Dexing Shen

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.

Organo-soluble segmented rigid-rod polyimide films. Part 4.—Anisotropic structure and properties

A series of fully aromatic copolyimides have been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in m-cresol, (BPDA–PFMB)x-(PMDA–PFMB)y. The molar composition ratios are 100 : 0, 85 : 15, 70 : 30 and 50 : 50, respectively. The unoriented polyimide films with a thickness ranging between 10 and 20 µm have been characterized by an in-plane orientation of the c axis in the crystals as well as the molecular chain axis relative to the film surface. This in-plane orientation has been studied by transmission and reflection geometrical modes of wide-angle X-ray diffraction (WARD) and polarized Fourier-transform infrared spectroscopy (FTIR), and can be attributed to the rigidity and linearity of the chain molecules. The structural anisotropy leads to anisotropic behaviour in the glass-transition temperatures (Tg), the linear coefficients of thermal expansion (α), the dielectric behaviour (Iµ′ and Iµ″) as well as the refractive indices (n‖ and n⊥) parallel and perpendicular to the copolyimide film surface. Possible structure–property relationships are also discussed.

Organo-Soluble Segmented Rigid-Rod Polyimide Films Part 3: Effects of Copolymer Composition on Thermal Expansivity and on Relaxation Processes

A series of copolyimides based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA)(1,2,4,5,-benzenetetracarboxylic dianhydride) and 2,2′-bis(trifluoromethyl) benzidene (PFMB) have been synthesized with varying BPDA–PFMB and PMDA–PFMB compositions. A systematic change in the linear coefficient of thermal expansion (CTE) and the glass-transition temperature (Tg) is observed by changing the ratios of the BPDA–PFMB and PMDA–PFMB compositions. Both the CTEs and Tgs have been found to follow the molar-additivity rules. Dynamic mechanical experiments manifest two relaxation processes above room temperature. A subglass β relaxation, which is independent of the copolymer composition relative to the varying dianhydride composition, is found in the copolyimides studied. This β transition exhibits the same temperature and frequency dependence in all the copolyimides. An α transition (corresponding to the glass-transition temperature) is observed and is dependent on the PMDA–PFMB composition within the system. The possible molecular origin of the β relaxation is also discussed.

Organo-soluble segmented rigid-rod polyimide films. Part 5.—Effect of orientation

A series of semi-rigid aromatic polyimides have been synthesized via a one-step polymerization in which the poly(amic acid) precursors were not isolated. The polyimides were synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in refluxing m-cresol. Uniaxially oriented films were utilized in order to investigate the chain orientation in the ordered state. Wide-angle X-ray diffraction (WAXD) patterns showed an increase in the crystal orientation and the crystallinity for oriented CPI(100/0) films with increasing draw ratio. Dichroic ratios obtained under polarized Fourier transform infrared spectroscopy (FTIR) represented overall orientation factors attributed to both ordered and non-ordered regions, and they were a function of the incident angle and draw ratio. This indicates that the conformational arrangement and chain packing changed during deformation. The orientation along the molecular chain axis and the lateral orientation of the imide planes became increasingly enhanced with increasing draw ratio, an indication of planar alignment of the imide planes as well as uniaxial orientation of the molecular axis. The surface morphology of unoriented and uniaxially oriented films was also studied via transmission electron microscopy (TEM). The lateral crystal dimension of the ordered regions observed from TEM corresponded well to the apparent crystallite sizes of the (310) crystalline plane in BPDA–PFMB crystals obtained from WAXD experiments.

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.

A new semicrystalline, thermoplastic polyimide for carbon fiber-reinforced composites

Abstract A new semicrystalline polyimide (PI-2) has been prepared from 1,3-bis(4-aminophenoxy)-2,2-dimethylpropane and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride. The polymer, which has a glass transition temperature of 230°C and a melting point of 335°C, displays outstanding toughness with a fracture energy of 4·28 KJ/m 2 . The toughness may be associated with the polymers unique morphology, which consists of randomly stacked, ribbon-like lamellae. A reactive precursor approach has been used to prepare carbon-fiber reinforced composites that display interlaminar fracture energies ( G 1C ) as high as 1·81 KJ/m 2 .