School of Fashion Design & Engineering, Zhejiang Sci-Tech University, China 3. Japan Corresponding author: Kazumasa Hirogaki;

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1 SYNTHESIS OF NANO-POROUS ARAMID AEROGEL FIBRE THROGH SUPERCRITICAL DRYING Kazumasa Hirogaki 1, Lei Du 1,2, Yumiko Suzuki 1, Isao Tabata 3, Teruo Hori 4 1 Frontier Fibre Technology and Science Course, Graduate School of Engineering, University of Fukui, Japan 2 School of Fashion Design & Engineering, Zhejiang Sci-Tech University, China 3 Technical Division, University of Fukui, Japan 4 Headquarters for Innovative Society-Academia Cooperation, University of Fukui, Japan Corresponding author: Kazumasa Hirogaki; hirogaki@u-fukui.ac.jp Abstract In order to overcome shortages such as fragility and inflammability of a typical aerogel based on silica or cellulose, it is reasonable to consider the use of paraaramid fibre which has high modulus, high strength and flame retardance. Paraaramid fibre used as the precursor was dissolved in the solution of dimethyl sulfoxide / tetrabutylammonium fluoride at 80 C, then it was cooled to room temperature and regenerated in a poor solvent and dried by supercritical CO2 to obtain the aerogel [1]. Acetone was the appropriate solvent for regenerating. Four kinds of solvents were used to regenerate the wet gel. It was found that the wet gel in the solvent of methanol, ethanol or water induced large shrinkage and opaque appearance, while the sample in acetone had less shrinkage in size and remained transparent (Fig. 1). Figure 1. Photographs of the wet gels: (a) solution before regeneration, (b) the wet gel substitution in the acetone

2 In Tab. 1, the most suitable condition of temperature was 80 C and the pressure was 10 MPa. The depressurization time was 3 hours with setting the releasing rate (delta p / MPa) as Under this condition, the volume of the gel remained as 43.5 % (shrinkage: 56.5 %) after drying. The critical point and the density of CO2 could significantly affect the drying of the samples. It shows that it is effective in preserving the nanoporous structure to keep the process of depressurization staying away from the critical temperature of 31.1 C and using the smaller density change at the critical pressure of 7.4 MPa of CO2. Rapid density change (Fig.2 No. 6, 7) would result in destruction of the nano-porous structure. Table 1. Conditions of supercritical CO2 drying and properties of aramid aerogel No. Temperature ( o C) Pressure (MPa) Depressuriza tion time (h) Shrinkage (%) Density (g/cm 3 ) Specific surface area (m 2 /g) Modal pore size (nm) Average pore size (nm) a a a The sample was dried at 40 o C for 4 hours and then the temperature was risen to 80 o C for releasing CO2. Figure 2. Density behavior of CO2 calculated with compressive factor at releasing CO2: (1) the condition of No. 1, (2) No. 3, (3) No. 6, (4) No. 7 in Tab. 1 The aerogel was characterized by the apparent density measurement, the scanning electron microscope observation, and the nitrogen adsorption method. The aerogel, which was prepared in the optimal condition of No. 3 in Tab. 1, has regular bundles

3 of molecular chains with the width about 20 nm (Fig.3 (b), (c)). The bundles of molecular chains cross-linked each other and formed the 3D-network structure. In this structure, there are lots of nanopores with the average pore size of 11.4 nm, and with the modal pore size of 0.77 nm. Besides, the nanopores were open-pores cross-linked with each other. The aerogel has large specific surface area of 510 m 2 / g and density is as low as g / cm 3 (up to 94.3% of porosity). It also exhibited yellow translucent appearance. We read a text through the aerogel with the thickness of 0.5 mm on the paper (Fig.3 (a)). (a) (b) (c) Figure 3. (a) photograph of the aerogel with the thickness 0.5 mm, (b) and (c) SEM micrographs of the aerogel prepared under the condition of No. 3 in Tab. 1 To overcome the fragility of the traditional aerogel, the para-aramid aerogel fibre with the diameter about 100 µm was prepared under the optimal drying condition of No. 3 in Tab. 1. The obtained aerogel fibre has translucent appearance and it was constructed from monofibrillar network of molecular chain bundles similar to the bulk aerogel as shown in Fig. 3. We bended and stretched the aerogel fibre freely without deforming or breaking of the gel. (a) (b) 100 nm 100 µm Figure 4. (a) Photograph and (b) SEM micrograph of the aerogel fibre prepared under the condition of No. 3 in Tab. 1 References 1. L. Du, I. Tabata, K. Hirogaki, Sen i Gakkaishi, (2014), 70(),

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