The Influences of Coagulation Conditions on the Structure and

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The Influences of Coagulation Conditions on the Structure and Properties of Regenerated Cellulose Filaments via Wet-spinning in LiOH/Urea Solvent Cuibo Qiu, Kunkun Zhu, Xin Zhou, Longbo Luo, Jie Zeng, Rui Huang, Ang Lu, Xiangyang Liu, Feng Chen, Lina Zhang, and Qiang Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04429 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Sustainable Chemistry & Engineering

The Influences of Coagulation Conditions on the Structure and Properties of Regenerated Cellulose Filaments via Wet-spinning in LiOH/Urea Solvent

Cuibo Qiu1‡, Kunkun Zhu2‡, Xin Zhou1, Longbo Luo1, Jie Zeng1, Rui Huang1, Ang Lu2 Xiangyang Liu1, Feng Chen1, Lina Zhang2* , Qiang Fu1* 1

College of polymer science and engineering, Sichuan University 2

College of Chemistry and Molecular Sciences, Wuhan University ‡ These authors contributed equally to this paper.

Corresponding author E-mail: [email protected](Q. Fu) College of polymer science and engineering, Sichuan University, No 24 South Section-1,Yihuan Road, Chengdu 610065, P. R. China E-mail: [email protected] (L. Zhang) College of Chemistry and Molecular Sciences, Wuhan University, Bayi Road, Wuhan 430072, P. R. China

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ABSTRACT: Dissolving cellulose rapidly at low temperature by using solvents (NaOH/urea, NaOH/thiourea, LiOH/urea or NaOH/thiourea/urea) opens a new chapter for the preparation of high-performance cellulose filaments, for the unique structure containing nanofibers. In our previous work, it was found that the coagulation rate is a key to construct the nanofiber structure and thus to achieve high performance of regenerated cellulose filament (RCF) via wet-spinning. In this work, phytic acid salt was used to further adjust the coagulation rate for a better control of the structure of RCF. It was found that adding small amount of salt would promote a rapid diffusion of phytic acid from the skin to the core of cellulose filament, resulting in a relatively compacted structure accompanying an increasing skin yet decreasing core. However, excessive salt would result in a decrease of neutralization capability of phytic acid, leading to a less compacted structure in the filament. The optimum salt concentration was found to be 5% at which phytic acid could permeate to the center of the filament and maintain good neutralization capability. The best tenacity of the filaments achieved is about 3.43 cN/dtex, which preponderates over that of commercial viscose rayon. A comparison study between commercial viscose rayon and the obtained filament was carried out to further demonstrate the importance of nanofibers for the enhancement of cellulose filaments. In this work, we systematically investigated the influence of phytic acid salt concentration on the structure and properties of novel regenerated cellulose filaments, which provides a positive guidance on the environmentally friendly cellulose industrialization.

KEYWORDS: Cellulose filaments, Nanofibers, Salt concentration, Compacted structure, Mechanical properties

■ INTRODUCTION As the most abundant biomass resources on earth, cellulose is widely used in the production of green bio-based products due to its unique and fascinating properties.1,2 Viscose, one of the oldest manufactured cellulose fibers, has been occupying an important position in our daily life on account of its good hygroscopicity, comfortable wearing, strong permeability and easy dyeing.3,4 However, the


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large emission of CS2 and H2S pose serious environmental and human health hazards in viscose processing.5 In order to solve the problem of fussy process and severe pollution in this technology, a series of new solvents have been developed, such as N-methylmorpholine-N-oxide (NMMO),6 ,7 lithium chloride/N,N-dimethylacetamide (LiCl/DMAc)8 and ionic liquids (ILs).9-11 Moreover, with the improvement of spinning process, these novel cellulose fibers show outstanding mechanical properties.12-14 However, the fabrication processes mentioned still suffer from some disadvantages like the high-cost, high temperature as volatility, complexity of the dissolution or the difficulty in solvent recovery.15

