Improving UV Resistance of Aramid Fibers by Simultaneously Synthesizing TiO2 on Their Surfaces and in the Interfaces Between Fibrils/Microfibrils Using Supercritical Carbon Dioxide

Aramid fibers with low density and high strength, modulus, and thermal resistance are widely used in applications such as bulletproof vests and cables. However, owing to their chemical structure, they are sensitive to ultraviolet light, which degrades the fibers’ useful mechanical properties. In this study, titanium dioxide (TiO2) nanoparticles were synthesized both on the aramid III fiber surface and in the interfacial space between the fibrils/microfibrils in supercritical carbon dioxide (scCO2) to improve the UV resistance of aramid fibers. The effects of scCO2 treatment pressure on the TiO2 structure, morphology, surface composition, thermal stability, photostability, and mechanical properties were investigated using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, ultraviolet–visible spectroscopy, and single-fiber test. The results show that amorphous TiO2 formed on the fiber surface and the interface between fibrils/microfibrils, and decreased the photodegradation rate of the aramid III fiber. Moreover, this modification can also improve the tensile strength via treatment at low temperature and without the use of a solvent. The simple synthesis process in scCO2, which is scalable, is used for mild modifications with a green solvent, providing a promising technique for synthesizing metal dioxide on polymers.


Introduction
Aramid III fiber (AF-III) has high strength, i.e., up to about 6 GPa, a modulus of up to 230 GPa, and a density of 1.44 g/cm 3 ; it is widely used in many applications, such as bulletproof vests, firefighting gear components, ropes and cables, and fiber-reinforced composite applications. Therefore, it is an indispensable strategic material for national security, construction, and scientific and technological progress [1,2]. Unfortunately, it is sensitive to ultraviolet (UV) radiation, which leads to serious degradation. It has been found that exposing the fiber to UV radiation results in significant damage, such as mass loss, deterioration of mechanical properties, color changes, and changes in the morphology and structure, that in turn lead to a reduction in performance, causing premature failure and limiting

Synthesis of TiO2 in scCO2
The synthesis process was carried out in a 500-mL stainless-steel high-pressure reactor with a syringe pump to pump CO2. The scCO2 treatment device is shown in Figure 1a. The process comprises two steps. The first is to deposit the TBT on the fiber surface and impregnate it into the interior of the fibers. After being cleaned with acetone, the AF-III fiber was rolled on a stainless-steel framework and placed in the reactor. An appropriate amount of TBT precursors was first placed on a glass filter at the bottom of the reactor. The reactor was pressurized with CO2 up to the desired temperature and pressure under stirring. After 2 h, the system was depressurized nearly instantaneously to atmospheric pressure. With the help of the swelling and dissolution of scCO2, the TBT was dissolved and impregnated into the fiber; the obtained fiber was called TBT-fiber.
The second process comprises hydrolysis in scCO2 with 30 mL ethanol, 0.1 mL acetic acid, and 3 mL distilled water to obtain TiO2-modified aramid fiber. Ethanol and acetic acid were mixed and put in a glass beaker at the bottom of the reactor, and CO2 was pumped again to the former pressure. Once the hydrolysis of the TBT deposited on the surface of the fiber or impregnated into the fibers had occurred, TiO2 was formed in the interface between the fibrils/microfibrils region in the fiber and on the fiber surface.
A schematic illustration of a two-step process is shown in Figure 1b. The products were washed with ethanol, acetone, and distilled water (cleaned more than three times). Then, the products were dried in the oven at 100 °C for 2 h. Both steps were carried out in scCO2, concluding the penetration and hydrolysis of TBT precursors.

Characterizations
The surface functional groups were studied using Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher Company, New York, NY, USA) in the spectral region of 4000-400 cm −1 with a resolution of 2 cm −1 at 32 scans. The samples were powdered, mixed with KBr, and processed into pellets. The surface morphology and fracture morphology of the modified aramid III fibers were observed using scanning electron microscopy (SEM) (S-4800, Tokyo, Japan). Before SEM The second process comprises hydrolysis in scCO 2 with 30 mL ethanol, 0.1 mL acetic acid, and 3 mL distilled water to obtain TiO 2 -modified aramid fiber. Ethanol and acetic acid were mixed and put in a glass beaker at the bottom of the reactor, and CO 2 was pumped again to the former pressure. Once the hydrolysis of the TBT deposited on the surface of the fiber or impregnated into the fibers had occurred, TiO 2 was formed in the interface between the fibrils/microfibrils region in the fiber and on the fiber surface.
A schematic illustration of a two-step process is shown in Figure 1b. The products were washed with ethanol, acetone, and distilled water (cleaned more than three times). Then, the products were dried in the oven at 100 • C for 2 h. Both steps were carried out in scCO 2 , concluding the penetration and hydrolysis of TBT precursors.

