Nano titanium dioxide/PAoQ-coated polybenzoxazol fibers for enhancing anti-ultraviolet performance

Abstract

Inspired by the composition of adhesive in mussels, polydopamine has been widely used for surface modification of various materials. In accord with the formation mechanism of polydopamine coating, the catechol containing two o-phenol groups and triethylenetetramine (TETA) containing two primary amine groups were used to copolymerize and deposit a polyamine-o-benzoquinone polymers (PAoQ) film on the polybenzoxazol (PBO) fibers. In order to enhance the anti-ultraviolet performance of PBO fibers, rutile nano titanium dioxide particles (TiO₂) were also decorated on the PBO fibers by the layer-by-layer self-assembling technique. The optimum modification conditions were obtained by orthogonal method. Morphological structure and chemical composition of the modified fibers were studied using scanning electron microscopy with energy dispersive X-ray, and Fourier transform infrared spectroscopy. The UV-aging test results showed that under 144 h UV-light exposure at 340 nm, the modified PBO fibers’ strength retention was promoted to 80.8%, 34.4% higher than that of the original PBO fiber. The thermal stability of the modified fibers had no obvious change after modification with TiO₂/PAoQ, while their carbon residue rate increased slightly.

Keywords

polybenzoxazol fiber ultraviolet resistance modification catechol polyamine layer-by-layer self-assembly technique

Polybenzoxazol fiber (PBO fiber) is a kind of high-performance special fiber which has high strength, high modulus, high temperature resistance and flame-retardant properties.^1–4 These excellent properties make PBO fiber widely applicable in the military, aviatic and astronautic industries, and sports apparatus, etc. However, PBO fibers have poor anti-UV stability and show low performance in hot and humid environments. Data from the Toyobo Corporation revealed that the mechanical properties of Zylon fiber (PBO fiber) remarkably decrease with exposure to sunlight or a Xenon Weather-Ometer.⁵ In order to avoid the application of PBO fiber being limited, these weaknesses should be overcome.

Studying on enhancing the resistance of PBO fiber to UV degradation,⁶⁷ thermal degradation,⁸⁹ temperature and humidity aging10 has become a hotspot in recent years. Some researchers have also undertaken a lot of work to find effective methods to improve the light stability of PBO fibers. The use of exfoliated graphite, carbon black, and glassy titanium dioxide (TiO₂) as UV-vis blockers was not successful.¹¹ However, TiO₂ nanoparticles has outstanding performance and wide applications, such as photocatalysts, photosensitive reaction,¹² and the TiO₂ was considered an effective UV-attenuating agent.¹³ TiO₂ nanoparticles were deposited on the aramid fiber, indicating that the UV-resistance properties of the aramid fiber had demonstrably improved.¹⁴

Inspired by the composition of adhesive in mussels, which can endure repeated buffeting from strong waves by attaching themselves tightly to rocks making them attach themselves to rocks tightly, dopamine auto-polymerization could be used to synthesize surface adhesion polydopamine films on a wide range of materials¹⁵ such as noble nanoparticle metals, ¹⁶ polymers,¹⁷ semiconductors, and ceramics.¹⁸ Researchers have conducted similarity methods for polymer materials. The polyimide films were covered with polydopamine, and then the polydopamine coating was used for the electrolessly deposited silver as an adhesion promotion layer.^19, ²⁰ Polydopamine coating and titanium dioxide films were successfully deposited onto the polymer bases of polytetrafluoroethylene (PTFE), polyethylene (PE), and polyethylene-terephthalate (PET), successively, improving the vitro cytocompatibility.²¹ However, a dopamine monomer is easily oxidized and auto-polymerized in the presence of moisture and air, moreover its raw material is very expensive. Recently, many researchers have studied catechol (CAT)/polyamine, which can be identified as polymers with amine and CAT groups that may possess a similar adhesive ability, and the polymers could be used to functionalize the hydrophobic polypropylene (PP) separators, protecting the pore structure and mechanical property of the separator and improving the uptake of electrolytes.²²

