D.J. Johnson

Electron-microscope studies of structural heterogeneity in pan-based carbon fibres

Thin longitudinal and transverse sections of Type I (2500°C), Type II (1500°C), and Type III (1000°C) PAN-based carbon fibres have been examined by dark-field and lattice-fringe electron microscopy. Skin/core heterogeneity has been found in Type I, but not in Type II or Type III fibres. Time of oxidation (5 hr or more) had no effect on the extent of the skin, which generally comprised about 5% of the fibre and was 100–150 nm in thickness. No evidence for the popular concept of a thick circumferentially organized sheath surrounding a radially arranged core has been found in our specimens. Crystallite organisation was parallel to the surface in the skin, and random in the core, thus the c axes of the layer planes are normal to the surface in the skin. Transverse sections show the convoluted nature of the surface region and the extent of layer folding in the transverse direction, but there was no evidence for discrete fibrils. Characterization parameters have been evaluated from selected-area electron-diffraction patterns after the application of a peak-resolution procedure. The mean defect-free distance Dc (perfect Lv) was 10.0 nm in the skin and 3.3 nm in the core, for a 3 denier fully stabilized fibre (Type I); lattice distortion was 0.6% and 1.5% and preferred orientation 14° and 26°, in skin and core respectively. The change in stacking size with aximuthal angle has been measured for all fibre types and is a function of heat-treatment temperature. Large misoriented crystallites have been observed in the skin of Type I fibres and are considered to be features limiting the intrinsic strength of this material.

Crystallinity and crystallite size measurement in polyamide and polyester fibres

Abstract X-ray diffraction methods for evaluating crystallinity and crystallite size in fibres generally neglect the possibility of peak overlap, and separate peaks and background by arbitrary procedures. The method described here is based on the resolution of normalized diffraction peaks in terms of combined Gaussian-Cauchy profiles for each peak, together with a polynomial background. Peak area crystallinity is then measured as the total area under the resolved peaks over a defined range. Measurements of apparent crystallite size are obtained from the widths of the resolved peaks. Application of this method to high tenacity nylon-6, nylon-6,6, and PET fibres after annealing, indicates that peak area crystallinity is about 100% for the polyamides and 85% for the polyester. The increase in apparent crystallite size is considered to be due to both an increase in the true mean size and to improvements in local lattice order.

Compressional behaviour of Kevlar fibres

Abstract The compressional deformation of poly(p-phenylene terephthalamide) (PPT) fibres of the Kevlar type has been followed by scanning and transmission electron microscope methods in order to explain changes in mechanical properties. A structural mechanism describing the mode of deformation is proposed which is based on the initial formation of kink-bands. The propagation of the latter appears to be unaffected by the presence of the axially pleated-sheet structure exhibited by Kevlar fibres. The proposed mechanism is consistent with an observed loss in tensile strength after compression. It is considered that the relatively poor compressional behaviour of aramid-type fibres arises from the weak lateral cohesion between their essentially rigid molecular chains, in agreement with the findings of Greenwood and Rose.

Crystallinity and crystallite size measurement in cellulose fibres: 1. Ramie and Fortisan☆

Abstract A new method for assessing crystallinity in cellulosic materials has been established after a rigorous computational analysis of X-ray diffraction traces from the highly crystalline fibrous forms of cellulose I and II, Ramie and Fortisan. The method involves resolution of the diffraction peaks into profiles which are part Gaussian (f) and part Cauchy (1—f); crystallinity is then a parameter dependent on f. A plot of the residual sum of squares gives a clear indication of the optimum f values, 0·7for Ramie and 0·4 for Fortisan in terms of the resolved profiles at these f values, Ramie has a crystallinity of 72% and Fortisan a crystallinity of 74%. Optimum crystallite sizes obtained from integral-breadth measurements are 53 A for Ramie and 37 A for Fortisan, and from peak-width measurements sizes are 64 A for Ramie and 50 A for Fortisan. The latter appear to overestimate the crystallite width when compared with fibrils observed in electron micrographs obtained at very high resolution with little phase-contrast enhancement; here electron-scattering units vary in width with mean values of around 50 A for Ramie and 40 A for Fortisan. There is no evidence for the concept that the elementary fibril in these celluloses is a discrete unit 35 A in width.

