Aggregation structure characterization of silk fibroin–crosslinking oxidized bamboo pulp fibers

Abstract

Bamboo pulp fiber as one of new promising green fiber was widely used in clothing, medical, automotive, construction, transportation, and many other areas. However, bamboo pulp fiber also has many defects such as larger shrinkage, lower wet strength, fabric wrinkle, and the poor ability to keep type. In this study, a green modification method for bamboo fiber was applied to overcome above-mentioned drawback and verified by scanning electron microscopy, Fourier-transform infrared spectrometry, X-ray diffraction, and solid-state CP/MAS 13C nuclear magnetic resonance respectively. The scanning electron microscopy results showed that oxidation treatment can cause fibers edge damage and denudation after modification, but can introduce the carboxyl groups to C₆ position of pure bamboo cellulose macromolecular. The X-ray diffraction results revealed that the fibers’ crystalline structure was not changed throughout the modification process. The oxidation treatment processes can be interpreted as following: amino groups of silk fibroin first react with the carboxyl group and are connected to the fibers though C–N covalent bond, and then a smooth silk fibroin film was formed on the surface of fibers by crosslinking reaction of itself.

Introduction

China has the biggest bamboo forest area in the world and is rich in bamboo resources, which provides good condition for the development of the bamboo pulp fibers.^1–3 Bamboo pulp fiber could be easily prepared via viscose spinning process or Lyocell spinning process with bamboo. ⁴ Bamboo pulp fiber as one kind of regenerated cellulose fiber has many excellent properties including dyeability, antibacterial property, hygroscopicity, and moisture regain. Therefore, it was widely used for making underwear, bedding, sanitary materials, and so on. However, bamboo pulp fiber also has many defects such as larger shrinkage, lower wet strength, fabric wrinkle, and the poor ability to keep type. Therefore, many efforts are devoted to overcome above-mentioned drawbacks by various physical and chemical means.^5–10 At present, the textile modification technology is using acid, alkali, cross-linking agent, and other chemical reagents or ultraviolet and gamma radiation to treat bamboo pulp fiber in order to change the bamboo pulp fiber apparent morphology and internal structure and improve the performance of bamboo pulp fiber.^11–13 But these chemicals will also have some adverse effects on the human body, and serious environmental pollution. These conventional modification technologies usually use toxic chemical agents and have a negative impact on the ecological environment and human health; thus, it is difficult to meet the green textile requirements advocated today. In our experiment, pure bamboo pulp fibers were oxidized by HNO₃/H₃PO₄-NaNO₂ system selectively. In this process, the hydroxyl of cellulose molecules position on C₆ was first oxidized into lively carboxyl groups. Then, the carboxyl groups could react with amino groups which are on the silk fibroin (SF) molecules when the bamboo fibers were modified directly in SF aqueous solution.^14–16 SF is one kind of natural polymer protein and is superbly suitable to our skin. What’s more, in the process of modification, it did not need to use any other crosslinking agent for chemical additives. Therefore, a green modification process could be achieved.

In this study, a comparative analysis about the effect of oxidation treatment and SF modifications on bamboo pulp fibers was performed. We applied four technical methods to characterize aggregation structure of the modified bamboo pulp fibers. The morphology changes of fibers’ longitudinal surface were observed by scanning electron microscopy (SEM). The X-ray diffraction (XRD) analysis was carried out to explain the modification effect on the fibers’ chemical structure in terms of crystalline. And the transformation of chemical bond in the modification process was determined by Fourier-transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy.

Materials and methods

Materials

The following are the materials used in this study: Bamboo pulp fiber yarns (28tex, supplied by Anhui LiangLiang textile Corporation); HNO₃ (68% w/v, analytical purity; supplied by Ying Fengda Textile Auxiliaries Industrial Co., Ltd.); H₃PO₄ (85% w/v, analytical purity; supplied by Linyi Green Sen Chemical Co., Ltd); NaNO₂ and HCl (analytical purity; supplied by Sinopharm Chemical Reagent Co., Ltd); and deionized water (supplied by Tianjin Zhiyuan Chemical Reagent Co., Ltd.).

