Antidegradation Property of Alginate Materials by Riveting Functionalized Carbon Nanotubes on the Sugar Chain.

Alginate materials with the advantages of being renewable, inexpensive, and environment-friendly have been considered promising fiber materials. However, they are prone to degrade under UV light, limiting their large-scale application in the textile field. Herein, the fracture of glycosidic bonds during the degradation process is revealed clearly by Fourier transform infrared (FT-IR) and 1H NMR. To effectively inhibit this process, functionalized multiwalled carbon nanotubes (MWCNTs) are chosen as dopants and used to interact with the sugar chain via hydrogen bonds. The results demonstrate that alginate materials with functionalized MWCNTs exhibit slower degradation rates. The intermolecular energy transfer between functionalized MWCNTs and sodium alginate (SA) is proposed for the antidegradation effect of functionalized MWCNTs, which is supported by the experiments. Moreover, SA/MWCNT fibers also show enhanced mechanical properties compared with pure alginate fibers. The appealing effect of the degradation inhibition feature makes the composite alginate materials very promising candidates for their future use in textile material development.

Introduction

Nowadays, alginate fibers are considered as one of the most promising alternative fibrous materials due to their superior properties, such as biocompatibility, intrinsic flame retardancy, radioresistance, antibacterial properties, etc.[1−8] Alginate, composed of a (1–4)-linked β-d-mannuronic acid (M) block and α-l-guluronic acid (G) blocks with a random arrangement in the backbone, is an important component of these fibrous materials.[9−17] However, alginate alginate macromolecules are easy to degrade in the presence of sunlight and microorganisms, which has a great impact on the spinning technology. Generally, degradation of natural polymers can be an undesirable or desirable process depending on their application fields. For example, natural polymer plastics and pesticides should be used as environmentally friendly products and the degradation process is permitted. While strength and flexibility properties are the prerequisite for natural polymer fibers, the degradation process is detrimental to their commercial applications and should be prevented.[18,19] Therefore, for the rapid development of the natural polymer textile industry, the degradation of alginate materials should be inhibited. Also, the degradation mechanism needs to be studied intensively.

Many strategies have been employed to inhibit the degradation of alginate materials. Metal ions have been successfully applied to cross-link alginate chains; the degradation of these ionically cross-linked alginate aqueous solutions could be inhibited effectively.[20−25] Unfortunately, the cross-linked alginate solutions lack flowability, which are difficult to process further. In comparison, the reinforcement of a secondary phase is an alternative method to avoid their rapid degradation in the ambient environment and maintain favorable processibility of the alginate solutions.[26−29] Among these materials, multiwalled carbon nanotubes (MWCNTs) show good biocompatibility, excellent electrical conductivity, and advanced mechanical properties. They are widely used in the fields of electronics and textiles.[30−38] Studies also suggest that MWCNTs can adsorb bacteria and exhibit strong antibacterial properties.[39] Generally, to make the original MWCNTs soluble in aqueous solutions, water-soluble groups are needed to be introduced on the MWCNTs. In addition, the substituted groups would also contribute to form hydrogen bonds between alginate and MWCNTs via the O–H···O interaction.[40] Thus, the MWCNTs can effectively prevent the degradation of the alginate polymer framework. However, the details about the antidegradation and the corresponding mechanism are not involved.

In this article, alginate materials show inhibited degradation rates by encapsulating COOH- or NH2-functionalized MWCNTs. Compared to pure alginate materials, SA/MWCNTs show slower degradation rates with their degradation rate constants of 0.57 × 10–4 and 2.44 × 10–4 h–1 for SA/COOH-MWCNTs and SA/NH2-MWCNTs, respectively (Figure ). Moreover, these SA/MWCNT fibers endow enhanced tensile modulus and strength. All these results indicate that MWCNTs significantly inhibit the degradation of alginate materials. Fourier transform infrared (FT-IR) (Figure ) and 1H NMR spectra data indicate that only the sensitive glycosidic bond in the backbone of alginate is broken and other chemical structures in the main chain are reserved. Based on the fluorescence spectra, the functionalized MWCNTs are used as energy acceptors, which can take the high energy away from the UV-excited SA macromolecule and stabilize the SA chemical structure (Figure ). It is well concluded that the entire path of the degradation decreases by controlling this step of degradation. The antidegradation results are mainly ascribed to the bound hydrogen bonds and the resulting intermolecular energy transfer between alginate and MWCNTs.

