Abstract
Cellulose nanocrystals (CNCs) were produced with different pre-mixing times between the cotton fiber and the sulfuric acid at room temperature, prior to the reaction at 45 °C. The CNC0 and CNC60 films were prepared using vacuum filtration methods. Based on transmission electron microscopy observations, the dimension and yield of CNCs gradually decreased with increasing pre-mixing time. Considering the balance of yield and quality of CNCs, CNC0 was chosen as the optimal product. The synthetic process played an important role in the production of CNCs. Various CNCs had similar crystallinity index values with the increased pre-mixing time. The decreased contact angle was the result of decreased dimensions of CNCs or the additional sulfate group at the surface of the CNCs. Both thermogravimetric and contact angle analysis are sensitive for the constituents of CNCs.
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The Influence of Pre-treatment Time and Sulfuric Acid on Cellulose Nanocrystals
Ya-Yu Li,a,b,c Bin Wang,a Ming-Guo Ma,a,* and Bo Wang a
Cellulose nanocrystals (CNCs) were produced with different pre-mixing times between the cotton fiber and the sulfuric acid at room temperature, prior to the reaction at 45 °C. The CNC0 and CNC60 films were prepared using vacuum filtration methods. Based on transmission electron microscopy observations, the dimension and yield of CNCs gradually decreased with increasing pre-mixing time. Considering the balance of yield and quality of CNCs, CNC0 was chosen as the optimal product. The synthetic process played an important role in the production of CNCs. Various CNCs had similar crystallinity index values with the increased pre-mixing time. The decreased contact angle was the result of decreased dimensions of CNCs or the additional sulfate group at the surface of the CNCs. Both thermogravimetric and contact angle analysis are sensitive for the constituents of CNCs.
Keywords: Cellulose nanocrystals; Sulfuric acid; Pre-mixing time; Characterization
Contact information: a: Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, PR China; b: College of Chemical Engineering, Xinjiang Agricultural University, Urumqi 830052, Xinjiang Province, PR China; c: Xinjiang Blue Ridge Tunhe Profiles Co., Ltd., Changji 831100, Xinjiang Province, PR China; * Corresponding author: mg_ma@bjfu.edu.cn
INTRODUCTION
Cellulose nanocrystals (CNCs) have been developed for their distinct properties, e.g., biocompatibility, high elastic modulus (Wang et al.2012; Zhang et al. 2016), high specific surface area (Huang et al.2016; Li et al. 2017), distinct optical properties (Csiszar and Nagy 2017; Zhang et al. 2017), and high aspect ratio (Chen et al. 2015). CNCs have been added into matrix to prepare mechanically enhanced materials (Gawryla et al. 2009; Pei et al. 2011; Meesorn et al. 2017; Sapkota et al. 2017; Nicharat et al. 2017), mechanically-adaptive materials (Mendez et al. 2011; Way et al. 2012; Biyani et al. 2013; Biyani et al. 2014), self-healing materials (Imato et al. 2017), synthesis templates (Padalkar et al. 2010), biosensors (Schyrr et al. 2014), and so on. Sobolciak et al. (2017) modified co-polyamide nanofibers using CNCs and a one-step method from date palm leaves, which showcased the increased mechanical properties and hydrophilicity. Yan et al. (2017) prepared bacterial cellulose nanocrystals and verified their high thermal stability and good emulsifying performance compared with bacterial cellulose. Multifunctional composites with enhanced mechanical strength and antibacterial properties were prepared by melt extrusion of three components including poly(lactic acid), CNC, and silver nanoparticles in food packages application (Fortunati et al. 2012). Shanmuganathan et al. (2010) introduced percolating networks of CNCs into polymer matrix as adaptive substrates for intracortical electrodes. When this nanocomposite was immersed into emulated physiological solution, its tensile storage moduli can be reduced significantly from 40 MPa to 5 MPa with only about 28% w/w swelling. Yu et al. (2017) obtained electrospun composite nanofibers by adding CNCs/polyethylene glycol (PEG) to poly(lactic acid) (PLA). In comparison to neat PLA, the addition of CNCs/PEG reduced fiber diameters, enhanced fiber uniformity, and decreased the water contact angle (CA) of composite nanofibers with 10 wt.% CNCs/PEG from 117.3° to 98.0°.
