Sodium alginate, the raw material of alginate fibers, is a natural polysaccharide. Its molecule is composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G) connected by 1-4 glycosidic bonds. Because of its good biocompatibility, natural degradability, water absorption, and non-toxicity, it is widely used in biomedicine, textile, and sewage treatment fields, such as medical wound dressings (Zheng et al. 2021). In the long run, the oil reserves on the earth are gradually decreasing, and the sustainable natural resources of bio-based fibers are more worthy of research by scholars than oil-based synthetic fibers (Ma et al. 2017).
Conventional pure alginate fibers prepared by wet spinning technology have poor mechanical properties because the wet spinning process conditions determine that there are fewer gaps in the supramolecular structure of alginate fibers (Potiwiput et al. 2019). When the external force is stretched, the fiber will break due to stress concentration. However, the crystallinity and orientation of alginate are low. The force between the macromolecules is weak, and the initial modulus is small. When a large external force acts on the alginate fiber, the macromolecular chain will relatively slip, which will also lead to the fiber fracture (Smyth et al. 2017). In a word, the structural characteristics of alginate fiber make it have the disadvantages of low strength, high brittleness, and poor toughness. It is easy to be squeezed and deformed during processing, production, and application, which affects its further application in medical, textile, and other fields.
In order to overcome these limitations, some nano, natural, synthetic polymers such as nanomaterials (nanocellulose) (Park et al. 2021), natural polymer materials (gelatin, chitosan) (Dong et al. 2006; Mikos et al. 1994), synthetic polymers (polyvinyl alcohol) (Fan et al. 2005), and other mixing was used to improve the mechanical properties and adsorption properties of the alginate fiber. Liu J et al. (2019) reported that the breaking strength of alginate fiber can be significantly improved by the addition of CNCs (2 wt%) because the nano-size of CNCs makes it easier to diffuse into the alginate matrix and act as enhancers. Simultaneously, more hydrogen bonds can be formed between the alginate matrix and the hydroxyl groups on CNCs, which improves the stability of the macromolecular network structure and improves the fiber load capacity. However, in some hydrophobic or low-polar polymers, nano-cellulose is prone to agglomeration due to intermolecular hydrogen bonding, which greatly reduces the strengthening effect of nanocellulose (Musa et al. 2015). In our previous research, we found that CNCs were added as effective nucleating agents to the poly(L-lactic acid)/poly(D-lactic acid) (PDLA/PLLA) matrix to promote the mechanical properties and thermal stability of PLLA/PDLA composites films (Cao et al. 2021). However, the elongation at the break of these composites films is slightly decreased. In this regard, there are still some challenges in using CNCs to strengthen and toughen polymer materials, simultaneously. The modification of CNCs is an optional way to make it compatible and dispersed in the polymer matrix, so as to achieve a better enhancement effect (Lu et al. 2016; Sahlin et al. 2018). Ten et al. (2010) took 30% polyethylene glycol (PEG) as a compatibilizer to effectively increase the dispersibility of CNCs in a poly(hydroxybutyrate-co-hydroxyvalerate) matrix. The mechanical properties of the modified composite material have been significantly improved. PEG is a widely used high molecular polymer with good water solubility and good compatibility with many organic components and aliphatic polyesters such as PLLA (Pivsa-Art et al. 2016; Lai et al. 2004; Hu et al. 2003). It can effectively enhance the obdurability and tractility of polymer materials. Moreover, PEG has good biocompatibility, biodegradability, non-toxicity, and other characteristics, making it widely used in the fields of cosmetics, textiles, biomedicine, and food processing. (Min et al. 2015; Gui et al. 2012). In our study, PEG is chosen to modify CNCs by the grafting method. This way not only retains the excellent biodegradability and biocompatibility of nanocellulose but also improves the dispersion stability of CNCs in the alginate matrix. And, CNCs used in this research are obtained from waste cotton fabrics. (Cao et al. 2021) As is known to all, no investigation has ever been covered on this.
Here, we report a series of CNC-g-PEG modified alginate composite fibers prepared by wet spinning with CNC-g-PEG as a bifunctional enhancer into alginate fibers, in which the CNC fillers are used as green enhancers, and PEG molecular chain is used as a plasticizer while increasing the dispersibility of CNCs. The morphology, microstructure, and mechanical properties of the prepared alginate composite fibers were analyzed in detail. As expected, the incorporation of CNC-g-PEG can synergistically strengthen and toughen alginate fibers. The prepared alginate composite fibers are green biomass fibers, which have great application prospects in the field of medical dressings.