In

2000,

novel

and

green

solvents

(NaOH/urea,16

NaOH/thiourea,17

or

NaOH/thiourea/urea18 ) have been discovered to dissolve cellulose rapidly at low temperature for the first time. This provides an entirely new path for its non-toxic and inexpensive process. Nevertheless, spinning dope stability19 and relatively weak strength (tensile strength and elongation reaching 1.1–2.2 cN/dtex, 2-10% respectively20-22 ) of the obtained filaments largely limits its practical application in industry. Necessarily, the improvement in intensity of cellulosic filaments has become a priority. In previous work, 23,24 it has been found that the single chains as an inclusion complex associated with NaOH and urea (UACC) and their aggregates co-exist in the cellulose/alkali/urea aqueous solution. The experimental results have indicated that two peaks corresponding to the single cellulose chains and their aggregates in the hydrodynamic radius distributions [f(Rh)]25 and the images of atomic force microscopy (AFM) has also supported the existence of the cellulose aggregates as nanofibers in the dilute solution. Thus, we developed an approach for the direct construction of nanofibrous cellulose multifilament fibers from alkali/urea aqueous solution by regenerating in phytic acid (PA) in coagulation bath. The results demonstrated that nanofibers array in an orderly fashion became the key factor toimprove the properties of regenerated cellulose filaments. It has been reported that mild exchange rate usually form a homogeneous and dense structure,while fast and violent regeneration process result in an inhomogeneous and loose structure.26 Thus the balance between the diffusion rate and neutralization rate is beneficial for the formation of a uniform and tight structure with parallel arrangement of nanofibers in the filament. On the basis of the above, the coagulation and regeneration conditions are important on the improvement of the structure and properties of the materials. In this work, the effect of the concentration



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of phytic acid salt on the diffusion and neutralization of phytic acid in coagulation bath was investigated. In order to eliminate the influence of system instability and make the salt content become the only variable, we prepared cellulose filaments via wet spinning from a LiOH/urea solvent system on an experimental machine in coagulation bath containing 15 wt% phytic acid at 5 °C then second regeneration both at 60 °C in favor of drawing. The structure and properties of the obtained filaments were characterized using polarizing microscope (POM), scanning electron microscope (SEM), CP/MAS solid nuclear magnetic resonance (13C NMR), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), two-dimensional wide-angle X-ray diffraction (2D-WAXD) and small angle X-ray scattering (SAXS). Our goal was to find out the optimum salt concentration both for the diffusion and neutralization of phytic acid and further demonstrate the importance of the coagulation and regeneration conditions in the enhancement of cellulose filament by comparison of the obtained filaments with commercial viscose rayon.

■ EXPERIMENTAL SECTION Materials.The cellulose (cotton linter pulp) with an α-cellulose content of more than 95% and viscose rayon were provided by the Jilin Chemical Fiber Group Ltd. (China). The viscosity-average molecular weights (Mη) of the cellulose was 7.5×10 4 g/mol (degree of polymerization, DP 460). All chemical reagents were of analytical grade and were purchased from commercial producers in China. Preparation of spinning dope. According to previous work,16 an aqueous solution was prepared for the cellulose solvent by directly mixing LiOH, Urea and distilled water (4.6:15:80.4 in weight). Afterwards, precooled to -12 °C, then the fluffy cellulose in a desired amount was immediately dispersed into the solvent system (1.2 L) under vigorous stirring for 5 min to obtain a transparent cellulose dope. The spinning dope (SP) was centrifuged via 8500r/min for 30 min at 5 °C to remove the impurities and air bubbles. Wet spinning. The filaments were made by wet spinning on a pilot scale spinning machine manufactured by Chengdu Science and technology Co., Ltd. The spinning diagram is shown in Figure 1. The spinneret cylinder (50 orifices; diameter, 160 µm) was immersed directly into the first coagulation bath with 15 wt% phytic acid /0-10 wt% phytic acid lithium (Li-PA) at 5 °C, in a flow rate of 12m/min.


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And the resulting gelation filaments were partly solidified in the first coagulation bath, then taken up on the Nelson-type roller I and drawn to the Nelson-type roller II in order to give a jet stretch (0.63 times). Subsequently, the ascent gel were further drawn from the Nelson-type roller III into the second coagulation bath with deionized water at 60 °C. The deionized water had plasticizing effect and could remove some salts as well as acids from the filaments, which was good for drawing. The drawing ratio is adjusted to 2.0 to ensure orientation and continuity. Following this process, the residual salts and acids of resultant multifilaments were washed out by flowing distilled water at 30 °C until the conductivity of the lotions was less than 100. Finally, the filaments were dried using two heating rollers (surface temperature, 60 °C) and wound on a spool to obtain multifilaments. In all the cases, the filament was straight to avoid orientation loss. RCFs were obtained and were coded according to the concentration of Li-PA, as RCF-0, RCF-1, RCF-3, RCF-5, RCF-7 and RCF-10 (0, 1, 3…represents the concentration of Li-PA), respectively. Commercial viscose rayon provided by Jilin Chemical Fiber Co. Ltd. (China) was used for comparison.

