Abstract
Lignocellulose nanofibrils (LCNFs) were prepared using Liriodendron tulipifera L. wood flour. Electrospun nanofibers were fabricated by mixing the LCNFs with poly(vinyl alcohol) (PVOH). The lignin and hemicellulose contents of the wood flour were controlled with an alkaline-peroxide treatment at a pH of 11.5 using various hydrogen peroxide concentrations. The morphological characteristics, mean diameter, and filtration time of the LCNFs subjected to wet disk milling (WDM) and high-pressure homogenization were determined. Furthermore, the spinning suspension viscosity was measured with various LCNF concentrations and PVOH/LCNF addition ratios. After the alkaline-peroxide treatment, the lignin and hemicellulose contents decreased with an increasing hydrogen peroxide concentration and reaction time. As the lignin content decreased, the nanofibril diameter decreased and the filtration time increased. The diameter decreased further after the homogenization treatment following WDM. The viscosity of the mixed solution increased with an increasing PVOH and LCNF mixed solution concentration and LCNF addition ratio, and decreasing lignin content. Scanning electron micrographs revealed that the diameter of the electrospun nanofibers increased as the mixed solution concentration and LCNFs addition increased, the lignin content decreased, and with the homogenization treatment.
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Preparation and Characterization of Electrospun Composite Nanofibers from Poly(vinyl alcohol) /Lignocellulose Nanofibrils with Different Chemical Compositions
Pureun-Narae Seo,a Song-Yi Han,a Chan-Woo Park,a Sun-Young Lee,a,* Nam-Hun Kim,a and Seung-Hwan Lee b,*
Lignocellulose nanofibrils (LCNFs) were prepared using Liriodendron tulipifera L. wood flour. Electrospun nanofibers were fabricated by mixing the LCNFs with poly(vinyl alcohol) (PVOH). The lignin and hemicellulose contents of the wood flour were controlled with an alkaline-peroxide treatment at a pH of 11.5 using various hydrogen peroxide concentrations. The morphological characteristics, mean diameter, and filtration time of the LCNFs subjected to wet disk milling (WDM) and high-pressure homogenization were determined. Furthermore, the spinning suspension viscosity was measured with various LCNF concentrations and PVOH/LCNF addition ratios. After the alkaline-peroxide treatment, the lignin and hemicellulose contents decreased with an increasing hydrogen peroxide concentration and reaction time. As the lignin content decreased, the nanofibril diameter decreased and the filtration time increased. The diameter decreased further after the homogenization treatment following WDM. The viscosity of the mixed solution increased with an increasing PVOH and LCNF mixed solution concentration and LCNF addition ratio, and decreasing lignin content. Scanning electron micrographs revealed that the diameter of the electrospun nanofibers increased as the mixed solution concentration and LCNFs addition increased, the lignin content decreased, and with the homogenization treatment.
Keywords: Lignocellulose nanofibril; Alkaline-peroxide treatment; Wet disk milling; High-pressure homogenization; Electrospinning
Contact information: a: Department of Forest Biomaterials and Engineering, Kangwon National University, Chuncheon 200-701, Republic of Korea; b: Department of Forest Products, Korea Forest Research Institute, Seoul 130-713, Republic of Korea;
* Corresponding authors: nararawood@korea.kr; lshyhk@kangwon.ac.kr
INTRODUCTION
Electrospinning is a method employed for fabricating micro- and nanometer-scale fibers, and it has been highlighted as a more efficient process than conventional spinning methods (Liao et al. 2018; Guo et al. 2019; Huang et al. 2019). Many variables are considered in electrospinning, such as the solution concentration, viscosity, surface tension, electric field intensity, spinning speed, spinning time, and collector distance. The shape of the fibers obtained by electrospinning differs with the manner in which each of these variables are controlled (Sun et al. 2014). Electrospun nanofibers have a large specific surface area and fiber-to-fiber voids, unlike those prepared by other spinning methods, and hence they can be developed into porous fibers (Wang and Hsiao 2016; Wang et al. 2019). Because of this feature, electrospun nanofibers have widespread applications in filters, sensors, catalysts, and tissue engineering. Thus, research into the use of various polymers to prepare electrospun fibers is actively being performed (Yun et al. 2007; Liu et al. 2009; Fang et al. 2011; Hu et al. 2014; Alvarado et al. 2018).
