Effect of Processing Parameters on Morphology and Mechanical Properties of Hollow Gel-Spun Lignin/Graphene Oxide/Poly (Vinyl Alcohol) Fibers

JIN Yanhong(金艳红)， HU Wenxin(胡文鑫)， CHENG Yu(程 毓)， LIN Jiaxian(林佳娴)， YANG Xiaona(杨晓娜)， JING Yuanyuan(景媛媛)， ZHANG Kun(张 坤)， LU Chunhong(陆春红)*

1 Key Laboratory of Textile Science & Technology， Ministry of Education， Donghua University， Shanghai 201620， China

2 College of Textiles， Donghua University， Shanghai 201620， China

Abstract: To investigate the influence of various processing parameters on the mechanical properties of fibers， lignin/graphene oxide (GO)/poly(vinyl alcohol) (PVA) fibers with different mass ratios of lignin and GO to PVA were prepared by gel spinning technique. Air drawing process and spinneret diameters were tuned as the main factors. The tensile strength increased up to 472 MPa with air drawing process applied in 30L0.05GO0.72D-A PVA fibers (air-drawn PVA fibers reinforced by 30% lignin and 0.05% GO spun with a spinneret diameter of 0.72 mm)， indicating 17.4% higher than that of the fiber without air drawing process (402 MPa). Similarly， at least a 14.6% increase in Young’s modulus has been achieved for 30L0.05GO0.72D-A fiber. In addition， a smaller spinneret diameter(0.72 mm) also led to a 24.9% increase in tensile strength and a 7.7% increase in Young’s modulus in comparison with those of 5L0.05GO0.84D-A fibers.

Key words: gel spinning； poly(vinyl alcohol) (PVA)； mechanical property； spinning condition； morphology

Introduction

Poly(vinyl alcohol) (PVA) is a non-toxic polymer that can be applied in sizing， packaging films， and hydrogel[1]. Similar to polyethylene (PE)， PVA has a planar zigzag polymer chain structure， making it a perfect candidate for producing high modulus and high strength fibers[2-3]. The theoretical lattice modulus of PVA is 250-300 GPa if the polymer chain has a perfect alignment[4-6]. Even though various attempts (i.e., polymerization control， spinning techniques， and filler reinforcement) have been made to obtain high-performance PVA fibers， there is still a large gap between the actual value of mechanical properties and the theoretical value[7].

Different factors can influence the mechanical performance of fibers. For instance， polymers with lower molecular weights and more chain terminals cannot transfer load efficiently， which easily causes stress concentration and fiber breakage[8]. In other words， higher molecular weights and fewer chain terminals in the polymer structure improve mechanical properties to a certain degree. Moreover， the spinning method has a significant impact on fiber structure and performance. Several methods have been developed to prepare PVA fibers， such as wet spinning， dry spinning， melt spinning， and gel spinning[9]. Among all these spinning methods， gel spinning is the most effective one to produce high-performance fibers[10]since the molecular chain entanglement is greatly reduced， which further facilitates fiber alignment in the drawing process. Processing parameters of gel spinning， including solvents for spinning solution preparation， coagulation temperature， and drawing ratios[11]， have a great influence on the mechanical performance of gel-spun fibers. For instance， Chaetal.[9]explored the effect of coagulation temperature on PVA fibers properties. Gel spinning at low coagulating temperature (-20 ℃) yielded fibers with high drawability (45 times) and mechanical properties (tensile strength of 2.8 GPa and Young’s modulus of 64 GPa). The low coagulation temperature leads to faster gelation， macromolecular networks with relatively low entanglement density， and favorable intermolecular topology that remains intact during the solvent removal process of the gel fiber.