Figure 1

(a) Degradation ratio of SA and its composite solutions under sunlight. (b) Degradation rate constant of SA and its composite solutions under 365 nm UV irradiation.

Figure 2

FT-IR spectra of pure SA as well as SA/NH2-MWCNTs and SA/COOH-MWCNTs composites before and after degradation.

Figure 3

(a) UV–vis absorption and (b) fluorescence spectra of pure SA, COOH-MWCNTs, and the composite in aqueous solution. (c) Schematic diagram of the SA/MWCNTs composite and the energy transfer process in the SA/MWCNTs composite.

Results and Discussion

The cleavage of glycosidic bonds in the backbone has a great impact on the molecular weight of sodium alginate molecules. Therefore, gel permeation chromatography (GPC) and a Ubbelohde capillary viscometer are used, and the measured number average molecular weight (Mn), polydispersity index (PDI), and intrinsic viscosity ([η]) are listed in Table . After 24 days of illumination, the degraded SA/MWCNTs exhibit higher Mn (154 and 121 kDa) and [η] (6.9 dL/g and 6.0 dL/g) as well as narrower distribution (PDI = 1.62 and 1.72) than the degraded SA with lower Mn (65 kDa) and [η] (4.1 dL/g) and broader distribution (PDI = 1.86).

Table 1

[η], MW, Mn, and PDI of Alginate and Its Composite Solutions before and after Degradation

	[η] (dL/g)	MW (kDa)	Mn (kDa)	PDI
pure SA	8.2	325	224	1.45
degraded SAa	4.1	109	65	1.86
degraded SA/COOH-MWCNTsa	6.9	249	154	1.62
degraded SA/NH2-MWCNTsa	6.0	208	121	1.72

Degraded for 24 days.

The degradation behaviors under sunlight are further evaluated and the results are shown in Figure a. The degradation rate of the pure SA solution is much larger than that of SA/MWCNTs, and the SA/COOH-MWCNTs composite shows the slowest degradation process. Moreover, their degradation kinetics is studied in the accelerated degradation process with 365 nm UV radiation. Figure b is plotted according to the equation 1/Mt – 1/M0 = kt/m. In this equation, Mt and M0 are the viscosity average molecular weights of alginate macromolecules at different times, k is the rate constant, and m is the molecular weight of the alginate monomer.[41] All samples show linear correlations, indicating the first-order degradation kinetic reaction. The rate constant and half-life are calculated and listed in Table . Compared to pure SA, the composites exhibit smaller rate constants and longer half-lives, which also indicates that functionalized MWCNTs could effectively inhibit the degradation of SA under UV light.

Table 2

Rate Constant (k) and Half-Life (t1/2) of SA, SA/NH2-MWCNTs, and SA/COOH-MWCNTs

	rate constant (×10–6)	t1/2 (day)
SA	437	70
SA/NH2-MWCNTs	244	116
SA/COOH-MWCNTs	57.4	519

FT-IR spectra are employed to confirm the variation of SA before and after degradation. As shown in Figure , the strong adsorption of 3390 cm–1 is attributed to the stretching of hydroxyl groups. Compared with undegraded materials, the peaks of degraded samples at 3390 cm–1 become stronger and broader. The result indicates that the degradation happens at the position of glycosidic bonds and numerous additional hydroxyl groups are formed. In addition, the spectra of degraded samples exhibit most of the characteristic adsorption peaks of the undegraded counterparts, but some tiny differences can be observed. The −C–OH stretching vibrations of degraded SA/NH2-MWCNTs and SA/COOH-MWCNTs composites at 1061 cm–1 are shifted to a low energy region of ca. 20 cm–1 compared to that of pure SA, which is due to the interactions of −OH groups of SA and −C=O/–NH2 groups of MWCNTs.