Figure 1. Schematic Diagram of the Pilot Scale Spinning Machine: (a) Stainless Steel Reservoir, (b) Metering Pump, (c) Spinneret, (d) Coagulation Bath, (e) Plasticizing Bath, (f) Water Washing, (g) Drying Roller, and (h) Take-Up Roller.

Characterization. AFM observations were carried out on a commercial AFM (Cypher ES, Asylum Research) in a MAC mode at 35 oC. Silicon probes with a spring constant of 2 N/m and resonance frequency of 70 kHz were employed. The average tip radius was 28 nm. TEM observations were performed on a Tecnai G2 F20 transmission electron microscopy. Drops of cellulose solution prepared (0.06mg/mL) were placed onto copper grid with a glass capillary. The redundant liquid was absorbed by a piece of filter paper and the remaining film of stain was allowed to dry naturally. The samples were observed at an acceleration voltage 200 kV. The cross section of the RC filaments was observed on an optical polarizing microscope equipped



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with a Linkam CSS-450. A bundle of fibers was embedded in the mixed solution of gelatin with glycerol and left to harden for approximately 10 min at -20 °C. Cross sections of the filament axis of about 2 mm thickness were prepared using a Struers Accutom. The sample for cross-section features were prepared in liquid N2 and sputtered with gold in vacuum prior, as well as surfaces were observed by scanning electron microscopy (SEM, NOVA NANOSEM 450) at an accelerating voltage of 5 kV. Small angle X-ray scattering (SAXS) experiments were recorded on the GeniX 3D beam delivery system using a MP-Xeuss 2.0, SAXS (BRUKER AXS, Inc.) with Cu Kα radiation (λ= 0.154 nm). The generator was operated at 40 kV and 650 µA. 2D SAXS patterns were obtained using a HI-STAR detector and the specimen to detector distances were 1074 mm. The filament bundles were dried before the measurement and straightened. A typical image acquisition time was 45s. X-ray diffraction (XRD) patterns of cellulose filaments were obtained in a Ultima IV diffractometer (Rigaku, Japan) with the Cu Kα radiation (λ = 0.15406 nm, kV and 40 mA ) at a scanning rate of 2°/min in the region of 2θ from 5° to 50°. The samples were ground into powders to avoid the influence of orientation and then vacuum-dried for 12h before testing. The degree of crystallinity (χc) was calculated by the following equations :27 χ =

× 100%

(1)

where Sc and Sa are the areas of the crystal and amorphous regions from the one-dimensional WAXS integral curve that the background and overlapping peaks were separated by peak separation and analysis software PeakFit, respectively. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) patterns were obtained by a Bruker D8 Discover X-ray diffractometer equipped with a Vantec 500 detector. The sample to detector distance was 8.3 cm. Straight specimens of filament fasciculus were placed perpendicular to the beam. The degree of orientation (Π) and Herman’s orientation parameter (f), f = 1 corresponds to perfect orientation parallel to the stretching direction, whereas f = 0 revealed completely random orientation in the fibrils, were quantified using the following equations:28 Π=

180 - FWHM 180


（2）

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f =

= 2

∫

3 − 1 2

（3）

π

2 0

I ( ϕ ) si n ϕ cos2 ϕd ϕ

∫

π

2 0

（4）

I ( ϕ ) si n ϕd ϕ

Where FWHM, φ, and I resprent the full width at half-maximum, the azimuthal angle defined as zero at the meridian and the corresponding integral intensity, respectively. The thickness of the fibers (linear density) was calculated in terms of tex, defined as the weight in grams per 1000 meters of the filament. Tensile mechanical properties were measured at 23 °C and humidity was 65% using an LLY-06 electronic monofilament strength tester (Textile Instruments Laizhou Co., LTD., China) at a stretching velocity of 20 mm/min. For each fiber prototype, the σb and εb values were obtained from at least 20 independent specimens with a gauge length of 20 mm under a certain pre-tension. In order to determine the nitrogen and sulfur content of the RC filaments and viscose rayon, X-ray photoelectron spectroscopy (XPS) experiments using an Axis Ultra DLD spectrometer (KratosCo., UK) were performed on the dried filament powder by focused monochromatized Al Kα radiation (15 kV) at room temperature. Solid state

