Effect of the Coagulation Bath on the Structure and Mechanical

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Effect of Coagulation Bath on the Structure and Mechanical Properties of Gel Spun Lignin/Polyvinyl Alcohol Fibers Chunhong Lu, Charles Blackwell, Qingyuan Ren, and Ericka Ford ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02423 • Publication Date (Web): 12 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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ACS Sustainable Chemistry & Engineering

Effect of Coagulation Bath on the Structure and Mechanical Properties of Gel Spun Lignin/Polyvinyl Alcohol Fibers Chunhong Lu, Charles Blackwell, Qingyuan Ren, Ericka Ford* Department of Textile Engineering, Chemistry and Science, The Nonwovens Institute, North Carolina State University 1020 Main Campus Drive, Raleigh, NC 27606 Raleigh, NC, 27695 U.S.A *[email protected]

ABSTRACT

Gel spinning was investigated as an alternative approach to melt spinning lignin-based fibers. Lignin/polyvinyl alcohol (PVA) composites, at varying weight percentages of lignin, were gel spun into high strength fibers. Although lignin is an amorphous biopolymer, incorporation of the rigid filler enhanced the mechanical properties of PVA fibers and affected their structure. Lignin stabilized the gel structure of thermoreversible PVA gel, as noted by higher gel melting temperatures. Methanol/acetone coagulation baths at high acetone content rendered gel spun fibers more drawable, helped to maintain lignin within gel fibers and increased their gel melting 1

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point. The best mechanical performance was observed among 5% lignin fibers, having an average tensile strength of 1.1 GPa, Young’s modulus of 37 GPa, and toughness of 17 J/g. Structural analysis of 5% lignin fibers has shown them to possess to the highest index of PVA crystallinity. Fibers were more drawable at higher weight percentages of lignin. This plasticizing behavior at elevated temperatures of drawing led to stronger lignin-based fibers. Evidence of hydrogen bonding between lignin and PVA within gel-drawn fibers was observed by infrared spectroscopy, and substituents of lignin biopolymer were mildly aligned along the fiber axis. Lignin/PVA fibers resisted dissolution in boiling water unlike neat PVA fibers. But, fiber swelling increased with lignin content. Structurally, gel spun composites must have regions of PVA laced between lignin. In summary, biobased fibers of 5-50% lignin were gel spun into composites having mechanical properties that are suitable for industrial and high performance fiber applications.

KEYWORDS Gel Spinning, Lignin, Polyvinyl Alcohol, Coagulation Bath, Raman Anisotropy INTRODUCTION High performance fibers are versatile reinforcing materials for different industries and applications. With the high demand for cost-effective high performance fibers, recent efforts have focused on the development of high-end materials from biopolymers, namely lignin. Lignin is the second most abundant biopolymer next to plant-based cellulose and a waste by-product of the pulp and paper industry1-2. Lignin has been investigated as a filler for biobased fibers. However, melt extruded fibers of lignin/polypropylene (PP) and lignin/polyethylene (PE)3 showed significantly less tensile strength with the addition of lignin. Lignin is a candidate for precursor carbon fiber due to its 2


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high carbon yield; however, lignin derived fibers typically demonstrate mechanical properties that are inferior to conventional carbon fibers4-5. The mechanical properties of both precursor6 and carbonized fibers7 from melt extruded lignin/polylactic acid (PLA) were less than fibers prepared from PLA and carbon fiber from wholly lignin, respectively. Lignin substituents branch out three dimensionally2, which could hinder its molecular compatibility with other polymers, especially in the melt. Melt spun fibers often show evidence of phase separation6, 8. The low molecular weight of lignin and its lack of crystalline order9 are likely causes of poor mechanical performance among carbonized fibers. Nevertheless, lignin has the potential to blend with polymers. The polar nature of lignin and its tendency to hydrogen bond with molecular groups aid its miscibility with polar polymers10. Kubo et al reported strong interactions between lignin and PVA in melt spun fibers, but the mechanical properties of lignin/PVA fibers were not reported10. Improved interactions between lignin and polymer were observed among solvent cast blends. Xu et al and Su et al have increased mechanical performance among solution cast lignin/PVA membranes in the range of 10-20% lignin.11-12 Insufficient studies in the area of solution spun, lignin-based fibers motivated this work that uses PVA as the matrix polymer. PVA staple fibers are used as concrete reinforcement. These fibers prevent crack propagation and their hydroxyl groups promote molecular adhesion between the fibers and concrete. Solution spun PVA fibers have high tenacity values in the range of 14-18 g/den13 (1-2 GPa). In comparison to melt spinning, solvent is used to homogenize blends used for solution spinning. High performance PVA fibers are produced by the gel spinning technique, wherein the Young’s modulus values of gel spun PVA fibers range between 20-115 GPa14. The mechanical

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performance of gel spun fibers are influenced by several processing parameters: solvent mixture for dissolution, gelation temperature15, drawing temperature and ultimate draw ratio16. Motivating this work is the prospect of fabricating sustainable, biobased fibers whose mechanical performance and low-cost are ideal for use in industrial and high performance applications. To our knowledge, this is the first study that investigates the fabrication of lignin/PVA fibers by the gel spinning technique. The structure-property relationships of gel spun lignin/PVA fibers were investigated in consequence to lignin content, composition of solvent coagulation bath for gelation, drawing temperature and draw ratio. EXPERIMENTAL SECTION Materials PVA, having a molecular weight of 146-186 kg/mole and 99% hydrolysis, was purchased from Sigma Aldrich. Aqueous raw pine sawdust lignin paste (Project # L28) at pH 3 was provided by Pure Lignin Environmental Technology (PLET) LLC. Lignin was extracted from wood pulp using a weak-acid hydrolysis treatment. Solvents were used as-received: dimethyl sulfoxide (DMSO-from Sigma-Aldrich), acetone, and methanol (both from BDH Chemicals). Spinning Dope Preparation Aqueous PLET lignin paste was filtered to remove low molecular weight fractions. Lignin was extracted from paste using acetone and vacuum filtration. Afterwards, lignin was dried at 85 °C for 24 hr and finely ground into powder using a mortar and pestle. Spinning dopes of PVA and lignin/PVA were prepared. 10 g PVA powder was dissolved in 100 ml of 80/20 (v/v) DMSO/distilled water under constant stirring at 85 °C for 1 hr. 4