There are various strategies to produce CNCs. Diluted sulfuric acid was firstly used for CNCs preparation in 1951 by Rånby. Concentrated sulfuric acid was determined to be a better option in CNC production by Mukherjee and Woods (1953), and actually their procedure is the precursor for the ubiquitous sulfuric acid method still in use today. Chieng et al. (2017) produced CNCs by H2SO4 acid hydrolysis of oil palm mesocarp fiber after the removal of hemicellulose and lignin. Sampath et al. (2017) prepared CNCs of approximately 200 nm to 300 nm in length and 40 nm to 50 nm in width from microcrystalline cellulose (MCC) via sulfuric acid hydrolysis. This process was used to fabricate CNCs-chitosan hydrogels with improved mechanical properties and pH sensitivity. The shape and size distribution of CNCs extracted from cotton, Avicel, and tunicate have been investigated comprehensively using transmission electron microscopy, atomic force microscopy, and X-ray scattering (Elazzouzi-Hafraoui et al. 2008). In addition to sulfuric acid hydrolysis method, Kontturi’s group developed a hydrogen chloride acid vapor hydrolysis method followed by TEMPO oxidation towards more effortless isolation of cellulose nanocrystals (Kontturi et al. 2016; Lorenz et al. 2017; Lee 2018). Besides acid hydrolysis, Trache et al. (2017) summarized six categories of methods including mechanical treatment, oxidation method, enzymatic hydrolysis, ionic liquid treatment, subcritical water hydrolysis, and combined process.
There are several strategies to optimize the synthetic condition of CNCs using the sulfuric acid hydrolysis method. Beck-Candanedo et al. (2005) examined the effects of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp. They found that longer hydrolysis times produced shorter and less polydisperse black spruce CNCs. Bondeson et al. (2006) optimized the production of CNCs from MCC with a sulfuric acid concentration of 63.5% (w/w) and a reaction time of 2 h using response surface methodology. Ioelovich (2012a, b) reported the optimal conditions for obtaining CNCs and studied carefully the dissolution process of cellulose in various concentrations of sulfuric acid and temperatures. The study found that the optimal conditions of the acidic treatment for the isolation of CNCs were the sulfuric acid concentration of 57 wt.% to 60 wt.%, acid to cellulose ratio of 8 to 10, and temperature of 45 °C to 55 °C for 40 to 60 min.
The mechanism of sulfuric acid hydrolysis of cellulose is usually explained by selective acid hydrolysis of disordered (amorphous) domains of cellulose nano-fibrils; the more resistant nanocrystallites remain intact and can be isolated in a form of rod-like particles (Habibi et al. 2010). Camacho et al. (1996) analyzed the effects of temperature (25 °C to 40 °C), H2SO4 concentration (31% to 70% (w/v)), and the acid/substrate relationship (1 cm3 to 5 cm3 of H2SO4per g of cellulose) on the solubilization rate of MCC and glucose production. They found that the solubilization followed zero-order kinetics and the production of glucose was a two part consecutive first-order pseudo-homogeneous reaction. Wang et al. (2014) used three phenomenological reactions to express the solubilization of cellulose fibers, which could be described by pseudo-homogenous first-order kinetics.
Nevertheless, the interaction between cellulose and sulfuric acid is not completely understood (Moon et al. 2011). For example, the process of hydrolysis of cellulose can be divided into two sequential processes, specifically of the solubilization and the depolymerization of cellulose. The solubilization process of cellulose in sulfuric acid includes the swelling and dissolving of cellulose, meaning that the acid penetrates the noncrystalline and crystalline domains of cellulose and forms complexes (Ioelovich 2016). The depolymerization process means the decrease of degree of polymerization (DP) of cellulose macromolecular, particularly the decomposition of noncrystalline domains and finally, CNC is produced. Similar to the results of Camacho et al. (1996), these two processes may have different kinetics too. So at room temperatures and 45 °C, the relative rate of the two processes may vary. In order to verify the effect of this temperature assembly on yield and uniformity of CNC, the different pre-mixing times at room temperature prior to acid hydrolysis at 45 °C were implemented. This approach has not been reported yet. The purpose of this paper is to investigate the influences of pre-treatment with sulfuric acid prior to the hydrolysis. Herein, two levels of temperature were chosen, room temperature (~25 °C) and 45 °C, to investigate the effects of pre-mixing between cotton and sulfuric acid on the yield and properties of CNCs.
EXPERIMENTAL
Materials
Cotton was harvested from Xinjiang province, PR China. The cotton fiber was separated from the seed by rolling mill and was used as the raw cotton material. Analytical grade sulfuric acid was purchased from Beijing Chemical Factory (Beijing, China). The content of sulfuric acid was 95% to 98%. Deionized water (3 μS cm-1) was prepared with a Heal force Water Purification System (Shanghai, China).
Sulfuric Acid Hydrolysis of Cellulose
First, 10 g raw cotton fiber was added to a 500 mL beaker containing 200 mL of 64.1% (w/w) sulfuric acid. To investigate the effects of pre-mixing time at room temperature on the yield and properties of CNCs, the mixture was stirred at 100 rpm with a magnetic rotor for 0, 30, 60, 120, and 180 min. The beaker was transferred to a water bath at 45 °C for 60 min. Immediately following the acid hydrolysis, the suspension was diluted with 10-fold deionized water to quench the reaction. To remove the unreacted cotton fiber and some leaves, the solution was passed through nylon filters of 100-mesh and 500 mesh. The suspension was centrifuged at 10,000 rpm for 10 min to concentrate the cellulose and remove excess aqueous acid. The resultant CNCs precipitate was rinsed, re-centrifuged three times, and dialyzed against water for one week until the electrical conductivity outside the dialysis bag was below 10 μS cm-1. The resultant CNCs aqueous suspension was approximately 3% to 5% by weight. The produced CNCs with pre-mixing times of 0, 30, 60, 120, and 180 min were coded as CNC0, CNC30, CNC60, CNC120, CNC180, respectively. A 15 mL CNCs aqueous suspension was freeze dried for analysis.