13

C NMR spectra were performed on an Infinity Plus 400 spectrometer (Varian,U.S.A.,

magnetic field = 9.4T,

13

C frequency = 100.12 MHz) with a CP/MAS unit at room temperature. The

spinning rate and contact time were 5.0 kHz and 5.0 ms, respectively. The pulse width, spectral width, and acquisition time were 2.10 µs, 50.0 kHz, and 20.48 ms, respectively; 2000 scans were accumulated for each sample. The CH signal of 6-methylbenzene was used as an external reference for the determination of chemical shifts. FTIR spectra were determined on Nicolet FTIR spectrometer (Nicolet FTIR 6700, Thermo Electron Co., USA) at room temperature. The test specimens were ground blending KBr after vacuum-dried for 24 h. The data were gathered from 4000 to 400 cm−1 with a resolution of 4 cm−1 under reflection mode. Thermogravimetric analysis (TGA) was achieved on a Q500 analyzer (TA Instruments) under a nitrogen atmosphere. The samples were heated from 25 to 100 °C and maintained at 100 °C to remove the water at a heating rate of 10 °C/min, and then continued to be heated from 100 to 600 °C. For each



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thermogravimetric analysis, around 4 mg of specimen was used.

■ RESULTS AND DISCUSSION Morphology of dissolved cellulose. To further validate concerning the existence of the cellulose nanofibers, atomic force microscope (AFM) and transmission electron microscope (TEM) were used to study the structure of dissolved cellulose, the results are shown in Figure 2 (a, b). AFM result (Figure 2a) presents cellulose chain aggregates with an mean diameter of about 20-30 nm and average apparent length of 200-300 nm, and TEM observation (Figure 2b) also confirms a worm-like pattern having an average diameter of about 20 nm and average length of about 300 nm, both of them consistent with the literature.29-31 Early studies showed that the forming of an inclusion complex (IC) hosted by urea surrounded the LiOH could encage the cellulose macromolecule, leading to the dissolution of cellulose at low temperatures.16, 25 Figure 2c shows an IC piping model: the intra- and inter-molecular hydrogen bonding of cellulose has been destroyed completely and LiOH/urea prevented the approach toward each other of the cellulose chains, resulting in an uniform dispersion of cellulose.

Figure 2. (a) AFM images of the extremely dilute cellulose solution in alkaline system (2×10-7 g/ml), (b) TEM images of cellulose solution at 2mg/ml in 4.6wt% LiOH/15 wt% urea aqueous solution pre-cooled to −12 °C and (c) scheme of the worm-like cellulose complexes.

Morphology of RC Filaments. Here, cellulose solution containing nanofibers was easily spun as RC filaments via wet-spinning. The cross sections of obtained filaments was examined by POM micrograph, and the result is shown in Figure 3. The shape of cross sections is changed as change of salt concentration in coagulation bath. As the salt concentration increases, the section of filaments is varied from lobulate shape (Figure 3a-b) to elliptic (Figure 3c), and finally nearly circular (Figure 3d-f).


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Interestingly, when the concentration of salt reaches to 5 wt%, the shape of the section is no longer changed, keeping suborbicular. This could be due to the reduced osmotic pressure difference between inside and outside of cellulose as increase of salt concentration, leading to a relatively uniform coagulation in the whole area of RC filament. This is similar to viscose process that the concentration of Na2SO4 and certain metals in the coagulation bath played an important role in the micro-structure and mechanical properties of viscose.32,33

Figure 3. POM images of the cross section of the RC multifilament under different coagulation bath: (a) RCF-0, (b) RCF-1, (c) RCF-3, (d) RCF-5, (e) RCF-7, and (f) RCF-10.

For the purpose of more detailed information, scanning electron microscope (SEM) was used to observe the section structure of representative filaments, as shown in Figure 4. For filament coagulated by using 15 wt% phytic acid without salt (RCF-0), an irregular lobulate shape with a dense skin but a loose core is observed (Figure 4a-c), even a porous structure is seen in the core (Figure 4c). For filament coagulated with 15 wt% phytic acid containing 5 wt% salt (RCF-5), a homogeneous and compact structure is observed both in the skin and in the core (Figure 4d-f). However, with further increase of salt concentration, for RCF-10, an uniform but loose structure is observed (Figure 4g-i). A lot of voids can be seen in both skin and core area. Obviously, salt concentration plays a vital role in microstructure of RCFs. It can be concluded that phytic acid lithium could reduce the pressure difference of the cellulose



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gel between inside and outside, and promote the penetration of acid into the core part, leading to an uniform solidification of cellulose gel, it could be explained from the fine structure of viscose.3,34 However, excessive salt will decrease the concentration of lithium phytic acid, leading to a slow and incomplete neutralization of cellulose gel thus a loose structure. In a word, controlling the concentration of salt and keeping a balance between diffusion and neutralization rate is the key to obtain high-performance RCFs with dense structure, as RCF-5, which could be attributed to the ordered arrangement of nanofibers in a mild coagulating environment without severe fluctuation.