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Lignin/PVA dopes, at weight ratios up to 50% (w/w) lignin to polymer, were also dissolved in 80/20 DMSO/distilled water at 85 °C. The final concentration of PVA in spinning dopes was 10 g/dL. The homogeneity of lignin/PVA dopes were inspected visually and optically with a Nikon Eclipse 50i POL optical microscope. Gel Spinning Lignin/PVA dopes were dark brown in color (see Figure 1a, b). Optical micrographs of dopes having 0, 5, and 30% lignin (Figure 1b) showed homogenous solutions that were absent of aggregation. This suggests lignin and PVA were completely dissolved in DMSO/water- an ideal case for any type of solution spinning. A schematic of the gel spinning process is shown in Figure 1c. The lignin/PVA spinning dopes were dispensed from a steel high pressure syringe. The syringe was heated to 85 °C before extruding dopes through a 19-gauge syringe needle (0.69 mm inner diameter). Afterwards, the dope gelled in a -25 °C coagulation bath. Syringe tip to coagulation bath distance was 3-5 mm. The resulting as-spun gel fibers were collected onto a rotating winder and later immersed in the 5 °C coagulation bath for 24 hr. Fibers were drawn through 1-4 stages of silicone oil at elevated temperatures of 90-240 °C (Step 3 of Figure 1). Draw ratios (DR) at each stage of fiber drawing was calculated as

=

(1)

where V1 is the velocity of fiber feeding winder and V2 is the velocity of fiber take-up winder. Coagulation baths contained mixtures of methanol/acetone. Lignin/PVA gelation (in particularly gel opacity) and lignin leaching were initially tested in 20 mL of solvent. Spinning dopes at 85 °C were added dropwise into -25 °C mixtures of methanol/acetone. An ideal solvent system would yield a clear gel and negligible amounts of lignin leached into the coagulation 5



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bath. Gel opacity and lignin diffusion into the coagulant was observed after 10 min and 24 hr of aging spinning dopes in -25 and 5 °C baths. PVA gels are thermoreversible. At elevated temperatures, they will return to liquids of dissolved polymer. Gel melting of lignin/PVA gels was measured according to the method described by Ryan et al.17 Their test method was slightly modified and described in Figure 1S of Supporting Information. Lignin/PVA dopes were loaded into capillary tubes, having one end capped, and gelled in -25 °C 100/0 and 15/85 methanol/acetone baths for 10 min. The capillary tube of gelled polymer and a thermocouple were placed in a test tube that was positioned within the center of a Thiele. The Thiele filled with silicone oil was heated by a Bunsen burner. Gel melting points were identified as the stationary gel transformed into a flowing liquid.

Figure 1. The homogeneity of lignin/PVA dopes is visible within (a) photographs and (b) optical micrographs. Dopes of lignin/PVA contained 1) 0%, 2) 5% and 3) 30% lignin. The gel fiber spinning process is illustrated in (c) as three steps: as-spun gel fiber formation (step 1), gel fiber aging (step 2) and multi-stage fiber drawing (step 3). 6


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Mechanical Testing The mechanical properties of fibers were tested on the MTS-Q according to ASTM D 3379. Test parameters included strain rate of 15 mm/min, gauge length of 25 mm, and sample size of 10. To normalize data, effective cross-sectional area A was calculated gravimetrically from A=d/ρ

(2)

where d is the linear density of fiber and ρ is the density of composite fiber. Linear density is measured by weighing a known length of fiber. Before weighing, fibers that were hot-drawn in oil were rinsed with isopropyl alcohol to remove residual silicone oil. The density of composite fibers was determined from ρ=ρPVA 1- +ρ

lignin

(3)

where wf is the weight fraction of lignin. PVA’s density18 (ρPVA) and lignin’s density19 (ρlignin) have the same value. Therefore, the composite fiber’s density was ρ=1.3 g/cm3. Tensile toughness (Ut) was calculated by integrating stress-strain curves, which represents the energy absorbed before fiber breakage20. Equation 4 represents tensile toughness: Ut = σi εi

(4)

where σi, εi refer to stress and strain values at each data point, respectively. Imaging Analysis Fibers were embedded in synthetic cork. Thin, perpendicular mounts revealed fiber cross sectional areas that were later imaged by LEXT OSL4000 3D measuring laser confocal microscope. Fiber fracture tips from mechanical testing were sputter coated with gold and imaged by the FEI Verios 460L Scanning Electron Microscopy (SEM) using an accelerating voltage of 2 kV. 7



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Structural Characterization Infrared spectra of as-received PVA powder, lignin powder, and lignin/PVA fibers were obtained by the NICOLET iS50 spectrophotometer. Spectra were collected at 128 scans and a spectral resolution of 4 cm-1. Absorbance at 1144 cm-1 represents symmetric C-C stretching21 along the polymer chain, wherein neighboring hydroxyl (-OH) groups engage in intramolecular hydrogen bonding22. Absorbance spectra of PVA were normalized to the peak intensity of reference band 854 cm1 22-24

.