Characterizations
The surface functional groups were studied using Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher Company, New York, NY, USA) in the spectral region of 4000-400 cm −1 with a resolution of 2 cm −1 at 32 scans. The samples were powdered, mixed with KBr, and processed into pellets. The surface morphology and fracture morphology of the modified aramid III fibers were observed using scanning electron microscopy (SEM) (S-4800, Tokyo, Japan). Before SEM analysis, all samples were coated with a thin conductive layer of gold via vapor deposition to minimize the charge. X-ray diffraction (XRD, Rigaku D/MAX-2550, Tokyo, Japan) was used to determine the crystalline phase of AF-III. The measurement was conducted with a Cu Kα radiation source (λ = 1.5406 Å) with 40 kV and 200 mA), followed by a scanning range of 5.0 • -90.0 • at a speed of 20 • /min. The element composition of AF-III was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xielectron spectrometer, Thermo Fisher Scientific, New York, NY, USA) with a monochromatic Al Kα source. The pressure in the analysis chamber was controlled at 5 × 10 −8 Pa. The deconvolution curve fitting of the C1s and Ti2p peaks for fibers was performed using Peakfit software. A UV-visible (UV-Vis) spectrophotometer (UV3600, Shimadzu, Japan) equipped with an integrating sphere was used to measure the absorbance of the fibers in the wavelength range of 200-450 nm. The AF-III fibers were cut into powders. Then, the powder of the fiber was pressed into a thin layer at the center of the sphere and the measurements were performed. A thermogravimetric analyzer (TGA4000, Perkin Elmer, Waltham, MA, USA) was used to measure the thermal stability of the fibers. Temperature ramp measurements were conducted in an inert atmosphere of N2 from 30 to 850 • C at 20 • C·min -1 . The mechanical properties of the single fibers were carried out using a tensile-strength tester (XQ-1A, Shanghai New Fiber Instrument Co., Ltd., Shanghai, China) with a head speed of 10 mm/min at a gauge length of 10 mm. The average values of the tenacity and modulus were calculated from at least 15 samples. The linear density of the fiber was measured using an XD-1 vibrating fiber-fineness instrument (Shanghai New Fiber Instrument Co., Ltd., Shanghai, China). The unit conversion of Pa and CN/dtex was based on Equation (1).
where σ is the tensile strength of Pa unit, ρ is the density of the fiber as 1.44 g/cm 3 , dtex is the average linear density for 10,000 meters of fiber. Fibers were exposed to UV radiation using a weathering test machine with a UV lamp (UVB 280-315 nm, 40 W, lamp length of 1220 mm, Dongguan Instrument Co. Ltd., Guangzhou, China) to study the effect of UV aging over time on the mechanical properties of the fibers. The distance between the fiber sample and the UV lamp was 20 cm. The fibers were placed in parallel rows in the sample tray which was exposed to 40 W/m 2 and held at a relative humidity of 60% for 168 h, according to the Chinese Standard GB/T 14522-93.

Crystalline Structure
XRD was applied to detect the TiO 2 crystal phase. As shown in Figure 2, there are two diffraction peaks at 20.1 • and 21.9 • due to the (110) and (200) planes, respectively, for the untreated AF, corresponding to the crystalline domains of aramid fibers [39]. The crystalline phases of TiO 2 were not found in the XRD curves of the modified fibers, which demonstrated that the TiO 2 formed on the fibers was amorphous. At the same time, the more TiO 2 particles contained on the fiber surface and in the interfacial space between the fibrils/microfibrils, the lower the crystallinity of the fiber; this is related to the change of pressure. With the increase of pressure, the content of TiO 2 prepared on the surface and in the interfacial space increased gradually. As shown in Figure 2, the crystallinity of the sample prepared under 15 MPa was the lowest, which indicated that the particles of TiO 2 prepared under 15 MPa were most abundant. However, when the pressure is too high, i.e., up to 20 MPa, the content of TBT is also too high. TiO 2 particles aggregate under the influence of intermolecular force, forming uneven, large particles, which leads to shedding on the surface of the fiber. Due to its excellent ability of absorbing and shielding from ultraviolet light, TiO 2 particles were shown to play an important role in protecting aramid fiber from ultraviolet aging [40].