Figure 4. SEM images of the cross section of the representative filaments: (a,d,g) RCF-0, (b,e,h) RCF-5, and (c,f,i) RCF-10.

Structure of RC Filaments. The pores structure of the obtained RCFs can be quantified by using SAXS measurement, as shown in Figure 5. SAXS patterns exhibit a sharp and elongated equatorial streak superimposed with a relatively weak and short meridional intensity distribution (Figure 5a-c). For wet spinning fibers, previous studies have made it clear that the elongated equatorial streaks in the SAXS maybe give the credit to two possibilities: (1) the formation of a fibrillar superstructure or (2) the existence of microvoids.35-37 Zhang and co-workers implied the equatorial streak in SAXS is principally on account of the existence of microvoids in the filaments during the dual diffusion process in wet-spinning.38 Thus the elongated shape of the equatorial streak in the obtained RCFs suggests


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needle-shaped microvoids which are aligned parallel to the filaments direction. Figure 5d shows the Guinier plots along the equatorial direction. The radius of the microvoids (R) could be obtained by Guinier functions as follows39

Iq = I

(5)

where q ( q = 4π sinθ/λ ) is the scattering vector, 2θ and λ represent the scattering angle and wavelength, respectively. The average microvoids’ length ( L ) is closely related to misorientation BΦ that is parallel to the fiber axis from the following equation39 !

s = # $% "

(6)

where Bobs is the full width at half maximum of the azimuthal profile form, which could be obtained from the Figure4e. It can be seen that the full width gradually increases as the enlargement of q value. As is listed in the Table 1, the microvoids’ width ( R ) and microvoids’ length ( L ) of RCFs are in the range of 31-33 and 115-167 nm, respectively. Particularly, R and L gradually decrease when the salt concentration increases, nevertheless, a reversal appears when the salt content is 5 wt% (RCF-5), where the minimum microvoids’ width (31 nm) and the shortest microvoids’ length (115 nm) are obtained, which further demonstrates its tight internal structure, in well agreement with POM as well as SEM images.



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Figure 5. SAXS patterns of some representive: (a) RCF-0, (b) RCF-5 and (c) RCF-10. (d) Guinier plots of the scattered intensities along the layer line in the horizontal direction. (e)Schematic presentation of azimuthal scans of the equatorial streak of RCF-5. Table 1. The Structural Parameters of Novel RC Filaments

Sample

R (nm)

L (nm)

BΦ (º)

Π（）（%）

f

RCF-0

33

145

20.63

79

0.76

RCF-1

33

137

20.67

79

0.78

RCF-3

32

126

15. 57

80

0.80

RCF-5

31

114

15. 45

84

0.81

RCF-7

32

122

19.97

79

0.79

RCF-10

33

167

21.20

78

0.75

R, microvoids width; L, microvoids length; BΦ, misorientation of microvoids; Π, degree of orientation; f, Hermans’ orientation parameter for RCFs.

The properties of filaments are not only related to microstructure, but also to the orientation and crystallinity. Thus 2D-WAXD was carried out to determine the orientation of obtained filaments, and the result is shown in Figure 6. It is obvious that all the RCFs present the cellulose II structure disregarding the salt concentration used. The appearance of broad arcs is observed for all the RCFs, indicating a oriented structure. The strongest arcs are observed for RCF-5 (Figure 6c) , indicating a highest orientation in this case. Then they become weak again for RCF-10 (Figure 6d), suggesting a decreased orientation with further increase of salt concentration. Moreover, the degree of orientation and Hermans’ orientation function (f) in crystalline region calculated from the (020) are also listed in the last rows in Table 1. The degree of orentation (84%) and Hermans’ orientation parameter (0.81) for RCF-5 indicated maximum orientation and Hermans parameter, compared with other RCFs in the ranges of 79-80% and 0.75-0.80, respectively. The results of 2D-WAXD is corresponding to images of SEM. Additionally, we will use a micron-size X-ray to observe thin sections from skin and core of filaments, and then further indicates the internal structure of the filaments in future work.


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Figure 6. 2D WAXD patterns of some representive: (a) RCF-0, (b) RCF-3, (c) RCF-5, and (d) RCF-10.