The band at 854 cm-1 (C-C stretching) was chosen as the reference band since its

absorbance is not affected by processing22. Percent crystallinity (α) of polymer is presented by the following equation22-24, α (%)= +

× 100%

(5)

where B and C are constants. Values for B and C are calculated from known values of percent crystallinity, as by X-ray diffraction.23 Absorption intensities for A1144/A854 were obtained from Infrared spectra23. The absorbance intensity of crystalline PVA (A1144) at 1144 cm-1 is normalized by the reference band intensity at 854 cm-1, A854. Ratios of A1144/A854 are indices of fiber crystallinity. These were used to compare the relative crystallinity of each fiber. Polarized Raman spectroscopy was used to quantify the molecular anisotropy of lignin embedded in lignin/PVA fibers. Using the Bayspec Normadic Confocal Raman Microscope, polarized spectra were collected parallel and perpendicular to the axis of fiber bundles (containing ~50 fibers). Fiber measurements were taken with a 10 X objective lens and 785 nm laser at 104 mW (based on filter setting). Sample exposure time was 1 sec with an acquisition number of 10. 8


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Water Dissolution and Swelling To investigate the water resistance of composite fibers, 3 mg bundles of neat PVA, 5% lignin, and 50% lignin fibers were placed in 75 ml of water and gradually heated from room temperature to 100 °C on a hot plate. After immersion, fibers were imaged by confocal microscopy. Fiber swelling was tested among lignin/PVA fibers. Fiber bundles of 0, 5% and 30% lignin were immersed in vials of distilled water for 24 hr at 25 °C. Samples were taken from vials and excess water was removed with filter paper. Fiber swelling ratio (S) was calculated using Equation 6: % =

× 100%

(6)

where wd is the dry weight of fiber and ww is the weight of fiber after wetting. RESULTS AND DISCUSSION Methanol/Acetone Ratio on Lignin/PVA Gelation In general, methanol facilitates PVA gelation15. Wholly acetone coagulation baths were not optimum for PVA gelation. When exposed to acetone, neat PVA dopes turned opaque. PVA gels from acetone coagulation were brittle and less drawable than the transparent gels formed from PVA in chilled methanol. However, adding acetone to the methanol-based coagulation bath was beneficial

to

lignin/PVA

gelation.

Lignin/PVA

dopes

were

added

dropwise

into

methanol/acetone mixtures at different volume ratios (see Figure 2). Yellowing of the solvent mixture was indicative of lignin diffusion into methanol from gel polymer. Because lignin is soluble in methanol and insoluble in acetone, more lignin diffused into the coagulation bath at higher ratios of methanol/acetone, greater percentages of lignin/PVA and longer coagulation 9



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times. Negligible amounts of leaching were observed among baths having more than 80% acetone by volume. Further, these lignin/PVA gels remained soft and flexible.

Figure 2. Coagulation of lignin/PVA dopes, having (a) 5 and (b) 50% lignin in methanol/acetone mixtures of (A) 100/0, (B) 90/10, (C) 80/20, (D) 70/30, (E) 60/40, (F) 50/50, (G) 40/60, (H) 30/70, (I) 20/80, (J) 10/90 and (K) 0/100 (v/v) for 1) 10 min at -25 °C and 2) 24 hr at 5 °C. Gel structure, as interpreted from the results in Figure 2, at low to high lignin content are illustrated in Figure 3. At the onset of gelation, polymer dope phase separates into polymer-rich and solvent-rich domains25. Crystalline regions of PVA function as physical junctions for the gel network, and solvent domains are spaced throughout the network. Based on observations in Figure 2, lignin more readily diffuses into methanol-rich coagulation baths at high weight percentages of lignin and high ratios of methanol/acetone in the bath. Therefore, lignin must preferentially reside in polymer-rich domains at low lignin content. Once lignin has saturated polymer-rich domains, lignin will then reside in solvent-rich domains of the gel network. Acetone in the coagulation bath suppresses lignin’s diffusion, from both the polymer-rich and solvent-rich domains of the PVA gel, into the coagulation bath. Even at high lignin content, 8090% acetone in the bath maintained the higher amount of lignin in the gel structure.

10


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Figure 3. Illustration of fiber microstructures at (a) low lignin and (b) high lignin content in 1) as-spun gel fiber or 2) gel-drawn fibers. Lignin Content on Gel Melting Based on Table 1, lignin effectively increased the melting temperature of gels coagulated in media of 100/0 and 15/85 methanol/acetone. Gels coagulated in 15/85 methanol/acetone exhibited even higher melting temperatures at the same lignin concentration. Therefore, higher temperatures were expected for the drawing of gel fibers produced from 15/85 methanol/acetone coagulation. Crystalline regions in the gel structures affected gel melting temperature. As shown in Figure 3, lignin resided in semi-crystalline polymer-rich domains. Its presence enhanced the thermal resistance of PVA; thereby increasing the gel melting point of lignin/PVA gels. Pure methanol coagulation reduced lignin content in gel fiber. As a result, gels formed in 100/0

11



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methanol/acetone had lower gel melting temperatures than those formed by 15/85 methanol/acetone. Table 1. Lignin/PVA Gel Melting Points in Response to 100/0 and 15/85 Methanol/Acetone Coagulation Lignin/PVA Gel

Gel melting Temperature (°C) Gel melting Temperature (°C) 100/0 Methanol/Acetone Bath 15/85 Methanol/Acetone Bath

0% Lignin

101 ± 2

103 ± 3

5% Lignin

108 ± 6

114 ± 4

30% Lignin

111 ± 4

118 ± 5

50% Lignin

119 ± 2

122 ± 2

Effect of Coagulation Bath on Fiber Drawing Based on our study of solvent systems for coagulation (see Figure 2), the 15/85 methanol/acetone system was identified as an appropriate bath for lignin/PVA gelation. To understand the effects of coagulating solvents on the processing of gel spun fibers, parameters for lignin/PVA fibers that were drawn in 100/0 and 15/85 methanol/acetone baths were summarized in Table 2. The following discussion will describe the effects of coagulation bath, gel melting point, and lignin content on drawing parameters. Changes in stage drawing temperature and draw ratio were observed. Overall, the total draw ratio of fibers increased with lignin content (Table 2). Fibers were drawn in multiple stages after gel formation in 100/0 methanol/acetone. During stage 1 drawing, gel fibers solidify. In general, most of the imbibed solvent diffuses from gel 12