In addition to the study of gel spinning parameters， various types of nanofillers have been incorporated into the gel-spun PVA fiber. Carbon-based materials[i.e., single-walled carbon nanotube (SWCNT)[12-13]and graphene oxide (GO)[14]] and bio-based materials (i.e., chitin[15]， cellulose[16]and lignin[17]) are frequently used since the formation of strong interactions between matrix and fillers effectively leads to the enhancement of mechanical properties of PVA fibers. However， the maximum drawing ratio of the fibers is slightly reduced after the introduction of carbon-based materials such as GO and SWCNT possibly due to the physical barrier effect which hinders the free movement of polymer segments[12-14]. Among those natural materials， lignin， as the second most abundant biopolymer next to cellulose， has attracted considerable attention[17]. Despite that lignin has an amorphous structure， incorporation of lignin as the filler has a positive influence on the mechanical properties of the PVA fibers, and the main reason is that lignin acts as the small molecular plasticizer which leads to higher drawing ratios of PVA fibers[17]. However， high lignin content normally results in aggregation of lignin and decreasing mechanical performance of the fiber， which may hinder the value-added application of lignin. Although Huetal.[18]reported the synergistic effects of GO and lignin on mechanical and thermodynamical properties of PVA films， insufficient studies of gel-spun lignin/GO/PVA fibers motivated the present work.

In this work， we present the preparation of lignin/GO/PVA composite fibers via gel spinning technique. Through the high-ratio drawing process， the PVA molecular chain is stretched， oriented， and crystallized in the axial direction of the fiber. To further enhance the mechanical properties of the fibers， air drawing process and smaller spinneret diameters are applied. Morphological and mechanical properties of lignin/GO/PVA composite fibers have been characterized in order to study the effects of different processing parameters on the formation of gel-spun fibers. By the combination of process condition optimization and filler reinforcement， the low cost and sustainable composite PVA fibers with good mechanical properties might provide insight in the fabrication of engineering fibers in industrial applications.

1 Experiments

1.1 Materials

PVA powder (a molecular weight of 146-186 kg/mol and a hydrolysis of 99%) was purchased from Sigma-Aldrich， USA. Lignin (a purity of 90%) was purchased from Hubei Yunmei Technology Co.， Ltd.， China. GO aqueous dispersion (a concentration of 10 mg/mL and a GO sheet diameter of 5-8 μm) was purchased from Hangzhou Gaoxi Technology Co.， Ltd.， China. Dimethyl sulfoxide (DMSO) from Sigma-Aldrich， acetone， and methanol from Shanghai Titan Technology， Inc. (China) were used as received.

1.2 Spinning solutions preparation

Lignin was first filtered to remove low-molecular-weight fractions. In detail， it was dissolved in acetone， then extracted by vacuum filtration and washed by distilled water for multiple times. Finally， the lignin was dried in a vacuum oven at 65 ℃ for 4 h and finely grounded into powder by using a mortar and pestle.

Solutions of lignin/GO/PVA were prepared for spinning. PVA powder， lignin and GO were dissolved in DMSO/distilled water (a volume ratio of 80∶20) by constant stirring at 85 ℃ for 2 h. Three types of solutions were mixed by sequent magnetic stirring at 85 ℃ for 2 h to form homogenous spinning solutions. The final concentration of PVA in the spinning was 100 g/L. In the previous study[17]， lignin/PVA fibers incorporated with 5% lignin in weight percent increased mechanical properties in comparison with neat PVA fibers. However， aggregation was observed in 30% lignin reinforced PVA fibers， but the fibers still exhibited satisfactory mechanical performance， showing potential applications of low-cost lignin in engineering. Based on this phenomenon， this paper selected 5% lignin/PVA fibers and 30% lignin/PVA fibers as control fibers. To explore the synergistic effects of GO and lignin on PVA fibers， GO concentrations of 0.05% and 0.10% were selected since higher concentrations of GO might cause severe aggregation of GO in the spinning solution. All prepared solutions were maintained at 65 ℃ before spinning.

