Regulation mechanism of graphene oxide on the structure and mechanical properties of bio-based gel-spun lignin/poly (vinyl alcohol) fibers

Lignin has been used as a sustainable and eco-friendly filler in composite fibers. However, lignin aggregation occurred at high lignin content, which significantly hindered the further enhancement of fiber performance. The incorporation of graphene oxide (GO) enhanced the mechanical properties of the lignin/poly(vinyl alcohol) (PVA) fibers and affected their structure. With the GO content increasing from 0 to 0.2%, the tensile strength of 5% lignin/PVA fibers increased from 491 to 631 MPa, and the Young's modulus increased from 5.91 to 6.61 GPa. GO reinforced 30% lignin/PVA fibers also showed the same trend. The tensile strength increased from 455 to 553 MPa, and the Young's modulus increased from 5.39 to 7 GPa. The best mechanical performance was observed in PVA fibers containing 5% lignin and 0.2% GO, which had an average tensile strength of 631 MPa and a Young’s modulus of 6.61 GPa. The toughness values of these fibers were between 9.9 and 15.6 J/g, and the fibrillar and ductile fracture microstructure were observed. Structure analysis of fibers showed that GO reinforced 5% lignin/PVA fibers had higher percent crystallinity, and evidence of hydrogen bonding among GO, lignin, and PVA in the gel fibers was revealed. Further, water resistance and swelling behavior of composite PVA fibers were studied to further evidence the structure change of composite fibers.

5.39 to 7 GPa. The best mechanical performance was observed in PVA fibers containing 5% lignin and 0.2% GO, which had an average tensile strength of 631 MPa and a Young's modulus of 6.61 GPa. The toughness values of these fibers were between 9.9 and 15.6 J/g, and the fibrillar and ductile fracture microstructure were observed. Structure analysis of fibers showed that GO reinforced 5% lignin/PVA fibers had higher percent crystallinity, and evidence of hydrogen bonding among GO, lignin, and PVA in the gel fibers was revealed. Further, water resistance and swelling behavior of composite PVA fibers were studied to further evidence the structure change of composite fibers.

Introduction
Poly(vinyl alcohol) (PVA) is a biopolymer with high hydrophilicity and good mechanical properties (Lu and Ford 2018), which is widely used as a raw material for packaging film, textile sizing, and hydrogel (Iwaseya et al. 2005;Luo et al. 2018), etc. Similar to polyethylene (PE), the planar zigzag structure of PVA favors fiber-formation and makes it an ideal material for high-performance fibers (Kaufmann and Hesselbarth 2007;Sun et al. 2001). Fiber spinning techniques of PVA (i.e. wet spinning, dry spinning, melt spinning, and gel spinning, etc.) have been well developed in the past few decades (Cha et al. 1994). It is worth mentioning that gel spinning effectively reduces molecular chain entanglement during the spinning process, further promotes the fiber alignment in the drawing process, making it an efficient fabrication method for high-performance PVA fibers (Smith and Lemstra 1980). Moreover, filler (i.e. lignin, carbon nanotubes, cellulose whiskers, etc.) reinforcement has been investigated as a promising way for further improvement of fiber properties (Lu et al. 2017a;Uddin et al. 2011;Xu et al. 2010).
Lignin, as the second most abundant biopolymer on earth, is present in all fibrous plants. Nearly 50 million tons of lignin are produced each year as a byproduct of pulp and paper-making industry, while only 2% is utilized commercially (Laurichesse and Avérous 2014). Due to its high char yield of * 50% (Thakur et al. 2014), lignin has been proposed as a costeffective and renewable feedstock for carbon fibers (Baker and Rials 2013). However, compared to conventional polyacrylonitrile (PAN)-based carbon fibers, lignin-based carbon fibers have relatively lower mechanical properties (tensile strength of 400-700 MPa, Young's modulus of 40-95 GPa) (Kubo and Kadla 2005;Liu et al. 2015;Sudo and Shimizu 1992). Nevertheless, lignin has the potential to be used as an eco-friendly and sustainable filler in polymer due to its rigid structure. Lignin/PVA fibers have been spun by gel spinning (Lu et al. 2017a, b). With the incorporation of 5% lignin, PVA fiber exhibited a tensile strength of 1.1 GPa and a Young's modulus of 36 GPa. Even at up to 50% lignin, lignin/ PVA fibers had better mechanical properties than that of neat PVA fibers spun under optimum conditions (Lu et al. 2017a). As hydrogen bonding occurred between lignin and PVA hydroxyl groups, the molecular mobility of PVA enhanced, and fiber draw ratios increased. However, at lignin content of more than 20%, filler aggregation and poor alignment of lignin along the fiber axis were observed, which impeded the fabrication of even stronger lignin reinforced PVA fibers (Lu et al. 2017a). To ultimately achieve highperformance lignin-based fibers, the intermolecular compatibility between lignin and PVA, as well as the molecular alignment in fiber structure should be further optimized.
