Toughen and strengthen alginate fiber by incorporation of polyethylene glycol grafted cellulose nanocrystals

Excellent strength regenerated alginate composite fibers mixed with Poly (ethylene glycol) grafted cellulose nanocrystals (CNC-g-PEG) were prepared through a simple and scalable blending strategy. Firstly, high modulus cellulose nanocrystals (CNCs) were fabricated based on waste cotton fabrics by the chemical treatment. Poly (ethylene glycol) (PEG) with different molecular weight was grafted onto the CNC surface. The composite fibers were prepared from CNC-g-PEG/sodium alginate spinning dope by wet spinning. The CNC-g-PEG/Alginate fibers were characterized using FTIR, XRD, scanning electron microscopy, and mechanical testing. The morphology of the SEM image showed that CNC-g-PEG partially eliminated the defects of the alginate matrix and increased the roughness of the fracture cross-section, constructing material with a denser microstructure. Under the same load, the addition of CNC-g-PEG has better effects on the mechanical properties of alginate fibers than CNCs. The maximum strength and elongation at the break for alginate composite fibers were increased by 56.3% and 81.6% as a result of the CNC-g-PEG reinforcement, respectively. Besides, the salt tolerance and antibacterial activity of the CNC-g-PEG/Alginate composite fibers were improved as the molecular weight of PEG increased. This study provides the possibility for the development of high-performance, green alginate fibers.


