Hongqing Niu

Structures and properties of polyimide fibers containing fluorine groups

A series of copolyimide (co-PI) fibers containing fluorine groups were successfully obtained based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), p-phenylenediamine (p-PDA), 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-oxydianiline (ODA) via a typical two-step wet-spinning method. The increased 6FDA moieties in the system resulted in unexpected great changes on the structures and properties of the resultant PI fibers. Regarding mechanical performances of the PI fibers, the tensile strength and initial modulus of the fibers decreased from 2.56 to 0.13 GPa and 91.55 to 2.99 GPa, respectively. Two-dimensional wide angle X-ray diffraction (2D WAXD) confirmed the existence of highly oriented structures along the fiber axial direction, while this feature gradually disappeared after the introduction of bulky trifluoromethyl pendant groups. SEM results suggested the presence of defects such as macrovoids structures with the increased 6FDA moieties. Besides, the dielectric permittivity was found to decrease from 3.46 to 2.78 in the frequency of 10 MHz as a result of the incorporation of 6FDA. Moreover, the co-PI fibers possessed excellent thermal-oxidative stabilities with the 5% weight loss temperature ranging from 495 to 552 °C under nitrogen atmosphere.

Enhanced surface free energy of polyimide fibers by alkali treatment and its interfacial adhesion behavior to epoxy resins

Abstract In this article, polyimide (PI) fibers were modified by alkali treatment, and PI fiber-reinforced epoxy composites were fabricated. The effects of different alkali treatment times on the surface properties of the PI fibers and the adhesion behaviors of PI fiber/epoxy composites were studied. The surface morphologies, chemical compositions, mechanical properties, and surface free energy of the PI fibers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, single-fiber tensile strength analysis, and dynamic contact angle analysis, respectively. The results show that alkali treatment plays an important role in the improvement of the surface free energy and the wettability of PI fibers. We also found that, after the 3 min, 30 °C, 20 wt% NaOH solution treatment, the fibers possessed good mechanical properties and surface activities, and the interlaminar shear strength of the composites increased to 64.52 MPa, indicating good interfacial adhesion properties.

Effect of pre-imidization on the structures and properties of polyimide fibers

A series of polyimide (PI) fibers derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-oxydiphthalic anhydride (ODPA) and p-phenylenediamine (p-PDA) were successfully prepared through a partially pre-imidization process, and the effects of different amount of dehydration reagents and initial poly(amic acid) (PAA) concentrations on the structure–property relationship of the resultant PI fibers were systematically investigated. The results showed that both the increased amount of dehydration reagents and PAA concentration could result in the ordered molecular packing arrangement of the polymer chains and gradually formed homogeneous structures in the fibers, which are proposed to be essentially dominated for the effectively enhancement in the mechanical properties of PI fibers. Moreover, the PI fibers obtained through the pre-imidization process still exhibited excellent thermal-oxidative stabilities, although the 5% weight loss temperature of the PI fibers was slightly decreased compared with that of the pure PI fibers as a result of the residual dehydration reagents in the fibers. Consequently, the present work provided a new approach in preparing high-performance PI fibers through a partially pre-imidization process.

Surface modification of polyimide fibers by oxygen plasma treatment and interfacial adhesion behavior of a polyimide fiber/epoxy composite

Abstract Oxygen plasma was used to enhance the surface behavior of polyimide (PI) fibers and PI fiber-reinforced epoxy composites were prepared in our present work. The effects of plasma treating times on the surface properties of PI fiber and the interfacial adhesion of PI fiber/epoxy composites were investigated. Surface chemical composition, surface morphologies and surface free energy of the fibers were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy and dynamic contact angle analysis, respectively. The results suggest that some oxygen functional groups were introduced onto PI fiber surfaces, and the surface roughness of fibers was enhanced. Resultantly, the surface free energy of fibers and the interfacial adhesion of composites were improved by the oxygen plasma treatment. The interlaminar shear strength of the composites increased to 70 MPa when the fibers were treated for 10 min, which proved good interfacial adhesion properties.