Graphene is a two-dimensional carbon material with exceptional inherent mechanical properties, including extremely high tensile strength of 125 GPa and Young's modulus of 1000 GPa (Lee et al. 2008;Zhu et al. 2010). However, the weak interfacial interaction between the graphene nanofillers and the polymer matrix hinders the preparation of a homogeneous dispersion and further fibers with excellent mechanical performance. Graphene oxide (GO), with oxygen-containing groups (i.e. hydroxyl, carbonyl, carboxyl, and glycidyl groups) on the surface of nanosheets, has attracted considerable attention as filler for polymeric composites (Rourke et al. 2011;Wilson et al. 2009) due to its low cost, good mechanical performance and excellent compatibility with polymers (Liang et al. 2009). For instance, GO/ PVA nanocomposite hydrogels with aligned hierarchical microstructure and anisotropic mechanical properties had been reported (Luo et al. 2018). Gelspun GO/PVA nanocomposite fibers with GO nanosheets highly oriented along the fiber axis were also prepared (Hu et al. 2017). The tensile strength increased 46.15% after only 0.5 wt% GO nanosheets were incorporated into fibers. Although Hu et al. prepared GO/lignin/PVA films, the tensile strength of 100-125 MPa is far below the standards of industrial high-performance applications (Hu et al. 2019). Therefore, insufficient studies employ GO as filler in lignin/PVA gel-spun fibers to overcome the structural impediments of lignin/PVA fibers at high lignin contents for further enhancement in fiber performance or investigate the synergistic effects of GO and lignin on the mechanical properties and structure of PVA fibers.
To better address the regulation mechanism of GO on the structure and mechanical properties of biobased gel-spun lignin/PVA fibers, lignin/PVA, GO/ PVA and GO/lignin/PVA composite fibers with varying lignin and GO contents utilizing gel-spinning technique were fabricated in this work. The prepared fibers were characterized structurally by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and Fourier transform infrared spectroscopy (FTIR). Mechanical performance, water resistance, and swelling ratios were also evaluated, and the effects of lignin and GO on the fiber structure were discussed. This study will possibly expand the application of low-cost bio-based high-performance fibers in industry.

Preparation of spinning dopes
To remove low-molecular-weight fractions, lignin was first dissolved in acetone and then purified by vacuum filtration. Later, it was washed by deionized (DI) water for multiple times and dried in a vacuum oven at 65°C for 4 h. Finally, lignin was ground into fine powder by the mortar and pestle.
Three types of solution mixtures (GO/PVA, lignin/ PVA, and GO/lignin/PVA) were prepared for spinning. To prepare GO/PVA solution, GO (at weight ratios of 0.2% (w/w) GO to polymer) and PVA powder (10 g) were dissolved in 100 mL of 80/20 (v/v) DMSO/DI water under constant stirring at 85°C for 2 h. For the lignin/PVA solutions, PVA powder (10 g) and lignin, at weight ratios of 5% and 30% (w/w) lignin to polymer, were dissolved in 100 mL of 80/20 (v/v) DMSO/DI water under constant stirring at 85°C for 2 h, respectively. To prepare GO/lignin/PVA dopes, at weight ratios of 5% and 30% (w/w) lignin to polymer, concentrations of 0.05%, 0.1% and 0.2% (w/w) GO to polymer were also dissolved in 80/20 (v/ v) DMSO/DI water at the same condition, respectively. All prepared solutions were maintained at 65°C before spinning.

Gel spinning
The gel spinning process is illustrated in four steps (Fig. 1). Steps 1 involves the loading of polymer dope (50 mL) into a syringe maintained at 60°C and the extrusion of the polymeric jet through a 0.72 mm inner diameter syringe needle. Lignin/PVA and GO/lignin/ PVA polymeric jets were injected into acetone/ methanol (85/15 v/v) coagulation solvents maintained at -25°C with an air gap of 3-5 mm due to that methanol could facilitate PVA gel fiber formation but partially dissolve lignin while acetone could effectively hinder the lignin diffusion from fiber structure into coagulation solvents (Lu et al. 2017a). GO/PVA fibers were spun into 100% methanol coagulation solvent for good fiber formation under the same spinning condition. All as-spun fibers which underwent 1 m length coagulation bath were later collected onto a rotating take-up winder. The collected as-spun fibers were air drawn at room temperature (Step 2) before storage in coagulation solvents with the same compositions as the ones in Step 1 for gel aging at 5°C for 24 h (Step 3).