Introduction
Sodium alginate is a naturally occurring polysaccharide that serves as the basic material for alginate fibers. Its molecule is constituted of β-D-mannuronic acid (M) and α-L-guluronic acid (G), which are joined through 1-4 glycosidic linkages. Alginate fibers are extensively employed in biomedicine, textiles, and sewage treatment areas due to their biocompatibility, natural degradability, water absorption, and nontoxicity. Examples include medical wound dressings (Zheng et al. 2021a, b). In the long term, the Earth's oil supplies are depleting, and researchers should focus their attention on the sustainable natural resources of bio-based fibers rather than oil-based synthetic fibers (Ma et al. 2017).
Pure alginate fibers produced conventionally have low mechanical qualities since the wet spinning method results in fewer gaps in the supramolecular structure of alginate fibers (Potiwiput et al. 2019). Hence, when an external force strains the alginate fiber, the stress concentration causes it to rupture. Additionally, the reduced crystallinity, orientation degree, and weaker force between the macromolecules of the alginate fiber result in a low initial modulus, and the alginate fibers are readily broken when In this sense, there are still significant issues associated with employing CNCs to simultaneously strengthen and toughen polymer materials. CNCs may be modified to improve their compatibility and dispersion in the polymer matrix, hence achieving a greater enhancing effect (Lu et al. 2016;Sahlin et al. 2018). Ten et al. (2010) successfully increased the dispersibility of CNCs in a poly (hydroxybutyrateco-hydroxy valerate) matrix by using 30% polyethylene glycol as a compatibilizer. Consequently, the redesigned composite material's mechanical qualities have been greatly enhanced. As is well known, PEG is a commonly used high molecular weight polymer that exhibits excellent compatibility with a broad variety of organic compounds and aliphatic polyesters such as PLLA (Pivsa-Art et al. 2016;Lai et al. 2004;Hu et al. 2003). It may significantly improve the abrasion resistance and tractability of polymer materials. Due to PEG's excellent biocompatibility, biodegradability, and non-toxicity, it is extensively utilized in cosmetics, textiles, biomedicine, and food processing (Min et al. 2015;Gui et al. 2012). Additionally, the high solubility of PEG in water should be noted to avoid the PEG dissolving when the spinning solution reaches the coagulation bath. Instead of adding PEG directly to the CNC/Alginate spinning dope, we initially graft it to the CNC surface to create a synchronously strengthened and toughened filler before adding it to the alginate matrix. This method preserves the nanocellulose's outstanding biodegradability and biocompatibility and enhances the dispersion stability of CNCs in the alginate matrix.
Additionally, the CNCs employed in this study were derived from discarded cotton textiles (Cao et al. 2021). The critical point is that, as far as we are aware, several prior studies have been conducted to raise the strength of alginate fibers, but there are relatively few studies on simultaneously increasing the strength and toughness of alginate fibers. The fabrication of alginate fibers using CNC-g-PEG has not been previously described.
Thus, the purpose of this work was to develop toughened and reinforced alginate composite fibers using wet spinning with CNC-g-PEG as a bifunctional enhancer. The morphology, microstructure, and mechanical characteristics of the produced alginate composite fibers were thoroughly studied. Additionally, we investigated the antimicrobial characteristics of the composite fibers in anticipation of their use in medical textiles. Extraction and modification of CNCs and their chemical grafting of PEG with different molecular weights CNCs were prepared from waste cotton fabric according to our previous methods (Cao et al. 2021). In brief, the 10 g of waste cotton fabrics crushed by pulverizer were boiled in 350 mL of 3% sodium hydroxide solution for 6 h and washed with deionized water until neutral. Next, the treated fibers were stirred in 200 mL of a mixed acid aqueous solution containing at the ratio of 1:2:2 (v/v) of nitric acid, hydrochloric acid, and deionized water at 60 °C for 1 h to obtain microcrystalline cellulose (MCC). Then, 5 g of MCC was added into 50 mL of 64 wt % sulfuric acid and reacted for 3 h at 44 °C. The hydrolysis process was terminated by diluting with water to approximately a fivefold volume of the reaction solution. The suspension was washed with deionized water 5 times followed by centrifugation at 7000 rpm for 15 min. The supernatant liquid was replaced by distilled water and collected from the third centrifugation. Finally, the CNC suspension obtained after centrifugation was dialyzed to neutrality, and freeze dried to obtain CNCs.
The pH value of the suspension was adjusted to 10.2 with the dilute hydrochloric acid solution, 5 mL of sodium hypochlorite was added with slow stirring, and then the pH value of the reaction system was maintained at 10-10.5 with dilute sodium hydroxide solution (Saito et al. 2006). After fully reacting for 3 h, about 5 mL of ethanol was added to terminate the reaction. The oxidation product was thoroughly filtered and washed with deionized water to neutrality. The oxidized CNCs (OCNCs) will be used in the subsequent experiments.
The 10 g of PEG1000, PEG2000, and PEG4000 were separately put into an Erlenmeyer flask containing 50 mL of 1 wt% CNC dispersion and stirred vigorously. PEG1000, PEG2000, and PEG4000 were coded according to the different molecular weight of PEG. Then 0.1 mL of Tin 2-ethyl hexanoate (Sn (Oct) 2 ) was added to the reaction system. The reactant solution was heated to 90 °C for 6 h under stirring at 800 r/min. Lastly, ethanol was added to terminate the reaction. The remaining Sn (Oct) 2 and un-grafted PEG in the product were removed by centrifugation. The centrifuged product was dialyzed for one week and then freeze-dried to obtain the graft copolymer. The products were coded as CNC-g-PEGX (X refers to the molecular weight of PEG). The grafting rate of PEG measured by the weighing method was 31.3%.

Preparation of the alginate composite fibers
In our previous work, alginate composite fibers containing CNCs (8 wt%) showed better performance. Therefore, the content of CNCs or CNC-g-PEG used in the preparation of sodium alginate spinning dope in this study was 8 wt%. Firstly, CNC or CNC-g-PEG powder was dispersed in water by sonication method for 1 h. Then 4 wt% sodium alginate was added with stirring at 400 r/min for 6 h at 45 °C until a homogeneous mix of spinning dope was acquired. The spinning solution was vacuum defoamed before spinning. The spinning dope was injected into the coagulation bath of the calcium chloride aqueous solution through a self-made wet spinning machine. After the spinning was completed, the formed fiber was cleaned with deionized water to remove sodium chloride and calcium chloride on the fiber surface, and then continuously immersed in alcohol with increasing volume fractions (20 vol.%, 30 vol.%, and 50 vol.%) to remove the water inside the fibers, and finally dried and collected under natural conditions. The alginate composite fibers contained CNCs and CNC-g-PEGX were recorded as CNC/Alginate fibers and CNC-g-PEGX/Alginate fibers, respectively. The schematic diagram of the entire preparation process and the reactions involved are shown in Fig. 1.

