Isolation and Characterization of Cellulose Microfibrils and Nanocrystals from Corn Silk

MENGISTU Tessema， YANG Xue( )， HASSAN Mussana， JIANG Shuai( )， YU Jianyong()， LIU Lifang()*

1College of Textiles，Donghua University，Shanghai201620，China2The Key Lab of Textile Science&Technology，Ministry of Education，Donghua University，Shanghai201620，China3Innovation Center for Textile Science and Technology，Donghua University，Shanghai201620，China

Abstract：For the value-added utilization of discarded agricultural wastes，corn silk(CS)obtained abundantly in the farming field has been tested as a new source of cellulosic materials.Cellulose microfibril(CMF)and cellulose nanocrystal(CNC)were isolated from CS by ethanol and alkali pretreatments，and acid hydrolysis.The characterization was performed by scanning electron microscopy(SEM)，F(xiàn)ourier transform infrared spectroscopy(FT-IR)，X-ray diffraction(XRD)，thermogravimetric analysis(TGA)and transmission electron microscopy(TEM).After chemical pretreatments，the lignin，hemicelluloses and other non-structural components were removed.The degree of crystallinity and thermal stability of CMF and CNC were increased compared to raw CS.The crystallinity indexes of CS，CMF and CNC were 45.90%，65.77%，and 73.75%respectively.The CNC was flat and rod like shape with diameter and aspect ratio range of 13.96-33.69 nm and 34.34-23.02 nm respectively.The nanocrystals had an alternative potential to be used as reinforcing filler for bio-nanocomposites preparation.

Key words：agricultural waste；corn silk(CS)；cellulose microfibril(CMF)；cellulose nanocrystal(CNC)；new cellulose source

Introduction

In recent decades， many researchers focused on different agricultural and industrial wastes. Among those wastes， cellulose is remarkably abundant， renewable， biodegradable， low cost and non-toxic in nature. Nowadays cellulose has good potential for the isolation of cellulose microfibril(CMF) and cellulose nanocrystal(CNC)[1-2]. According to the demention， the diameters of CMF and CNC are both less than 100 nm， but the length of CMF is several micron and the length of CNC is 100-300 nm[3]. The CMF and CNC have special features， such as high surface area to volume ratio， biodegradable， excellent mechanical properties， low coefficient of thermal expansion， non-toxicity， chemically functional and easy modified. Due to these advantages， they are applied in wide range of fields such as nanocomposites， functional surfaces， biomedical， bioimaging and others[1-2， 4-6]. Non-conventional sources of cellulose from agriculture wastes have been extensively studied for CNC isolation[7]. However， there are still various unexplored and underutilized valuable cellulose sources from agricultural residues； among those sources, corn silk(CS) is one of the promising materials. CS is a thread like strands from the female flower of corn and it consists of crude fiber， extractives(such as flavonoids， steroids， alkaloids and terpenoid)， protein， wax and ashes. On the CS， there are many studies regarding utilization of bioactive extractive components for pharmaceutical and traditional health remedies[8-9]. However， there is a knowledge gap about CS used as new sources of cellulose materials for isolation of nanocrystals and their application. The aim of this study is to research isolation and characterization of CMF and CNC from CS to test their potential as a new source of cellulose for CNC.

1 Experimental

1.1 Materials

The dried CS was obtained from Shandong Province， China. Ethanol， Sodium hydroxide(NaOH)， hydrogen peroxide(H2O2) and Sulphuric acid(H2SO4) chemicals were purchased from Sinopharm Chemical Reagent Co.， Ltd.， Shanghai， China.

1.2 Isolation of CMF and CNC

Isolation processes of nanocellulose from ligno-cellulosic biomass mainly depends on plant cell walls， the contents of non-cellulosic components of plant cell wall， availability of processing equipments and chemicals， and final application of nanocelloses. According to literature， alkaline and bleaching chemical treatments are effective method to get pure CMF and to enhance CNC isolation. Sulphuric acid CNC isolation is the most extensively used to breakdown amorphous domains and local interfibrillar contacts of cellulose[3， 10]. Based on these perspectives， the CS was cleaned with tap water at first and then dried in an air oven at 60 ℃. To isolate CMF and CNC， CS was treated with 60% (volume fraction) of ethanol for 1 h at 70 ℃ to remove extractives[11]. After drying， the rest of CS was milled with a milling machine mainly and sieved with 80 mesh sieve. Hemicelluloses， lignin， and remaining non-cellulosic components were removed by alkali and bleaching treatments[10]. Alkali treatment was performed with 4% (mass fraction) of NaOH for 1 h at 80 ℃ and then washed with distilled water. After filtering alkali treated CS， bleaching was achieved by 4% (mass fraction) of NaOH and 4% (mass fraction) of H2O2for 20 min at 70 ℃. The bleached CS was washed with distilled water till the pH values reached 7 and filtered to get CMF for further hydrolysis. The acid hydrolysis treatment was performed with sulphuric acid(60%, mass fraction solution of H2SO4with CS to liquor ratio of 1∶20) for 2 h at 60 ℃ under continuous stirring to get CNC. After 2 h， hydrolysis was quenched by adding an excess of distilled water to the reaction mixture and the resulting mixture was cooled to room temperature. Then， the suspension was centrifuged using H1650 supercentrifuge(Hunan Xiangyi Laboratory Instrument Development Co.， Ltd.， Changsha， China) at 5 000 r/min for 15 min and the supernatant was discarded until it became turbid. The colloidal suspension was then homogenized for 5 min using an IKA T25 Digital Ultra Turrax( Laboratory Technology Co.， Ltd.， Staufen， Germeny). This centrifugation step was repeated several times before the suspension was dialyzed against distilled water for several times until pH value reached neutral.