Poly(vinyl alcohol) (PVOH) has been highlighted as one of the most commonly used polymers for electrospinning. PVOH is a hydrophilic polymer produced by the hydrolysis of poly(vinyl acetate) and is used in a wide range of applications, such as tissue support, drug release, and filters because of its semi-crystallinity, environmental friendliness, and biodegradability (Wu et al. 2005; Bolto et al. 2009; Islam et al. 2015). Because PVOH has a high solubility and surface activity, it can be easily mixed with other natural polymers, thereby improving the mechanical properties of the latter (Folkes and Hope 1993). Nanocellulose, by virtue of its biodegradability, renewability, and excellent mechanical properties, has been attracting attention as a PVOH reinforcing material. PVOH, which is a water-soluble polymer, is hydrogen-bonded to the hydroxyl group of nanocellulose to obtain a composite material with an excellent thermal stability and compatibility. Therefore, many studies have been conducted to prepare composite nanofibers by electrospinning a mixed suspension of PVOH and nanocellulose (Lu et al. 2008; Medeiros et al. 2008; Peresin et al. 2010; Sutka et al. 2013; Park et al. 2014).
In this study, Liriodendron tulipifera L. wood flour was subjected to alkaline-peroxide (AP) treatment to prepare lignocellulose nanofibrils (LCNFs) with controlled amounts of lignin and hemicellulose, and defibrillated to a nanosize scale by a wet disk milling (WDM) treatment. Furthermore, high-pressure homogenization of the WDM-treated LCNFs was conducted to prepare LCNFs with uniform dimensions. Then, the PVOH/LCNF suspension was prepared by mixing LCNF with PVOH at various concentrations and ratios; thus, electrospun composite nanofibers were prepared. The effects of the chemical composition of the LCNFs, additional homogenizer treatment, and viscosity of the PVOH/LCNF suspension on the properties of the electrospun composite nanofibers were investigated.
EXPERIMENTAL
Materials
Liriodendron tulipifera L. was provided by the Experimental Forest Kangwon National University (Chuncheon, South Korea), ground to 50-mesh size wood flour, and used as the study material. Hydrogen peroxide (H2O2, 35%), sodium hydroxide (NaOH, 50%), sulfuric acid (H2SO4, 95%), sodium chlorite (NaClO2), acetic acid (C2H4O2), and tert-butyl alcohol were purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, South Korea). Poly(vinyl alcohol) ([CH2CH(OH)]n) (n ≈ 2000) was purchased from Tokyo Chemical Industry (Tokyo, Japan).
AP treatment and WDM defibrillation
The lignin and hemicellulose contents were adjusted by treating the wood flour with AP, as described in Seo et al. (2019) by applying the Gould’s method (Gould 1984). For the alkali pretreatment, 5% wood flour and 0.4% NaOH suspensions were prepared and reacted in a water bath at 60 °C for 1 h. For the AP treatment, the alkali-pretreated wood flour was added to 0.2% and 12% H2O2 solutions, and the suspensions with a solids content of 2% were stirred in a water bath at 80 °C for 1 h and 5 h, respectively, with a pH of 11.5. The untreated and AP-treated wood flour were repeatedly defibrillated with 15 passes of WDM to prepare the LCNFs. Based on the lignin content, the LCNFs were named LCNF-32, LCNF-30, and LCNF-13, and their chemical compositions and reaction conditions are shown in Table 1.
Table 1. Chemical Compositions of the AP-treated Products
High-pressure homogenization
The LCNF suspension prepared by the WDM treatment was diluted to 0.3% and subjected to high-pressure homogenization (HPH) (M-110 EH-30, Microfluidics, Newton, MA, USA) three times at a pressure of 1300 bar. The obtained LCNFs were named WDM-LCNF and HPH-LCNF to differentiate the LCNFs according to the treatment method.
