Preparation of para-aramid aerogel fiber structurally colored by light scattering through wet spinning and supercritical drying

This research is an investigation of the preparation conditions for para-aramid aerogel fibers that are structurally colored by Rayleigh scattering, an angle-independent structural color. It is well known that structural color has the angle dependence of colors. However, there is currently a desire for materials with angle-independent structural color. During the fibers’ spinning process, the microstructures of the fibers are generated. The effect of spinning conditions is investigated by measuring the optical properties of the wet gel fibers. The neutralization speed of negatively charged PPTA fibrils has a significant impact on the formation of the dense microstructure. Spinning under the fast neutralization speed condition makes it possible to achieve the blue-colored fiber derived from Rayleigh scattering. The influence of neutralization speed on the microstructure of the fibers is confirmed by FE-SEM characterization. Furthermore, spinning using a narrow inner diameter needle decreases the whiteness component derived from Mie scattering of the fibers, resulting in a more vivid blue color. The angle-independent structurally colored fibers hold great promise for application in clothing, interiors, and industrial materials.


Introduction
A common method for coloring fibers is using pigments. However, this method has various problems concerning the regulation of the product lifetime by sunlight and has a harmful influence on the environment because it releases a considerable amount of wastewater [1]. Therefore, conventional dyeing processes require technology that can solve environmental and energy conservation problems. In addition to pigment colors based on chemical phenomena, structural colors are based on physical phenomena. Structural colors are chromogenic phenomena by diffraction, refraction, interference, and scattering of light that occur when light is irradiated on the microstructure of the substance itself [2]. It is a common phenomenon in nature [3]. Examples include morpho butterflies, peacock feathers, soap bubbles, and blue skies. Because of the absence of pigments, structural colors are environmentally friendly and have great sensuousness. Additionally, they have superior characteristics; for example, their colors do not fade away, provided the microstructure of the substance is maintained. Structural color is researched worldwide, and various coloring theories have been proposed [4][5][6]. Among these theories, the bestknown coloring theory of structural colors is the crystallization of colloidal particles [7,8]. A colloidal crystal is a three-dimensional structure comprising a regular array of colloidal particles with uniform particle sizes, and it selectively reflects the light of a specific wavelength that satisfies the following equation, which considers Bragg's law and Snell's law [9].
Here, λ is the wavelength of the incident light, d is the lattice spacing, m is the Bragg's reflection order, n a is the refractive index of the crystal, and θ is the incident and reflective angle, respectively. The wavelength of the reflected light by colloidal crystal depends on the viewing angle. Therefore, it is unsuitable for the application of colored materials [10].
(1) = 2d m n 2 a − sin 2 1 2 This study focuses on angle-independent structural colors by light scattering. Based on the relationship between the wavelength of light and the size of the scatterer, there are different scattering modes, such as Rayleigh and Mie scattering. Scattering by a scatterer whose size is much smaller than the wavelength of light is called Rayleigh scattering, and the scattering intensity is inversely proportional to the fourth power of the wavelength, resulting in a blue color. However, scattering by a scatterer of the same size as the wavelength of light is called Mie scattering, and the scattering intensity is constant and white regardless of the wavelength [11]. Among the scattering modes, this study focuses on Rayleigh scattering. Light scattering is easy to construct compared with other structural color theories because it depends on only the size of the scatterer and does not consider the arrangement of the scatterer. Herein, we propose the microstructural formation by aerogel fibers [12], which are nanoporous materials with characteristics, such as high specific surface area, low density, low thermal conductivity, and high permeability, as fiber materials that exhibit coloration by Rayleigh scattering. Silica aerogel is a typical aerogel [13][14][15]. However, it has been reported that it is difficult to make silica gel because it is rigid and fragile [16,17]. Then, we proposed preparing aerogel fibers using polyparaphenylene terephthalamide fibers (PPTA), which have high strength, elastic modulus, and flame retardancy. In our previous study [18], we successfully prepared para-aramid aerogel fibers. However, the fibers are white by Mie scattering derived from pores of the same size as the wavelength of light. Herein, we investigated the effects of various spinning conditions on the color and structure of the fibers, assuming that aerogel fibers with a blue color can be obtained by controlling the pore size. If aerogel fibers that show a blue color are realized, they could be applied to clothing, interior, and industrial materials [16].

