Heterocyclic aromatic polybenzoxazole(PBO) is a well-known frame retardant polymer which exhibits a superior thermo-oxidative stability, chemical resistance and good mechanical properties. However, drawback of PBO is soluble only in strong acids such as sulfuric acids and polyphosphoric acids but not organic solvents. Also, a poor solubility of PBO leads to the corrosion of the equipment and vicious processability. Therefore many researchers have studied to improve solubility of the disadvantage of the PBO.
Recently, Kantor''s group has reported a new way to obtain high performance flame retardant polymers using precursors to heterocyclic aromatic polymers. Polyhydroxyamide(PHA) of these categories have good mechanical properties and can be easily converted to PBO when ignited.
PHA solution in N, N-dimethyl acetamide (DMAc) containing lithium chloride (LiCl) was prepared for wet-spinning PHA fiber. Before wet-spinning, to understand the coagulation behavior, diffusion rate test of PHA dope was performed by Ziabicki theory with various coagulants and coagulation bath temperatures and the morphological features and mechanical properties of wet-spun PHA fibers were also investigated.
The best SEM observation indicated that wet-spun PHA fibers at 20℃ in EtOH had regular cross-section and then tensile strength and initial modulus were showed 130 MPa and 7.34 GPa, respectively.
Thus, structure features-mechanical properties-initial diffusion rate relationship were characterized and identified PHA fiber was well spun Ξ (×10-6cm2/s) ranging from 2.11×10-6cm2/s to 3.11×10-6cm2/s. Among these, it was adjusted that 2.11×10-6cm2/s was ideal conditions.
The structure features and mechanical properties and WAXS analysis of PHA fibers were investigated as a function of SDR ratio during spinning process at 20℃ in EtOH. The molecular orientation of PHA fibers was consistently enhanced with increasing SDR ratio. Also, The tensile strength and initial modulus of the PHA fibers increased with SDR ratio from 84.88 to 139.85 MPa and from 3.42 to 7.77 GPa, respectively. It was resulted that intensity peaks of PHA fibers were enhanced remarkably with increasing SDR ratio with annealing. After wet-spun PHA fiber at 20℃ in EtOH was annealed at over 350℃, the tensile strength of PHA and PBO fibers increased SDR 4.4 ratio from 139.85 MPa to 162.39 MPa and initial modulus of PHA and PBO fibers decreased SDR 4.4 ratio from 7.77 GPa to 6.96 GPa.