Dopamine is easily oxidized for forming semiquinones and quinones with high reactivity. Meanwhile, dopamine self-polymerizes with its own amine by the Michael addition reaction and the Schiff base reaction.¹⁵ CAT and triethylenetetramine (TETA) contain the characteristic groups of -OH and -NH₂, respectively, which can also react with each other by the Michael addition reaction and the Schiff base reaction, according to the reported research.¹⁷ Therefore, in our work, CAT and TETA were used as monomers to form polyamine-o-benzoquinone polymers (PAoQ). The possible polymerization mechanism of CAT and TETA is schematically presented in Scheme 1. In order to obtain a stable anti-ultraviolet coating, rutile TiO₂ nanoparticles were assembled with PAoQ and decorated on the surface of the PBO fibers by using the layer-by-layer (LBL) self-assembling technique, which is a new way of modifying PBO fibers. This method is versatile because of its simple ingredients, mild reaction conditions and applicability to many types of materials with complex shapes. The optimum modification conditions were obtained by orthogonal method. The results showed that under UV-light exposure at 340 nm, the modified PBO fibers’ strength retention was higher than that of the initial PBO fiber. There was no obvious change in the thermal stability of the modified fibers after modification with TiO₂/PAoQ, although the carbon residue rate had increased slightly.

Material and methods

Material

The PBO fiber was supplied by Toyobo Co. Ltd., Japan. The CAT (AR, purity 98.5%), TETA (AR, purity 98%), tris(hydroxymethyl)metyl aminomethane (Tris), hydrochloric acid (HCl, AR, 36.0%–38.0%) and ammonia water (AR, content of NH₃: 25%–28%) were purchased from Chengdu Kelong Chemical Reagent Factory in China. Acetone (AR, content CH₃COCH₃ > 99.5%) was purchased from Chengdu Changlian Chemical Reagent Co. Ltd, China. Nano rutile titanium dioxide (TiO₂, particle size 20–30 nm, purity 98%) was purchased from Shanghai Jianghu Chemical Products Co. Ltd., China.

Preparation of anti-ultraviolet PBO fibers

Modification of PBO fibers

The schematic illustration of the modification process of the PBO fiber is shown in Scheme 1 . The PBO fiber was firstly ultrasonicated for 15 mins in acetone to remove any organic contamination. The cleaned PBO fibers were immersed in a Tris-HCl buffer solution (pH = 8.5, 10 mM) containing the monomers of PAoQ (the molar ratio of CAT and TETA was 2:1) for 10 min, and then were taken out and placed in air for 1 min to ensure the formation of PAoQ. After that the fibers were rinsed with distilled water and then immersed in a solution with a pH value of 3, where the positive charges were taken onto the fiber surface.²³ Subsequently, these fibers were dipped into the negatively charged TiO₂ suspension, which was prepared by dissolving aqueous ammonia in the 250 ml TiO₂ aqueous suspension with ultrasonic dispersion, with a pH of 7∼8. Then the positive charges on the modified fiber surface formed static adsorption with the negative charges taken by TiO₂ and formed the first self-assembling bilayer. These PBO fibers were washed with distilled water for 1 min to remove excess TiO₂ particles. By repeating the above procedures, the modified fibers with the desired number of multilayers were prepared via the LBL self-assembling technique. Each washing process was followed with drying by a stream of N₂. After assembling per 5 bilayers of film, the TiO₂ aqueous suspension and PAoQ solution were reprepared. The liquor-to-fiber-ratio was 20 ml PAoQ solution per 0.05 g PBO fibers and 20 ml TiO₂ aqueous suspension per 0.05 g PBO fibers. Afterward, the modified PBO fibers (TiO₂/ PAoQ @PBO fibers) were obtained by this LBL process. For comparative analysis, only PAoQ-coated PBO fibers (PAoQ @PBO fiber) were prepared in a manner identical to that of the model system without TiO₂.