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fiber and into the high temperature oil bath. The stage 1 drawing temperature decreased from 100 to 90 °C for pure methanol coagulated gel fibers as the lignin concentration increased above 10% lignin to polymer. Further, stage 1 draw ratios decreased with additional lignin. Draw ratios also increased with lignin content during stage 2 and 3 heat drawing after 100/0 methanol/acetone coagulation. Based on results from the gel melting study (in Table 1), stage 1 drawing temperatures were expected to increase with lignin content. Lower drawing temperatures were also expected for 100/0 methanol/acetone coagulated gel fibers. Lignin leached into the 100/0 methanol/acetone bath and stage 1 drawing oil from structures of 15-20% lignin gel fibers (as represented by Figure 3b1). At 15-20% lignin, drawing temperatures decreased from 100 to 90 °C for fibers containing 0-10% lignin. Acetone in the 15/85 methanol/acetone coagulation bath precluded lignin leaching into gel spinning baths and drawing oil. As-spun draw ratios decreased with increasing lignin content among gels from 15/85 methanol/acetone baths. In contrast to pure methanol coagulation baths, more lignin would reside in as-spun gels from 15/85 methanol/acetone baths. Since lignin remained in thermally drawn fibers, temperatures at consecutive stages of drawing were significantly higher for gel fibers coagulated in 15/85 methanol/acetone and with high lignin content. Further, stage 1 drawing temperature increased from 100 to 120 °C by 50% lignin, and stage 1 draw ratio had increased from 5.8 X for 5% lignin to 8.5 X for 50% lignin. Higher values of draw ratio at elevated temperatures were attributed to the plasticizing effect of lignin on PVA. Similarly, Kubo, Kalda and Baumberger et al generalized the improved processing of PVA as due to lignin plasticization of PVA/lignin blends.10, 26 Lignin has a lower molecular weight than PVA, and it can disrupt hydrogen bonding between PVA molecules. 13



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Lignin’s glass transition temperature (Tg of 120-150°C27) is expected to influence the fibers’ molecular mobility at drawing temperatures close to Tg. As a result, the extension of matrix PVA chains was more feasible at higher temperatures. Table 2. Drawing Parameters and Draw Ratio (DR) for Gel Spun Fibers Coagulation Bath

100/0 Methanol/Acetone Bath

15/85 Methanol/Acetone Bath

Lignin concentration

0%

5%

10% 15% 20%

5%

20%

30%

50%

As -spun DR

2.5

2.2

2.3

2.5

2.4

2.6

1.9

1.4

1.4

Stage 1 Temperature (°C) Drawing DR

100

100

100

90

90

100

100

100

120

4.2

4.0

4.0

4.0

3.3

5.8

6.8

7.0

8.5

130

150

150

160

160

180

180

190

190

1.4

1.5

1.5

1.5

2.1

1.7

1.5

1.5

1.4

180

180

190

190

220

210

210

210

225

1.3

1.3

1.5

1.4

1.5

1.3

1.3

1.4

1.5

Stage 2 Temperature (°C) Drawing DR Stage 3 Temperature (°C) Drawing DR Stage 4 Temperature (°C) Drawing DR

-

-

-

-

-

230

230

230

240

-

-

-

-

-

1.2

1.2

1.2

1.2

Total Heat DR

7.6

7.8

9.0

8.2

10.1

14.7

16.0

17.5

20.6

Effective diameter (µm)

52

52

52

51

35

32

41

48

40

Linear Density (dtex)

27

27

27

26

13

10

17

23

17

“- “: Not Applicable: methanol coagulated fibers melted and relaxed at high temperatures Total Heat DR: cumulative draw ratio from stages 1 to 4 Effect of Coagulation Bath on Fiber Mechanical Properties The effect of solvent coagulation on the tensile strength and Young’s modulus of lignin/PVA fibers (up to 50% lignin) are shown in Figure 4. Neat PVA fiber was spun using 100/0 14


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methanol/acetone coagulation; however, it was not possible to spin neat PVA fiber using 15/85 methanol/acetone coagulation. Acetone is attributed with turning neat PVA gel fibers opaque and too brittle for spinning. As lignin content increased among the 100/0 methanol/acetone coagulated fibers in Figure 4a, lignin did aid fiber strengthening among some PVA fibers. Maximum tensile strength was 0.61 GPa at 5% lignin, and maximum modulus was 24 GPa at 10% lignin. Higher lignin concentrations did not yield further increases in fiber mechanical performance. Instead, fiber properties diminished drastically to 0.37 GPa in tensile strength and 11 GPa in Young’s modulus with 20% lignin. Voids were observed throughout the fiber cross sections of 20% lignin fibers that were coagulated in the 100/0 methanol/acetone (Figure 6). Fibers from 15/85 methanol/acetone coagulation had significantly higher mechanical properties than those from 100/0 methanol/acetone coagulation (Figure 4). The average tensile strength and Young’s modulus for 5% lignin fibers were 1.1 GPa and 37 GPa, respectively. These values exceeded that of commercially available, high strength KuralonTM PVA staple fiber that is 0.88 GPa in tensile strength and 23 GPa in modulus. Above 5% lignin, fibers exhibited lower mechanical properties. Tensile strength values were 0.74 GPa for 20% lignin, 0.77 GPa for 30% lignin, 0.76 GPa for 50% lignin. The Young’s modulus was in the range of 31-36 GPa, which is still higher than that of commercial KuralonTM PVA staple fiber from Kuraray. Fiber mechanical properties, from 15/85 methanol/acetone coagulation, were superior to those from 100/0 methanol/acetone coagulation for the following reasons. The 15/85 methanol/acetone bath suppressed lignin leaching during gelation and fiber drawing. At 20% lignin, voids were not observed at the fiber cross sections of 15/85 methanol/acetone gelled fibers. Further, gel fibers from the 15/85 methanol/acetone bath were more drawable. The total draw ratio for 15/85 15



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methanol/acetone fibers were higher than for 100/0 methanol/acetone fibers (see Table 2). Higher fiber draw ratios typically facilitate polymer chain alignment along the fiber axis for higher mechanical performance. Further, molecular interactions between lignin and matrix polymer can disrupt hydrogen bonding between PVA chains, so that lignin plasticizes PVA’s mobility during high temperature drawing. Evidence of PVA/lignin bonding will be discussed with infrared analysis.