According to Petro Lutsyk’s report,[42] MWCNTs can serve as energy acceptors. As shown in the UV–vis absorption spectra (Figure a), SA has no obvious absorption peak (Figure S2), while the SA/COOH-MWCNTs composite exhibits slightly higher intensities at 250 and 300 nm than those of COOH-MWCNTs, which is attributed to the contribution of the weak absorption of SA. To verify the intermolecular energy transfer process of SA/MWCNTs composites, their fluorescence spectra under the same concentration are measured with SA and MWCNTs as control (Figure b). The emission curves at 350 and 500 nm are chosen for comparison, as the former is mainly attributed to SA and the latter mainly belongs to MWCNTs. Compared to SA and COOH-MWCNTs, the emission of the SA component (350 nm) has decreased in the mixture system, while that of the MWCNTs component (500 nm) is enhanced. The phenomenon supports the energy transfer process from SA to MWCNTs in the SA/COOH-MWCNTs composite. Similar results can also be found in the SA/NH2-MWCNTs system (Figure S3). Thus, the ultraviolet energy absorbed by alginate macromolecules can be transferred to MWCNTs, which is supposed to suppress the cleavage of glycosidic bonds in SA chains and beneficial to stabilize the molecular structure of SA under ultraviolet light (Figure c).

The polymer microstructures with and without MWCNTs are also determined by 1H NMR spectra. The additional peaks of degraded SA with the chemical shifts in a low-field regime are dependent on the chemical identity of neighboring residues. Thus, multiple peaks for the protons in monosaccharide, disaccharide (MM, MG, GM, and GG), and trisaccharide (GGG, GGM, MGG, and MGM) segments are observed.[43] As shown in Figure b, the proton peaks at 5.2–6.0 ppm for pure SA present the produced oligomers after irradiation (Figure a). The anomeric protons at δ = 5.04 ppm come from guluronate residues. The overlapping peaks ranging from 4.76 to 4.64 are GGM, MGM, MG, and MM signals. Combining the FT-IR and 1H NMR results, it can be concluded that the degradation happens at the position of the glycosidic bonds and results in additional hydroxyl groups.

Figure 4

(a) Cleavage of the glycosidic bonds and the resulting oligomers under UV irradiation. (b) 1H NMR spectra of degraded SA (h), degraded SA/COOH-MWCNTs (i), degraded SA/NH2-MWCNTs (j), and pure SA (k).

Figure shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the dispersibility of functional MWCNTs in different states. As observed in Figure a,d, both NH2-MWCNTs and COOH-MWCNTs are uniformly dispersed in aqueous solutions through ultrasonication. The diameter of the MWCNTs is between 10 and 25 nm (Figure b,e). Moreover, MWCNTs can be observed in the longitudinal section of alginate composite fibers, resulting in a much higher degree of tube arrangement. The ordered orientation of MWCNTs in the fibers is very desirable for its application in textiles. Figure g–i shows the SEM images of pure calcium alginate fibers (CA) and CA/MWCNTs composite fibers. The surface morphology of pure CA fiber displays a smooth structure (Figure g), while those fibers with MWCNTs reveal a rather rough morphology with salient banded structures (Figure h,i). The results show that MWCNTs affect the morphology of these fibers.

Figure 5

TEM images of COOH-MWCNTs (a–c) and NH2-MWCNTs (d–f) in aqueous solutions (a, d), alginate solutions (b, e), and alginate fibers (c, f). SEM images of pristine CA (g), CA/COOH-MWCNTs (h), and CA/NH2-MWCNTs (i).

As the inhibited degradation of SA/MWCNTs is promising in alginate fibers, the corresponding fibers are prepared from their solutions with and without one month’s degradation. Representative stress–elongation curves of the fibers are presented in Figure . The pristine CA and its composite fibers exhibit higher breaking strength, elongation, and tensile modulus than that of the degraded counterparts. Moreover, the fibers loaded with nanotubes exhibit enhanced tensile strength. Therefore, the CA/MWCNT fibers exhibit higher mechanical properties than pristine CA fibers, which is consistent with the ordered orientation of MWCNTs in TEM images. However, the degraded CA/MWCNT fibers exhibit much higher mechanical properties than the degraded CA fibers, demonstrating the excellent antidegradation effect of MWCNTs. In addition, compared to pristine CA fibers, CA/COOH-MWCNTs and CA/NH2-MWCNTs show higher limit oxygen index (LOI) values of 41 and 40%, respectively (Figure S4). It is noteworthy that the functionalized MWCNTs have improved the flame retardant effect of alginate materials. These results indicate that MWCNTs are effective additives to inhibit the degradation of alginate materials. Moreover, they endow the materials with potential excellent properties.