Step 4 involves the hot drawing of fibers through silicone oil bath for four stages at high temperature (100-220°C). The draw ratio (DR) of each drawing stage was calculated by Eq. (1): where V 1 represents the velocity of the fiber feeding winder and V 2 represents the velocity of the fiber takeup winder, respectively ( Fig. 1, Step 2 & 4). Drawing parameters for gel-spun fibers are listed in Table 1, which will be discussed later.

Mechanical testing
The mechanical properties of fibers were measured with the XQ-1C tensile testing system according to ASTM D3379. Mechanical testing was performed with a 20 mm gauge length, a strain rate of 15 mm/ min, and a sample size of 10. Values of fiber crosssectional area A were determined gravimetrically from measurements of linear density d, and the density of the composite fiber q, using Eq. (2): where d values were obtained by weighing the mass of 30 cm composite fibers. Before weighing, the fibers were rinsed by isopropyl alcohol to remove residual silicone oil on the fiber surface and dried at room temperature for 24 h. Values of q were calculated by Eq. (3): where w is the weight fraction of PVA, lignin, and GO, respectively. The density of both PVA (Luo et al. 2013) and lignin (Hu 2002) is 1.3 g/cm 3 , and the density of GO is 2.2 g/cm 3 (Hu et al. 2017). The weight fraction of GO in the composite fibers was so small that it could be negligible. Therefore, the density of each composite fiber was approximately q = 1.3 g/ cm 3 . Tensile toughness (U t ) was calculated from the integration of the stress-strain curve of each composite fiber. It represents the energy absorbed during the fiber breakage process (Song et al. 2013), which can be expressed by Eq. (4): where r i and e i are the stress and strain at each data point i, respectively.

Imaging analysis
A SU8010 scanning electron microscopy was used to study the morphology of the fiber facture tips after mechanical testing. Fractured fiber samples were sputter coated with gold and imaged by SEM at 5 kV accelerating voltage.
Fiber structural analysis X-ray Diffraction patterns of fibers were collected by Rigaku D/max-2550 PC X-ray Diffractometer using Cu K a radiation (k = 1.541 Å ) at voltage of 40 kV and operating current of 150 mA. Shredded fibers were scanned at a step size of 0.05°with 2h between 5°and 60°. Peak fitting was performed by MDI Jade 6 software. The percent crystallinity (X c ) of each composite fiber was calculated based on crystalline (A c ) and amorphous (A a ) peak areas (Minus et al. 2006): Aligned fiber bundles of more than 100 fibers with 3 cm in length were placed on the sample holder and scanned for molecular anisotropy. Cu K a radiation (k = 1.541 Å ) at voltage of 40 kV and operating current of 150 mA were applied. Fibers were scanned in the equator and meridian directions at a step size of 0.05°with 2h between 5°and 60°. Then the detector was fixed at the 2h position of the strongest peak, and the sample holder rotated from -90°to 270°along the azimuth angle to test the intensity distribution of the diffraction peak. Peak fitting was performed by MDI Jade 6 software. The orientation (y) is calculated by Eq. (6) (Zhu et al. 2009): where H i is the half-height width at peak i. The structural analysis of fibers was performed for 128 scans at 4 cm -1 spectral resolution on the NEXUS-670 Fourier transform infrared spectrophotometer equipped with attenuated total reflection. FTIR spectra in the 800-4200 cm -1 range were normalized to the 854 cm -1 band (C-C stretching) (Peppas 1977;Tretinnikov and Zagorskaya 2012). The C-C stretching peak is chosen as a reference due to that its absorbance is not significantly affected by processing. Percent crystallinity (X c ) of polymer is expressed in Eq. (7): where a = 14.40 and b = 24.09 are constants whose values are calculated from known values of percent crystallinity from X-ray diffraction patterns (Fig. S1, S2 and Table S1 in Supplementary Information, SI). Absorbance values for A 1144 and A 854 were calculated from infrared spectra.