FTIR Analysis
The FTIR spectra of untreated and treated AF-III fibers are presented in Figure 3. The spectrum of all fibers showed three peaks at 3300 cm −1 (-NH stretching), 1640 cm −1 (-C=O-group), and 1540 cm −1 (-NH group of bending). After modification by TiO2 in scCO2, the peak intensity at 1640 cm −1 became stronger than that of the untreated fiber. Compared with pure TiO2, we know that stretching vibrations of Ti-O occur at 1396 cm −1 . It was shown that TiO2 was successfully synthesized on the fiber surface.

FTIR Analysis
The FTIR spectra of untreated and treated AF-III fibers are presented in Figure 3. The spectrum of all fibers showed three peaks at 3300 cm −1 (-NH stretching), 1640 cm −1 (-C=O-group), and 1540 cm −1 (-NH group of bending). After modification by TiO 2 in scCO 2 , the peak intensity at 1640 cm −1 became stronger than that of the untreated fiber. Compared with pure TiO 2 , we know that stretching vibrations of Ti-O occur at 1396 cm −1 . It was shown that TiO 2 was successfully synthesized on the fiber surface.

FTIR Analysis
The FTIR spectra of untreated and treated AF-III fibers are presented in Figure 3. The spectrum of all fibers showed three peaks at 3300 cm −1 (-NH stretching), 1640 cm −1 (-C=O-group), and 1540 cm −1 (-NH group of bending). After modification by TiO2 in scCO2, the peak intensity at 1640 cm −1 became stronger than that of the untreated fiber. Compared with pure TiO2, we know that stretching vibrations of Ti-O occur at 1396 cm −1 . It was shown that TiO2 was successfully synthesized on the fiber surface.   The surface morphology of aramid fibers was studied using SEM. In Figure 4, the SEM images of the untreated and modified fibers are shown under various pressures in scCO 2 . It can be seen that the surface of the untreated fibers ( Figure 4a) is smooth and clean. Compared to untreated aramid fiber, the surface of the modified aramid fiber is rough and has many obvious nanoparticles on the surface, as shown in Figure 4b-e. Combining the results of the XPS and FTIR, we can conclude that these nanoparticles are TiO 2 . When the treatment is at a pressure of 10 MPa, less TiO 2 forms on the surface, and the distribution and size are uneven. The results showed that, when treated at a pressure of 15 MPa, TiO 2 is well synthesized and packed on the surface. At the same time, the TiO 2 particles formed on the fiber surface were most abundant, and agglomeration was reduced. However, when the treatment pressure increased to 20 MPa, fewer particles attached to the surface and the size of the TiO 2 particles was largely due to the agglomeration. The main cause of this phenomenon is related to the solubility of scCO 2 ; the higher the pressure, the denser the distribution of the molecules. As the pressure increases, the CO 2 concentration also increases, leading to an increase in the dissolution amounts and diffusion rate of the TBT in scCO 2 . But when the pressure is too high, the TBT content will also be too high. When the CO 2 is released, the rapid relative motion between the particles and the gas causes the TiO 2 particles to agglomerate into large particles under the intermolecular force, and adhere unevenly to the surface of the fiber [41]. Since the large particles easily fall off the surface of AF-III; under external force, the content of nano-TiO 2 particles under a pressure of 20 MPa was less than that at 15 MPa. The above results are consistent with those of XPS.