Methods
LCNF morphological analysis
To prepare scanning electron microscopy (SEM) samples for morphological analysis of the LCNFs, a 0.001 wt% LCNF concentration was prepared and treated using an ultrasonicator (VCX130PB, Sonics and Materials Inc., Newtown, MA, USA) for 60 s. Then, the suspension was filtered through a PTFE membrane filter (ADVANTEC®, Toyo Roshi Kaisha Ltd., Tokyo, Japan), and the filtration time was measured. The residue obtained on the filter was immersed in tert-butyl alcohol for 20 min three times and then dried at -55 °C in a freeze dryer for 2 h. The LCNF samples were coated with iridium using a sputter coater (EM ACE600, Leica Microsystems, Seoul, South Korea) and observed with field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi, Tokyo, Japan). The LCNF diameters were measured at least 100 times using image analysis software (Version 1.45, Windows, ImageJ, U. S. National Institutes of Health, Bethesda, MD, USA).
Preparation of the Composite Nanofibers
Preparation of the PVOH/LCNF mixed suspension and viscosity measurement
To prepare an aqueous PVOH solution, 10 g of PVOH were added to 90 g of distilled water and stirred at 80 °C until it completely dissolved. Then, the PVOH/LCNF mixed suspensions of various concentrations were prepared by mixing the PVOH solution and the LCNFs water suspension at various ratios. The PVOH/LCNF ratios were set to be 99/1, 97/3, and 95/5. The concentration of the mixed suspensions ranged from 2% to 8%. The PVOH and LCNFs were mixed and stirred at 60 °C for 1 h and cooled at room temperature. The viscosity of the mixed suspension was measured using a viscometer (DV-II+, Brookfield Engineering, Inc., Middleborough, MA, USA) equipped with a SC4-18 spindle at shear rates ranging from 0.4 s-1 to 132 s-1 at 25 °C.
Electrospinning
The PVOH/LCNF mixed suspension was placed in a 15-mL syringe and mounted on an electrospinning pump (ESR200RD, NanoNC, Seoul, South Korea), which was operated at a voltage of 15 kV and spinning rate of 10 μL/min. A 21G metal nozzle with an inner diameter of 0.5 mm was used. The collector was covered with aluminum foil, and the tip-to-collector distance was fixed at 15 cm. For morphological characterization of the electrospun nanofibers formed on the aluminum foil, the electrospun nanofibers were cut and stuck onto the SEM grid. Then, the SEM samples were coated with iridium using the sputter coater and observed with FE-SEM. The fiber diameters were measured using the image analysis software.
RESULTS AND DISCUSSION
Figure 1 shows the SEM images of the LCNFs prepared by the WDM and HPH treatments. The WDM treatment time was 6.02 h/kg, 5.42 h/kg, and 7.3 h/kg for LCNF-32, LCNF-30, and LCNF-13, respectively, and the HPH treatment was then performed three times. In the case of LCNF-32, which was not subjected to the AP treatment, agglomerated fibers were observed. This would be mainly due to the high lignin and hemicellulose contents. The LCNF-30 and LCNF-13 showed more uniform fibers, compared to LCNF-32. The diameters of the separated fibers are summarized in Table 2. The diameters decreased to 37.5 nm, 34.4 nm, and 24.4 nm after the WDM treatment and to 31.1 nm, 27.2 nm, and 21.3 nm after the HPH treatment for LCNF-32, LCNF-30, and LCNF-13.
Table 2. Average Diameters of the WDM-LCNF and HPH-LCNF with Different Chemical Compositions
Figure 2 shows the filtration times of the six types of LCNFs subjected to the WDM and HPH treatments with various lignin contents. After the HPH treatment, the filtration time for LCNF-32 was largely unchanged, but that for LCNF-30 and LCNF-13 increased, which may have been because of the increase in the surface area from the decreased LCNF diameter.