Water dissolution and swelling
To investigate the water resistance of PVA fibers with different lignin and GO contents, fiber bundles (2 mg) designated as (lignin to PVA)/(GO to PVA) ratios of 0/ 0.01, 0/0.02, 5/0, 5/0.05, 5/0.1, 5/0.2, 30/0, 30/0.05, 30/0.1, and 30/0.2, were placed in 25 mL of water and gradually heated from 25 to 85°C. Optical images of fibers were obtained by optical microscope (ECLIPES LV100N POL) after water immersion of fibers. To study the swelling behavior of GO/lignin/PVA fibers, fiber bundles were immersed in DI water for 24 h at room temperature. Post immersion, the fibers were blotted with filter paper to remove excess water before being weighed. The fiber swelling ratio (S) was calculated according to Eq. (8): where m d and m w represent the mass of the fiber before and after wetting, respectively.

Results and discussion
Effect of GO on lignin/PVA fiber drawing process In this section, the drawing parameters for all gel-spun composite fibers, including drawing ratios, drawing temperature, effective diameters, and linear density, were summarized in Table 1. Changes in drawing temperature and drawing ratios of each stage were observed. It should be well noted that drawing ratios and drawing temperature were optimized for each fiber composition to obtain fully drawn fibers with optimized microstructure and mechanical properties. Ideal fibers are those with all polymer chains perfectly aligned along fiber axis to exhibit high mechanical performance (Chae and Kumar 2008). In reality, fibers are always composed of crystalline and amorphous regions, and high fiber orientation and percent crystallinity are preferred to achieve high mechanical properties (Hearle 2001). High ratio of fiber extension is crucial to obtain highly crystalline and oriented fiber microstructure, which contributes to the superior mechanical performance of gel-spun fibers (Pakhomov et al. 2006). Moreover, filler concentration has an impact on the microstructure and physical properties of gel fibers (Hu et al. 2017;Lu et al. 2017a). This results in difference in drawing temperature of fibers as filler concentration varies. Instead of setting the drawing temperature and draw ratios to the same values for all fibers, the influence of filler on the optimized process parameters (drawing ratios and temperature) are investigated to yield fibers with optimized microstructure and mechanical properties in this work. The method of determining the optimized drawing temperature and draw ratio in each drawing stage is described as follows. To determine the drawing temperature for each fiber, a temperature slightly higher than that of previous study (Lu et al. 2017a) was used to start. If the fiber did not melt immediately, an initial draw ratio slightly higher than that of the reference in each stage was employed. If continuous drawn fibers were obtained, the temperature and draw ratio were determined for each stage. However, in most cases, fiber melted at the initial temperature or fiber broke instantly at the initial draw ratio. Thus, optimum drawing temperature was determined by lowering it every 1-2°C each time and observing fiber status in the hot oil bath at each temperature until the fiber was drawable without melting. As for the determination of optimized draw ratio, the feeding roller's speed was always set to the lowest constant value according to the the equipment capacity, and the takeup winder's speed was lowered step by step of every 0.04 m/min until no frequent fiber breakage in * 2 m length occurred and continuous fibers were obtained.
As the first drawing process after the as-spun fibers being collected from the coagulation bath, air drawing facilitates the alignment of molecular chains, the reduction of the fiber diameter, and promotes the solvent exchange in the subsequent gel aging process (Zhang et al. 2011). With the air drawing process applied, the subsequent thermal drawing of gel-spun fibers is more stable, which may contribute to the enhanced strength, toughness, and dimensional stability of the gel-spun fibers, which will be further evidenced in the later section.
After gel aging of the air-drawn fibers in the coagulation solvents for 24 h, gel fibers were further drawn in hot oil for multiple stages at elevated temperatures. In the first stage, the drawing temperature of fibers was between 105 and 120°C. It was observed that solvent diffused from the fibers into the high-temperature oil during the first stage of thermal drawing, which facilitated the conversion of gel fibers into solid fibers. The temperature was increased to 180-190°C in the second stage of thermal drawing. There was still slight solvent diffusion during this stage, which was attributed to the residual DMSO (with a boiling point of 189°C) in the fibers. In the third and fourth stages, the fibers were further drawn to obtain finer structure at increased drawing temperature. Effective diameters and linear density of fibers are shown in Table 1. Changes of GO/lignin/PVA fibers in stage drawing ratios were slight. The draw ratio of 0.2%GO/PVA fiber could reach 16.3, while the total draw ratios of GO/lignin/PVA fibers were in the range of 9.8-12.9 regardless of the content of lignin. It is possible that the intermolecular interaction in the GO/lignin/PVA ternary system, which will be shown in the later section, impedes the enhancement of fiber draw ratios.