Surface and Interior Morphology of the Fibril/Microfibril
The surface morphology of aramid fibers was studied using SEM. In Figure 4, the SEM images of the untreated and modified fibers are shown under various pressures in scCO2. It can be seen that the surface of the untreated fibers ( Figure 4a) is smooth and clean. Compared to untreated aramid fiber, the surface of the modified aramid fiber is rough and has many obvious nanoparticles on the surface, as shown in Figure 4b-e. Combining the results of the XPS and FTIR, we can conclude that these nanoparticles are TiO2. When the treatment is at a pressure of 10 MPa, less TiO2 forms on the surface, and the distribution and size are uneven. The results showed that, when treated at a pressure of 15 MPa, TiO2 is well synthesized and packed on the surface. At the same time, the TiO2 particles formed on the fiber surface were most abundant, and agglomeration was reduced. However, when the treatment pressure increased to 20 MPa, fewer particles attached to the surface and the size of the TiO2 particles was largely due to the agglomeration. The main cause of this phenomenon is related to the solubility of scCO2; the higher the pressure, the denser the distribution of the molecules. As the pressure increases, the CO2 concentration also increases, leading to an increase in the dissolution amounts and diffusion rate of the TBT in scCO2. But when the pressure is too high, the TBT content will also be too high. When the CO2 is released, the rapid relative motion between the particles and the gas causes the TiO2 particles to agglomerate into large particles under the intermolecular force, and adhere unevenly to the surface of the fiber [41]. Since the large particles easily fall off the surface of AF-Ш under external force, the content of nano-TiO2 particles under a pressure of 20 MPa was less than that at 15 MPa. The above results are consistent with those of XPS.
To investigate the formation inside the fibers, the fiber was longitudinally torn along the fiber direction. As shown in Figure 5, there are many nano-TiO2 particles attached to the supercriticaltreated aramid fiber compared to the untreated aramid fiber. Figure 5b shows the internal structure of aramid fibers with precursor, and Figure 5c shows the morphology after hydrolysis. As shown in Figure 5b, the particles impregnated the interspace of the fibrils/microfibrils. In Figure 5c, the particles become smaller and had a regular morphology with the TBT fiber after hydrolysis. The results show that nano-TiO2 particles had formed inside the aramid fiber.  To investigate the formation inside the fibers, the fiber was longitudinally torn along the fiber direction. As shown in Figure 5, there are many nano-TiO 2 particles attached to the supercritical-treated aramid fiber compared to the untreated aramid fiber. Figure 5b shows the internal structure of aramid fibers with precursor, and Figure 5c shows the morphology after hydrolysis. As shown in Figure 5b, the particles impregnated the interspace of the fibrils/microfibrils. In Figure 5c, the particles become smaller and had a regular morphology with the TBT fiber after hydrolysis. The results show that nano-TiO 2 particles had formed inside the aramid fiber.

XPS Analysis
To investigate the influence of supercritical pressure on the surface of aramid fibers, the surface composition of the fibers under different pressures was determined using XPS. It was shown that the oxygen contents of the surfaces of aramid fibers prepared under different pressures were higher than those of untreated fibers. As shown in Figure 6, C1s, O1s, and N1s peaks appear at 284.7, 531.8, and 400.3 eV, respectively, which represent the presence of C, N, and O in the aramid. Compared with the untreated aramid fiber, the modified aramid fiber showed a new peak of titanium atoms at 453.4 eV, which means that TiO2 was successfully prepared on the surface. The results of the elemental content changes are shown in Table 1. Compared with the untreated AF-III fibers, the contents of C and N in the modified aramid decreased, while those of O and Ti increased. When the treatment pressure was 15 MPa, the O and Ti contents were the greatest, and the ratio of O:C increased from 0.2269 to 0.3711; also the content of elemental Ti was the highest, i.e., up to 2.66%. The results were consistent with the results shown in Figure 4, which indicated that the content of TiO2 prepared at 15 MPa was the highest. This means that TiO2 can be synthesized as much as possible on the surface of AF-Ш when the appropriate supercritical pressure is reached. Figure 7 depicts the XPS spectrum of Ti2p in the treated fibers in scCO2 under a pressure of 15 MPa. The peaks at 464.5 eV for Ti2p 1/2 and at 458.6 eV for Ti2p 3/2 can be observed in Figure 7. These are typical binding energies for the Ti 4+ species, showing good agreement with those of the ordinary TiO2 [42][43][44]. These results further confirmed that TiO2 was synthesized on the surface of fiber.