Figure 4. Tensile strength and Young’s modulus of lignin/PVA fibers at 0-50% lignin. Coagulation baths of (a) 100/0 and (b) 15/85 methanol/acetone were used during gel fiber spinning. The effect of coagulation bath on lignin/PVA fiber toughness is shown in Figure 5. Characteristics of tough fibers include high strength and ductility. Strain at break values for all lignin/PVA fibers ranged between 3.1-4.2% regardless of the coagulation bath. The lignin/PVA fibers were tougher than neat PVA fiber, as observed for fibers from pure methanol coagulation. 15/85 Methanol/acetone coagulation yielded tougher fibers than those obtained from wholly methanol baths. Maximum toughness value 8.5 J/g was observed at 5% lignin upon 100/0 methanol/acetone coagulation. An even higher value of 17 J/g was obtained 16


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after 15/85 methanol/acetone coagulation for 5% lignin fibers. As lignin content increased, toughness values slightly decreased. Toughness values of 10-12 J/g were reported at 20-50% lignin and 15/85 methanol/acetone coagulation- values that are greater than graphene oxide reinforced PVA composites (6 J/g)28. The toughness value of 12 J/g for 50% lignin fiber was greater than at 20-30% lignin- although its tensile modulus at 50% lignin was less than values at those lower weight fractions. By incorporating lignin at 50% of the matrix polymer, its rigid structure increased fiber toughness. A discussion of fiber morphology is later used to explain the apparent differences in mechanical performance.

Figure 5. Toughness of lignin/PVA fibers having increasing amounts of lignin that were coagulated in 100/0 (▲) or 15/85 (●) methanol/acetone. Effect of Lignin on Fiber Morphology Neat PVA and 5% lignin fibers from 100/0 methanol/acetone coagulation were smooth and had round cross-sections. At higher lignin content, fiber cross sections were less circular (Figure 6) owing to non-uniform solvent removal. Kidney-bean cross-sections were observed among 17



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fibers having 20% lignin and coagulation in 100/0 methanol/acetone. Similarly, 15/85 methanol/acetone coagulation also resulted in kidney-bean shaped fibers. At 5% lignin, fiber from coagulation in 15/85 methanol/acetone was finer than fiber from the 100/0 methanol/acetone bath. Gel fibers from 15/85 methanol/acetone had higher total draw ratios and lower values of linear density (Table 2), which correspond to smaller diameter fibers. Coagulation solvents and lignin content affected solvent removal from gel spun fibers. Solvent removal occurred in the coagulation bath and high temperature drawing oil. Voids throughout the fiber cross-section of 20% lignin fibers had resulted in significant losses in mechanical performance (Figure 4a). These voids were likely caused by the high content of lignin in the solvent-rich domains of gel fiber (Figure 3b1), which was followed by the aggressive diffusion of lignin from as-spun gel to methanol and from gel fiber to high temperature drawing oil. Fibers, having 20% lignin and from 15/85 methanol/acetone coagulation, were finer than 20% lignin fibers from 100/0 methanol/acetone. Also, their fiber cross sections were rougher and without voids. Methanol/acetone coagulation suppressed voiding while enabling better mechanical performance (Figure 6). The reduction in mechanical performance among 15/85 methanol/acetone coagulated fibers, having more than 20% lignin, was caused by other differences in fiber morphology, namely lignin aggregation.

18


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Figure 6. Confocal micrographs of (a) 0, (b) 5 and (c) 20% lignin/PVA fiber cross sections from coagulation baths of (1) 100/0 and (2) 15/85 methanol/acetone. After mechanical testing, fiber fracture tips were imaged with SEM, as shown in Figure 7. Neat PVA fibers showed a smooth fracture tip (Figure 7a). Its microstructure contrasts with the fibrillar, more ductile fracture tip of 5% lignin fiber (Figure 7b). PVA fibrils are associated with highly oriented and ordered chains of polymer. (In the next section, indices of crystallinity among gel spun PVA fibers versus lignin content was measured with infrared spectroscopy.) The fibrillar microstructure appeared due to lignin plasticization and is responsible for high mechanical properties. PVA fibrils were noticeable among 30% lignin fibers (Figure 7d). At 50% lignin (Figure 7e), fiber was less fibrillar and lignin aggregation, in the form of small beads, were observed at high resolution (Figure 7e2). Lignin aggregation is attributed to its concentration within polymer-poor domains of the gel fiber at high lignin content (Figure 3b1). 19



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As a result, lignin was not homogenously dispersed throughout the polymer. In summary, lignin promoted the formation of fibrillar PVA at high fiber draw ratios; however, lignin aggregates were structural defects that limited the mechanical performance of fiber at more than 20% lignin.

Figure 7. SEM images of fracture tips of (a) neat PVA fiber from 100/0 methanol/acetone and (b) 5, (c) 20, (d) 30 and (e) 50% lignin fibers from 15/85 methanol/acetone coagulation and after mechanical testing at 1) low and 2) high resolution.

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Effect of Lignin Content on Fiber Microstructure Since 15/85 methanol/acetone coagulation yielded the highest values of mechanical performance, the effect of lignin content on PVA crystallization was investigated per IR spectroscopy (Figure 8). Amorphous PVA is associated with the C-O vibrational mode at 1094 cm-1 and PVA crystallinity affected 1144 cm-1 absorbance.

Figure 8. IR absorbance spectra of lignin powder, PVA powder, neat PVA fiber coagulated in 100/0 methanol/acetone and lignin/PVA fibers coagulated in 15/85 methanol/acetone between a) 4200-1000 cm-1 and b) 1250-800 cm-1. Values of A1144/A854 are indices of fiber crystallinity (Table 3). After gel fiber spinning, the crystallinity of PVA fiber was greater than for as-received powder. Fiber drawing caused the dense packing of polymer chains. The highest degree of crystallinity occurred at 5% lignin, but PVA crystallinity decreased from this value at more than 20% lignin. Fibers having 30% lignin were slightly more crystalline than those containing 20 and 50% lignin. However, lignin aggregation and phase separation from PVA, as observed among fiber fracture tips, was detrimental to the properties and crystallinity of PVA fibers. These trends in fiber crystallinity

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agree with changes in the mechanical properties of lignin/PVA fibers from 15/85 methanol/acetone coagulation (Figure 4). Table 3. Indices of Lignin/PVA Fiber Crystallinity Using A1144/A854 Infrared Absorbance Peaks Materials