XRD was carried out to determine the crystallinity of obtained filaments, as illustrated in Figure 6. The patterns of RC filaments

(Figure 7a) show broad peaks at 2θ = 12.2°, 20.2°, and 21.9°, which are

assigned to the (11̅0), (110), and (020) planes of the cellulose II crystalline form, respectively,40 echoing the 2D-WAXD data. The crystallinity (χc ) values calculated from the XRD patterns for the RC filaments are presented in Figure 7b. On the whole, the crystallinity of the filaments fluctuated between 56% and 65%. As the salt concentration changes, a slightly increased χc values are observed in RCF-0 (60%), RCF-1(62%), RCF-3(63%), RCF-5(65%), suggesting a relatively denser structure41 by adding salt. However, as the salt concentration increases from 5 wt% to 10 wt%, the χc value decreases from 65% to 56%, indicating a descending order of the crystals.

Figure 7. (a) XRD patterns and (b) the corresponding crystallinity of the RCFs (RCF-0, RCF-1, RCF-3, RCF-5, RCF-7 and RCF-10).

Through the above research, the structure of RCFs, including micro-and macro-structure, could be proposed as shown in Figure 8. For filament coagulated without salt (RCF-0), too much free hydrogen ions provide a rapid solidification on the surface of the filament, which prevents a further diffusion of hydrogen ions into the core, leading to a thin and ordered skin yet a thick but loose core. Besides, the large density difference between the inside and outside makes the filament present irregular section. By



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adding salt, a rapid diffusion of phytic acid takes place from the skin to the core of cellulose filament, resulting in a thicker skin but thinner core layer. 42,43 At the salt concentration of 5 wt%, an homogenous filament is obtained that cellulose nanofibers are in order and the whole structure is compacted. Nevertheless, excessive phytic acid litium slows down the rate of reaction, making the neutralization be not completed in the first coagulation bath, resulting in that nanofibers can not be orientated that high temperature destroyed the cellulose-alkali-urea in the second coagulation bath, which decreases orientation and crystallinity,44 leading to a uniform but loose structure.

Figure 8. Schematic diagram of the RCFs’ forming process in different coagulation baths：(a) cellulose solution, (b) cellulose hydrogels, (c) the internal structure of RCFs.

Mechanical Properties of RCFs.The typical stress-strain curves of RCFs are shown in Figure 9a. With adding salt, an increase of strength and modulus is observed, companied with a decrease of breaking elongation. Figure 9b shows the specific values of tenacity and modulus of the obtained RCFs as function of salt concentration. The tenacity and the modulus increase from 2.48 cN/dtex and 72


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cN/dtex for RCF-0, to 3.43 cN/dtex and 144 cN/dtex for RCF-5, respectively, which preponderates over that of commercial viscose rayon. A further increase of salt concentration to 10 wt%, a decreased tenacity and the modulus is observed to 2.21 cN/dtex and 68 cN/dtex for RCF-10, respectively. Clearly, RCF-5 has the best mechanical properties (tensile strength of 3.43 cN/dtex, break elongation of 10.2%, the modulus of 144 cN/dtex), which could be explained from the dense fine structure, higher degree of orientation and crystallinity compared to other RCFs. Besides, the performance of several fibers regenerated from environmental friendly solvents are summarized in Table 2. The mechanical properties of filaments solidified in 15 wt% phytic acid (RCF-0) are improved effectively compared to U-3-i solidified in 10 wt% H2SO4/8 wt% Na2SO4.22 Particularly, RCF-5 shows excellent mechanical properties which is very similar to that of Ionicell fibers prepared from ionic liquid45 and Lyocell fibers prepared from NMMO.46

Figure 9. (a) Stress-strain curves; (b) Tenacity and modulus.

Table 2. Physical properties of several novel regenerated cellulose fibers. Sample

Solvent

σb (cN/dtex)

εb (%)

Ref.

U-3-i

NaOH/urea

1.9

2

22

RCF

NaOH/ZnO

1.69~2.36

15~21

41

RCF-0

LiOH/urea

2.48

24

This work

RCF-5

LiOH/urea

3.43

10.2

This work

Ionicell

Ionic liquids

2.31~3.84

8.8~11.3

45

Lyocell

NMMO

2.47~3.11

6.8~7.9

46



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The Contrast between RCF and viscose rayon. A comparison study between commercial viscose rayon and the obtained filament was carried out to demonstrate the importance of nanofibers for the enhancement of cellulose filaments. We first compared the photograph of the RCF and viscose rayon, as presented in Figure 10. Both RCF-5 and viscose rayon show superb flexibility and a silk-like luster in their appearance, but the diameter of RCF-5 is 4 times bigger than that of viscose rayon. Optical microscopy analysis make it clear that RCF-5 is with relative homogeneous quasicircular cross section while viscose rayon has a lobed shape cross section.32

Figure 10. Photograph of the novel RCF-5 spun from cellulose solution in a LiOH/Urea aqueous system on an extended laboratory scale and viscose rayon (provided by Jilin Chemical Fiber Co. Ltd.).