XPS Analysis
To investigate the influence of supercritical pressure on the surface of aramid fibers, the surface composition of the fibers under different pressures was determined using XPS. It was shown that the oxygen contents of the surfaces of aramid fibers prepared under different pressures were higher than those of untreated fibers. As shown in Figure 6, C 1s , O 1s , and N 1s peaks appear at 284.7, 531.8, and 400.3 eV, respectively, which represent the presence of C, N, and O in the aramid. Compared with the untreated aramid fiber, the modified aramid fiber showed a new peak of titanium atoms at 453.4 eV, which means that TiO 2 was successfully prepared on the surface. The results of the elemental content changes are shown in Table 1. Compared with the untreated AF-III fibers, the contents of C and N in the modified aramid decreased, while those of O and Ti increased. When the treatment pressure was 15 MPa, the O and Ti contents were the greatest, and the ratio of O:C increased from 0.2269 to 0.3711; also the content of elemental Ti was the highest, i.e., up to 2.66%. The results were consistent with the results shown in Figure 4, which indicated that the content of TiO 2 prepared at 15 MPa was the highest. This means that TiO 2 can be synthesized as much as possible on the surface of AF-III when the appropriate supercritical pressure is reached. Figure 7 depicts the XPS spectrum of Ti 2p in the treated fibers in scCO 2 under a pressure of 15 MPa. The peaks at 464.5 eV for Ti 2p 1/2 and at 458.6 eV for Ti 2p 3/2 can be observed in Figure 7. These are typical binding energies for the Ti 4+ species, showing good agreement with those of the ordinary TiO 2 [42][43][44]. These results further confirmed that TiO 2 was synthesized on the surface of fiber.

XPS Analysis
To investigate the influence of supercritical pressure on the surface of aramid fibers, the surface composition of the fibers under different pressures was determined using XPS. It was shown that the oxygen contents of the surfaces of aramid fibers prepared under different pressures were higher than those of untreated fibers. As shown in Figure 6, C1s, O1s, and N1s peaks appear at 284.7, 531.8, and 400.3 eV, respectively, which represent the presence of C, N, and O in the aramid. Compared with the untreated aramid fiber, the modified aramid fiber showed a new peak of titanium atoms at 453.4 eV, which means that TiO2 was successfully prepared on the surface. The results of the elemental content changes are shown in Table 1. Compared with the untreated AF-III fibers, the contents of C and N in the modified aramid decreased, while those of O and Ti increased. When the treatment pressure was 15 MPa, the O and Ti contents were the greatest, and the ratio of O:C increased from 0.2269 to 0.3711; also the content of elemental Ti was the highest, i.e., up to 2.66%. The results were consistent with the results shown in Figure 4, which indicated that the content of TiO2 prepared at 15 MPa was the highest. This means that TiO2 can be synthesized as much as possible on the surface of AF-Ш when the appropriate supercritical pressure is reached. Figure 7 depicts the XPS spectrum of Ti2p in the treated fibers in scCO2 under a pressure of 15 MPa. The peaks at 464.5 eV for Ti2p 1/2 and at 458.6 eV for Ti2p 3/2 can be observed in Figure 7. These are typical binding energies for the Ti 4+ species, showing good agreement with those of the ordinary TiO2 [42][43][44]. These results further confirmed that TiO2 was synthesized on the surface of fiber.      Table 2 shows the TiO2 at different masses added under the different treatment pressures. It was found that the mass of the added TiO2 increases with increasing fiber mass. The results agree with the observed higher area covered by TiO2. However, the weight of added TiO2 varied with pressure. When the pressure was 15 MPa, the TiO2 added was 2.38%, which was the largest increase. This may be because with increasing pressure, the dissolution and impregnation of TBT in scCO2 increased. However, the pressure is too high at 20 MPa, and the amount of added TiO2 was not the largest. The reason for this is that TiO2 would agglomerate to form large particles at the higher pressure that cannot uniformly coat the fiber surface, and which would easily be removed by washing or further characterization.   Table 2 shows the TiO 2 at different masses added under the different treatment pressures. It was found that the mass of the added TiO 2 increases with increasing fiber mass. The results agree with the observed higher area covered by TiO 2 . However, the weight of added TiO 2 varied with pressure. When the pressure was 15 MPa, the TiO 2 added was 2.38%, which was the largest increase. This may be because with increasing pressure, the dissolution and impregnation of TBT in scCO 2 increased. However, the pressure is too high at 20 MPa, and the amount of added TiO 2 was not the largest. The reason for this is that TiO 2 would agglomerate to form large particles at the higher pressure that cannot uniformly coat the fiber surface, and which would easily be removed by washing or further characterization.