Materials
Para-aramid fibers (Kevlar 29) were purchased from the DuPont-Toray Co. Ltd. Potassium hydroxide (KOH) was purchased from Nacalai Tesque. Dimethyl sulfoxide (DMSO), acetone, hydrochloric acid, and ethanol were obtained from Wako Pure Chemical Co. All chemicals were used without further purification.

Preparation of para-aramid fibril dispersion
The aramid fibers were washed with deionized water and ethanol, and then dried in an oven at 120 °C for ~ 3 h. Next, KOH was put into DMSO with weight ratios of 0.0037, 0.0042, 0.0045, 0.0053, and 0.0071 wt%, respectively. Then, the dried aramid fibers were put into the DMSO/KOH solution with weight ratios of 0.50, 0.75, 1.00, 1.25, and 1.50 wt%, respectively. The hydrogen bonds of aramid fibers are hampered by the base effect [19,20]. Afterward, the solution was stirred for 48 h to fibrillate the aramid fibers. This mixture of fibrils was denoted as the aramid dispersion.

Wet gel spinning
The aramid dispersion was put into a syringe attached to a 22-g needle. Afterward, wet spinning was performed in a mixture of acetone and hydrochloric acid using a syringe pump YSP-101 (YMC, Japan). The weight concentrations of the added hydrochloric acid to acetone were 5, 15, 25, and 35 wt%. The temperature of the coagulation bath was adjusted to 42.5 °C using a mini cool block bath MyBL-10C (As one, Japan). The shear rate of spinning using a 22-g needle was 1145.7 s −1 . The spun aramid wet gel fibers were solvent replaced with acetone.

Supercritical drying of wet gel fiber
Several aramid wet gel fibers, which completed the solvent replacement with acetone, were supercritically dried. Afterward, we achieved para-aramid aerogel fibers. Wet gel fibers were put into a TSC-WC-0025 autoclave (Taiatsu Techno Corp, Japan) and heated to 80 °C. Next, CO 2 was injected into the autoclave, and the device interior was pressurized to 10 MPa using an SCF-Get high-pressure pump (JASCO Corp, Japan). Furthermore, acetone was replaced with supercritical CO 2 by holding it in this condition for 3 h. Next, BP-2080 (JASCO, Japan) was used to set the pressure in the autoclave at 0.01 MPa intervals for the exhaust pressure, and CO 2 was released to atmospheric pressure over a 3-h period.

Analytical methods
The optical characteristics of the spun para-aramid wet gel fibers were investigated from the absolute transmittance spectrum using an angle-resolved absolute reflectance measurement device (V-650, JASCO, Japan) and an automatic reflectance measurement unit (ARMV-650, JASCO, Japan). In this experiment, acetone and wet gel fibers were put into quartz cells. Next, we measured the optical characteristics of the fiber strands. The optical characteristics of the achieved para-aramid aerogel fibers were investigated from the relative reflection spectrum using a spectrometer for measuring the reflection spectrum (USB-4000, Ocean Photonics, Japan). In this experiment, we selected the intersection point between the two fibers as a measurement point. The surface microstructures of several para-aramid aerogel fibers obtained by supercritical drying were observed using field-emission scanning electron microscopy (SEM, JSM-7600F, JEOL, Japan) at magnifications of 100,000×. The shape of the wet gel and aerogel fibers were observed using optical microscopy (VB-7010, Keyence, Japan) at magnifications of 100×. Using the obtained fiber images, we measured fiber diameters. The shrinking rates of the fibers were measured via supercritical drying. Finally, absorption isotherm and pore size distribution were determined by BET method and BJH method using pore size distribution analyzer (BELSORP-mini II, MicrotracBEL, Japan).