1.3 Gel spinning

A schematic of the gel spinning process is shown in Fig. 1. The spinning solutions were first dispensed from a 50 mL syringe heated to 60 ℃ into an acetone/methanol (a volume ratio of 85∶15) coagulation bath maintained at -25 ℃ with an air gap of 3-5 mm[shown in Fig. 1(a)]. The collected as-spun fibers were either drawn at room temperature by air drawing process[shown in Fig. 1(b)] or underwent no air drawing process before storage in a coagulation bath at 5 ℃ for 24 h[shown in Fig. 1(c)]. All the fibers were subsequently drawn for several stages in silicone oil at elevated temperatures of 100-220 ℃[shown in Fig. 1(d)]. The drawing ratioRDof each drawing stage was calculated by

(1)

whereV1represents the velocity of the fiber feeding winder andV2represents the velocity of the fiber take-up winder. Various processing parameters including drawing steps and spinneret diameters are listed in Table 1.

Fig. 1 Gel spinning process： (a) as-spun gel fiber formation； (b) air drawing (V2>V1)；

Table 1 Fibers with different processing parameters

1.4 Mechanical testing

The mechanical properties of fibers were tested by the XQ-1C testing system according to the standard ASTM D3379. The strain rate was 15 mm/min， and the gauge length was 20 mm. Ten specimens were tested for each sample. The effective cross-sectional areaAwas calculated by

A=d/ρ，

(2)

wheredrepresents the linear density of the composite fiber andρrepresents the density of the composite fiber. The linear density was measured by weighing fibers with known length. Before being weighed， the fibers were rinsed with isopropanol to remove residual silicone oil on the surface. The density of composite fibers was calculated as

ρ=ρPVAwPVA+ρligninwlignin+ρGOwGO，

(3)

wherewPVA，wligninandwGOare weight fractions of PVA， lignin， and GO， respectively. PVA and lignin have the same density of about 1.3 g/cm3[19-20]. The weight fraction of GO (0.05% or 0.10%) is so small that it can be negligible. Therefore， the density of the composite fiber was approximately 1.3 g/cm3.

1.5 Imaging analysis

Fibers were sputter-coated with gold， and imaged by an SU8010 scanning electron microscope (SEM) at an accelerating voltage of 5 kV. After mechanical testing， fiber fracture tips were also sputter-coated with gold and imaged by an SEM.

1.6 Water dissolution and swelling

To investigate the moisture resistance of fibers prepared with various processing parameters， fiber bundles (2 mg) were placed in 20 mL water and gradually heated from 25 ℃ to 85 ℃ on a hot plate. An optical microscope was used to image fibers after immersion. Fiber swelling ratios were tested among lignin/GO/PVA fibers. Fiber bundles of 30L0.05GO0.72D， 30L0.05GO0.72D-A， 5L0.05GO0.84D-A and 5L 0.05GO0.72D-A were immersed in distilled water for 24 h at room temperature. After samples were taken out from distilled water， residual water on the fiber surface was removed with filter paper. The fiber swelling ratioSwas calculated by

(4)

wherewdandwwrepresent the weight of dry fibers and wet fibers， respectively.

2 Results and Discussion

2.1 Effect of processing parameters on fiber drawing

Based on the study of fiber drawing conditions， an air drawing process is identified as an appropriate process for gel-spun fibers[21]since the air drawing process leads to higher drawing ratios and increasing mechanical performance of fibers. In addition， the size of the spinneret also has a great influence on the fiber formation process as well as the linear density of the fiber[22]， which ultimately affects the stress of fibers. To better understand the effect of these spinning conditions on the processing of gel-spun fibers， different parameters in the lignin/GO/PVA fibers’ drawing process were used and summarized in Table 2. Changes in the drawing temperature and the drawing ratios were observed. Overall， the total drawing ratios of fibers increased with an air drawing process and a smaller spinneret diameter (shown in Table 2). The following discussion will describe the effects of air drawing process and a spinneret diameter on the drawing ratio and the drawing temperature.