Orthogonal experimental design

Orthogonal experimental design plays an important role in dealing with multiple factors. This method can guarantee we select the reasonable and representative levels of the factors. In this work, this analytical methodology was used to determine the optimum conditions of the LBL process under different parameters such as the concentration of the PAoQ monomers solution, the concentration of the TiO₂ aqueous suspension, and the self-assembling layers. The different parameters such as the concentration of CAT (20 mM, 40 mM, 60 mM), concentration of TiO₂ (1 g/L, 2 g/L, 3 g/L), and number of self-assembling layers (5 layers, 10 layers, 15 layers) were chosen as the critical variables and designated as A, B, and C, respectively. Nine LBL surface modification systems were tested by L₉(3⁴) orthogonal test design (Table 1). After the preparation of modified PBO fibers, according to the retention of tensile strength under 144 h exposure in UV light, the best modification conditions were selected. PBO fibers under the optimal conditions were used for the performance test and characterization.

Table 1.

Orthogonal designing of factors and levels

Factors	Concentration of CAT (A) mM	Concentration of TiO₂ (B) g/L	Number of self-assembling layers (C)
1	20	1	5
2	40	2	10
3	60	3	15

UV-accelerated aging

UV-accelerated aging of PBO fibers was carried out in a self-made UV-aging test chamber equipped with four UV 340 nm lamps as the radiation source. The fibers were held at 25℃ and exposed to UV light with a density of about 50 W/m² for 24 h, 48 h, 72 h, 96 h, and 144 h.

Characterization

Tensile test

The mechanical properties of a single original PBO fiber, PAoQ @PBO fiber and TiO₂/ PAoQ @PBO fiber were measured by single fiber tension tester LLY-06B (Laizhou Electronic Instrument Co., Ltd., China). The tension values of all these fibers were tested before and after exposure to UV light for 24 h/48 h/72 h/96 h/120 h/144 h, with a speed of 5 mm/min and a gauge length of 20 mm. Ten fibers of each kind of sample were tested to obtain the average tensile value. Testing accuracy was 1%. The standard deviations of all tensile measurements were below 10%.

Thermogravimetric (TG) measurements

The TG technique was used to analyze the thermal stability of the fiber. The measurements were carried out with a DTG-60 analyzer (Shimaduz Co., Japan). An approximately 5 mg sample was heated from ambient temperature to 900℃ at a rate of 10℃/min under a nitrogen atmosphere.

Fourier transform infrared spectroscopy (FTIR)

FTIR spectrum was used to characterize the chemical groups of PBO fibers before and after modification by PAoQ and TiO₂/PAoQ and pure PAoQ, and pure TiO₂. The measurements of fibers were conducted at room temperature in transmission mode by a Tracer-100 spectrometer (Shimaduz Co., Japan). Spectra were obtained in the region of 4000–650 cm⁻¹.

Scanning electron microscopy with energy dispersive X-ray (SEM-EDX)

The original PBO fiber, PAoQ @PBO fiber and TiO₂/PAoQ @PBO fiber were investigated to analyze the surface morphology of the fibers with a scanning electron microscope using a JSM-5900LV analyzer (JEOL, Japan) at an accelerating voltage of 10–20 KV and a working distance of 12 mm.

Results and discussion

Optimization of surface modification conditions

A suitable condition selection is vital for improving the anti-UV performance of PBO fibers. In this work, orthogonal analytical methodology^24, ²⁵ was used to determine the optimum conditions of the LBL process. The orthogonal design table L₉(3⁴) was determined according to previous studies.²⁶ As shown in Table 2, the main factors and levels are PAoQ monomer concentration, TiO₂ concentration, and number of self-assembling layers. The self-polymerization process of PAoQ was affected by the concentration of monomers. The TiO₂ content of the modified PBO fibers was influenced by the concentration of TiO₂ and the number of self-assembling layers. The retention of tensile strength of the PBO fibers was obtained on the basis of the experimental design presented in Table 3.