Film Preparation
Approximately 10 mL of a CNCs aqueous suspension (~3 wt%) was added to the vacuum filtration apparatus mounted with a 0.22 μm PTFE membrane. After pumping for 8 h, the CNCs film had formed on the PTFE membrane and had a smooth surface. A micrometer was used to measure the thickness of the film. The yield of CNCs was obtained by weight from both the lyophilized sample and the film. The production process is illustrated in Fig. 1.
Fig. 1. The production process of CNC suspension and films with different pre-mixing times
CNCs Characterizations
Transmission electron microscopy
Drops of 0.001 wt.% CNC suspensions were deposited on glow-discharged carbon-coated copper grids (300 mesh). The specimens were then negatively stained with 1% phosphotungstic acid and observed using a Philips CM200 transmission electron microscope (TEM; Philips, Netherlands) operating at 80 kV. For each CNCs sample, the width and length of about 100 particles were measured from the TEM images using the Image J 1.46r software (https://imagej.nih.gov/ij/index.html).
X-ray diffraction (XRD) analysis
X-Ray diffraction measurements were conducted using a Panalytical X’Pert PRO MPD (Karlsruhe, Germany). The diffracted intensity of the Cu Kα radiation (0.154 nm, 40 kV, and 40 mA) was measured in a 2θ range between 10° and 50°. The CNCs films samples were characterized, and the crystallinity index (CrI) was determined by an empirical method using the following equation (Segal et al. 1959),
CrI(%)=(I002-Iam)/I002×100 (1)
where I002 is the maximum intensity of the (002) lattice diffraction peak and Iam is the intensity scattered by the amorphous part of the sample. The diffraction peak for the plane (002) is located at a diffraction angle of around 2θ = 22.7°, and the intensity scattered by the amorphous part was measured as the lowest intensity at a diffraction angle of around 2θ = 18.7°.
Fourier transform infrared (FT-IR) spectroscopy
Fourier transform infrared spectroscopy was carried out on an FT-IR spectrophotometer (TENSOR II, Bruker Optics, Karlsruhe, Germany) using the KBr disk method. Before testing, the raw cotton fiber or CNCs was dried at 60 °C for 24 h and then ground in an agate mortar with KBr. The mass ratio between the sample and the dried KBr was 1:300. Thirty-two scans were carried out of each sample recorded from a range of 4000 cm-1 to 400 cm-1 at a resolution of 4 cm-1 in the transmission mode.
Thermogravimetric analysis (TGA)
The thermal stability of the CNCs was characterized using thermogravimetric analysis on a Shimadzu DTG-60 instrument (Kyoto, Japan). The samples (~ 5 mg) were heated from room temperature to 800 °C at a rate of 10 °C min-1 under a nitrogen flow rate of 50 mL min-1.
Measurement of contact angle
The contact angle (CA) of the CNCs films with deionized water was measured using the dynamic contact angle meter (KINO SL200KS, KINO Industrial company ltd., City, America) mounted with CAST2.0 software (KINO Industrial Co., Ltd., America).
Birefringence of CNCs suspension
Approximately 0.2 mL of the CNCs suspension was injected into a rectangular vessel with a length of 40 mm, width of 10 mm, and a thickness of 1 mm. This vessel was ultra-sounded for 20 min at room temperature. Then this vessel was observed by a crossed polarizing microscope (model 58XC, Shanghai Optical Instrument Factory, Shanghai, China).
RESULTS AND DISCUSSION
Particle Morphology and Yield Analysis
Transmission electron microscopy (TEM) images of CNCs prepared by pre-mixing for 0 min and 60 min at room temperature prior to the reaction at 45 °C are shown in Fig. 2. The CNCs had a needle-like shape. The lengths and widths were obtained by measuring more than 100 individual CNCs rods in several images, as listed in the statistic results in Table 1 and Fig. 3. The length and width of the CNC60 was lower than those of CNC0. This result suggests that longer time spent by the mixed cotton with sulfuric acid resulted in smaller CNCs (Jiang and Hsieh 2013; Chen et al. 2015). In the case of CNCs produced from Whatman filter paper, Dong et al. (1998) showed that the length of the nanocrystals steadily decreased from 390 nm to 177 nm as the hydrolysis time increased from 10 min to 240 min (Dong et al. 1998). The standard deviation of distributions of length and width of CNC60 also decreased. Similar results were reported by Beck-Candanedo et al. (2005). The CNC60 had a more uniform dimension than CNC0. In CNC180, no needle-like shape was found.