Lignin/GO/PVA fibers were drawn in multiple stages after gel formation and 24 h gel-aging in the acetone/methanol coagulation bath. The gel fibers solidified during the first stage of hot drawing. In general， most of the imbibed solvent diffused from the fibers into the high-temperature oil bath[17]. The temperature in the first stage of drawing increased from 110 ℃(or 120 ℃) to 160 ℃ for gel fibers with the lignin concentration increasing from 5% to 30%. For 30% lignin reinforced PVA fibers， hot drawing temperature in the fourth stage of drawing increased from 215 ℃ to 220 ℃ with air drawing process employed. The temperature in both the third stage and the fourth stage for 5% lignin and 0.05% GO reinforced PVA fibers increased with the decrease of the spinneret diameter.

With air drawing process， the total drawing ratios of gel-spun 30L0.05GO PVA fibers and 30L0.10GO PVA fibers increased from 11.3 to 11.8， and 11.4 to 13.1， respectively. The phenomenon suggested that fibers with air drawing process had better flexibility of polymer chains[17]. During the spinning process， as-spun fiber contained a large amount of solvents， which might hinder the macromolecular chain of the development from a disordered state to an ordered state. The air drawing process is the first drawing process after the as-spun fiber is collected from the coagulation bath. This additional process further stretches polymer chains， reduces the fiber diameter， and facilitates the gel-aging process， which reduces the solvent amount in the fiber structure. After air drawing， the strength， the toughness， and the dimensional stability of the fiber can be appropriately enhanced， and the subsequent thermal drawing can be more stable.

In the previous study， the total drawing ratio of PVA fibers was reduced slightly after introducing GO， and the reduction of the drawing ratio might be attributed to the physical barrier effect of graphene that impeded macromolecular segment movement freely[14]. However， the results in Table 2 showed that the total drawing ratio increased from 11.8 to 13.1 with GO content increasing from 0.05% to 0.10% for 30 lignin PVA fiber possibly due to the strong interfacial interaction between filler/filler and filler/polymer， and the synergistic effect of GO and lignin as plasticizers on the polymer chain stretching in the drawing process[18].

As for the impact of the spinneret diameter on the drawing process， it was shown in Table 2 that 5L0.05GO0.84D-A PVA fibers have larger fiber diameters (73 μm) than 5L0.05GO0.72D-A PVA fibers (62 μm)， whereas the drawing ratios of the two fibers are similar (12.1 and 12.9， respectively). This indicates that the spinneret diameter may have a negligible influence on the maximum drawing ratio of fibers.

Table 2 Drawing parameters and drawing ratios of fibers

2.2 Effect of processing parameters on fiber mechanical properties

2.2.1Airdrawing

The effect of air drawing on the tensile strength and Young’s modulus of lignin/GO/PVA fibers is discussed in this section. Fibers with air drawing process significantly enhanced mechanical properties when compared with those fibers that underwent no air drawing process. For 30% lignin and 0.05% GO reinforced PVA fibers， the tensile strength increased from 402 MPa (without air drawing) to 472 MPa (with air drawing)[shown in Fig. 2(a)]， indicating about 17.4% enhancement when air drawing process was used. Similarly， Young’s modulus increased from 4.94 GPa to 5.66 GPa after air drawing process was employed[shown in Fig. 2(b)]. For 30% lignin and 0.10% GO reinforced PVA fibers， the enhancement in the tensile strength (24.9%) and Young’s modulus (7.7%) was also observed. In addition， with GO content increased from 0.05% to 0.10%， 30% lignin reinforced PVA fibers exhibited higher tensile strength (from 472 MPa to 488 MPa)， which might be attributed to better alignment of GO nanosheets in the PVA matrix[14]. However， the Young’s modulus decreased from 5.60 GPa to 4.90 GPa with GO percentage increasing from 0.05% to 0.10%， as shown in Fig. 2(b).