Table 2.

UV-accelerated aging test of L₉(3⁴) orthogonal design and analysis (exposure 144 h)

Test numbers	Factors						Retention of tensile strength (%)
Test numbers	PAoQ concentration (mM)		TiO₂ concentration (g/L)		Self-assembling layer		Retention of tensile strength (%)
1	A1	20	B1	1	C1	5	78
2	A1	20	B2	2	C2	10	72
3	A1	20	B3	3	C3	15	74
4	A2	40	B1	1	C2	10	75
5	A2	40	B2	2	C3	15	75
6	A2	40	B3	3	C1	5	77
7	A3	60	B1	1	C3	15	73
8	A3	60	B2	2	C1	5	78
9	A3	60	B3	3	C2	10	79

Table 3.

Analysis of L₉(3⁴) orthogonal test results

	PAoQ concentration (mM)	TiO₂ concentration (g/L)	Self-assembling layer
	A	B	C
K1^a	2.24	2.26	2.33
K2	2.27	2.25	2.26
K3	2.3	2.3	2.22
k1^b	0.747	0.753	0.777
k2	0.757	0.750	0.753
k3	0.767	0.767	0.740
R^c	0.02	0.017	0.037
Optimal level	A3	B3	C1

^aK1, K2, and K3 are the sums of the test values with each factor on different levels. ^bThe values of k1, k2, and k3 are the average numbers of corresponding test values. ^cThe R values are the maximum ranges of the average numbers for each factor.

The test indicators are shown in Table 3. The average test values for each factor on a different level performance index were respectively calculated through the columns of A, B and C. We found that the self-assembling layer showed great influence on the content of TiO₂ on the surface of the PBO fibers. When the layers were increased from 5 to 15, the retention of tensile strength was 78%, 72% and 74%, respectively. Thus, the suitable layer was 5 due to the highest retention of tensile strength. The TiO₂ concentration had the main influence on the anti-UV performance of the PBO fiber. The retention of tensile strength of PBO fibers was 73%, 78% and 79% when the TiO₂ concentrations were increased from 1 g/L to 2 g/L to 3 g/L. Though the monomer concentrations of PAoQ had little influence on the retention of tensile strength, k3 of factor A was still higher than k1 and k2. So, 60 mM was chosen as the suitable CAT concentration. From the analysis of the above results, we knew that C was the main influencing factor, and the influence order of various factors was C > A > B. In order to attain highest anti-UV properties, the optimum condition was A3B3C1.

Mechanical properties of modified PBO fibers

According to the optimized conditions, the modified PBO fibers, TiO₂/PAoQ @PBO fibers, were obtained. For comparative analysis, only PAoQ-coated PBO fibers (PAoQ @PBO fibers) were also prepared using the same procedure without adding TiO₂. The tensile test results of the original PBO fibers, PAoQ @PBO fibers and TiO₂/PAoQ @PBO fibers are shown in Figure 1 after having been irradiated in UV light for different lengths of time. The tensile strength retention and tensile strength decreased with increasing exposure time. After 72 h of UV irradiation, the tensile strength retention (Figure 1a) and tensile strength (Figure 1b) of all the fibers decreased dramatically. For the original PBO fibers, the tensile strength retention (Figure 1a) decreased to 63.3%, while those of PAoQ @PBO and TiO₂/PAoQ @PBO fibers were 75.6% and 82.4%, respectively. This indicates that PAoQ modification can significantly improve anti-UV performance. What is more, after 144 h UV irradiation, the tensile strength retention values of the PBO fiber, PAoQ @PBO fiber and TiO₂/PAoQ @PBO fiber were 60.1%, 66.8% and 80.8%, respectively. The tensile strength retention values of PAoQ @PBO fibers and TiO₂/PAoQ @PBO fibers were larger, 11.0% and 34.4%, than that of the original PBO fibers. This result was consistent with the research of Jin et al.²⁷ The results shown in Figure 1b demonstrate that both the coatings of PAoQ and TiO₂/PAoQ can effectively improve the anti-UV properties of PBO fiber. This may be due to the following two reasons. Firstly, PAoQ, which is dark brown, can absorb UV light. Before UV radiation, the color of the original PBO fibers is brownish-yellow, and the color of modified PBO fibers is light brown. After UV radiation, the color of the original PBO fibers darkens, but the color variation of the modified PBO fibers is not obvious. Secondly, TiO₂ plays a critical role in improving the anti-UV properties of PBO fibers, which was confirmed in many researches about improving the ultraviolet-aging-resistance performance of other polymer fibers.²⁸

