dissociated and then fibrillated.As shown in Fig.4(c)and Fig.4(d),CNFs with diameters of 30-200 nm were easily produced.This process involved the combined action of chemical dissolution and mechanical treatment.DMAc/LiCl is an important solvent system wherein the tight binding of Li+and DMAc carbonyl leaves free Cl-.Subsequently,the interaction of Cl-with hydroxyl protons in cellulose leads to competitive new Hbonding and destruction of the existing intermolecular hydrogen bonds among cellulosic chains[27].Parts of the hydrogen bonds are destroyed in partially dissociated cellulose,which is beneficial to physical treatment.Accompanied by the compression and friction between cellulose and zirconia balls,BPPC was successfully defibrillated,and consequently,nanosized CNFs were obtained[28].The TEM image in Fig.5(a)and the AFM image in Fig.5(b)again confirm that the diameter of the CNFs was between 30-200 nm,and that their lengths were several micrometers.

Fig.4 SEM images of BPPC(a and b)and CNFs(c and d)

Fig.5 TEM image(a)and AFM image(b)of CNFs

Fig.6 illustrates the stability of BPPC and CNF suspensions(1 wt%and 2 wt%in DMAc).After being obtained through ball milling,CNFs were dispersed as nanofibers that were stably suspended in DMAc after immobility for 24 h.In contrast,BPPC tended to be aggregated and precipitated soon owing to the existence of macro-sized BPPC flakes.

Fig.6 Suspensions of BPPC and CNFs in DMAc

As illustrated in Fig.1,a wet-stable nanopaper was formed during regeneration of the mixture of CNFs and dissolved cellulose in a water bath with sorbitol added as a plasticizer.As shown in Fig.7(a)of the digital photo and UV-vis spectrum,the nanopaper had a high transparency of up to 95.8%at 800 nm.To explain this phenomenon,SEM images were used to observe the nanopaper morphology.As shown in Fig.7(d),a nanonetwork structure formed by the CNFs was observed.Further,the nanoscale network structure ensured that the nanopaper has high transparency and tensile properties.Because of the short diameter of CNFs(30-200 nm)and the nano-network structure described above,the fibers and voids between them were much smaller than the wavelength of visible light[10],rendering the nanopaper transparent.Fig.7(e)shows an SEM image of a cross-section of the nanopaper.The smooth and tight morphology of the nanopaper resulted in less light scattering and allowed more light to pass through,resulting in high transmittance[29-30].

Fig.7 UV-vis spectrum and digital image(the inset image)of nanopaper(a);stress-strain curve of nanopaper(b);tensile properties and Young's modulus of the nanopaper compared with other cellulose film materials(c);SEM images of longitudinal(d)and cross-section(e)of the nanopaper

The nanopaper exhibited an excellent strain of 101%and a low Young's modulus of 4.3 MPa as illustrated by the stress-strain curve in Fig.7(b)and Fig.7(c).Fig.7(c)shows the Young's modulus and tensile properties of the nanopaper compared with those of other cellulose films (pine cellulose sheets (PCS)[31],eucalyptus cellulose sheets (ECS)[31],corn stalk cellulose sheets(CTSCS)[31],wood cellulose composite hydrogel (WCCH)[32], all-cellulose cross-linked hydrogels(A-CCH)[33],carrot nanofibers networks(CNN)[34],and nanofibers networks from brewer spent grains(BSGNN)[34]).The nanopaper had the highest strain and lowest Young's modulus among the films included,owing to the addition of sorbitol and the nano-network structure[35-37].The plasticizer has been reported to enter cellulose chains owing to its small molecule size,which enhances the flexibility,ductility,and extensibility of the nanopaper by decreasing the intermolecular forces among cellulose chains[38].The interference of sorbitol with the chain-to-chain hydrogen bonding of polysaccharides increases the chain mobility of cellulose[39].Thus,the resulting nanopaper becomes stretchable.

4 Conclusions

Cellulose nanofibrils (CNFs) were successfully obtained from parenchyma cellulose(PC)of bagasse pith(BP)via one-step mechano-partial dissolution by ball milling in N,N-dimethyl acetamide(DMAc)with a low concentration of LiCl.The as-obtained CNFs and dissolved cellulose formed a wet-stable and highly transparent nanopaper in a sorbitol aqueous solution bath.Fourier transform infrared (FT-IR)spectra indicated that lignin and hemicellulose from BP were removed,and pure BPPC was obtained.With the removal of lignin and hemicellulose from the amorphous region,the crystallinity index(CrI)of cellulose I gradually increased from 50.4%to 60.6%.Owing to the partial dissolution process,the nanopaper showed two crystal forms,cellulose I and cellulose II,and theCrIwas 58.9% and 6.16%,respectively,indicating the co-existence of cellulose I and cellulose II in the nanopaper.Scanning electron microscope(SEM),transmission electron microscopy(TEM),and atomic force microscopy(AFM)images of the CNFs proved that BPPC dissociated into CNFs with a diameter of 30-200 nm and a length of several micrometers.Owing to the reduction in size,CNFs showed good stability after being dispersed in DMAc for 24 h.The nanoscale network structure provided the nanopaper with a high transparency of up to 95.8%.Addition of the plasticizer—sorbitol,resulted in a high strain of 101%and a low Young's modulus of 4.3 MPa.Therefore,the nanopaper has potential applications in wearable technology and stretchable electronics.

Acknowledgments

The authors are grateful for the financial support from the National Natural Science Foundation of China(Grant No.51963012)and Special Foundation for Excellent Youth Scholars of Yunnan Province,China(Nos.YNWR-QNBJ-2020-039 and YNWR-QNBJ-2020-045).