Methods

Oxidation treatment of bamboo pulp fibers

HNO₃ and H₃PO₄ were mixed in 2:1 (v/v) ratio. According to the bath ratio 1:30 (the weight ratio of bamboo fiber to HNO₃/H₃PO₄ mixed solution is 1:30), high-quality bamboo pulp yarns were immersed into mixed solution completely. Then NaNO₂ was added one time into mixed solution until the solution concentration reached to 1 g/100 mL. At room temperature, pure bamboo pulp yarns were oxidized for 1 h away from light, then flushed by deionized water, and then soaked in a certain concentration of ethanol solution for 20 min. Finally, it was washed with deionized water and dried at 60°C. ¹⁷

SF modification treatment of bamboo pulp fibers

First, silkworm cocoon shells were dissolved in ternary solution of CaCl₂/H₂O/C₂H₅OH (1:8:2, n/n/n), then the SF solution was dialyzed for 72 h to reach the concentration of 30 g/L. According to the bath ratio 1:50 (the weight ratio of bamboo fiber to CaCl₂/H₂O/C₂H₅OH mixed solution is 1:50), oxidized bamboo pulp fibers were soaked in SF solution for 1.5 h at the temperature of 40°C. Finally, it was kept in drying oven at 80°C for 2 h.

SEM microscopy

First, the fiber samples were fixed to conductive double-faced adhesive tape and sputtered with gold nanoparticles, then the longitudinal surface was observed on a Sirion 200 scanning electron microscope at scanning voltage of 3 kV, temperature of 20°C, and relative humidity of 65%.

XRD patterns

XRD patterns were conducted on a model MXPAHF X-ray diffraction system at voltage of 36 kV, current of 20 mA, scan rate of 2°/min, and the 2θ sweep range from 5° to 50°.

FT-IR spectroscopy

Through the method of KBr pellets, the FT-IR spectra were acquired on a Nicolette iS50 FT-IR spectrophotometer at resolution of 4 cm⁻¹, 128 times scanned.

NMR spectrum

The samples were analyzed by AV AVANCE 400 type Superconducting Digital Fourier NMR spectrometer, under conditions of 4-mm CP/MAS broadband probe, operating frequency of 75.5 MHz, and speed of 5 kHz.

Breaking strength test

Using the YG021-A electronic single yarn strength tester to test the yarn strength, the average value of 10 yarns is taken as the test result. Test condition: initial tension 0.1 cn/dtex, working length 200 mm, and tensile speed 500 mm/min.

Capillary effect

In the temperature of 25°C, at 80% humidity, the test fluid capacity is 3000-mL distilled water (containing 5-g/L potassium permanganate), recording distilled water in 30 min along the height of the fabric rising.

Results and discussion

SEM analysis

A direct visual assessment to the surface of modified samples was conducted, which can obtain important information about surface topography, structure, and the presence of impurities or uniformity of deposited materials. ¹⁸ Figure 1 shows SEM images of bamboo pulp fibers before and after the modification. As shown in Figure 1(a), it is clear that the surface of pure bamboo pulp fibers is smooth and there are many longitudinal slender grooves. Figure 1(b) shows that the fiber’s surface becomes rough, and obvious etching trace could be seen after oxidation treatment. Figure 1(c) and (d) show that SF is adhered on the fiber surface unevenly in the initial stage of crosslinking, which makes the fiber surface rougher. When the crosslinking process is over, it can be clearly seen that a uniform SF layer is coated onto the surface of the fiber. Once the SF molecules enter the amorphous region of bamboo pulp fibers, SF molecules react with the carboxyl groups first, and then fill up the grooves. As the chemical crosslinking reaction is saturated, SF will attach on the fibers surface in a physical way, and finally, a uniform layer of SF membrane was achieved to the self-crosslinking reaction.

Figure 1.

SEM images of (a) pure bamboo pulp fibers, (b) oxidized bamboo pulp fibers, (c) incomplete SF-crosslinking oxidized bamboo pulp fibers, and (d) complete SF-crosslinking oxidized bamboo pulp fibers.