Fourier transform infrared (FTIR) measurement
The FTIR spectra were recorded on a TENSOR-27 type Fourier transform infrared spectrometer (Bruker Co, USA) with a resolution of 2 cm −1 in the wavenumber range of 400-4000 cm −1 . The samples were prepared for analysis by grinding the dry blends with KBr and compressing the mixtures to form sheets.

Scanning electron microscope (SEM)
The morphology of the prepared fibers was observed using a JEOL JSM-5600LV scanning electron microscope (SEM, Japan). The accelerating voltage was 20 kV. The surfaces of the samples were subjected to a 10 nm gold spray treatment.

Powder X-ray diffraction analysis (XRD)
The crystal structures of the samples were studied using a Riguaku X-ray diffractometer (Ultima VI, Japan) with a Cu Kα radiation (k = 0.15418 nm) over a 2θ range of 5-80° with a step size of 0.05°. The degree of crystallinity (X c ) was calculated according to the deconvolution method by using the Eq. (1): where A c represents area under crystalline peaks and A a is area under amorphous region.

Rheological behavior
The rheological behavior of the spinning dope was measured by an advanced rotational rheometer (AR-2000ex, US). Steady-state flow measurement was carried out at room temperature with the shear rate from 0.1 to 1000 s −1 . (1)

Mechanical property
The mechanical properties of the dried fibers were tested according to GB/T 14,337-2008 on an XQ-1 fiber tensile tester (Shanghai New Fiber Instrument Co., Ltd., China). The fibers were first preconditioned for one day at room temperature and 65% room humidity (R.H.). When testing, a test length of 20 mm was used for fibers at a crosshead speed of 10 mm/ min at 20 °C and 65% relative humidity. The pretension was 0.3 cN. At least 20 independent samples of each specimen were tested and averaged.

Water absorbency
The fiber samples were dried at 50 °C in a vacuum dryer for 24 h and weighed (W 0 ). Then, the dried sample was immersed in distilled water to swell at 25 °C. After swelling equilibrium, the samples were taken out and weighted (W t ) after removing the excess water on the surface by filter paper. The water absorbency was calculated as follows: The results were expressed as an average value from at least three tests.

Salt tolerance
The fiber samples were treated in a series of NaCl solutions of different concentrations (10 g/L, 15 g/L, 20 g/L, 25 g/L, 30 g/L) for 1 h and taken out. When there were no drops on the fiber surface, the weight was weighed. Then the fiber samples were dried at 50 °C in a vacuum dryer for 24 h and weighed. The swelling ratio (S) of the fiber was calculated according to Eq. (3): where W s was the sample's weight after absorbing NaCl and W d was the sample's weight after drying. The final result was the average of three sets of data.

Biodegradability
The in vitro degradation of alginate fiber was carried out using phosphate-buffered saline (PBS). About 150 mL of PBS solution was added to a 250 mL Erlenmeyer flask, sealed, and sterilized for 30 min. Then about 0.5 g of dry sample was added and shook well. The Erlenmeyer flask containing PBS and sample fibers was placed at about 37 °C. The sample fiber was collected on the 5 th, 10 th, 15 th, 20 th, 25 th, and 30 th days, and was washed with distilled water several times to remove the salt. The precipitate was dried in a vacuum to a constant weight and then weighed. The weight loss rate of the sample fiber was calculated according to Eq. (4): where W was the weight loss percentage of the sample, and M 0 (g) and M 1 (g) were the dry sample weight before and after degradation, respectively.