The mechanical properties of fibers with air drawing process were superior to those of fibers without air drawing process for the following possible reasons. After the air drawing process， the relatively finer fiber has a larger contact area with the acetone/methanol coagulation bath， which is beneficial for the extraction of DMSO/water solvent from the fiber inner structure by acetone/methanol coagulation solvent during the gel-aging process. Furthermore， gel fibers with air drawing process are more drawable. The total drawing ratios for air-drawn fibers are higher than those for fibers without air drawing process (shown in Table 2). Higher fiber drawing ratios typically promote polymer chain alignment along the fiber axis for better mechanical performance.

The effect of the air drawing process on toughness of lignin reinforced PVA fibers is shown in Fig. 2(c). A maximum toughness value of 11.54 J/g was observed for 30L0.05GO0.72D-A PVA fibers. Toughness values range from 6.71 J/g to 11.54 J/g， which are greater than those of reported GO/PVA composites fibers (6.00 J/g)[23]. The strain at breakage for all of the lignin/GO/PVA fibers ranges from 5.5% to 7.0% regardless of the air drawing process.

Fig. 2 Mechanical properties of 30L0.05GO0.72D and 30L0.10GO0.72D PVA fibers with and without air drawing：

2.2.2Spinneretdiameter

Mechanical properties of lignin/GO/PVA fibers with different spinneret diameters are shown in Table 3. With the spinneret diameter decreasing from 0.84 mm to 0.72 mm， tensile strength， Young’s modulus， and toughness of 5L0.05GO0.72D-A PVA fibers increased accordingly. Typically， the macromolecular chains are oriented with the flow of the solution when the spinning solutions are extruded through the spinneret. As shown in Fig. 3， the small spinneret diameter facilitates the orientation of polymer chains in the extrusion process[22]. Although the spinneret inner diameter has a negligible influence on the fiber ultimate drawing ratio of drawn fibers (shown in Table 2)， the polymer orientation in the as-spun fiber may greatly determine the chain alignment of the ultimate solid fibers. Thus， fibers from small spinnerets may have better polymer orientation， small fiber diameters and ultimately better mechanical performance.

Table 3 Mechanical properties of fibers with different spinneret diameters

Fig. 3 Alignment of polymer chains： (a) large spinneret diameter； (b) small spinneret diameter

2.3 Fiber morphology

SEM images of lignin/GO/PVA fibers[shown in Figs. 4(a)， (b) and (c)] revealed that the diameters of the fibers were in the range of 50-75 μm， which were in accordance with the calculated diameters in Table 2. It was shown that the air drawing process and the smaller spinneret diameter facilitated the formation of finer fibers. SEM images showed that these fibers， regardless of the processing parameters， exhibited a smooth surface.

After mechanical testing， fiber fracture tips were also imaged by SEM， as shown in Figs. 4(d)-(i). 30L0.05GO0.72D PVA fibers and 30L0.05GO0.72D-A PVA fibers showed a fibrillar and ductile fracture tip[shown in Figs. 4 (g) and (h)]， which were associated with highly oriented and ordered chains of the polymer. The fibrillar microstructure appeared due to lignin plasticization[17]and was responsible for the good mechanical properties (tensile strength of 475 MPa for fibers with air drawing). However， 5L0.05GO0.84-A PVA fibers[shown in Fig. 4(i)] exhibited a smooth and less fibrillar fracture tip， indicating less oriented structure and interior mechanical performance (tensile strength of 429 MPa).