Evaluation method
The intensity of light per scatterer scattered by Rayleigh scattering is expressed as follows: where I(θ) is the intensity of the scattered light, I 0 is the intensity of incident light, d is the particle size, r is the distance from the particle to the observation point, m is the relative refractive index, λ is the wavelength, and θ is the scattering angle, respectively. Equation 2 indicates that when light is irradiated on the same scatterer under the same conditions, the light with a shorter wavelength is scattered more strongly. Thus, when Rayleigh scattering is observed, the transmittance and reflectance decrease and increase as the wavelength decreases [21]. Then, we defined and evaluated the transmittance of the wet gel fiber at an 800-nm wavelength as the maximum transmittance (T max ). The difference between the transmittance of the wet gel fibers at a 460-nm wavelength and T max is the transmittance difference (ΔT max − T 460 nm ). Regarding the evaluated wavelength range, although the visible light wavelength range is ~ 400 to 800 nm, the wavelength was set from 460 to 800 nm because acetone, which was used to measure the transmittance spectrum, absorbs light in the region ~ 460 nm (Fig. 1).

Influence of hydrochloric acid on fiber microstructure and optical characteristics
We thought that the mass concentration of hydrochloric acid used for the coagulation bath affected the neutralization speed of the aramid fibers defibrated by the base. Thus, hydrochloric acid was added to acetone, and the mass concentrations were adjusted to 5, 15, 25, and 35 wt%, respectively. The solution was used as a coagulation bath for wet spinning. The following conditions were set: coagulation bath temperature = 42.5 °C, mass  Figure 2a is transmittance spectrum of wet gel fibers obtained under each hydrochloric acid concentration. As the mass concentration of hydrochloric acid used in the coagulation bath increases, the maximum transmittance increases, peaking at ~ 90% (Fig. 2b). The maximum transmittance is an indicator for evaluating the transparency of wet gel fibers. Thus, we achieved extremely transparent fibers. Furthermore, the transmittance difference, which indicates the blueness of the fibers, also increases with the increasing mass concentration of hydrochloric acid and culminates at 25 wt% (Fig. 2c). The fibers indicate a ~ 10% transmittance difference in the range of low mass concentrations of hydrochloric acid, but the fibers appear white. Herein, even if the transmittance difference was high, the blue color could not be confirmed unless the fibers showed a maximum transmittance of ≥ 85%. These results imply that a dense network structure was formed due to a faster neutralization speed, resulting in the spinning of fibers with many small pores, possibly because the time required for structural fixation is shorter due to a faster neutralization speed. The structural formation starts at the same time as spinning and forms a dense structure, but under the low mass concentration of hydrochloric acid conditions, where it takes longer to fix the structure, phase separation may progress, resulting in a network structure with large pore size. However, it was assumed that under the high mass concentration of hydrochloric acid conditions, the initially formed dense structure could be fixed before phase separation occurred.

