Development and antiultraviolet properties of epoxidized styrene–butadiene–styrene nanofibers loaded with nanometer titania dioxide

Materials

All of the anatase and rutile nano-TiO₂ and SBS were commercially available and used in form of powder. THF and DMF were purchased from National Chemical Co., Ltd, China. Formic acid (99%) and hydrogen peroxide (30%) were reagent grade for the epoxidation of SBS. All other reagents and solvents such as toluene, ethanol, hydrochloric acid, and acetone were commercially available and were of analytical grade.

Preparation of ESBS nanofibers

As shown in Figure 1, the 100 g/l SBS solution was prepared by dissolving the SBS powder into toluene by stirring. Then the temperature was raised to 70℃ and the formic acid (V_{formic acid}/V_SBS = 1/40) was added. The H₂O₂ (V_{hydrogen peroxide}/V_SBS = 3/40) was added into the mixture by pouring H₂O₂ aqueous solution slowly. The mixture was stirred at 70℃ for 2 h, and the production was agglomerated by ethanol after the solution got cold. Finally, the prepared product was washed to neutral (pH = 7.0 ± 0.5) by distilled water and dried at 60℃ in vacuum. The reaction equation is shown in Figure 1(b), and the formic acid was reacted with hydrogen peroxide under 70℃ as the step 1. Then the SBS was epoxidized by the peroxy acid which is prepared by step 1 at the same temperature as the step 2. OPEN IN VIEWER

The two different crystalline structures of nano-TiO₂ were dried at 70℃ and dispersed by ultrasonication with the same solvent (V_THF/V_DMF = 3/1) for 30 min, respectively. All of the SBS spinning solution, ESBS solution, and nano-TiO₂ dispersions with the same mass fraction of 12% were prepared by the same complex solvent (V_THF/V_DMF = 3/1). The volume ratio of nano-TiO₂ dispersion to ESBS solution was 1/4, and the two different kinds of nano-TiO₂/ESBS spinning solutions were dispersed by ultrasonication at room temperature for 1 h. Then, the spinning solutions were put into a plastic syringe with a stainless steel needle 0.7 mm in inner diameter, and then the syringe was placed into the high voltage electrospinning machine (NEC-10, Japan). And all of the solutions were controlled at the same spinning condition. The distance between the needle tip and the aluminum-covered collector was 200 ± 10 mm, the applied voltage was 23 ± 1 kV, and the flow rate of the spinning solution was controlled at 0.4 ± 0.05 ml/h by a syringe pump (Figure 1(c)). After electrospinning for 8 h, the nanofiber membranes were taken off and dried in vacuum at room temperature for 5 h, respectively.

Characterization of ESBS nanofibers

Structural analysis

The infrared spectrums of SBS and ESBS were collected by Fourier transform infrared spectrometer (IR Prestige-21, Shimadzu, Japan). The epoxy value of the ESBS was tested by hydrochloric acid–acetone method [23]. Twenty-five milliliters mixed testing solvent of hydrochloric acid and acetone (1/80, v/v) was added into a solution of 1.0 g ESBS or SBS sample in 25.0 ml toluene and reacted at 20℃ for 2 h. Then three drops of phenolphthalein indicator and 0.1 mol/l potassium hydroxide–ethanol solution were dropped into the mixture to get the light pink color using base buret. The test and blank test were repeated for three times and the average epoxy value (E) was calculated by equation (1)

E = ( V 0 - V 1 ) × M / 10 W

(1)

where V₀ and V₁ are the consuming volume (ml) of the potassium hydroxide for blank (SBS) and test (ESBS) samples, respectively, M is the concentration of potassium hydroxide, and W is the weight (g) of sample.