Mechanical Properties of AF-III
The effects of treatment pressure on the mechanical properties of aramid fibers are shown in Figure 8. It was found that both the tensile strength and modulus of AF-III fibers treated at different pressures were higher than those of the untreated fibers, and the strength and modulus of aramid Polymers 2020, 12, 147 9 of 15 fibers increased with an increase in reaction pressure. When the reaction pressure was 15 MPa, the strength and modulus of the fibers reached the maximum value. When the pressure was 20 MPa, the strength and modulus showed a lesser increase, but it was still higher than that of untreated fiber. With the increase of pressure, the CO 2 molecular density also increased and more TBT was dissolved in scCO 2 , which infiltrated the fiber and adhered to the fibrillar elements, thus providing a secondary means by which to improve the mechanical properties. Moreover, the tensile modulus clearly improves because the molecular chains tend to arrange themselves with the help of the plasticization of scCO 2 and tension; however, when the pressure is too high, it may decrease in volume and mobility, so the strength and modulus are not noticeably improved.

Mechanical Properties of AF-III
The effects of treatment pressure on the mechanical properties of aramid fibers are shown in Figure 8. It was found that both the tensile strength and modulus of AF-III fibers treated at different pressures were higher than those of the untreated fibers, and the strength and modulus of aramid fibers increased with an increase in reaction pressure. When the reaction pressure was 15 MPa, the strength and modulus of the fibers reached the maximum value. When the pressure was 20 MPa, the strength and modulus showed a lesser increase, but it was still higher than that of untreated fiber. With the increase of pressure, the CO2 molecular density also increased and more TBT was dissolved in scCO2, which infiltrated the fiber and adhered to the fibrillar elements, thus providing a secondary means by which to improve the mechanical properties. Moreover, the tensile modulus clearly improves because the molecular chains tend to arrange themselves with the help of the plasticization of scCO2 and tension; however, when the pressure is too high, it may decrease in volume and mobility, so the strength and modulus are not noticeably improved.  Figure 9 shows that the UV-Vis spectrum of AF-III is in the range of 250-600 nm, of which 250-400 nm is the UV-absorption region and 400-600 nm is the visible absorption region. Compared to the untreated fibers, treated fibers have significant UV shielding because the nano-TiO2 could absorb the UV light such that the anti-UV ability of modified fiber is enhanced. The results demonstrate that the ultraviolet absorption effect at 15 MPa is the best, which might be due to the high levels of loaded TiO2. However, the UV absorption of fibers at 12 and 20 MPa is slightly different; this trend is not in agreement with the mass of the loaded TiO2. The results showed that the UV-Vis absorption is not only related to the mass of the TiO2, but also to the size and distribution of its particles.  Figure 9 shows that the UV-Vis spectrum of AF-III is in the range of 250-600 nm, of which 250-400 nm is the UV-absorption region and 400-600 nm is the visible absorption region. Compared to the untreated fibers, treated fibers have significant UV shielding because the nano-TiO 2 could absorb the UV light such that the anti-UV ability of modified fiber is enhanced. The results demonstrate that the ultraviolet absorption effect at 15 MPa is the best, which might be due to the high levels of loaded TiO 2 . However, the UV absorption of fibers at 12 and 20 MPa is slightly different; this trend is not in agreement with the mass of the loaded TiO 2 . The results showed that the UV-Vis absorption is not only related to the mass of the TiO 2 , but also to the size and distribution of its particles. The mechanical properties of AF-III fibers modified in scCO2 under different pressures irradiated by a UV lamp for different times were studied. As shown in Figure 10, there is a clear decreasing trend in the tensile strength and modulus for these fibers, both treated and untreated,

Mechanical Properties
The mechanical properties of AF-III fibers modified in scCO 2 under different pressures irradiated by a UV lamp for different times were studied. As shown in Figure 10, there is a clear decreasing trend in the tensile strength and modulus for these fibers, both treated and untreated, with increasing exposure time. With a treatment pressure at 15 MPa, the fibers retained better tensile strength than those at pressures of 10, 12, and 20 MPa. The results are consistent with the UV-Vis spectroscopy results.
irradiated by a UV lamp for different times were studied. As shown in Figure 10, there is a clear decreasing trend in the tensile strength and modulus for these fibers, both treated and untreated, with increasing exposure time. With a treatment pressure at 15 MPa, the fibers retained better tensile strength than those at pressures of 10, 12, and 20 MPa. The results are consistent with the UV-Vis spectroscopy results.
The tensile strength and modulus of untreated aramid fibers remained 81.48% and 51.41%, respectively, after 168 h of UV irradiation. The fibers modified with TiO2 in scCO2 showed less strength and modulus loss than untreated fibers after exposure to UV-Vis radiation, especially at 24 h. The fibers treated at 15 MPa showed 86.34% tensile strength and 65.03% modulus after 168 h, which is a 5%-14% improvement compared to those of untreated AF-III. Furthermore, as Figure 8 shows, the tensile strength and modulus of the modified fiber were improved, so after exposure to UV-Vis, they all showed better mechanical properties. The results showed that an in situ preparation of titanium dioxide on the surface and in the interface between fibrils/microfibrils in scCO2 improved not only the tensile strength, but also the UV-Vis resistance.  The tensile strength and modulus of untreated aramid fibers remained 81.48% and 51.41%, respectively, after 168 h of UV irradiation. The fibers modified with TiO 2 in scCO 2 showed less strength and modulus loss than untreated fibers after exposure to UV-Vis radiation, especially at 24 h. The fibers treated at 15 MPa showed 86.34% tensile strength and 65.03% modulus after 168 h, which is a 5%-14% improvement compared to those of untreated AF-III. Furthermore, as Figure 8 shows, the tensile strength and modulus of the modified fiber were improved, so after exposure to UV-Vis, they all showed better mechanical properties. The results showed that an in situ preparation of titanium dioxide on the surface and in the interface between fibrils/microfibrils in scCO 2 improved not only the tensile strength, but also the UV-Vis resistance.