Influence of aramid fiber concentration on fiber microstructure and optical characteristics
We thought that the aramid fiber concentration added during the preparation of the dispersion liquid affects the number of aramid fibrils per space. Therefore, the mass concentration of the added aramid fibers was set at five conditions: 0.50, 0.75, 1.00, 1.25, and 1.50 wt%. Then, the dispersion liquid was wet spun used, which was obtained under such conditions. The following conditions were set: mass concentration of hydrochloric acid = 5 or 25 wt%, coagulation bath temperature = 42.5 °C, mass concentration of KOH = 0.0042 wt%, and shear rate = 1145.7 s −1 . A 22-g needle was used here. In our previous study, aramid fibril dispersion liquid was prepared under 1 wt% aramid fiber concentration conditions. However, we used tetrabutylammonium fluoride (TBAF) as a fibrillation agent in our previous research [22], so we have no data about prepared aramid dispersion liquids using KOH as a fibrillation agent. Therefore, using a concentration of ~ 1 wt% as a criterion, the dispersion liquid conditions and the availability of spinning at each aramid fiber concentration set in this experiment are presented in Table 1.
The dispersion liquid could not be prepared under too high aramid fiber concentration conditions because aramid fibers do not fibrillate and disperse. However, continuous spinning was too difficult under too low aramid fiber concentration conditions because the aramid fibers were completely dispersed, but the viscosity of the dispersion liquid decreased significantly. Therefore, we could spin wet gel fibers under 25 wt% hydrochloric acid concentration conditions only, and the squeezed dispersion liquid could not maintain the fiber shape under 5 and 0.5 wt% hydrochloric acid and aramid fiber concentration conditions, respectively. Therefore, we investigated the optical characteristics of aramid wet gel fibers from 0.50 to 1.25 wt% aramid fiber conditions. The transmittance spectrum, maximum transmittance, and transmittance difference obtained under each condition are shown in Fig. 3.
(C) in Fig. 3c is an identification symbol used in later discussions. Figure 3a, b are transmittance spectra of wet gel fibers obtained under each aramid fiber concentration. In terms of the maximum transmittance, the maximum value was obtained at an aramid fiber concentration of 0.75 wt% in both hydrochloric acid concentration conditions. As the aramid concentration increases, the transmittance gradually decreases. The difference in transmittance is greatest at an aramid fiber concentration of 1.00 wt% in both hydrochloric acid concentration conditions. These behavioral characteristics suggest that the number of fibrils increases with increasing aramid concentration, which reduces the pore size and thus suppresses the Mie scattering effect. However, when the concentration exceeded 1.00 wt%, the number of pores decreased due to too many fibrils, and both transmittance and scattering intensity decreased.

Influence of KOH on fiber microstructure and optical characteristics
The mass concentration of KOH added during the dispersion preparation affects the diameter of aramid fibril and the magnitude of intermolecular interactions. Therefore, the mass concentration of KOH added was set at five conditions: 0.0037, 0.0042, 0.0045, 0.0053, and 0.0071 wt%. Then, the dispersion liquid was wet spun used, which was obtained under such conditions. The following conditions  were set: mass concentration of hydrochloric acid = 5 or 25 wt%, coagulation bath temperature = 42.5 °C, mass concentration of aramid fiber = 1 wt%, and shear rate = 1145.7 s −1 . A 22-g needle was used. Since KOH contributes to the dissociation of the aramid fibers, a small concentration change is expected to cause a large change in the dispersion state. The dispersion liquid conditions and spinning availability are presented in Table 2. As shown in Table 2, under a KOH concentration of 0.0071 wt%, KOH was incompletely dissolved and remained in the dispersion at a visible size. Because of these dispersion liquid conditions, continuous spinning was impossible. Therefore, we investigated the optical characteristics of the wet gel fibers obtained under KOH concentrations ranging from 0.0037 to 0.0053 wt% conditions. The transmittance spectrum, maximum transmittance, and transmittance difference obtained under each condition are shown in Fig. 4. Fig. 4c is an identification symbol used in later discussions. Figure 4a, b are transmittance spectra of wet gel fibers obtained under each KOH concentration. Regardless of the hydrochloric acid concentration, both the maximum transmittance and the transmittance difference decrease as the KOH concentration increases within the range of 0.0042-0.0045 wt% (Fig. 4c, d). As the KOH concentration increases, the fibril diameter decreases, which is thought to increase the number of pores and the pore size. Therefore, we thought that the graphs of maximum transmittance and transmittance difference indicated the above behavior due to the increased number of pores that would exhibit Mie scattering. Several of the transmittance spectra as shown in Figs. 2, 3, and 4 have a maximum at low wavelength. It was the influence of hydrochloric acid used in the coagulation bath. The transmittance spectrum of mixed liquid of acetone and hydrochloric acid is shown in Fig. S1.