A1144/A854

PVA powder

2.92

Neat PVA fiber

3.34

5% Lignin/PVA

4.67

20% Lignin/PVA

2.88

30% Lignin/PVA

2.98

50% Lignin/PVA

2.60

Note: Neat PVA fiber was fabricated from 100/0 methanol/acetone coagulation, and 5-50% lignin/PVA fibers were from 15/85 methanol/acetone coagulation. Intermolecular bonding between lignin and PVA induced molecular adhesion. IR absorbance spectrum (from 3000-3700 cm-1 in Figure 8a) provided insight into hydrogen bonding for lignin powder (3384 cm-1) and PVA (3345 cm-1). Among 5% lignin fibers, the -OH band shifted toward lower frequencies of 3333 cm-1. This behavior was suggestive of hydrogen bonding between lignin and PVA.10 As a result, 5% lignin exhibited higher mechanical performance and a fibrillar morphology. At high lignin content, peak positions were 3347 cm-1 for 20% lignin, 3342 cm-1 for 30% and 3360 cm-1 for 50% lignin. Those wavenumbers were associated with less intermolecular bonding between lignin and PVA. Lignin orientation within composite fibers was also studied to understand its structure in fibers. Raman anisotropy (R in Equation 7) has been used to quantify the molecular orientation of conjugated polymer29: I

R= I ∥

(7)

⊥

22


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where I∥ and I⊥ represent peak Raman intensities at polarization angles that are parallel (∥) or perpendicular (⊥) to the fiber axis. This technique was used to measure lignin’s molecular orientation within fibers coagulated in 15/85 methanol/acetone. The Herman’s orientation factor30 f is often used to quantify molecular orientation in the direction of fiber drawing. Parameters f and R are related by Equation 829: $%

# = $&'

(8)

For perfectly aligned molecules, R=∞ and f=1. For randomly aligned molecules, R=1 and f=0.

Figure 9. Polarized Raman spectra are of 5% lignin fiber from 15/85 methanol/acetone coagulation- parallel (∥ or 0°) and perpendicular (⊥ or 90°) to the fiber axis. The peak at ~1560 cm-1 was assigned to phenol in-plane stretching, and the peak at ~1650 cm-1 was assigned to conjugated bonds in lignin’s structure31 (Figure 9). Orientation parameters R and f were determined for both peaks for all lignin/PVA fibers (Table 4). Parameters R and f were the greatest for 5% lignin fiber although its anisotropy value was very low, R~2. The amorphous structure of lignin impedes its axial alignment; nevertheless, our results suggest interactions between PVA and lignin have caused lignin segments to align along the fiber axis. Lignin 23



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aggregation at higher weight fractions decreased interactions between the biopolymer and PVA. Nevertheless, highly drawn fibers of fibrillar PVA resulted from the plasticizing behavior of lignin and interactions between aligned PVA with lignin segments. Values of R and f were the lowest for 20% lignin, but these increased slightly at 30-50% lignin in fiber.

Table 4. Orientation Parameters for Lignin Embedded in Lignin/PVA Fibers from 15/85 Methanol/Acetone Coagulation Lignin Concentration in Fiber

5%

20%

30%

50%

Raman Anisotropy (R) Benzene ring

1.65

1.10

1.45

1.27

Conjugated C=C

1.62

1.16

1.50

1.33

Benzene ring

0.12

0.02

0.08

0.05

Conjugated C=C

0.11

0.03

0.09

0.06

Orientation factor (f)

Moisture Resistance of Lignin/PVA Fibers Carbon nanotube (CNT) fillers are known to enhance the solvent resistance of PVA composite fibers against DMSO32. In this study, lignin had enhanced the thermal resistance of composite PVA fibers to dissolution in water, while enhancing its swelling behavior at room temperature. Lignin/PVA fiber structures were implied from these studies of their susceptibility to water dissolution at elevated temperatures (Figure 10). The dissolution of neat PVA, 5% lignin and 50% lignin composites fibers was tested. All three fibers remained intact at room temperature (25 °C). In 85 °C water, neat PVA fiber had dissolved, whereas lignin-based fibers exhibited diameter increases from swelling.

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At 5% lignin, fiber had partially dissolved in boiling water. Fibrils of lignin/PVA were suspended in water, leaving behind a fine fiber, as seen in Figure 10b3. Hydrogen bonding between lignin and PVA (as previously determined from IR spectra in Figure 8) stymied fiber dissolution. Fibrils of lignin/PVA remained after boiling in water. Their presence at elevated temperatures suggests the PVA interfacing lignin is highly crystalline. Lignin locale in the polymer-rich domains of gel fiber (Figure 3a2) later causes the crystalline regions of fibrillar polymer to resist dissolution at elevated temperatures, while amorphous polymer dissolved in boiling water. At 100 °C, 50% lignin fiber behaved like a swollen gel (Figure 10c3). Its structure remained mostly intact. Intermolecular interactions between lignin and PVA have caused fibers to resist dissolution. Also, water insoluble aggregates of lignin can hinder the dissolution of crystalline and amorphous polymer.

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Figure 10. Confocal micrographs of (a) PVA from 100/0 methanol/acetone coagulation, (b) 5% lignin and (c) 50% lignin-based fibers from 15/85 methanol/acetone coagulation after water immersion at 5, 85 and 100 °C. The results from immersing lignin/PVA fibers in room temperature water are in Table 5. Increasing lignin content influenced moisture swelling within gel spun lignin/PVA fibers. Although lignin inhibits the thermal dissolution of PVA, it permits PVA swelling in confined regions. This behavior suggests the structure of composite fibers have water insoluble lignin that is dispersed throughout PVA and in intimate contact with the matrix polymer.

Table 5. Swelling of Lignin/PVA Fibers After 24 Hours of Water Immersion

S(%)

Neat PVA

5% Lignin

30% Lignin

14

19

82

Note: Neat PVA fiber was fabricated from 100/0 methanol/acetone coagulation. Fibers having 5 and 30% lignin were from 15/85 methanol/acetone coagulation.

