The Relationship Between Structure Regulation And Properties Of Polyimide Fibers

Polyimide (PI) fibers have deserved much attentions recently owing to the unique rigid heterocyclic structures in the polymer backbone and therefore, they have been considered as one of the most promising high-performance polymeric fibers. Due to the outstanding mechanical, thermal as well as dielectric performances, PI fibers are widely utilized in aerospace, high-temperature resistance, microelectric industry and so on. Currently, a two-step technique is mainly adopted in preparing PI fibers, in which the poly (amic acid) (PAA) precursor is initially obtained through the polycondensation of dianhydrides and diamines in polar solvents. Accordingly, the as-spun PAA fibers are prepared via the wet-spinning method, and then are converted to final PI fibers by thermal or chemical imidization process. By means of the convenience during the procedures, the large-scale industrial production of PI fibers is more likely to be realized by the two-step technique. Unfortunately, the defects such as microvoids structures are readily generated in the fibers during dual-diffusion coagulation and thermal imidization process, which have strongly affected the mechanical performances of the PI fibers. In this regard, the comprehensive properties of PI fibers are modified by changing the chemical structure, spinning technology and imidization process. Meanwhile, the relationship between chemical structure, aggregation structure and properties of PI fibers is systematically evaluated, which will serve as a general rule in preparing high-performance PI fibers.With the purpose of obtaining high-performance PI fibers, a series of 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA)/4,4’-oxydiphthalic anhydride (ODPA)/p-phenylenediamine (p-PDA) (BOP) copolyimide (co-PI) fibers were obtained by incorporating the flexible monomer ODPA into the BPDA/p-PDA backbone, which had also enhanced the processability of the resulting PI fibers. It was found that with the increased ODPA contents, the mechanical properties of the PI fibers increased initially and then decreased. The optimum mechanical properties were obtained at a BPDA/ODPA molar ratio of 7/3 with the tensile strength of 1.58 GPa and initial modulus of 67.75 GPa. The introduction of ODPA led to the improved mobility of the polymer chains, which restricted the regular molecular arrangement with external drawing and consequently, resulted in the decreased molecular orientation of the macromolecules. Meanwhile, the homogeneous and impact structures in the fibers were obtained along with the enhanced mobility of the polymer chains, namely, the size of the microvoids in the fibers decreased simultaneously, which favored the drastically improved mechanical properties of the PI fibers. On the other hand, the 5% weight temperature of the PI fibers was in the range of 572-599 ℃ and 535-596 ℃ under nitrogen and air atmosphere, respectively, suggesting that the prepared PI fibers exhibited excellent thermal stabilities.On the basis of the structure-property relationship of BOP fibers, another flexible monomer 4,4’-oxydianiline (ODA) was introduced into the rigid BPDA/p-PDA polymer backbone. Subsequently, the BPDA/p-PDA/ODA (BPO) co-PI fibers were fabricated via a typical two-step wet-spinning method. It was identified that the optimum mechanical properties of the resultant PI fibers were achieved when the molar ratio of p-PDA/ODA was 5/5, with the tensile strength and initial modulus of 2.53 and 53.10 GPa, respectively. In the same manner, the addition of flexible monomer ODA destroyed the regular molecular packing patterns of the polymer chains, and thus leading to the reduced molecular orientation of the macromolecules along the fiber axial direction. However, the size of the microvoids structures decreased at the same time, attributing to the explicitly improvement in the mechanical performances of the BPO fibers. Moreover, the PI fibers possessed superior thermal properties with the 5% weight loss temperature ranging from 563 to 596 ℃ and 536 to 586 ℃ under nitrogen and air atmosphere, respectively.Moreover, the 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) was incorporated into the BPDA/p-PDA/2-(4-aminophenyl)-5-aminobenzimidazole (BIA)/ODA polymer chains, which was expected to reduce the dielectric permittivity of the resulting PI/epoxy composites. With the increased 6FDA moieties, the dielectric permittivity of the composites decreased from 3.46 to 2.78 in the frequency of 10 MHz. Also, the large free volume of -CF3 groups and strong electronegativity of F atom resulted in the poor intermolecular associations of the polymer chains, which inhibited close packing of the macromolecules and inevitably affected the mechanical as well as thermal performances of the PI fibers. Accordingly, it was found that with the increase of 6FDA contents, the mechanical properties of the PI fibers were strongly impaired with the tensile strength decreased from 2.56 to 0.13 GPa and initial modulus decreased from 91.55 to 2.99 GPa. With regard to fractured cross-section morphologies of the PI fibers, the defects such as macrovoids generated in the fibers along with the increased 6FDA moieties, which was of great significane in governing the comprehensive properties of the PI fibers. On the other hand, the 5% weight loss temperature of the PI fibers was found to decrease from 552 to 495℃ under nitrogen atmosphere, indicating that excessive amount of 6FDA contents have strongly influenced the mechanical and thermal properties of the PI fibers.Furthermore, the chemical and thermal imidization process were combined in preparing BPDA/ODPA/p-PDA fibers via a two-step wet-spinning method. The effects of different amount of dehydration reagents and different PAA concentrations on the structures and properties of the PI fibers were systematically investigated. With the increased amount of dehydration reagents and PAA concentration, the order degree of the macromolecules in the PI fibers gradually increased. Meanwhile, some PAA macromolecules were converted to PI macromolecules in the solution state, which limited the removal of micromolecules such as H2O during thermal imidization process. Therefore, the average size of the microvoids in the fibers was reduced, which resulted in the remarkably enhanced mechanical properties of the PI fibers. Besides, the thermal-oxidative stabilities of the PI fibers through the pre-imidization process were decreased as compared with those of pure PI fibers. It was caused by the residual dehydration reagents in the fibers which facilitated the decomposition of PI macromolecules.