3.4. Preparation of RSSP(VLI) Fiber, Acetylated RSSP(VLI), and Acetylated RSSP(QQQ) Fibers

The preparation of RSSP(VLI) fiber was performed as follows. The RSSP(VLI) powder was dissolved in FA (conc. 20 w/v%) with a stirrer for 16 h at 40 °C and dissolution using rotation/revolution mixer (Awatori Rentaro^® ARE-310: THINKY Co., Ltd., Tokyo, Japan) within 4 h. After the FA solution of RSSP(VLI) was filtered with a 5 m steel filter, the FA solution was extruded through a stainless steel spinneret with 0.2 mm inner diameter into the MeOH coagulation bath at room temperature without an air gap [22,23,58,75,76,77]. The coagulation time of the fiber in MeOH was set for 1 min, and then, the fiber was stretched by 4 times with a manual uniaxial stretching machine (Imoto machinery Co. Ltd., Japan) and dried at room temperature. The fibers were used for further experiments, i.e., the determination of the mechanical properties, dimensional stability evaluation, water absorption experiments, and the ¹³C solid-state NMR measurements. The fibers of acetylated RSSP(VLI) and acetylated RSSP(QQQ) powders were prepared according to a similar method, mentioned above.