Surface Compositions
In order to further study the effect of the TiO 2 on the aging performance of the aramid fiber, the change of the surface composition before and after the aging of aramid fiber was determined using XPS. Figure 11 shows the C1s core-level spectra of AF-III fibers and UV irradiated AF-III fibers under different pressures. The components of untreated AF-III fibers (Figure 10a) can be divided into three peaks, i.e., 284.6 eV (C-C), 285.9 eV (C-N) and 287.8 eV (N-C=O). After 168h UV radiation, a new peak associated with the COOH group was observed at a binding energy of 289.8 eV for the irradiated untreated AF-III fibers (Figure 11b), demonstrating that aging absorbs UV energy, leading to the formation of the COOH group [17,45,46]. Compared with the COOH peak of AF-III-untreated-UV fibers, AF-III-scCO 2 -UV fibers (Figure 11c-f) shows relatively insignificant weak peaks. The results show that the nano-TiO 2 particles formed on the surface of the fiber can shield a large amount of ultraviolet light. Notably, this peak is almost invisible in AF-III-scCO 2 -15MPa-UV fibers (Figure 11e), which indicates that AF-III fibers are more effectively protected by scCO 2 treatment at 15 MPa. untreated AF-III fibers (Figure 11b), demonstrating that aging absorbs UV energy, leading to the formation of the COOH group [17,45,46]. Compared with the COOH peak of AF-III-untreated-UV fibers, AF-III-scCO2-UV fibers (Figure 11c-f) shows relatively insignificant weak peaks. The results show that the nano-TiO2 particles formed on the surface of the fiber can shield a large amount of ultraviolet light. Notably, this peak is almost invisible in AF-III-scCO2-15MPa-UV fibers (Figure 11e), which indicates that AF-III fibers are more effectively protected by scCO2 treatment at 15MPa. Figure 11. C1s core-level spectra of AF-III fibers and UV irradiated AF-III fibers. Figure 11. C1s core-level spectra of AF-III fibers and UV irradiated AF-III fibers. Figure 12 shows a decrease in the initial weight loss of the fiber below 850 • C. Before 500 • C, fibers treated at 10, 12, 15, and 20 MPa had higher residual masses than the original fibers. The results show that the heat resistance of the treated fiber was improved.

Thermogravimetric Analysis (TGA)
The first interval is the microgravity stage, which is mainly the process of losing the intermolecular bound water. From this range, it can be concluded that there is no significant difference in the content of molecular binding water between the two aramid fibers. The second stage is the thermal decomposition stage (500-600 • C), and the onset of the fiber decomposition is almost the same. The results demonstrate that there is less effect on the stability of the fiber after coating with TiO 2 .
The third zone (600-850 • C) is the stable stage of carbon formation, in which the fiber is carbonized and the weight loss of the residue is small. Compared to untreated aramid, as shown in Table 3, the residual mass of modified AF-III with TiO 2 in scCO 2 was higher than those of the untreated fibers due to the better thermal stability of TiO 2 . the same. The results demonstrate that there is less effect on the stability of the fiber after coating with TiO2.
The third zone (600-850 °C) is the stable stage of carbon formation, in which the fiber is carbonized and the weight loss of the residue is small. Compared to untreated aramid, as shown in Table 3, the residual mass of modified AF-III with TiO2 in scCO2 was higher than those of the untreated fibers due to the better thermal stability of TiO2.