Antibacterial activity
The antibacterial activities of the fibers were estimated for the Gram-positive bacteria S. aureus according to the colony count method. The detailed steps were described in our previous report (Cui et al. 2020). All instruments need to be sterilized before the antibacterial experiment. The 0.375 ± 0.005 g fiber was weighed and put in an Erlenmeyer flask containing 35 mL of PBS, one of the fiber-free samples was used as a blank sample. Subsequently, 2.5 mL of the diluted bacterial suspension was added, and the sample solution was shaken at 150 r/min for 18 h at 24 ± 1 °C. Then 100 μL of the treated sample solution was dropped on the surface of the agar plate and evenly coated with a spreader. After the bacterial solution was completely absorbed, it was then transferred to an incubator at 37 ± 1 °C for 36 h.
(4) where Y is the antibacterial rate of the sample, A is the viable cell count of the blank sample, and B is the viable cell count of the tested sample.`(

Results and discussion
Morphology and stability of CNCs and CNC-g-PEG As shown in Fig. 2a, b, CNCs obtained from waste cotton fabrics displayed rod-shaped nanocellulose with high aspect ratio. CNC and CNC-g-PEG suspensions were light blue and no obvious sedimentation was observed at the bottom of glass bottles after ultrasonication (0 day) (Fig. 2c). However, after 30 days later (Fig. 2d), a large number of CNCs presented obvious agglomeration, while CNC-g-PEG suspension still maintained good stability. CNCs prepared by sulfuric acid hydrolysis were electrostatically stable due to the negatively-charged sulfate groups on its surface (Habibi et al. 2010). However, for CNCg-PEG, the steric hindrance effect of PEG molecular chain on the CNC surface could replace the electrostatic stabilization to prevent agglomeration and precipitation. And as a hydrophilic and flexible long chain, PEG could better promote CNC-g-PEG suspension to form a stable solution with good dispersion performance (Zheng et al. 2021a, b). Furthermore, the additional COO − groups generated by the selective oxidation of TEMPO also played a positive role in the dispersibility of CNC-g-PEG suspension (Ma et al. 2017). The good stability of CNC-g-PEG suspensions could be preserved for several months at low temperature. The FT-IR spectrum of obtained CNCs, OCNCs, PEG, and CNC-g-PEG2000 are shown in Fig. 3. At about 3400 cm −1 , the intensity of -OH stretching vibration peak of OCNCs was weaker than that of CNCs, indicating that some hydroxyl groups on the CNC surface reacted. At 1640 cm −1 , the peak intensity of OCNCs increased, corresponding to the O-H bending vibration peaks of OCNCs for water absorption. The carboxyl peak at 1730 cm −1 was obtained from the hydroxyl group at the C 6 position of CNCs oxidized by TEMPO. After the grafting of OCNCs, the carboxyl peak at 1730 cm −1 decreased and disappeared. In particular, a new peak was formed at about 1656 cm −1 , which belonged to the carbonyl stretching vibration peak of the ester group of the graft copolymer (Gu et al. 2016). The results indicated that the PEG was grafted onto the OCNC surface. Figure 4a shows the XRD diffraction patterns studied for the waste cotton, CNCs, PEG, CNC-g-PEG, and CNC/PEG mixture. The XRD pattern of waste cotton displayed the two peaks at 2θ = 14.8°, 16.5°, and 22.7°, corresponding to (1 1 0), (110), and (200) crystal planes of cellulose I crystals crystallized in a monoclinic pattern (Yu et al. 2014). The crystal structure of CNCs prepared from waste cotton fabric retained the cellulose I crystal structures of natural biomass cellulose. The diffraction pattern of PEG was observed with two major crystalline peaks at 2θ = 19.34° and 23.13°and some minor peaks at 2θ = 25.06°. In the case of CNC-g-PEG, all characteristic peaks related to CNCs and two major crystalline peaks of PEG were found. In addition, CNCg-PEG exhibited cellulose I crystal structure. In the mixture of CNCs and PEG, the characteristic peak at 2θ = 19.34° of the mixture was significantly different from that of CNC-g-PEG. Some minor peaks of PEG could also be seen in the mixture.