To further study the structural difference between RCF-5 and viscose rayon, scanning electron microscope (SEM) was used to observe the detailed structure of RCF-5 and viscose rayon, as shown in Figure 11. Obviously, the nanofibers with an average diameter of about 30 nm are observed in the section of RCF-5 (Figure 11a), similar to the cellulose nanofibril structure in wood.35 Whereas the section of viscose rayon only exhibits network (Figure 11b). Additionally, the samples were sliced along the fiber axis direction to expose the inner structure. Again cellulose nanofibers with average diameter of about 30 nm are observed for RCF-5, as shown in Figure 11d, which is very much different from that of


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viscose rayon Figure 11f. These results demonstrate strongly that the closely packed nanofibril bundles could be a good reason for the high performance of RCF-5.

Figure 11. SEM images of the cross section of (a) RCF-5 and (b) viscose rayon by freezing dry. The inner structure of (c,d) RCF-5 and (e,f) viscose rayon prepared by slicing along the fiber axis direction.

To evaluate the chemical composition of cotton pulp, RCF-5 and viscose rayon, XPS testing was carried out, as shown in Figure 12a. Obviously, there is no sulfur in the cotton pulp and sulfur content of the RCF-5 is determined to be essentially zero, which can be equivalent to that of viscose rayon in which sulfur has been to take off. On one hand, sulfur has a significant influence on the use of safe; On the other hand, the presence of sulfur in the filaments leads bad luster and feel. Thus, the novel cellulose filaments are safe and comfortable materials. The Figure 12b shows the solid-state

13

C CP/MAS NMR measurements of RCF-5 with different

conditions as well as cotton pulp in comparison to viscose rayon. The spectra of the RCF-5 performs four main peaks at 105.5, 87.8, 75.2, and 62.8 ppm, which is assigned to the C1, C4, C2,3,5 and C6,20, 47 respectively. The C6 peak that lies at 65.3 ppm for cotton pulp, whereas the C6 peak for the RCF-5 and viscose rayon exhibits a shift to a higher magnetic field at 62.8 ppm, as well as the bimodal of C2,3,5 peak in the cotton pulp. The results indicate that the ‘t-g’ conformation of the C6−OH group transformed into a ‘g-t’ conformation, which strongly manifests that the native cellulose could be dissolved and lead



Page 18 of 28

to the transformation of the crystalline structure through the coagulation bath (cellulose I transforms into cellulose II), in addition to simultaneously forming the intramolecular hydrogen bonds O6−H····O-2-’.16, 48 Comparing the spectrum of RCF-5 to that of viscose rayon, it is easily to find that the peak shoulder becomes higher, indicating an increase in the degree of anisotropy. Besides, on account of the existence of strong hydrogen bonding between PA and the –OH of cellulose, the chemical shifts of C2, C3 and C6 for cellulose in the filaments are seen to up change by about 1 ppm.49 The spectroscopic data on the XRD of cotton pulp, RCF-5 and viscose rayon is shown in Figure 12c. The diffraction peaks at 2θ = 14.8°, 16.3°, and 22.6° corresponding to (11̅0), (110), and (200) planes are characteristics for cellulose I crystal as well as those at 2θ=12.1°, 19.8°, and 22.0° assigned to the (11̅0), (110), and (020) planes of the typical cellulose II crystalline form, respectively.15, 41 The XRD patterns prove that cellulose I (cotton pulp) is turned to cellulose II (RCF-5 and viscose rayon) in the regeneration process of both viscose rayon and RCF-5,40 but a significant difference in their own intensity represents different crystallinity due to nanofibers promoted crystallization for RCF-5. From the IR spectrum in Figure 12d, a strong band is exhibited at 1120 cm−1 in the cotton pulp but the intensity of this band emerges a sharp drop in the RCF-5 and viscose rayon. In previous finding, a relatively strong band appeared at 1120 cm−1 in the spectrum of cellulose I, while is presented only as a shoulder in cellulose II.50 Therefore, the FTIR spectrum suggests again that cellulose is dissolved and regenerated into cellulose II. The absorption peak at 1431 cm-1 peak for the cotton pulp is weakened and shifted to 1422cm-1 for the regenerated filaments corresponding to the CH2 scissoring motion, suggesting the destruction of an intra-molecular hydrogen bond involving O6,51,52 which further verifies the transformation of crystal forms. Furthermore, the nitrogen content of the RCF-5 and viscose rayon is determined as essentially zero. This results proves that RCF-5 has the same crystalline structure as viscose rayon but higher crystallinity.