XRD diffraction analysis

In Figure 2, three curves (a, b, c) respectively represent the XRD patterns of pure bamboo pulp fibers, oxidized bamboo pulp fibers, and SF-crosslinking oxidized bamboo pulp fibers. It should be noticed that all the XRD patterns of a, b, and c have peaks located at 2θ = 12.5°, 20.2,° and 22.2°, belonging to the characteristic positions of cellulose II. ¹⁹ Curves indicate that the crystalline structure of bamboo pulp fibers is not altered by HNO₃/H₃PO₄-NaNO₂ oxidation system and SF modification process.

Figure 2.

X-ray diffraction patterns of (a) pure bamboo pulp fibers, (b) oxidized bamboo pulp fibers, and (c) SF-crosslinking oxidized bamboo pulp fibers.

FT-IR spectra analysis

Figure 3 shows FT-IR spectra of bamboo pulp fibers before and after oxidation treatment and SF modification. Compared with curve a, the spectrum curve of the pure bamboo pulp fibers, a new characteristic absorption band appears at 1737.9 cm⁻¹ on curve b, which belongs to the stretching vibration of the C=O double bond. It suggests that the lively carboxyl groups are introduced into the cellulose molecular chains after oxidation treatment by HNO₃/H₃PO₄-NaNO₂ system. But in the curve c, the stretching vibration of the C=O double bond appears at 1637.2 cm⁻¹, which can attribute to the ρ-π conjugation between carbonyl group and unshared electron pair on nitrogen atom of SF-crosslinking oxidized bamboo fiber, making frequency of the stretching vibration absorption center of the C=O decrease (called amide I). As can be seen, the N–H band of SF-crosslinking oxidized bamboo pulp fibers appears at 3437.0 cm^–1. ²⁰ Meanwhile, a new characteristic absorption peak appears at 1283.8 cm⁻¹, which is the stretching vibration band of the C–N (called amide III). Thus, the result proved that amidation reaction was occurred between carboxyl group of oxidized bamboo pulp fiber and amino of SF molecular chains.

Figure 3.

FT-IR spectra of (a) pure bamboo pulp fibers, (b) oxidized bamboo pulp fibers, and (c) SF-crosslinking oxidized bamboo pulp fibers.

NMR spectrum analysis

The solid-state CP/MAS 13C NMR spectra of raw bamboo pulp fibers, oxidized bamboo pulp fibers, and SF-crosslinking oxidized bamboo pulp fibers are illustrated in Figure 4. In the NMR spectrum of cellulose, the chemical shift can be divided into four regions: chemical shift 60–70 area belonging to C₆ on main hydroxyl; 70–81 area belonging to C₂, C₃, and C₅ on the ring carbon which is unconnected to the glucoside bone; and 81–93 area and 102–108 area respectively belonging to C₄ and C₁. ²¹ In curve b of Figure 4, there is an obvious carbon atom response peak of carboxyl group (–COOH) at chemical shift 175.34; meanwhile, the response peak intensity of the primary hydroxyl on C₆ decreases significantly. All that indicates that only the primary hydroxyl on C₆ is selective oxidized to carboxyl group in the bamboo cellulose macromolecules. The curve c shows that carbon atoms response peak of amide group (–CONH–) appears at chemical shift 171.83cm⁻¹, which demonstrates again that amidation reaction occurred between carboxyl group of oxidized bamboo pulp fibers and amino of SF molecular chains. ²² According to curves a, b, and c, C₅ characteristic peak of the raw bamboo pulp fibers, oxidized bamboo pulp fibers, and SF-crosslinking oxidized bamboo pulp fibers respectively appear at 74.35, 74.20, and 74.28cm⁻¹, suggesting modification makes chemical shift move to the low field. And it probably was caused by the shielding effect of carboxyl group upon C₅. Also, it can be seen that in curves b and c the response peak intensity of C₁ and C₄ decline significantly which is caused by hydrolysis reaction in processes of oxidation and SF-crosslinking modification.

Figure 4.

Solid-state CP/MAS 13C NMR spectra of (a) pure bamboo pulp fibers, (b) oxidized bamboo pulp fibers, and (c) SF-crosslinking oxidized bamboo pulp fibers.