Rheological property of the spinning dopes
The viscosity of the spinning dope is a key factor for fiber formation in wet spinning. The high-viscosity spinning dope is difficult to remove air bubbles while the low-viscosity spinning dope is not conducive to the formation of filaments. Figure 4b shows the relationship between the steady-state viscosity (η) versus the shear rate (γ) of different spinning dopes. The steady-state viscosity of all spinning dopes at low shear rates (0.1 to 1 s −1 ) was constant, and the solutions behaved as Newtonian fluids. As the shear rate increased from 1 to 1000 s −1 , all spinning dopes had exhibited shear thinning behavior. This was because the high shear rate destroyed the physical crosslinks of the interaction between alginate molecules, and they could not be reformed in time, resulting in a decrease in crosslink density and a drop in the viscosity. The plateau of steady-state viscosity of CNC/ Alginate and CNC-g-PEG/Alginate spinning dope at the low shear rate (0.1 to 1 s −1 ) was lower than that of pure alginate spinning dope. That was because the incorporation of these fillers reduced the entanglement between sodium alginate molecules (Ma et al. 2017). The steady-state viscosity of CNC-g-PEG/ Alginate spinning dope increased with increasing PEG molecular weight at the low shear rate (0.1 to 1 s −1 ). This was a logical observation since the viscosity of PEG increased with the increase of molecular weight. Low viscosity was more conducive to the defoaming step under low shear conditions before spinning. Thus, although the existence of CNCs and CNC-g-PEG led to a decrease in the viscosity of the spinning dope, it was conducive to the wet-spinning process.
Morphology of the alginate composite fibers SEM images (see Fig. 5) showed the surface of the fibers had obvious grooves and linear structures, which were caused by uneven shrinkage due to the different removal rates of large amounts of solvent on the fiber surface and inside the fiber during the coagulation and molding process (Zhang et al. 2018). Compared with pure alginate fibers, CNC-g-PEG/Alginate fibers exhibited more and deeper grooves on the surface, which was attributed to the formation of hydrogen bonds between CNCs and alginate macromolecules in the alginate fiber. It led to the increase of the number of wrinkles on the fiber surface. In addition, the tensile fracture cross-section of alginate fibers and CNC/Alginate fibers were relatively smooth, indicating that their ability to resist crack propagation was weak and brittle fracture may occur.
However, after CNC-g-PEG was added, the roughness of the fractured cross-section increased significantly with the increase of PEG molecular weight. There were increasing irregular zigzag stripes on the fractured cross-section of modified alginate fibers, which was the main feature of the matrix's transformation from brittleness to plastic deformation (Terentyev et al. 2019). It indicated that the ability of modified fiber to resist crack propagation was improved.

Aggregated structure of the alginate composite fibers
The XRD spectra of alginate fibers, CNCs, and CNCg-PEG modified alginate fibers are shown in Fig. 6a.
Alginate fibers saw a broad diffraction peak near 2θ = 13.2°and 22.6°, which were assigned to (110) plane from polyguluronate unit (G), and (200) plane from polymannuronate (M) (Sundarrajan et al. 2012). The characteristic peaks of pure alginate fibers were wide and diffuse, confirming that pure alginate fibers had weak crystalline properties. The intensity of the diffraction peak of CNC/Alginate fibers or CNC-g-PEG/Alginate fibers increased significantly, and the peak shape became narrower with the addition of CNCs or CNC-g-PEG. The characteristic diffraction peak of alginate fibers at 2θ = 13.2° demonstrated a higher diffraction peak intensity and a narrower peak shape. The crystallinity of the modified alginate composite fibers was found to increase compared with the pure alginate fibers (Fig. 6c). It was because of the participation of CNCs and CNCg-PEG in heterophase nucleation. At the same time, CNCs and CNC-g-PEG reacted with the hydroxyl group on the macromolecular chain. More hydrogen bonds were introduced into the alginate fibers, and the hydrogen bonding force between the molecules of the modified alginate fibers was enhanced, resulting in the improvement of the regularity and order of the molecular chain. The degree of crystallinity of alginate composite fiber was improved greatly.