Longitudinal compressive behaviour and microstructure of PAN-based carbon fibres

Abstract The longitudinal compressive behaviour of HS and HM PAN-based carbon fibres has been examined by means of a direct compression method which can be applied to single carbon fibres. Longitudinal compressive strength was found to be ∼30 to 50% of tensile strength depending on the modulus levels in carbon fibres used. Longitudinal compressive modulus was estimated making use of the Euler buckling formula applied to buckled samples; compressive modulus was about 50% of tensile modulus. SEM observations in-situ revealed distinct kink bands in HM fibres, which usually develop by splitting failure; this implies local crystallite buckling and the existence of needle-like pores in HM fibres. The compressive strength in HS fibres decreases with increased apparent porosity. HM fibres exhibit smaller amounts of disorder and larger crystallites, and have increased modulus; it may be deduced that large needle-like pores also exist. Therefore, a compressive failure mechanism which involves local buckling failure of large crystallites with insufficient lateral support of the large needle-like pores may be suggested in the case of HM fibres.

Microvoids in aramid-type fibrous polymers

Abstract The presence of microvoids in aramid fibres has been suspected for some time. This communication reports their detection together with an investigation of their distribution and size in Kevlar 29 and 49 by electron microscopy and X-ray diffraction techniques. The voids appear to be located mainly around the periphery of the fibres and their dimensions measured from direct microscope examinations are compared with those obtained from a Guinier-type analysis of the low-angle X-ray diffraction data.

Crystallinity and crystallite size measurement in cellulose fibres: 2. Viscose rayon☆

Abstract A new X-ray diffraction procedure is established for the determination of crystallinity, crystallite size, and the relative proportions of cellulose II and cellulose IV in viscose rayons. The method involves resolution of the diffraction traces into peaks which are described by combined Gaussian-Cauchy functions, fG +(1− f ) C . The profile-function parameter f is considered to be a useful new measure of order. Samples were heat treated in dry nitrogen gas and in glycerol for the preparation of the cellulose IV modification which is produced by a real lattice transformation from cellulose II.

Thermotropic polyesters: Synthesis, structure and thermal transitions of poly(p-oxybenzoate-co-p-phenylene isophthalate)

Abstract A range of wholly aromatic polyesters has been prepared from p-acetoxybenzoic acid, hydroquinone diacetate, and isophthalic acid, by a modified melt acidolysis. X-ray diffraction indicated the presence of four different types of ordered structure dependent upon composition and thermal history. The 100 0 homopolymer poly(p-oxybenzoate) has a structure described as type A, and the 0 100 homopolymer poly(p-phenylene isophthalate) a structure described as type B. The 75 25 copolymer has pseudohexagonal order termed modified type A, which persists in the 67 33 and 50 50 copolymers. These latter two copolymers also contain a modified type B structure. Copolymers within the composition range 67 33 to 50 50 exhibit a nematogenic temperature range of at least 50°C; consequently, these materials may be regarded as potentially useful thermotropic copolymers based on readily available monomers.

The fine structure of lignin-based carbon fibres

Lignin-based carbon fibres heat treated at 1500 and 2000°C have been characterised by high-angle X-ray diffraction together with high-resolution electron microscopy. The 1500°C material reveals a poorly developed lattice structure together with a few more perfectly ordered inclusions. After treatment at 2000°C a wide range of basic textures are developed together with a variety of continuous and discontinuous inclusions of a highly graphitic nature. Graphitization processes for these inclusions are discussed in terms of a mechanism which will be enhanced by the catalytic effects of impurities in the precursor. A reasonable comparison can be made between values of the parameters Lc and La as estimated from lattice-resolved electron micrographs, and as evaluated from the {002} and {10} diffraction profiles.

Structure-compressional property relations in carbon fibres

Abstract Microstructural features of carbon fibres based on both polyacrylonitrile (PAN) and mesophase pitch (MP) are discussed in relation to their compressional performance estimated by the recoil test. The compressive strength of carbon fibres is influenced by a combination of various structural features and not by any single feature. In particular, the compressive failure strength increases linearly with increasing amounts of both inter- and intracrystalline disorder. For a given amount of disorder, the more recently developed PAN-based fibres exhibited much higher compressive failure stresses than older PAN- and MP-based carbon fibres; this is due to the more homogeneous distribution of microstructural features. In order to promote compressive strength it is suggested that all crystallite dimensions should be limited to below 5 nm whilst maintaining good orientation.