Optical characteristics and microstructure of obtained aerogel fiber
Wet gel fibers obtained under the conditions described above were supercritically dried. We investigated the microstructure of these aerogel fibers to relate the scattering phenomenon and color of the aerogel fibers. SEM observations were performed to investigate the surface microstructure of the obtained aerogel fibers. The observation magnification was 100,000×. The conditions of the fiber samples observed are (A) and (B) in Fig. 2, (C) in Fig. 3, and (D) in Fig. 4. For   Fig. 3 Influence of aramid fiber concentration for optical characteristics of spun para-aramid wet gel fibers using 22-g needle: a transmittance spectrum (5 wt% hydrochloric acid), b transmittance spectrum (25 wt% hydrochloric acid), c maximum transmittance, and d transmittance difference these condition settings, using sample (A) as the criterion, the effects of the hydrochloric acid, aramid fibers, and KOH concentrations were investigated by comparing it with samples (B), (C), and (D), respectively. Next, the fiber diameter, density, porosity, and fibril diameter of aerogel fibers are presented in Table 3.
The SEM image of Fig. 5a shows that the pore size is tiny, and many pores are present. The scale bar in Fig. 5 is equivalent to 100 nm. Comparing the pore size with the scale bar shows that the pore size is ~ 30 nm, which satisfies the condition for Rayleigh scattering. In addition, we obtained the pore size distribution of aramid aerogel fibers by BET analysis. Absorption isotherm and pore size distribution are shown in Figs. S2 and S3, respectively. They showed that blue aramid aerogel fibers are composed of nano size pores, which size are approximately 10-20 nm. However, the pore size becomes huge when the hydrochloric acid concentration decreases (Fig. 5b). Furthermore, comparing the pore size with the scale bar indicates many pores exceeding 100 nm. It is thought that the pore size increased to the same size as the wavelength of visible light so that Mie scattering becomes dominant, resulting in cloudy fibers. Under high aramid fiber concentration conditions, the pore size does not change compared to the condition in Fig. 5a, but the number of pores decreases (Fig. 5c). It is thought that the number of aramid fibrils increased. Furthermore, the porosity of sample (C) decreased compared with sample (A). Under the high KOH concentration condition, the orientation of the aramid fibril changes, and the number of pores decreases (Fig. 5d). It was also observed that the fibril diameter decreased, and the fibril diameter could be controlled by varying the KOH concentration. SEM images show that the microstructure of sample (B) is different from the other samples. The process of forming crosslinks between fibrils at multiple points during fibril elongation is the same for all samples. In sample (B), the spacing of crosslinking points was wide, resulting in the fibrils forming an annular pore structure. In the other samples, the number of crosslinking points and the denseness of their spacing increased, forming a network that appeared to be a mixture of annular pore structures and oriented structures of fibrils. The difference in the topology among all samples is derived from the neutralization speed of aramid fibrils in the process of forming fiber structure. The optical characteristics of the aramid aerogel fibers were also investigated by the relative reflection spectrum (Fig. 6). The theoretical line derived from the theoretical equation of Rayleigh scattering by adjusting the number of scatterers is also shown in Fig. 4 Influence of KOH on optical characteristics of spun para-aramid wet gel fibers using 22-g needle: a transmittance spectrum (5 wt% hydrochloric acid), b transmittance spectrum (25 wt% hydrochloric acid), c maximum transmittance, and d transmittance difference  Fig. 6, assuming that the pores with the pore size obtained from Fig. 5a of the fiber surface exist throughout the fibers. The decrease in reflection from around a 450-nm wavelength can be attributed to the light absorption by the aramid fibers because the aramid aerogel fibers comprise only aramid fibrils, which are the backbone, and air. The reflection spectrum shape of each fiber sample showed that the reflectance increase as the wavelength decreases, and their behavior is similar to the theoretical line of the Rayleigh scattering. There are insignificant differences between the spectrum behavioral characteristics of samples (A) and (B), but they appear differently. Considering the SEM images and fiber appearance images shown before, sample (A) with a small pore size and dominant Rayleigh scattering appears bluish, whereas sample (B) with a large pore size and dominant Mie scattering has a cloudy appearance. As shown in the reflection spectrum of sample (A), it is thought that the background reflection of approximately 10% is the effect of Mie scattering. Therefore, we believe that suppressing it will result in more blue-colored aramid aerogel fibers. These results showed that the effect of hydrochloric acid concentration was dominant and that aramid aerogel fibers with blue structural coloration could be obtained by increasing the hydrochloric acid concentration.