Mechanical properties of the alginate composite fibers
The breaking strength, elongation at the break, and work at break of alginate fibers and alginate composite fibers are shown in Fig. 6b, d, f. As expected, pure alginate fibers owned the lowest tensile strength value. And the tensile strength of alginate composite fibers was significantly improved by CNCs and CNCg-PEG. In this blending system, CNCs and PEG acted as reinforcing agent and compatibilizer/toughing agent in alginate composite fibers, respectively. When CNCs was added to the sodium alginate spinning solution, the tensile strength of the alginate composite fibers reached the maximum, but its elongation at break was even than that of pure alginate fibers. However, when CNC-g-PEG was added, both elongation at break and tensile strength of alginate composite fibers were higher than that of pure alginate fibers. With the increase of PEG molecular weight, the tensile strength was gradually decreased, while the elongation at break was increased. The changing trend of 1 3 Vol:. (1234567890) Fig. 6 a XRD patterns of CNCs, PEG and different fibers; The tensile strength b, Xc c, elongation at break d and work at break f of different fibers; e The weight loss rate of differ-ent fibers in the PBS solution (I. Alginate fibers, II. CNC/Alginate fibers, III. CNC-g-PEG1000/Alginate fibers,IV. CNC-g-PEG2000/Alginate fibers,V. CNC-g-PEG4000/Alginate fibers) the work at break (Fig. 6f) was similar to that of the elongation (Fig. 6d). The maximum break elongation increases by 82% while the maximum work at break increases by 153%.
The increase of crystallinity, intermolecular force, and regularity of macromolecular chain arrangement could improve fiber mechanical properties Compared with CNC-g-PEG/Alginate fibers. And the tensile strength of CNCs was improved greatly under the same loading levels and same amount. High crystallinity and good interface adhesion (hydrogen bond) of alginate composite fibers contributed to the improvement of the tensile strength. In addition, CNC nanoparticles dispersed in the alginate matrix could serve as stress concentration centers and induce the generation of silver stripes and shear bands to consume a large amount of energy. Nevertheless, the grafted PEG molecular chains would replace hydroxyl groups on the CNC surface. What's more, the hydrogen bonds between CNCs and alginate chains were reduced. At the same time, CNC-g-PEG macromolecular chains contained flexible PEG side chains, which increased the distance between macromolecules and the mobility of molecular chains. The grafted PEG chain could be recruited as a bridge to enhance the interface characteristics/adhesion between CNCs and the alginate matrix, to enhance transfer stress and strain of the alginate chains along the tensile direction hauled by the CNCs during the stretching process (Zhang et al. 2015). Thus, the toughness of the alginate composite fibers had been greatly improved. The plasticization effect of CNC-g-PEG with larger PEG molecular weight also speeded up the mobility of the alginate chains along the tensile direction and enhanced the Alginate fibers, II. CNC/Alginate fibers, III. CNC-g-PEG1000/ Alginate fibers, IV. CNC-g-PEG2000/Alginate fibers, V. CNCg-PEG4000/Alginate fibers) toughness of the alginate fibers. With the comprehensive evaluation of tensile strength and elongation at break properties, the addition of CNC-g-PEG2000 showed the best effect of toughening and strengthening for the alginate composite fibers.

Biodegradability of the alginate composite fibers
The biodegradation behavior of prepared alginate composite fibers is shown in Fig. 6e. All samples were immersed in PBS for 30 days, and all fibers were degraded over time and the weight loss increased gradually. In the end, the biodegradability of pure alginate fibers reached 68% after 30 days, which was attributed to the hydrolysis of b-1,4 glycoside bonds in alginate molecule. The biodegradation rate of CNCs was lower than that of alginate (Cui et al. 2020;Doh et al. 2020), so the degradation of fibers became difficult when CNCs or CNC-g-PEG were added to the alginate matrix. Therefore, it could be concluded that the biodegradability of the fibers was decreased by the addition of CNC or CNC-g-PEG into the alginate matrix. Overall, it had a weaker effect on the degradation of alginate composite fibers.
Water absorbency of the alginate composite fibers Alginate fibers have many hydrophilic groups (e. g. hydroxyl and carboxyl groups) that can bind to water molecules to form hydrogen bonds. In addition, the alginate fibers generate many tiny voids in the wet spinning process. So, alginate fibers had excellent moisture absorption capacity (Jabeen et al. 2016). As shown in Fig. 7a, all fibers exhibited excellent water absorption. The water absorbency of all alginate composite fibers was gradually decreased with the addition of CNCs or CNC-g-PEG. With CNCs, the water absorbency was reduced by 19.7% in specific. Meanwhile, the reduction rates for the CNC-g-PEG1000/ Alginate fibers, CNC-g-PEG2000/Alginate fibers, CNC-g-PEG4000/Alginate fibers were greater than that of CNC/Alginate fibers, which were reduced by 41.5%, 36%, 29.1%, respectively. This might be due to the relatively strong interfacial interaction between CNC-g-PEG and alginate molecule, resulting in the increased crystallinity of the composite fibers and tortuosity of the transport path for small molecules to diffuse through the alginate matrix (See Fig. 7c).