CONCLUSION We have successfully gel spun biobased lignin/PVA fibers, from coagulation baths of 15/85 methanol/acetone that show promising results of high mechanical performance and water resistance. Even at 50% lignin, the mechanical strength and Young’s modulus of our biobased fibers were competitive with commercial PVA staple fiber. In summary, 5% lignin fiber possessed the highest value of mechanical strength. Structural enhancements at 5% lignin was evidenced by its high index of crystallinity, intermolecular bonding between lignin and PVA, 26


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and mild alignment of lignin upon fiber drawing. At 50% lignin, fibers exhibited improved toughness and resistance to water dissolution in comparison to neat fiber. Swelling results suggest water was capable of penetrating hydrophilic regions of PVA, but lignin kept the fibers’ structure loosely intact. Although lignin is an amorphous, three-dimensional biopolymer, this work has shown evidence of enhanced strength, which is generally observed with highly ordered fillers, such as one-dimensional CNTs32 and two-dimensional graphene oxide33. For instance: 0.3 wt% single walled carbon nanotubes (SWCNT)/PVA34 and 0.1 wt% reduce graphene oxide reinforced PVA35 fibers. Those carbon-based fibers had similar values of mechanical performance: 2.2 GPa in tensile strength and 36 GPa in Young’s modulus. At 5% lignin, a similar result in Young’s modulus was obtained although its tensile strength was half as much. For carbon-filler reinforced PVA, the draw ratio of gel spun fibers decreased from 13 X for neat PVA to 11 X for composites of 0.5 wt% reduced graphene oxide.36 Performance enhancements among gel-spun lignin/PVA fibers were thus attributed to high draw ratios (due to lignin plasticization) and molecular adhesion between PVA and lignin’s rigid, hydrophobic structure. At up to 50% lignin, the mechanical performance of gel spun lignin/PVA composites have potential use in industrial and high-performance applications. Thus, gel spinning is an attractive alternative to melt spinning for low cost lignin fiber production.

ASSOCIATED CONTENT Supporting Information Gel melting measurement of polymer gels AUTHOR INFORMATION 27



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Corresponding Author *Ericka Ford. E-mail: [email protected]. Tel:+1-601-329-1311 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The Nonwovens Institute (Project No. 14-176 NC) supported this research. This work was performed in part at the Analytical Instrumentation Facilities (AIF) at NC State University, which is supported by the state of North Carolina and the National Science Foundation (Award Number ECCS-1542015). AIF is a member of RTNN, a site in the National Nanotechnology Coordinated Infrastructure (NNCI). Pure Lignin Environmental Technology Ltd donated the lignin used in this study. LaTasha Nicholson is thanked for her experimental assistance with gel melting studies and Judy Elson for her expertise with confocal microscopy. REFERENCES: (1)

Sarkanen, K. V.; Ludwig, C. H. Lignins: Occurrence, Formation, Structure and Reactions. Wiley-Interscience: New York, 1971.

(2)

Sjöström, E. Wood Chemistry: Fundamentals and Applications. Academic Press: 1993.

(3)

Alexy, P.; Košı́ková, B.; Podstránska, G. The Effect of Blending Lignin with Polyethylene and Polypropylene on Physical Properties. Polymer 2000, 41, 4901-4908.

(4)

Luo, J. Lignin-based Carbon Fiber. The University of Maine, 2010.

(5)

Kadla, J.; Kubo, S.; Venditti, R.; Gilbert, R.; Compere, A.; Griffith, W. Lignin-based Carbon Fibers for Composite Fiber Applications. Carbon 2002, 40, 2913-2920. 28


Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6)

Thunga, M.; Chen, K.; Grewell, D.; Kessler, M. R. Bio-renewable Precursor Fibers from Lignin/Polylactide Blends for Conversion to Carbon Fibers. Carbon 2014, 68, 159-166.

(7)

Wang, S.; Li, Y.; Xiang, H.; Zhou, Z.; Chang, T.; Zhu, M. Low Cost Carbon Fibers from Bio-renewable Lignin/Poly(lactic acid) (PLA) Blends. Compos Sci Technol 2015, 119, 20-25.

(8)

Kubo, S.; Kadla, J. Lignin-based Carbon Fibers: Effect of Synthetic Polymer Blending on Fiber Properties. J Polym Environ 2005, 13, 97-105.

(9)

Oroumei, A.; Fox, B.; Naebe, M. Thermal and Rheological Characteristics of Biobased Carbon Fiber Precursor Derived from Low Molecular Weight Organosolv Lignin. ACS Sustainable Chem Eng 2015, 3, 758-769.

(10)

Kubo, S.; Kadla, J. F. The Formation of Strong Intermolecular Interactions in Immiscible Blends of Poly(vinyl alcohol)(PVA) and Lignin. Biomacromolecules 2003, 4, 561-567.

(11)

Xu, G.; Ren, S.; Wang, D.; Sun, L.; Fang, G. Fabrication and Properties of Alkaline Lignin/Poly(vinyl alcohol) Blend Membranes. BioResources 2013, 8, 2510-2520.

(12)

Su, L.; Xing, Z.; Wang, D.; Xu, G.; Ren, S.; Fang, G. Mechanical Properties Research and Structural Characterization of Alkali Lignin/Poly(Vinyl Alcohol) Reaction Films. BioResources 2013, 8, 3532-3543.

(13)

Lewin, M. Handbook of Fiber Chemistry. CRC Press: Boca Raton, 2007.

(14)

Kunugi, T.; Kawasumi, T.; Ito, T. Preparation of Ultra‐High Modulus Polyvinyl Alcohol Fibers by the Zone‐Drawing Method. J Appl Polym Sci 1990, 40, 2101-2112.

(15)

Cha, W. I.; Hyon, S. H.; Ikada, Y. Gel spinning of Poly (vinyl alcohol) from Dimethyl Sulfoxide/Water Mixture. J Polym Sci Part B: Polym Phys 1994, 32, 297-304.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16)

Yamaura, K.; Tanigami, T.; Hayashi, N.; Kosuda, K. I.; Okuda, S.; Takemura, Y.; Itok, M.; Matsuzawa, S. Preparation of High Modulus Poly(Vinyl Alcohol) by Drawing. J of Appl Polym Sci 1990, 40, 905-916.