Effects of using polyacrylonitrile on the thermal, morphological and mechanical properties of polyimide/polyacrylonitrile blend fibers

High-performance polyimide/polyacrylonitrile (PI/PAN) blend fibers were prepared through wet-spinning of the polymer blends of polyamide acid (PAA) and PAN solutions followed by a thermal treatment in air. During the heat treatment process, PAA was imidized into its final heteroaromatic PI form with the concomitant evolution of PAN into its ladder-like preoxidized form, thus yielding PI/PAN blend fibers with desirable properties. The structure evolution was traced by fourier transform infrared radiation spectrometer, differential scanning calorimetry and thermogravimetry, implying that the PAA moiety in the blend fibers played a role as initiator for the cyclization of PAN. The final PI/PAN blend fibers exhibited a single glass-transition peak at 326 °C and a smooth surface without any pores. Moreover, the PI/PAN blend fibers possessed the tensile strength of 1.06 GPa and modulus of 59.9 GPa.

Polyimide Fibers with High Strength and High Modulus: Preparation, Structures, Properties, and Applications

High strength and high modulus polyimide (HSHMPI) fibers are a type of novel high-performance organic fiber with an initial modulus higher than 90 GPa and extremely high tensile strength over 2.5 GPa realizing broad applications in the fields of electronic, engineering, aerospace, and atomic energy industries. There are currently two synthetic pathways, i.e., one-step and two-step methods, developed for the manufacture of HSHMPI fibers. An integrated fabrication process involving wet-spinning followed by thermal imidization is accepted as a typical two-step synthetic route for industrialization of HSHMPI fibers. In this article, the classification, synthetic method and technology, molecular structure, morphology, microstructures, and properties of HSHMPI fibers are summarized extensively. The effects of molecular structure and synthetic technology on the microstructures and overall performance of HSHMPI fibers are discussed. In addition, the trend in development and the application prospect of HSHMPI fibers are analyzed accordingly.

Structure–property relationship of carbon fibers derived from polyimide/polyacrylonitrile blends

A series of polyimide/polyacrylonitrile (PI/PAN) blend fibers with different PAN weight ratios were prepared through a two-step wet-spinning method and stabilization process and then were carbonized at 1500°C under high-purity nitrogen atmosphere, yielding in PI/PAN-derived carbon fibers. The effects of PAN content on the structures of the PI/PAN blend fibers were systematically investigated. The elemental composition, aggregation structure, carbon yields, and electrical properties of the PI/PAN-derived carbon fibers were also analyzed. The imidization degree and molecular orientation of the PI/PAN blend fibers increased first and then decreased with increasing PAN content, which directly affect the aggregation structures and properties of the corresponding PI/PAN-derived carbon fibers. As a consequence, the carbon fibers derived from PI/PAN-35% exhibited perfect graphite structure with a planar spacing d002 of 0.349 nm, high carbon content of 97.14%, and low electrical resistivity of 1.89 × 10−5 Ω·m, attributing to the high degree of orientation along the fiber axis and low value of φa/φc in the PI/PAN blend fibers.

High performance polyimide fibers

Abstract High-performance aromatic polyimide (PI) fibers are considered as one of the most promising engineering materials in the class of polymeric fibers because of their excellent mechanical properties, superior chemical and radiation resistance, outstanding thermal-oxidative stabilities, and unique electric as well as dielectric properties. With a number of great progresses in the past decades, they offered tremendous opportunities for PI fibers to be utilized in widespread uses such as electric, microelectronics, engineering, and aerospace applications. To further optimize the performances of the resulting PI fibers to meet the requirement for industrial development, the relationship between structures and properties of the PI fibers have been systematically investigated. Herein, the effect of preparation methods, spinning conditions, and chemical structures on the comprehensive properties of the resultant PI fibers are reviewed, which will provide useful information to establish a general rule in designing and preparing high-performance PI fibers.