Fracture strength analysis

As is shown in Figure 5, with the increase of oxidant concentration and the prolonging of oxidation time, the fracture strength of bamboo pulp fiber yarn decreases. When the oxidant concentration is not higher than 1%, when the oxidation time is less than or equal to 120 min, the breaking strength of the yarn can be kept above 75%.

Figure 5.

Fracture strength of bamboo pulp yarns in different oxidation conditions.

At the beginning of the oxidation reaction, the oxidant can quickly enter the amorphous area of the fiber, but the crystal structure of the fiber has little effect, and when the oxidant concentration is low, the oxidant can only make the amorphous part of the cellulose macromolecular chain break and the unit end hydrolysis, the fiber strength damage is small. Because of the oxidation reaction, the oxidant gradually spreads from the amorphous zone to the crystal region, and this reaction weakens the bonding of hydrogen bond and walls in the molecular chain between the bamboo pulp fiber and the molecular chain. At the same time, the end unit of cellulose macromolecular chain was increased, the fiber crystalline region was destroyed, and the fiber fracture strength was seriously damaged. The mechanical properties of yarns are determined by the friction between fiber strength and fiber, and when the mechanical properties of the fibers are damaged, the mechanical properties of the yarns become worse.

Wearability analysis

We compared the fabric woven by modified bamboo fiber with the original bamboo fiber fabric, and the wearability was improved. It can also be seen from Table 1 that the grafting treatment of SF has remarkable effect on crease resistance, abrasion resistance, and moisture absorption of bamboo fiber fabric. The crease recovery angle of SF-grafted bamboo fiber fabric has increased 54.4º than that of the original bamboo fiber fabric. The reason is that SF molecules into the human carboxyl-like bamboo fiber fabric internal micro-gap, and oxidized fibers in the amorphous region of carboxyl, hydroxyl, and other active groups to form covalent bonds, hydrogen bonds, salt bonds, and so on ²³ in the bamboo fiber surface forming cross-linked reticular structure film. In addition, the SF in the bamboo fiber film makes the fabric surface or interwoven point bond, and under the action of methanol in the bamboo fiber it forms a cross bond, thus restricting bamboo fiber fabric shrinkage and wrinkle, and improving bamboo fiber fabric crease resistance.

Table 1.

Wearability of cotton fabrics before and after SF graft.

Sample	Crease recovery angle/°	Wear loss/mg cm−2	Moisture absorption/(cm·30 min)
Original fiber fabric	113.1	1.72	13.38
Oxidized fiber fabric	167.5	1.43	16.14

At the same time, the bamboo fiber fabric after grafting SF has a great improvement in wear resistance; mainly after SF treatment, the bamboo fiber is glued to the bundle shape, and the surface of bamboo fiber fabric is coated with a tightly smooth SF film, which can be used as lubricant between the abrasive and fabric to a certain extent, creating a “boundary lubrication” state, which causes the “boundary friction” effect between the two objects. The resistance of abrasive and bamboo fiber fabric in relative motion is reduced, thus enhancing the wear resistance of bamboo fiber fabric. On the other hand, because of the amino acid in the fibroin protein, amino, carboxyl and other polar groups have strong affinity to water molecules, so the grafted SF fiber fabric moisture absorption performance increased.

Conclusion

In summary, a green modification method was applied to bamboo fibers and verified by SEM, FT-IR, XRD, and solid-state CP/MAS 13C nuclear magnetic resonance. SEM images show that oxidation treatment can etch the fiber surface and make it rough. After grafting the SF onto the fiber, there is a layer of uniform SF membrane on the fiber surface. XRD analysis indicates that the crystalline structure of bamboo pulp fibers is not changed after modification. Besides, FT-IR and NMR spectra analyses illustrate that first primary hydroxyl on C₆ of raw bamboo pulp fibers is selectively oxidized to carboxyl group, then reacts to amino groups of SF, and formed C–N covalent bond. All in all, SF is grafted onto bamboo pulp fibers successfully without changing its crystalline structure.