Effect of GO content on lignin/PVA fiber mechanical properties The effect of GO content on the tensile strength and Young's modulus of gel-spun lignin/PVA fibers is discussed in this section. Generally speaking, fibers with 5% lignin have significantly better mechanical properties than those with 30% lignin content, which is attributed to the higher crystallinity (as shown in Table 2), the stronger hydrogen bonding between lignin and PVA, and the better molecular orientation of 5% lignin/PVA fiber (Lu et al. 2017a). It is noted that the mechanical properties of the fibers in this work, for instance 5% lignin and no GO are lower that of the fiber achieved in previous work (Lu et al. 2017a). The main reason are follows: first, the sources or types of the lignin have an influence on fiber's chemical structures and properties (Baker et al. 2010). Lignin used in this work differs from the one used in previous work, which may lead to difference in the fiber performance. Second, the processing apparatus from pump, syringe needle to drawing winders are not the same. For instance, difference in shear stress due to the apparatus forces results in the change of the unentangled molecules' conformations and orientation under shear, particularly in a syringe with confined space and an impenetrable wall (Jeszka and Pakula 2006). Thus, it is believed that the polymeric jets ejected from syringe under different pressures have different microstructures (especially in terms of polymer chain orientation), which may have great influence on the subsequently processed fiber microstructure and properties. Moreover, coarse fibers' large diameter, less oriented polymer chain (Michud et al. 2016) and more solvent in the structure (Liu et al. 1997), together with less dwelling time in hot oil due to higher drawing speeds in this work contribute to inferior mechanical performance of fibers than that of fibers with similar concentrations in previous work (Lu et al. 2017a). Strictly speaking, 5% and 30% lignin/PVA fibers processed in previous (Lu et al. 2017a) and current work are quite different in raw materials, fiber spinning and fiber drawing process. Therefore, it is reasonable that fibers demonstrated different mechanical properties. Nevertheless, it does not affect the discussion of the effect of GO content on the mechanical performance of lignin/PVA fibers in this work. After adding GO, fibers containing both GO and lignin fillers exhibited superior mechanical properties compared to those containing only lignin fillers (Fig. 2). As the GO content increased from 0 to 0.2%, the tensile strength of 5% lignin/PVA fibers increased from 491 to 631 MPa, and Young's modulus increased from 5.91 GPa to 6.61 GPa. The 30% lignin/ PVA fibers showed the same trend as the GO content increased from 0 to 0.2%: the tensile strength increased from 455 to 553 MPa, and Young's modulus increased from 5.39 to 7 GPa. The maximum tensile strength was 631 MPa at 0.2%GO/5% lignin, and the maximum modulus was 7 GPa at 0.2% GO/30% lignin.
It was clearly shown that for GO/lignin/PVA composite fibers, both 5% and 30% lignin/PVA fibers had the best mechanical properties at 0.2% GO. To better verify the role of lignin in the ternary system of composite fibers, 0.2% GO/PVA fibers were fabricated and tested for comparison. Figure 2 showed that the tensile strength (529 MPa) and Young's modulus (6.3 GPa) of 0.2% GO/PVA fiber were lower than those of 0.2% GO/lignin/PVA fiber. It confirms that lignin may work as an efficient filler in GO/PVA fibers and that the synergistic effect of GO, lignin and PVA results in the enhancement of fiber properties.
The effect of GO content on gel-spun lignin/PVA fiber toughness is shown in Fig. 3. Toughness indicates fiber's ability to absorb mechanical energy before rupture (Shin et al. 2012). The strain at break values for all of the lignin/PVA and GO/lignin/PVA fibers ranged between 5.5 and 7.5% regardless of the filler content. Toughness values of GO/lignin/PVA fibers with varying GO and lignin contents ranged from 9.9 to 15.6 J/g, which were much greater than that of reported GO/PVA composites fiber (6 J/g) (Shin et al. 2012). A maximum toughness value of 15.6 J/g was observed in 0.1% GO/5% lignin/PVA fibers. As the GO content increased, the toughness of both 5% and 30% lignin/PVA fibers slightly increased. Compared with 0.2%GO/5% and 30%lignin/PVA composite fibers, 0.2%GO/PVA fiber had a lower toughness value (10.88 J/g). This indicates that lignin with a large amount of rigid benzene rings can more effectively increase the fiber toughness than GO.