Figure 6

Stress–elongation graphs of CA as well as CA/NH2-MWCNTs and CA/COOH-MWCNTs composites before and after degradation.

Conclusions

In summary, we have studied the degradation inhibition measure of alginate materials. The results show that the backbone cleavages of the materials happen on glycosidic bonds and the resulting oligomers promote their degradation further. As an efficient degradation inhibition phase, MWCNTs are added into the system. The excellent properties of MWCNTs endow the alginate materials with a mitigated degradation process and improved tensile strength. We have also studied the potential degradation inhibition mechanism. It shows that MWCNTs can form hydrogen bonds with alginate macromolecules, and the intermolecular energy transfer can suppress the cleavage of glycosidic bonds. These results suggest that MWCNTs are promising additives to endow the alginate materials with inhibited degradation and superior properties.

Experimental Section

Materials

Multiwalled carbon nanotubes (MWCNTs) that include amine-modified (NH2−) and carboxyl-modified (COOH−) multiwalled carbon nanotubes were provided by Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Science (Chengdu, China). Calcium chloride anhydrous (CaCl2) was purchased from Sinopharm Chemical Reagent (Shanghai, China). Sodium alginate (Mw = 296 kDa, PDI = 1.24) was purchased from the Hyzlin Biology Development Co., Ltd. (Qingdao, China). All of them were used without further purification.

Preparation of Materials

Sodium alginate (SA) with 1% NH2-MWCNTs or COOH-MWCNTs were dissolved into deionized water and their aqueous solutions were formed, separately. After vigorously stirred for 3 h, 5 wt % pure and composite sodium alginate solutions were prepared to form aqueous solutions of SA, SA/COOH-MWCNTs, and SA/NH2-MWCNTs. The solutions were extruded from a spinneret into a coagulating bath with a 5% calcium chloride aqueous solution, forming calcium alginate (CA), CA/COOH-MWCNT, and CA/NH2-MWCNT fibers.

Characterization

Nuclear magnetic resonance (1H NMR) spectra were measured with a Bruker AV-600 in D2O at 60 °C. The chemical shift is reported in δ ppm using D2O (4.79 ppm). UV–vis absorption spectra were recorded with a Shimadzu UV-3600 spectrometer. Fluorescence spectra were recorded with a Hitachi F-4500 spectrometer in spectral grade solvents. Infrared spectra were obtained using a Thermo Nicolet 5700 Fourier transform infrared spectrometer (FT-IR) with a resolution of 8 cm–1. The limit oxygen index test was carried out using an oxygen indexer (LOI HC-2). Thermal analyses were performed using differential scanning calorimetry (DSC 822e) under a nitrogen flow at a heating rate of 10 °C min–1. The scanning electron microscopy (JEOL Co., Japan) images were obtained with a JSM-840. The transmission electron microscopy (TEM) images were obtained with a JEM-1200 (JEOL Co., Japan) operated at an accelerating voltage of 100 kV. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) on a Viscotek TDA 302-type at a temperature of 35 °C. The viscosity average molecular weight (Mη) was calculated from intrinsic viscosities [η] measured by a Ubbelohde capillary viscometer. The apparent viscosities were determined by a Brookfield DV–II + programmable viscometer (Brookfield Engineering Labs, Inc.).

The degradation ratio of the alginate solution was calculated employing the equation of the degradation ratio (%) = [(X – Y)/X] × 100, where X is the initial apparent viscosity of the alginate solution and Y is the detected viscosity. Then, Mη was determined from the intrinsic viscosity of a 0.1 mol/L NaCl solution and 0.05% EDTA-2Na at 20 ± 0.05 °C. Under these conditions, the Mark–Houwink equation for sodium alginate is: [η] = K × Mηα (K = 2.0 × 10–5, α = 1).

Tensile Measurements

The CA, CA/COOH-MWCNT, and CA/NH2-MWCNT fibers were tested using an Instron Tensile Testing Instrument. The breaking strength, elongation, and tensile modulus were calculated from the relative data to evaluate the effect of degradation on their mechanical properties.