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Figure 12. (a) XPS, (b) 13C NMR, (c) XRD and (d) FT-IR spectra of cotton pulp, RCF and viscose rayon.

TGA was used to examine the thermal stability of cotton pulp, RCF-5 as well as viscose rayon, as presented in Figure 13. Figure 13a shows the loss of water in the samples. All of them having good hygroscopicity, the water content is about 5%, 7% and 9% for cotton pulp, RCF-5 and viscose rayon, respectively, which ensures the has good dyeing property. Figure11b exhibits the decomposition temperature of samples, they are 330 °C, 301 °C and 285 °C for cotton pulp, RCF-5 and viscose rayon, respectively. The result of TGA suggests RCF-5 has good temperature resistant and able to be used for a long time at 200-250 °C.



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Figure 13. Weight loss dependence on temperature of the cotton pulp, RCF and viscose rayon: (a) the loss of water and (b) decomposition.

Table 3 shows the differences in physical properties between RCF-5 and viscose rayon. The value of crystallinity (χc) of RCF-5 is determined to be 65.18%, obviously higher than those of the viscose rayon (44.62%), which is consistent with the literature report.53 The tenacity and initial modulus of RCF-5 in the dry state are tested to be 3.43 cN/dtex and 144, respectively, which is superior to those of viscose rayon. A smaller elongation at break (10.2%) for RF-5 is observed but it can still meet the use requirement. Overall, compared to viscose rayon, RCF-5 demonstrates better mechanical properties thanks to its nanofibers structure and higher crystallinity. Table 3. Physical properties of RCF-5 compared with viscose rayon

Sample

RCF-5

Viscose rayon

χc (%)

65

45

E (cN/dtex)

144

83

бb (cN/dtex)

3.43

2.69

10.2

15.7

Moisture rate (%)

7

9

Decomposition temperature (°C)

301

285

εb (%)

χc, crystallinity; E, modulus; бb, tenacity; εb, elongation at break for RCF-5 and viscose rayon.

■

CONCLUSIONS

In summary, high-performance regenerated cellulose filaments (RCFs) were successfully prepared from the cellulose solution in LiOH/urea aqueous system with cooling in an acid/salt coagulant bath via wet-spinning. Phytic acid salt was used to adjust the coagulation rate for a better control of the structure of RCF to improve mechanical properties. Adding small amount of phytic salt could promote a rapid diffusion of phytic acid from the skin to the core of cellulose filament due to the diffusion of water, while excessive salt would lead to a decrease of neutralization capability of phytic acid, resulting in a homogenous but loose structure in the filament. The optimum salt concentration was found to be 5% at


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which phytic acid could permeate to the center of the filament and maintain good neutralization capability, resulting in a uniform and compacted structure. The obtained filaments showed tight structure, favorable flexibility and a silk-like luster as well as the improved tenacity and breaking elongation. For RCF-5, the tenacity and breaking elongation reached to 3.43 cN/dtex and 10.2%, respectively, coming closer to those of the Lyocell. Additionally, compared with viscose rayon, RCF-5 presented similar crystal type to viscose rayon yet different intensity and micro-structure, bringing about a vast difference in mechanical performance. In this work, the mechanical properties of novel regenerated cellulose filaments are greatly improved by simple process, which provides a positive guidance on the environmentally friendly cellulose industrialization.

■ ACKNOWLEDGMENT We would like to express our sincere thanks to the National Natural Science Foundation of China for financial support (Grant No. 51573102 and 51421061), the Major Program of National Natural Science Foundation of China (21334005), the Major International (Regional) Joint Research Project of National Natural Science Foundation of China (21620102004).

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For Table of Contents Used Only A strong and green cellulose filaments with nanofibril-structure was obtained from LiOH/urea solvent using 15 wt% phytic acid/5 wt% salt as coagulator via wet-spinning.


The Influences of Coagulation Conditions on the Structure and

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