Salt tolerance property
The preparation of alginate fiber is reversible. When the content of Ca 2+ is high, Na + in alginate will be replaced to form cross-linking structure for coagulation. But, too much Na + will make Ca 2+ be replaced to gelation inversely . As shown in Fig. 7b, the swelling ratio of pure alginate fiber gradually increased as the concentration of NaCl solution increased and even dissolved. Because Na + causes greater dissociation of water molecules, eventually, pure alginate fibers dissolved. In addition, the salt tolerance of all alginate composite fibers was improved. Physical cross-linking between CNCs or CNC-g-PEG and the alginate matrix prevented Na + in NaCl solution from entering the fiber and protected the "egg-box" structure of fibers, and prevented water molecules from entering the fiber to reduce the dissociation effect (Chan and Neufeld 2009). With the same concentration of NaCl solution, the swelling ratio of CNC-g-PEG/Alginate fibers was lower than that of CNC/Alginate fibers. It may because that the good compatibility of the better dispersed CNC-g-PEG with the alginate matrix, which improved the interfacial cohesion between the two components and prevented the entry of Na + and water molecules. Moreover, the swelling ratio of CNC-g-PEG/Alginate fibers increased gradually with the increase of PEG molecular weight. Owing to the increase in molecular weight of PEG, the crystallinity of the CNC-g-PEG modified alginate fibers decreased slightly and allowed more water molecules to enter the amorphous region of the fiber, and the swelling ratio also increased. With the gradual increase of the Na + concentration, increasing Na + entered the fiber to exchange with Ca 2+ , and the swelling ratio of CNC/ Alginate fibers and CNC-g-PEG/Alginate fibers gradually increased.

Antibacterial activity
The antibacterial activity of fibers against S. aureus is shown in Table 1. Compared with pure alginate fiber, the antibacterial performance of CNC/alginate fiber was improved slightly. The inhibition rates of CNCg-PEG/Alginate fibers were much higher than that of alginate fibers, indicating that CNC-g-PEG/alginate fibers against S. aureus had better antibacterial activity. PEG combined with water molecules to form hydration layer by hydrogen bond. And the steric repulsion of molecular chains inhibited bacterial adhesion, especially PEG of high molecular weight, which could significantly reduce the amount of bacterial adhesion.

Conclusions
Alginate composite fibers blended with CNCs or CNC-g-PEG with different PEG molecular weights were successfully prepared through wet spinning. Compared with CNCs, the steric hindrance of CNCg-PEG made it owned better interfacial compatibility with alginate fibers. The tensile strength of alginate fibers reached the maximum with the addition of 8 wt % CNCs, but the elongation at break was even worse than that of pure alginate fibers. However, CNC-g-PEG showed a slightly lower reinforcing effect on the tensile strength of alginate fibers than CNCs under the same amount of addition. With the increase of the molecular weight of the grafted PEG, the elongation at break gradually reached 17.8%. Therefore, CNC-g-PEG nanofillers improved toughening and strengthening alginate composite fibers. Moreover, its toughening effect was enhanced by grafting a larger molecular weight of PEG into the CNC surface. Furthermore, the addition of CNCs and CNC-g-PEG reduced the water absorbency, while improved the salt tolerance of the alginate composite fiber. In addition, the biodegradability of CNC/Alginate fibers and CNC-g-PEG/Alginate fibers was slightly reduced, compared with the alginate fibers. This kind of green composite fibers with excellent performance could be regarded as new materials in the field of medical textiles.