Zongquan Wu

A high-performance aromatic polyimide fibre: 1. Structure, properties and mechanical-history dependence☆

Abstract A new segmented rigid-rod polyimide has been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB). This polyimide is soluble in hot m-cresol, allowing fibres to be spun from an isotropic solution using a dry-jet wet spinning method. The as-spun fibres have low tenacities and low moduli, but they can be drawn at high temperatures (> 380°C) under tension to large draw ratios (up to 10 times), which produces a remarkable increase in strength and modulus. Drawn fibres display a tensile strength of about 25 g den−1 (3.2 GPa) and an initial modulus higher than 1000 g den−1 (130 GPa). BPDA-PFMB fibres show excellent thermal stability and retain relatively high strength and modulus at elevated temperatures. Annealed BPDA-PFMB fibres display distinct wide-angle X-ray patterns, from which a monoclinic unit cell has been determined. Furthermore, changes in crystallographic c-axis, apparent crystal sizes, degree of crystallinity, crystal orientation and thermomechanical properties have been observed with different draw ratios.

High-performance aromatic polyimide fibres: 2. Thermal mechanical and dynamic properties

Abstract A family of high-temperature, high-modulus aromatic polyimide fibres has been dry-jet wet spun from either its gel state or isotropic solution, followed by high-temperature drawing. In this report, thermal and dynamic mechanical properties of one of the family members, a segmented rigid-rod polyimide synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), are presented in detail. Mechanical properties of these BPDA-PFMB fibres can be improved remarkably by drawing due to drastic increases in overall orientation, crystal orientation and crystallinity. These three structural parameters, however, do not show parallel changes with increasing draw ratio. It has been observed that the linear coefficient of thermal expansion (CTE) of BPDA-PFMB fibres after drawing generally show negative values in the solid state when low stresses are applied during measurements. For as-spun fibres, the CTEs are constant over a certain applied stress region, which is on the same order of magnitude as CTEs of in-plane oriented BPDA-PFMB films along the film surface. This may be an indication that within this region the stress applied is at the same level as the internal stress frozen into the fibres during spinning and drawing. Glass transition temperatures (Tg) of as-spun fibres show a linear decrease at low applied stress region, then level off when the applied stress becomes high. Dynamic mechanical data indicate two relaxation processes in as-spun fibres above room temperature: an α relaxation corresponding to the glass transition and a β relaxation which is a subglass transition. In the fibres with a draw ratio of above three times, the α relaxation is totally suppressed. This reveals a rigid fraction (above Tg) dependence of this relaxation in the fibres. The β relaxation is, on the other hand, crystallinity dependent. The Arrhenius activation energy (about 160 kJ mol−1) of the β relaxation in as-spun fibres is about 50 kJ mol−1 lower than that of drawn fibres, indicating that the cooperativity of molecular motion in the fibre changes with orientation and crystallinity.

X-ray analysis and molecular modelling of the structure of aromatic copolyimides

Abstract X-ray diffraction and molecular mechanics modelling have been used to investigate the structures of two families of aromatic copolyimides. The first of these is synthesized by the reaction of 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) with a mixture of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA); the second is synthesized from BPDA and a mixture of o -tolidine (OTOL) and p -phenylene diamine (PPD). The X-ray fibre diagram of 70/30 copoly(BPDA-PFMB/PMDA-PFMB) is very similar to that of homopoly(BPDA-PFMB), except that it is more diffuse. It appears that the structure is highly blocky, and the main effect of introduction of comonomer is to reduce the crystallinity of the poly(BPDA-PFMB) blocks. In contrast, the X-ray data for the copoly(BPDA-OTOL/BPDA-PPD) are non-periodic along the fibre direction, and the layer-line positions are reproduced by a model consisting of parallel arrays of extended chains of completely random sequence. Interestingly, the correlation length for the extended chain is larger in the copolymer than in homopolymer BPDA-OTOL, in contrast to those seen for the copoly(BPDA-PFMB/PMDA-PFMB) series. It is suggested that this is due to the fact that ‘kink’ distortions are less disruptive in copoly(BPDA-OTOL/BPDA-PPD).