It should be noted that the porous structure was observed in all of these fibers， as shown in red circles in Figs. 4(g)-(i). The pores in 30% lignin reinforced PVA fibers were obviously larger than those in 5% lignin reinforced PVA fibers. These pores， serving as the defects of the fiber， significantly hindered the mechanical properties of fibers. In the gel-aging and drawing process， solvent removal occurred with partial lignin diffusion. These pores were formed probably due to the aggressive diffusion of lignin and solvent from the as-spun gel to the coagulation bath in gel-aging. It may also be caused by the diffusion of the residual solvent from the gel fiber to high-temperature oil during the hot drawing stages[17， 24]. Moreover， the large temperature difference (at least 80 ℃) between the spinning solution (60 ℃) and the coagulation bath (-25 ℃) may accelerate the phase separation and formation of large pores[25].

Fig. 4 SEM images： (a)-(c) surface of fibers； (d)-(i) fracture tips of fibers

2.4 Moisture resistance of lignin/GO/PVA fibers

The moisture resistance of lignin/GO/PVA fibers at room and elevated temperatures was observed by an optical microscope (shown in Fig. 5). All fibers showed an intact structure after immersion in water at 25 ℃. With 5% lignin， lignin/GO/PVA fibers[shown in Figs. 5(i)-(l)] in water at 85 ℃ exhibited larger diameters due to swelling. Swollen and gel-like structures were more obviously observed in the lignin/GO/PVA fibers with an increasing lignin percentage of 30%[shown in Figs. 5(a)-(h)]. The neat PVA fibers can be dissolved in water at 85 ℃ according to the previous study[17]. With the addition of lignin， strong intermolecular bonding between lignin and PVA hindered PVA fibers dissolution in water at 85 ℃. With 30% lignin， fibers fabricated without air drawing process[shown in Figs. 5(b) and (f)] swelled more than those with air drawing[shown in Figs. 5(d) and (h)].

The swelling ratios of immersing lignin/GO/PVA fibers in water at room temperature are presented in Table 4. Luetal.[17]reported that swelling ratios of lignin/PVA fibers increased from 19% to 82% with lignin content increasing from 5% to 30%. The study proved that lignin permitted PVA swelling in confined regions. However， with incorporation of GO， the swelling ratios of both 5% lignin and 30% lignin reinforced PVA fibers decreased to nearly 10%.

Fig. 5 Optical micrographs of lignin/GO/PVA fibers with different processing parameters after immersion in water at 25 ℃ and 85 ℃： (a)-(b) 30L0.05GO0.72D； (c)-(d) 30L0.05GO0.72D-A； (e)-(f) 30L0.10GO0.72D； (g)-(h) 30L0.10GO0.72D-A； (i)-(j) 5L0.05GO0.84D-A； (k)-(l) 5L0.05GO0.72D-A

Table 4 Swelling ratio S of lignin/GO/PVA fibers after water immersion for 24 h

3 Conclusions

We have successfully spun lignin/GO/PVA fibers with different processing parameters and promising results in terms of mechanical performance and water resistance. By optimizing the spinning process， the mechanical properties of fibers with air drawing process were significantly improved in comparison with those of fibers without air drawing process. With air drawing process， the tensile strength of 30% lignin and 0.05% GO reinforced PVA fibers increased by 17.4%， and Young’s modulus increased by 14.6% (from 4.94 GPa to 5.66 GPa). For 5% lignin and 0.05% GO reinforced PVA fibers， the enhancements in tensile strength (24.9%) and Young’s modulus (7.7%) were observed with a spinneret diameter of 0.72 mm， implying that a smaller diameter of the spinneret had a positive effect on fiber properties. Furthermore， both lignin and GO are low-cost materials， which makes lignin/GO/PVA fibers a potential candidate for traditional high-performance fibers. The incorporation of GO which has excellent UV-blocking property and good antibacterial activity[14]， provides possibilities of PVA composite fibers for applications in textiles， biomedical fields， and biotechnological fields.

However， pores were observed in fibers， which significantly impeded the enhancement of mechanical properties. Further study should be concentrated on restricting lignin diffusion in coagulation bath and tailoring or optimizing the spinning parameters of the solvent and coagulant to produce high-performance functional fibers.