Effect of GO content on lignin/PVA fiber structure Fiber fracture tips from mechanical testing were imaged by SEM, as shown in Fig. 4. It was observed that all fibers had a dense structure without pores. Fiber diameters were in the range of 50-65 lm, which were consistent with the effective diameters calculated  Table 1. The fibrillar structure of the 0.2% GO/PVA fiber was also observed (Fig. S3 in SI). The addition of GO could effectively enhance the strength of the PVA fiber (Hu et al. 2017) and the fibrillar structure indicated the highly aligned polymer chains along the fiber axis. 5% lignin/PVA fiber showed a smooth fracture tip (Fig. 4a). With 0.2% GO incorporated, 0.2% GO/5% lignin/PVA fiber exhibited more fibrillar microstructure and more ductile fracture tip (Fig. 4b).
PVA fibrils are related to the highly oriented and ordered polymer chains, which are responsible for the good mechanical properties (tensile strength of 631 MPa, Young's modulus of 6.61 GPa). The fibrillar structure was observed in 30% lignin/PVA fiber (Fig. 4c), which was possibly due to the plasticizing effect of lignin (Lu et al. 2017a). The fibrillar structure was also observed in 0.2% GO/30% lignin/PVA fiber (Fig. 4d). Even in high-resolution images, no aggregation of lignin or GO was observed in the 0.2% GO/ 30% lignin/PVA fiber, which only demonstrated uniform structure without obvious defects. In summary, both lignin and GO promotes the formation of PVA fibrillar structure and are responsible for the enhancement of mechanical properties aforementioned.
To better understand the effect of GO filler on the mechanical properties of lignin/PVA fibers, fiber crystallinity, and molecular alignment will be discussed in this section. Both molecular adhesion and orientation of polymer chains contribute to the enhancement of mechanical performance of fibers (Gonzalez et al. 2014;Minus et al. 2009;Spitalsky et al. 2010). Thus both FTIR and XRD were used to indicate the structure change of gel-spun lignin/PVA fibers before/after the incorporation of GO.
FTIR spectra of PVA composite fibers in Fig. 5 together with XRD patterns (Fig. S1 in SI) were used to calculate the constants a and b in Eq. (7) for further confirmation of all of the percent crystallinity values of composite PVA fibers (Table 2). In pervious study, it was shown that the peak of pure lignin powder at 1144 cm -1 and 854 cm -1 in FTIR spectra were so weak that the overlapping peaks of lignin had negligible influence on the calculation of fiber crystallinity even at lignin content of 30% (Lu et al. 2017a). Besides, XRD was also used for the calculation of the percent crystallinity to create a correlation plot for accuracy. The percent crystallinity calculated by XRD is consistent with the relative crystallinity calculated by FTIR (Fig. S2), which verifies the accuracy of the crystallinity calculated by FTIR. Amorphous PVA is related to the C-O vibrational mode at 1094 cm -1 , and PVA crystallinity affects the peak at 1144 cm -1 (Lu et al. 2017a). The value of the A 1144 /A 854 ratio is an index of fiber crystallinity. The crystallinity of PVA composite fibers (Table 2) approximately agreed with overall trends shown for the mechanical properties of fibers (Fig. 2). In detail, fibers with 5% lignin were more crystalline than those containing 30% lignin. Both 5% and 30% lignin fibers with GO showed higher or close crystallinity values in comparison with fibers without GO. This is due to that the presence of GO in the crystalline region promotes the formation of crystals, which will be demonstrated later in Fig. 6. In detail, the optimized draw ratio values of the fibers were relatively close (Table 1), indicating GO content was the main factor which led to the difference in fiber structure and properties, especially in fiber crystallinity. Both amorphous and crystalline regions exist in lignin/PVA fibers (Fig. 6b 1 ). When GO nanosheets are well dispersed in the PVA matrix (Fig. 6c 1 ), they act as nucleating agents for polymer crystallization (Li et al. 2016). Hence, PVA molecular chains tend to form increased crystalline regions in the surrounding area of GO nanosheets. As a result, GO mainly exists in the crystalline region and its presence promotes the formation of crystals. The highest degree of crystallinity occurred at 0.05% GO/5% lignin, however, the value slightly decreased when GO content was more than 0.05% in 5% lignin/PVA fibers. Fibers with 30% lignin and 0.2% GO were obviously more Fig. 3 Toughness of gel-spun PVA fibers with 0/5/30% lignin and 0/0.05%/0.1%/0.2% GO crystalline than those containing 0/0.05% GO/0.1% GO, which had a similar value of crystallinity.
Moreover, FTIR spectra implied the molecular interaction among GO, lignin and PVA. The peaks at 2942 cm -1 , 1440 cm -1 , 1086 cm -1 , and 854 cm -1 were associated with the vibrations of C-H stretching, -CH 2 bending, C-O-C asymmetric stretching, and C-C stretching, respectively. It was obvious that in GO/ PVA, lignin/PVA, and GO/lignin/PVA composite fibers, no new absorbance peaks occurred with the incorporation of lignin or GO, suggesting that no new functional groups were formed.