1.3 Characterizations

1.3.1Compositionalanalysis

The compositional contents of α-cellulose， hemicelluloses， lignin and extractives of CS were determined according to woody and non-woody biomass analysis method[12].

1.3.2Apparentshapeandmorphology

The morphology of CS and freeze-dried CMF were characterized by JSM-5600LV scanning electron microscopy(SEM， JEOL Ltd.， Tokyo， Japan). The apparent shape and size of CNC were determined by JEM-2100 transmission electron microscopy(TEM， JEOL Ltd.， Tokyo， Japan).

1.3.3Fouriertransforminfraredspectroscopy(FT-IR)spectra

FT-IR spectra of the samples were obtained by Nicolet 8700 FT-IR spectrometer(Thermo Fisher Scientific Co., Ltd.， Waltham， MA， USA) in the range of 4 000-500 cm-1.

1.3.4Crystallinityindex

The X-ray diffraction (XRD) of the samples was examined by D/MAX 2550 PC XRD diffractometer(Rigaku， Tokyo， Japan) at room temperature with a monochromatic CuKα radiation source(λ=0.154 056 nm) in the step-scan mode with a 2θangle ranging from 5° to 90° with a step of 0.02 and scanning time of 5.0 min. The crystallinity index(Cr) was calculated using Eq.(1)[13].

Cr/%=(I002-Iam)/I002×100，

(1)

whereI002is the maximum intensity of lattice diffraction peak(at 2θ=22°) andIamis the intensity scattered by the amorphous part of the sample at 2θ=18°.

1.3.5Thermalstability

The thermal stability of the samples was tested with TG209F1 thermogravimetric analyzer(NETZSCH Instrument Trading Ltd.， Selb， Germany). About 5 mg of each sample was heated from 30 ℃ to 600 ℃ at a heating rate of 10 ℃/min under a nitrogen flow rate of 19.8 cm3/min.

2 Results and Discussion

2.1 Chemical analysis

The compositional analysis results revealed that the CS composed of cellulose(30.81±1.70)%， hemicelluloses(24.39±3.30)%， lignin(25.59±1.60)% and extractives(17.73±0.80)%. The cellulose was the dominant one in the CS. Therefore， the CS can be valuable new cellulose source potential for isolation of CMF and CNC， and as well as extraction of chemicals such as lignin and extractives.

2.2 Apparent shape and morphology

The apparent shape and morphology of the CS， CMF， and CNC are shown in Fig. 1. Originally the CS(Fig. 1(a)) is a brownish color， but both of the CMF(Fig. 1(b)) and CNC(Fig. 1(c)) are white. Morphologically， the CS(Fig. 1(d)) looks like thick and opaque due to bundles of the fibrils bundled with lignin. However， CMF(Fig. 1(e)) is thin and seems like a light transparent sheet of fibrils. This is mainly due to the alkaline and bleaching treatments which degrade and remove amorphous components of lignin， hemicelluloses and wax bound firmly to the α-celluloses of CS[8]. Hydronium ions from sulphuric acid induced into the amorphous region of CMF and cleaved down loosely held glycosidic bonds of celluloses to release nano-scale crystals[10]. Finally， the CNC(Fig. 1(f)) exhibits flat and road like shape with diameter and aspect ratio range of 13.96-33.69 nm and 34.34-23.02 nm respectively.

Fig. 1 Apparent shape and morphology of the CS， CMF and CNC:(a) raw CS；(b) CMF；(c)freeze dried CNC；(d)SEM image of CS；(e) freeze dried CMF; (f)TEM image of CNC

2.3 FT-IR analysis

Fig. 2 FT-IR spectra of CS， ethanol-treated CS， CMF and CNC

2.4 XRD analysis

The XRD patterns of CS， CMF and CNC are shown in Fig. 3. The diffraction intensity peaks of CMF and CNC at around 2θ=16°， 18° and 22° were distinctively changed. Especially at 22°， the peak of CNC was shapely increased. The changes were due to the removal of non-cellulosic components from CS and acid hydrolysis which induces hydronium ions to break amorphous regions of CMF and finally releases the individual crystallites[17]. Thus， the crystallinity index values of raw CS， CMF， and CNC calculated using Eq. (1) are 45.90%， 65.77% and 73.75% respectively.

Fig. 3 XRD patterns of CS， CMF and CNC of CS

2.5 TGA analysis

The TGA values of samples are shown in Fig. 4. All of the samples have exhibited multiple steps of thermal degradation. Initially， the weights of the samples were decreased around 100 ℃， which was due to the removal of moisture from the samples. The major thermal degradation temperature of the CMF was higher than that of CS. The increased thermal stability might be attributed to the removal of amorphous non-cellulosic components as evidenced by FT-IR and XRD results. Section 2.4 showed that the crystallinity of CNC was higher than CMF. However， the thermal stability of CNC was nearly similar to CMF. This was probably due to the introduction of sulfate groups into cellulose crystals during sulfuric acid hydrolysis. The sulfate groups introduced to the surface of cellulose crystals caused dehydration and reduced the CNC thermal stability[18].

(a)

(b)

3 Conclusions

The CMF and CNC were isolated from CS by alkali and acid hydrolysis. Morphologically， the CMF was flat and the CNC was flat and road like shape with diameter of 13.96-33.69 nm and 34.34-23.02 nm respectively. The degree of crystallinity and thermal stability of the CMF and CNC were increased. Conclusively， the CS can be considered as a new source of cellulose and is expected to have a high potential for the value-added utilization in the form of CMF and CNC. The CMF and CNC can be used as eco-friendly cellulosic nanofillers for the bionanocomposites for the use in diverse areas, such as caffold， drug delivery， biomedical food packaging and other applications. In addition to nanofillers， the CS can be used for chemical extractions such as lignin and extractives.