Figure 1.

Tensile strength retention (a) and strength (b) of PBO, PAoQ @PBO and TiO₂/ PAoQ @PBO fibers exposed to UV light.

In addition, the initial tensile strengths of PAoQ @PBO fibers and TiO₂/PAoQ @PBO fibers before UV exposure were 4.4% and 4.5% larger than that of the original PBO fibers, respectively. Similar to how polydopamine coated on glass fibers can improve the mechanical properties of the resulting composites,²⁹ the PAoQ coating, which was considered to form a cross-linked uniform composite layer on PBO fibers, can also improve the mechanical properties of modified PBO fibers.

Surface morphologies of modified PBO fibers

Figure 2 shows the surface morphologies of the original PBO fibers, PAoQ @PBO fibers and TiO₂/PAoQ @PBO fibers. It is clear that the surface of the original PBO fibers before modification was smooth, but had some tiny grooves (Figure 2b). However, there were some wrinkles on the surface of PAoQ @PBO fibers in Figure 2d. Combing the results of the EDX in Figure 3, the weight percentage of the nitrogen (N) element in PAoQ@ PBO fibers increases by 5.92. The results showed that PAoQ was successfully coated on the surface of the PBO fibers. As is shown in Figure 2f, after LBL self-assembling modification, the surface of TiO₂/PAoQ @PBO fibers became rough. There were many particulate matters on the surface of the modified fibers, which demonstrates that TiO₂ particles were successfully attached to the surface of the PBO fibers by PAoQ. Parts of the nano TiO₂ particles were embedded in PAoQ, while the others were adsorbed on the surface. In addition, some bigger particles formed by the agglomeration of nano TiO₂ particles were also attached to the fiber surface. The obvious morphological change and the remarkably enhanced tensile strength retention indicate that the TiO₂/PAoQ composite coating can effectively prevent the PBO fibers from being damaged by UV light.

Figure 2.

Scanning electron microscopy (SEM) image of the original PBO fibers (a, b), PAoQ @PBO fibers (c, d) and TiO₂/ PAoQ @PBO fibers (e, f).

Figure 3.

Energy dispersive X-ray (EDX) of the N element of the original PBO fibers, PAoQ @PBO fibers and TiO₂/ PAoQ @PBO fibers (a, b, c).

Chemical structure changes

The Attenuated total reflection fourier transform infrared (ATR-FTIR) spectra of PBO, PAoQ @PBO and TiO₂/PAoQ @PBO fibers are shown in Figure 4. From the chemical structure of the PBO fibers used, the peaks at 1625 cm⁻¹ and 1390 cm⁻¹ correspond to the stretching vibrations of C = N and C_Ar-N bonds present in the benzoxazole rings structure.³⁰ The peaks centered at 3500–3300 cm⁻¹ are attributed to the combination of stretching vibrations N-H and O-H. The peak of 3470 cm⁻¹ disappeared in the spectra of modified PBO fibers, which may be due to the influence of -NH₂ in PAoQ. The spectra of modified PBO fibers presented a few changes, including the peaks at 3235 cm⁻¹, 3125 cm⁻¹ and 1070 cm⁻¹, which corresponded to the primary amide introduced by TETA. The characteristic peaks of pure PAoQ at 3235 cm⁻¹ and 1095 cm⁻¹ are shown in Figure 4. These indicate that PAoQ was stably deposited on the surface of the PBO fibers. Meanwhile, the N-H deformation vibrations band (950 cm⁻¹) of PAoQ can be clearly observed in the spectra of modified PBO fibers. This suggests that the Michael addition reaction might have occurred during the polymerizing process. In addition, the O-H and C-O vibration bands of phenolic hydroxyl groups presented at 1380 cm⁻¹ and 1150 cm⁻¹. This was verified by Chen et al.³¹ As is shown in Figure 4a, the main broad peak at 700–650 cm⁻¹ is attributed to Ti-O stretching and Ti-O-Ti bridging stretching modes.³² But the Ti-O peak disappeared on the TiO₂/PAoQ @PBO fibers, which might be covered by the peak of the C-H bending vibration.³⁰ However, the SEM results proved that TiO₂ was successfully attached to the PBO fibers.