Intermolecular bonding among lignin, GO and PVA induces molecular adhesion. FTIR absorbance Fig. 4 (1) Low-and (2) high-resolution SEM images of fracture tips of (a) 5% lignin /PVA fibers, (b) 0.2% GO/5% lignin/PVA fibers, (c) 30% lignin/PVA fibers, (d) 0.2% GO/30% lignin/PVA fibers spectra from 3000 to 3700 cm -1 (Fig. 5a) provides insight into hydrogen bonding. The -OH stretching vibration peak of 5% lignin/PVA fiber was at 3343 cm -1 . With the addition of 0.05% GO, the peak shifted to lower frequencies, indicating shorter distances between oxygen atoms (OÁÁÁO) from different hydroxyl groups (Jiang et al. 2012). The shift was possibly the result of intermolecular hydrogen bonding formed among GO, lignin, and PVA (Kubo and Kadla 2003). With higher GO content, the OH group absorbance peak shifted slightly to the higher frequencies, which might be due to the dissociation of hydrogen bonding between PVA molecular chains. The -OH stretching vibration peak of 30% lignin/PVA fiber was at 3294 cm -1 , indicating that the hydrogen bonding between PVA and lignin was strong. With the incorporation of GO, the peak shifted to higher frequencies at different levels, indicating that no more hydrogen bonding formed. Strong molecular interactions among composite fibers indicates good compatibility among GO, lignin, and PVA. As a result, gelspun composite fibers showed no evidence of filler aggregation within the fiber microstructure (Fig. 4) and possessed good mechanical performance (Fig. 2). In addition to the percent crystallinity investigation of fibers, XRD patterns (Fig. S4, SI) were also used to investigate the orientation of crystallites in polymers. As shown in Table 3, all fibers exhibited a high orientation of [ 90%. This verifies that gel-spinning technique is an effective method of fabricating fibers with high molecular orientation (Smith and Lemstra 1980). Taken both percent crystallinity and orientation into consideration, the former might be the main factor that differentiated the mechanical performance of the fabricated fibers in this work since the molecular orientation of the polymer chains was similar.
The reinforcement mechanism of GO in lignin/ PVA fibers The microstructure models of spinning dopes and fibers are shown in Fig. 6 to better illustrate the structure of GO reinforced lignin/PVA fibers. The distribution of GO, lignin, and PVA in the homogenous spinning dope is random (Fig. 6a). Entanglements between the PVA macromolecular chains exist and hydrogen bonding can be formed among GO, lignin, and PVA since they all have oxygen-containing functional groups. After the gel-spun solid fiber is obtained, taking 5% lignin/PVA fiber as an example, both crystalline and amorphous regions exist in the fiber structure (Fig. 6b 1 ). PVA molecular chains are highly aligned along the fiber axis and partially crystallized, which are indicated by percent crystallinity and orientation results in Tables 2 and 3, respectively. Intramolecular bonding of PVA can be replaced by intermolecular interaction between lignin and PVA, which reduces the entanglement between macromolecules and is beneficial to the formation of crystalline regions. However, when the lignin content increased to 30%, the amorphous structure of excessive filler results in the decrease of fiber crystallinity. In addition, the aggregation (Fig. 6b 2 ) of lignin may hinder the alignment of the PVA chains and cause a further decrease of the crystallinity. With the incorporation of a small amount of GO, hydrogen bonding is formed between GO/lignin, GO/PVA in addition to lignin/PVA. More crystalline areas are formed in GO reinforced lignin/PVA fibers in comparison with that of 5% and 30% lignin/PVA fibers (Fig. 6c 1 and c 2 ). No lignin aggregation was observed in the SEM images of GO reinforced 30% lignin/PVA fiber (Fig. 4d), indicating that hydrogen bonding between GO/lignin, GO/PVA promotes the even distribution of lignin in the fiber structure and the formation of crystals. However, due to the plasticizing effect and amorphous structure of lignin, the crystallinity of GO reinforced 30% lignin/PVA fiber was lower than that of GO reinforced 5% lignin/PVA fiber (Table 2).