Morphology of ESBS nanofibers

The morphology and diameters of the ESBS electrospun fibers were analyzed before and after the nano-TiO₂ embedding process. SEM in Figure 2 shows that the two different crystal structures of nano-TiO₂ are combined with ESBS fibers. The ESBS nanofibers are entangled due to the hydrogen bonding between ESBS epoxy groups and hydroxyl groups in solution [24]. We can also observe that the ESBS nanofibers with nano-TiO₂ are slightly more entangled than the ESBS pure nanofibers. The reason is that the addition of nano-TiO₂, which can enhance the conductivity of spinning solution, may lead to the tendency of breakup of Taylor cone and promote the entanglement of fibers [25–27]. OPEN IN VIEWER

Structural analysis

The infrared spectrums of SBS and ESBS nanofibers at the range of 500–2500 cm⁻¹ are quite different as shown in Figure 3. The sharp peaks observed at 748, 962, and 908 cm⁻¹ in curve A are attributed to the double bond of the cis-1,4-polybutadiene, trans-1,4-polybutadiene, and the 1,2-polybutadiene. The peaks observed at 1600, 1448, and 693 cm⁻¹ in curve A are attributed to the styrene chain. However, the weakening of the peaks around 748, 962, and 908 cm⁻¹ in curves B, C, D indicates that the double bond of the SBS has been damaged. Simultaneously, the appearing of the peaks at 1248 cm⁻¹, 889 cm⁻¹ and 854 cm⁻¹ in curves B, C, D certify the existences of the epoxy group in the production. Furthermore, the broad peak of anatase-TiO₂ is observed from 623 to 537 cm⁻¹ in curve C and the broad peak of rutile TiO₂ is observed from 630 to 550 cm⁻¹ in curve D. It can be concluded that both the TiO₂/ESBS nanofibers were successfully prepared, and the epoxy value of the ESBS electrospun fibers measured using the hydrochloric acid–acetone method is 13.6%. OPEN IN VIEWER

Anti-UV property

Owing to the conjugate system and the intramolecular hydrogen bond between the epoxide group and the hydrogen of ESBS, the average UPF (31.6) of it is bigger compared to SBS (Figure 4). As shown in Figure 5, the anti-UV mechanism describes that when exposing on the sunlight, ESBS absorbs the UV lay, and the epoxy group may combine with the hydrogen bond on the o-position carbon, and then the structure will restore while the energy is transformed and released [28]. The circular reaction may lead to the anti-UV property of ESBS repeatable and durable. OPEN IN VIEWER

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Figure 5.

The antiultraviolet mechanism of ESBS nanofiber membrane. When exposing on the sunlight, the epoxy group may combine with the hydrogen bond on the o-position carbon, and then the structure will restore while the energy is transformed and released.

The nano-TiO₂/ESBS fiber membranes show much better anti-UV property as data given in Figure 4. It can be explained that the anatase nano-TiO₂/ESBS fiber membrane has double anti-UV effects: one is that the macromolecular structure of ESBS can absorb and scatter the ultraviolet ray; the other is that anatase nano-TiO₂ can also absorb the ultraviolet ray with a match or exceed its band gap energy [28–30]. Meanwhile, the average UPF (183.4) of rutile nano-TiO₂/ESBS fiber membrane shown in Figure 4 is the biggest one of the four fiber membranes and almost the four times of the UPF (47.1) of anatase nano-TiO₂. It also can be found that the T (UV-A) and T (UV-B) of rutile nano-TiO₂/ESBS reduce obviously, especially for the T (UV-B), which is less than 0.5%. It can be explained that as the higher of density and absorption limit and the narrower of the optical band gap, the light induced by electron/hole composite rate of rutile nano-TiO₂ is much higher. At the same time, rutile nano-TiO₂ possessing longer Ti–O bond length and weaker agglomeration tendency than anatase nano-TiO₂ can be dispersed much more evenly, which may lead to the narrower distribution of particle size. All of above factors endow the rutile nano-TiO₂ equip with lower refractive index and more suitable band gap energy corresponding to the wavelength of UV-A and UV-B. In summary, the anti-UV properties of different fiber membranes are decreasing in the sequence of rutile nano-TiO₂/ESBS, anatase nano-TiO₂/ESBS, ESBS, and SBS.