Blue-colored aerogel fibers can be obtained under high hydrochloric acid concentration conditions, that is, fast neutralization speed conditions. However, as mentioned above, the effect of hydrochloric acid concentration culminates at 25 wt%. Moreover, the cause of Mie scattering remains unclear. Therefore, achieving an even denser structure from the fibers obtained at this stage cannot be achieved by changing the hydrochloric acid concentration. Also, we tried observing the cross-sectional microstructure of the aerogel fibers to investigate the cause of Mie scattering. Such observation was challenging because the cross-sectional microstructure was broken when the aerogel fiber was cut  during the preparation step for SEM. Therefore, most of the cross-section microstructure of aerogel samples could not be observed. However, while observing the surface microstructure of a sample, it was found that cracks had developed on the surface. Thus, we successfully observed the internal microstructure of the aerogel fibers through the cracks on the surface. The SEM image of the internal microstructure of the aerogel fibers is shown in Fig. 7. Figure 7 reveals a large pore structure inside the blue aerogel fibers. The white color of the aerogel fibers is thought to be due to Mie scattering caused by the countless large pores inside it. Additionally, we assumed that the speed of the coagulation bath liquid penetration into the fibers inside is late because of the surface and interior microstructural differences. Then, wet gel fibers were spun using a 26-g needle and a 28-g needle to improve the permeation velocity of the coagulation bath liquid into them. The reflection spectrum of the aerogel fibers under 25 wt% hydrochloric acid concentration and the abovementioned condition is shown in Fig. 8.
As shown in Fig. 8, the background reflection of the aerogel fibers that used 26-g and 28-g needles approach 0% as possible. This phenomenon can be attributed to the formation of a denser microstructure due to the improvement in the speed of the coagulation bath liquid penetration into the fibers inside. The results above show that the neutralization speed of fibrils affects the size of formed pores. The faster the neutralization speed, the smaller pores formed, and the slower it the larger pores formed. Under the narrow needle condition, it takes a short time for the coagulation bath liquid to reach the center of fiber. Therefore, the neutralization speed is faster not only on the fiber surface but also on the fiber inside, forming a denser microstructure. The fiber images also show that the white component was suppressed with decreasing fiber diameter.

Conclusion
Here, we prepared blue aramid aerogel fibers colored by Rayleigh scattering and investigated the effect of spinning and dispersion liquid conditions on the microstructure of the aerogel fibers and the color. There were many conditions, among which the hydrochloric acid, aramid fiber, and KOH concentrations were influential. Moreover, the effect of hydrochloric acid concentration is dominant; it was found that by increasing the hydrochloric acid concentration and speeding up the neutralization speed, a microstructure with many dense and small pores could be formed, resulting in aerogel fibers with a blue color. However, the obtained aramid aerogel fibers were slightly cloudy and were not perfectly blue-colored. Then, to improve the velocity of the coagulation bath liquid penetration into the fibers, the inner diameter of the needle used for spinning was made smaller, and aerogel fiber samples with suppressed Mie scattering were obtained. In the future, it is necessary to find a way to suppress Mie scattering and emphasize the blue color more.
Funding Open access funding provided by University of Fukui. This work was supported by JSPS KAKENHI Grant Number 19K05611.

Availability of data and material
The datasets during and analyzed during the current study available from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests The authors declare no competing interests.
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