Thermal degradation mechanism and thermal mechanical properties of two high-performance aromatic polyimide fibers

Abstract Two organo-soluble aromatic polyimides have been synthesized by introducing pendant groups of trifluromethyl or methyl onto the 2- and 2′-positions of a biphenyl diamine and reacting each of them with biphenyltetracarboxylic dianhydride (BPDA): 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) or 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). These two polyimides, BPDA-PFMB and BPDA-DMB, are prepared via a one-step poly condensation method, and they can be fabricated into fibers, films, and other application forms. Our research shows that in fiber applications, these two fibers exhibit excellent mechanical properties and outstanding thermal and thermooxidative stability. Their long-term mechanical tensile performance at high temperatures is found to be critically associated with the type of side pendant groups at the 2- and 2′-positions of the diamines. High-resolution pyrolysis-gas chromatography/mass spectrometry (PyGC-MS) results show that the pyrorgams of these two polyimides possess more than...

High-performance aromatic polyimide fibers. 6. Structure and morphology changes in compressed BPDA-DMB fibers

Abstract A high-performance aromatic polyimide fiber has been spun from a high molecular weight polyimide synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). The fiber exhibits not only excellent tensile properties and high temperature resistance but also a high compressive strength of 655 MPa. Morphological observations of BPDA-DMB fibers indicate that the fiber shows a skin-core structure and microfibrillar textures. A banded texture can also be found with a spacing of about 2μm, which may be introduced by the liquid crystalline behavior that appears during processing. Compressed BPDADMB fibers form kink and microkink bands over different size scales. The detailed formation mechanism of these banded textures is discussed. The structure parameter changes during compression-including crystal unit cell parameters, apparent crystallinity, crystal and overall orientation, and apparent crystallite sizes-are monitored. It is found that after restr...

The crystal structures and thermal shrinkage properties of aromatic polyimide fibers

Three aromatic polyimides based on 3,3′,4,4′-biphenyl-tetracarboxylic dianhydride (BPDA) and three different diamines 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), 2,2′-dimethyl-4, 4′-diaminophenyl (DMB) or 3,3′-dimethylbenzidine (OTOL) have been synthesized. These polyimides are soluble in hotp-chlorophenol,m-cresol or other phenolic solvents. Fibers have been spun from isotropic solutions using a dry-jet wet spinning method. The as-spun fibers generally exhibit low tensile properties, and can be drawn at elevated temperatures (>380° C) up to a draw ratio of 10 times. Remarkable increases in tensile strength and modulus are achieved after drawing and annealing. The crystal structures of highly drawn fibers were determinedvia wide angle X-ray diffraction (WAXD). The crystal unit cell lattices have been determined to be monoclinic for BPDA-PFMB and triclinic for both BPDA-DMB and BPDA-OTOL. Thermomechanical analysis (TMA) was used to measure thermal shrinkage stress and strain. A selfelongation has been found in the temperature region around 450°C. This phenomenon can be explained as resulting from the structural development in the fibers as evidencedvia WAXD observations.

Unit-cell size change of the thermotropic copolyester-TPA/PHQ/PEHQ under different annealing conditions

Abstract Wide-angle X-ray diffraction (WAXD) experiments show an anomalous change of the crystal unit-cell parameters in the thermotropic copolyester, poly[phenyl- p -phenyleneterephthalate)-co-(1-phenylethyl- p -phenyleneterephthalate)], under different annealing conditions, namely, annealing the fibres with free ends, at fixed length, and under tension. The crystallographic volumes of the crystal unit-cells in the annealed copolyester fibres are larger than those in the as-spun fibres. Density measurements reveal a decrease of the overall density in the annealed fibres. This behaviour is attributed to a partial rearrangement of the large pendant side groups in this copolyester during annealing.

Thermal degradation of organo-soluble polyimides

The thermal degradation behavior of two organo-soluble polyimides was investigated by high resolution pyrolysis-gas chromatography/mass spectrometry. The pyrolyzates of the polymers at various temperatures were identified and characterized quantitatively. The relationship between the polymer structure and pyrolyzate distribution was discussed. The kinetic parameters of the thermal degradation were calculated based on thermogravimetric measurements. Finally, the thermal degradation mechanism for the polymers was suggested.