Potential Mechanism of Modification
Based on the results and discussions above, a model of the modification process is shown in Figure 13. The AF-III fiber comprised fibrils with diameters ranging from 100 nm to 2 μm, which, in turn, are comprised of microfibrils [37]. The microfibrils are comprised of a large, perfectly oriented crystallite phase and an irregular amorphous phase. The amorphous phase has a less regular macromolecule with some chain end or defects, which exist between the crystallites or at fibril interfaces. Using scCO2 as a carrier, TBT as the TiO2 precursor could be dissolved and impregnated into the fibril interfaces, and deposited on the surface of the fiber when the CO2 was depressurized. In the second process step, TiO2 nanoparticles were synthesized via hydrolysis with ethanol. Plentiful hydroxyl groups on the surfaces of the fibers and the fibril/microfibril interfaces adsorb TiO 2+ , and then TiO2 was formed by hydrolysis condensation, subsequently growing into TiO2 particles. Meanwhile, the -CONH-groups of AF-III fiber would preferentially form rutile TiO2 [47]. In addition, hydrogen bonding exists between the hydroxyl groups of TiO2 and AF-III molecular chains; the chain  end in the amorphous phase between the interfaces of the fibril will adhere to the fibrillar elements and provide a secondary interfibrillar force, which is useful for improving the mechanical properties.

Conclusions
In this study, a two-step process is proposed to synthesize nano-TiO2 on the surface of aramid fibers and in the interfacial space between the fibrils/microfibrils of AF-III in scCO2 to improve its UV-Vis resistance. The results of FTIR and XPS showed that the TiO2 particles were successfully prepared on the surface. In addition, the SEM results demonstrated that TiO2 was not only

Potential Mechanism of Modification
Based on the results and discussions above, a model of the modification process is shown in Figure 13. The AF-III fiber comprised fibrils with diameters ranging from 100 nm to 2 µm, which, in turn, are comprised of microfibrils [37]. The microfibrils are comprised of a large, perfectly oriented crystallite phase and an irregular amorphous phase. The amorphous phase has a less regular macromolecule with some chain end or defects, which exist between the crystallites or at fibril interfaces. Using scCO 2 as a carrier, TBT as the TiO 2 precursor could be dissolved and impregnated into the fibril interfaces, and deposited on the surface of the fiber when the CO 2 was depressurized. In the second process step, TiO 2 nanoparticles were synthesized via hydrolysis with ethanol. Plentiful hydroxyl groups on the surfaces of the fibers and the fibril/microfibril interfaces adsorb TiO 2+ , and then TiO 2 was formed by hydrolysis condensation, subsequently growing into TiO 2 particles. Meanwhile, the -CONH-groups of AF-III fiber would preferentially form rutile TiO 2 [47]. In addition, hydrogen bonding exists between the hydroxyl groups of TiO 2 and AF-III molecular chains; the chain end in the amorphous phase between the interfaces of the fibril will adhere to the fibrillar elements and provide a secondary interfibrillar force, which is useful for improving the mechanical properties.

Conclusions
In this study, a two-step process is proposed to synthesize nano-TiO 2 on the surface of aramid fibers and in the interfacial space between the fibrils/microfibrils of AF-III in scCO 2 to improve its UV-Vis resistance. The results of FTIR and XPS showed that the TiO 2 particles were successfully prepared on the surface. In addition, the SEM results demonstrated that TiO 2 was not only synthesized on the surface, but also in the interface between the microfibril/fibril. The effects of pressure during the preparation of TiO 2 in scCO 2 on the mechanical properties and morphology of aramid fibers were investigated. The results showed that, when treated at a pressure of 15 MPa, TiO 2 is well synthesized and packed on the surface. It was found that the tensile strength and modulus of fibers treated in scCO 2 were higher than those of untreated fibers. The tensile strength of TiO 2 -modified aramid fibers after photo-aging decreased less than that for untreated. This was because TiO 2 could absorb UV and reduce the UV absorption of the aramid fiber itself. These findings showed that the scCO 2 treatment process is a better method for improving the fiber's properties, and could be a potential application of metal oxides to modify AFs and other high-performance fibers, which might lead to the development of exciting, new processes for engineers interested in bulletproof vests, firefighting gear components, ropes, and cables.