Figure 4.

Attenuated total reflection fourier transform infrared (ATR-FTIR) spectra of PBO, PAoQ @PBO and TiO₂/ PAoQ @PBO fibers, pure PAoQ and pure TiO₂.

Thermal stability

The TG curves of PBO, PAoQ @PBO and TiO₂/PAoQ @PBO fibers are displayed in Figure 5. According to the curves, all fibers demonstrate excellent thermal stability, and the starting degradation temperatures of all fibers were higher than 650℃, where the weight loss rate of all fibers was above 5%. Similar results were observed by Zhu et al.³³ However, there was a small distinction in the starting degradation temperature between the original PBO fibers and TiO₂/PAoQ @PBO. The starting degradation temperature of the TiO₂/PAoQ @PBO fiber was 706℃, which was 11℃ higher than that of the original PBO fibers. This phenomenon indicates that TiO₂ can slightly improve the thermal stability of PBO fibers. This is because rutile TiO₂ has a high heat resistance temperature, which was confirmed by Chen and Mao.³⁴ The residue weight rates of the PBO fibers, PAoQ @PBO fibers and TiO₂/PAoQ @PBO fibers at 800℃ were about 60.8%, 62.2% and 63.9 %, respectively. These results illustrate that the method of coating PAoQ and TiO₂ on the surface of PBO fibers was an effective way to obtain modified PBO fibers with good thermal stability.

Figure 5.

Thermogravimetric (TG) curves of PBO fibers before and after modification, and inset for a clear observation around the plateaus.

Conclusion

In our study, the anti-UV agent TiO₂ was attached to the surface of PBO fibers by the LBL self-assembling technique using PAoQ as an adhesive coating to obtain UV-resistant PBO fibers, and an orthogonal method was introduced to obtain the optimal condition. The results showed that the tensile strength retention of TiO₂/PAoQ @PBO fibers went up to 80.8% after exposure to UV light at 144 h, which had increased by 34.4% compared to that of the original PBO fibers. The apparent morphologies and chemical structures of the modified PBO fibers showed that PAoQ and TiO₂ were successfully attached to the surface of the PBO fibers, both of which played important roles in preventing the PBO fibers from being damaged by UV light. In addition, the TiO₂/PAoQ @PBO fibers showed better thermal stability than that of the original PBO fibers. In conclusion, nano titanium dioxide/ PAoQ-coated PBO fibers had excellent UV-resistance and thermal stability.

Scheme 1.

Schematic illustration of the possible polymerization mechanism of catechol (CAT) and triethylenetetramine (TETA) in a weak alkaline Tris(hydroxymethyl)aminomethane-Hydrochloric acid (Tris-HCl) solution.

Scheme 2.

Schematic showing the preparation process of modified PBO fiber.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Open Project Program of High-Tech Organic Fibers Key Laboratory of Sichuan Province (Grant Number PLN2015-14) and the Sichuan Province Undergraduate Training Programs for Innovation and Entrepreneurship (Grant Number 201510610269).

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