(17)

Ryan, C. F.; Fleischer, P. C. The Gel Melting Point as a Measure of the Tacticity of Poly(Methyl Methacrylate). J Phys Chem 1965, 69, 3384-3400.

(18)

Luo, J.; Li, Q.; Zhao, T.; Gao, S.; Sun, S. Bonding and Toughness Properties of PVA Fibre Reinforced Aqueous Epoxy Resin Cement Repair Mortar. Constr Build Mater 2013, 49, 766-771.

(19)

Hu, T. Q. Chemical Modification, Properties, and Usage of Lignin. Springer US: 2012.

(20)

Song, P. a.; Xu, Z.; Guo, Q. Bioinspired Strategy to Reinforce PVA with Improved Toughness and Thermal Properties via Hydrogen-Bond Self-Assembly. ACS Macro Lett 2013, 2, 1100-1104.

(21)

Tadokoro, H.; Kǒzai, K.; Seki, S.; Nitta, I. On the Crystalline Band in the Infrared Absorption Spectrum of Polyvinyl Alcohol. J Polym Sci 1957, 26, 379-382.

(22)

Mallapragada, S. K.; Peppas, N. A. Dissolution Mechanism of Semicrystalline Poly(Vinyl Alcohol) in Water. J Polym Sci Part B: Polym Phys 1996, 34, 1339-1346.

(23)

Tretinnikov, O.; Zagorskaya, S. Determination of the Degree of Crystallinity of Poly(Vinyl Alcohol) by FTIR Spectroscopy. J Appl Spectrosc 2012, 79, 521-526.

(24)

Peppas, N. A. Infrared Spectroscopy of Semicrystalline Poly(Vinyl Alcohol) Networks. Die Makromolekulare Chemie (Macromol Chem Phys) 1977, 178, 595-601.

(25)

Takahashi, N.; Kanaya, T.; Nishida, K.; Kaji, K. Gelation-Induced Phase Separation of Poly(Vinyl Alcohol) in Mixed Solvents of Dimethyl Sulfoxide and Water. Macromolecules 2007, 40, 8750-8755. 30


Page 30 of 34

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Baumberger, S.; Lapierre, C.; Monties, B.; Lourdin, D.; Colonna, P. Preparation and Properties of Thermally Moulded and Cast Lignosulfonates-Starch Blends. Ind Crops and Prod 1997, 6, 253-258.

(27)

Sadeghifar, H.; Wells, T.; Le, R. K.; Sadeghifar, F.; Yuan, J. S.; Jonas Ragauskas, A. Fractionation of Organosolv Lignin Using Acetone:Water and Properties of the Obtained Fractions. ACS Sustainable Chem Eng 2017, 5, 580-587.

(28)

Shin, M. K.; Lee, B.; Kim, S. H.; Lee, J. A.; Spinks, G. M.; Gambhir, S.; Wallace, G. G.; Kozlov, M. E.; Baughman, R. H.; Kim, S. J. Synergistic Toughening of Composite Fibres by Self-Alignment of Reduced Graphene Oxide and Carbon Nanotubes. Nat Commun 2012, 3, 650.

(29)

Liem, H. M.; Etchegoin, P.; Whitehead, K. S.; Bradley, D. D. Raman Anisotropy Measurements: An Effective Probe of Molecular Orientation in Conjugated Polymer Thin Films. Adv Funct Mater 2003, 13, 66-72.

(30)

White, J. L.; Spruiell, J. E. The Specification of Orientation and its Development in Polymer Processing. Polym Eng Sci 1983, 23, 247-256.

(31)

Gierlinger, N.; Keplinger, T.; Harrington, M. Imaging of Plant Cell Walls by Confocal Raman Microscopy. Nat Protoc 2012, 7, 1694-1708.

(32)

Minus, M. L. The Study of Crystallization and Interfacial Morphology in Polymer/Carbon Nanotube Composites. GEORGIA INSTITUTE OF TECHNOLOGY, 2008.

(33)

Morimune,

S.;

Nishino,

T.;

Goto,

T.

Poly(Vinyl

Alcohol)/Graphene

Nanocomposites Prepared by a Simple Eco-Process. Polym J 2012, 44, 1056-1063.

31


Oxide


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(34)

Page 32 of 34

Xu, X.; Uddin, A. J.; Aoki, K.; Gotoh, Y.; Saito, T.; Yumura, M. Fabrication of High Strength PVA/SWCNT Composite Fibers by Gel Spinning. Carbon 2010, 48, 1977-1984.

(35)

Li, J.; Shao, L.; Zhou, X.; Wang, Y. Fabrication of High Strength PVA/rGO Composite Fibers by Gel Spinning. RSC Adv 2014, 4, 43612-43618.

(36)

Abdou, J. P.; Reynolds, K. J.; Pfau, M. R.; van Staden, J.; Braggin, G. A.; Tajaddod, N.; Minus, M.; Reguero, V.; Vilatela, J. J.; Zhang, S. Interfacial Crystallization of Isotactic Polypropylene Surrounding Macroscopic Carbon Nanotube and Graphene Fibers. Polymer 2016, 91, 136-145.

For Table of Contents Use Only

Effect of Coagulation Bath on the Structure and Mechanical Properties of Gel Spun Lignin/Polyvinyl Alcohol Fibers Chunhong Lu, Charles Blackwell, Qingyuan Ren, Ericka Ford*

32


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Synopsis: Biobased composite fibers of lignin/polyvinyl alcohol were produced by the gel spinning technique. This graphic illustrates the gel spinning process, fiber mechanical performance and microstructure of drawn composite fibers for high mechanical properties.

33



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Synopsis: Biobased composite fibers of lignin/polyvinyl alcohol were produced by the gel spinning technique. This graphic illustrates the gel spinning process, fiber mechanical performance and microstructure of drawn composite fibers for high mechanical properties. 274x150mm (96 x 96 DPI)


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Effect of the Coagulation Bath on the Structure and Mechanical

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