In summary, the enhancement of GO reinforced lignin/PVA fiber properties could be attributed to the following three reasons. First, GO has excellent inherent mechanical properties (tensile strength of 125 GPa and Young's modulus of 1000 GPa) (McAllister et al. 2007). When the composite fiber is under tension, the load is effectively transferred from the polymer matrix to GO, the mechanical properties of the fiber are therefore improved. Secondly, GO has good compatibility with PVA and lignin. Hydrogen bonding can be formed among them, which effectively avoids the aggregation of excessive lignin and the formation of fiber structure defects. Finally, the addition of GO favors the alignment of PVA molecular chains along the fiber axis, which is beneficial to the formation of crystalline regions (Table 2).
Water resistance of composite fibers PVA fibers are susceptible to water at elevated temperatures due to their polar chemical structure (Lu et al. 2017a). In this study, it was evidenced that lignin and GO could promote the water resistance behavior of composite PVA fibers since swelling behavior was observed at elevated temperatures.
The dissolution of GO/PVA, lignin/PVA, and GO/ lignin/PVA composite fibers were tested in water at different temperatures. Water resistance behavior of the composite fibers at room temperature and high temperature was observed by an optical microscope (Fig. 7). After being immersed in water at 25°C, all  (Lu et al. 2017a), the dissolution of PVA fibers in water at 85°C is hindered with the incorporation of lignin or GO due to the intermolecular hydrogen bonding among GO, lignin, and PVA. In detail, 0.2% GO/PVA fiber had an increase in diameter due to swelling in hot water at 85°C, and it still maintained its intact fiber structure (Fig. S5, SI). At 5% lignin, the GO/lignin/PVA fibers (Fig. 7a 2 -d 2 ) exhibited increased diameter by swelling in water at 85°C. At 30% lignin (Fig. 7e 2 -h 2 ), the GO/lignin/PVA fibers immersed in water at 85°C showed more significant swelling behavior and gel-like structure under the optical microscope. The possible explanation could be that PVA fibers with 30% lignin content had more amorphous region (lower crystallinity) than that of 5% lignin/PVA fibers, thus the amorphous region in PVA fibers absorbed more water. Moreover, with GO content increased from 0 to 0.2%, both 5% and 30% lignin/PVA fibers exhibited more obvious swollen structure, which was possibly attributed to that the hydrogen bonding formation of GO/lignin, GO/PVA and lignin/PVA promotes the structural network formation in the swollen fibers.
To further study the structural difference between fully drawn composite fibers, swelling ratio (S) revealing moisture uptake capability of fibers at room temperature was measured and presented in Table 4. GO content had no significant influence on the moisture uptake of fibers with 5% lignin and 30% lignin. At the same GO content (i.e. 0.2%), when lignin content increased from 0 to 30%, the swelling ratio increased slightly from 5.13 to 6.67%. This was due to that PVA fibers with 30% lignin were less crystalline so that the amorphous regions absorbed Fig. 7 Optical micrographs of (a) 5% lignin/PVA fiber, (b) 0.05% GO/5% lignin/PVA fiber, (c) 0.1% GO/5% lignin/ PVA fiber, (d) 0.2% GO/5% lignin/PVA fiber, (e) 30% lignin/ PVA fiber, (f) 0.05% GO/30% lignin/PVA fiber, (g) 0.1% GO/ 30% lignin/PVA fiber, (h) 0.2% GO/30% lignin/PVA fiber after water immersion at (1) 25°C and (2) 85°C more water than PVA fibers with 0% and 5% lignin. The most crystalline 0.05% GO/5% lignin /PVA fiber exhibited a swelling ratio of 4.09%, which was lower than other fibers. These results agree with the structural analysis of percent crystallinity shown in Table 2.

Conclusions
We have successfully fabricated gel-spun bio-based GO/lignin/PVA fibers which demonstrate promising results of mechanical performance. With the incorporation of GO, the aggregation of fillers was avoided and enhancement in fiber mechanical properties was observed. With the GO content increasing from 0 to 0.2%, the tensile strength of 5% lignin/PVA fiber increased from 491 to 631 MPa, and Young's modulus increased from 5.91 to 6.61 GPa. GO reinforced 30% lignin/PVA fibers also exhibited the same increasing trend. The tensile strength increased from 455 to 553 MPa, and Young's modulus increased from 5.39 to 7 GPa. Thus, GO favors the formation of defect-free fiber structure and the enhancement of fiber performance. The structural enhancement at 0.2% GO/5% lignin/PVA fiber was evidenced by its high crystallinity, intermolecular bonding among lignin, GO, and PVA, and good alignment of molecular chains. The good mechanical performance of gel-spun GO/ lignin/PVA fibers indicates their potential for use in industrial and high-performance applications.