《纺织复合材料》课程参考文献（Fibers and Composites）01 FORMATION OF MICROSTRUCTURE IN MESOPHASE CARBON FIBERS.pdf_大学文库

1 FORMATION OF MICROSTRUCTURE IN MESOPHASE CARBON FIBERS J.L.White',B.Fathollahi,and X.Bourrat 1 Introduction Mesophase carbon fiber was invented in the 1970s,independently and simultaneously,from our viewpoint today,by Leonard Singer in the US(Singer,1978)and by Sugio Otani in Japan (Otani,1981).Both based their concepts on the role of the liquid-crystalline car- bonaceous mesophase described by Brooks and Taylor in 1965.Both recognized two key steps:flow of the anisotropic liquid in the shear-stress field of the spinneret to align the disk- like molecules,and oxidation thermosetting to stabilize the shape and microstructure of the fiber prior to carbonization. These inventions led to high expectations in the carbon materials community for the rapid attainment of fiber with superior properties at the low costs anticipated for a pitch product. Vigorous research activities ensued,many under conditions of proprietary secrecy.An impor- tant advance,with potential for many carbon materials besides fiber,was the development of more satisfactory mesophase pitches,fully transformed to the liquid-crystalline state and of low viscosity (Lewis and Nazem,1987;Mochida et al.,1988;Sakanishi et al.,1992). However for mesophase carbon fiber,the results fell short of expectations.The mechan- ical properties were not competitive with PAN-based fiber,except for some high-modulus grades.Low costs were never achieved,apparently due to the lengthy process of stabiliza- tion and the early 1990's saw downsizing and abandonment of research programs,with only a few products commercialized. Nevertheless the prospects for a carbon fiber spun in the liquid-crystalline state contin- ued to fascinate carbon scientists.Comprehensive microstructural studies initiated by Hamada et al.in 1987 demonstrated the remarkable flow memory of viscous mesophase in a simple spinneret.Then Bourrat et al.(1990a-c)showed that the nanostructure of as-spun filaments can be described in terms of the basic microstructural features of liquid crystals: bend,fold,splay,and disclinations. Some practical results of manipulating the flow of mesophase in the spinneret became apparent.In a 1990 patent,Hara et al.showed that a fine-weave screen placed across the mesophase stream flowing to the spinneret can profoundly alter the microstructure of as-spun fiber.In 1993,Taylor and Cross reported their study of screened flow prior to spinning;their observations were rationalized in terms of an array of fine mesophase cylin- ders,leading directly to the concept of a filament comprised of a linear composite of near- nanotubes.Then rheologists entered the scene to study the flow instabilities of a discotic liquid crystal under the flow conditions in a spinneret(Singh and Rey,1995,1998;Didwania et al.,1998),thus providing basic guidance for spinning experiments. In memorial of Jack White deceased in 2002. ©2003 Taylor&Francis

1 FORMATION OF MICROSTRUCTURE IN MESOPHASE CARBON FIBERS J. L. White† , B. Fathollahi, and X. Bourrat 1 Introduction Mesophase carbon fiber was invented in the 1970s, independently and simultaneously, from our viewpoint today, by Leonard Singer in the US (Singer, 1978) and by Sugio Otani in Japan (Otani, 1981). Both based their concepts on the role of the liquid-crystalline carbonaceous mesophase described by Brooks and Taylor in 1965. Both recognized two key steps: flow of the anisotropic liquid in the shear-stress field of the spinneret to align the disklike molecules, and oxidation thermosetting to stabilize the shape and microstructure of the fiber prior to carbonization. These inventions led to high expectations in the carbon materials community for the rapid attainment of fiber with superior properties at the low costs anticipated for a pitch product. Vigorous research activities ensued, many under conditions of proprietary secrecy. An important advance, with potential for many carbon materials besides fiber, was the development of more satisfactory mesophase pitches, fully transformed to the liquid-crystalline state and of low viscosity (Lewis and Nazem, 1987; Mochida et al., 1988; Sakanishi et al., 1992). However for mesophase carbon fiber, the results fell short of expectations. The mechanical properties were not competitive with PAN-based fiber, except for some high-modulus grades. Low costs were never achieved, apparently due to the lengthy process of stabilization and the early 1990’s saw downsizing and abandonment of research programs, with only a few products commercialized. Nevertheless the prospects for a carbon fiber spun in the liquid-crystalline state continued to fascinate carbon scientists. Comprehensive microstructural studies initiated by Hamada et al. in 1987 demonstrated the remarkable flow memory of viscous mesophase in a simple spinneret. Then Bourrat et al. (1990a–c) showed that the nanostructure of as-spun filaments can be described in terms of the basic microstructural features of liquid crystals: bend, fold, splay, and disclinations. Some practical results of manipulating the flow of mesophase in the spinneret became apparent. In a 1990 patent, Hara et al. showed that a fine-weave screen placed across the mesophase stream flowing to the spinneret can profoundly alter the microstructure of as-spun fiber. In 1993, Taylor and Cross reported their study of screened flow prior to spinning; their observations were rationalized in terms of an array of fine mesophase cylinders, leading directly to the concept of a filament comprised of a linear composite of nearnanotubes. Then rheologists entered the scene to study the flow instabilities of a discotic liquid crystal under the flow conditions in a spinneret (Singh and Rey, 1995, 1998; Didwania et al., 1998), thus providing basic guidance for spinning experiments. † In memorial of Jack White deceased in 2002. © 2003 Taylor & Francis

The zigzag bands,only a fraction of a micron in width,consist of well-aligned layers within each band,and the boundaries are sharply defined.The zigzag angle,referred to later as the ripple angle,appears not to be fixed,but tends to 90 in the outer bands.Note the presence of many dots and short dashes appearing in reverse contrast to the bands in which they occur.These appear to be +/disclination loops(Zimmer and White,1982)inherited from the mesophase pitch as it enters the spinneret;the contrast is due to the local rotation of mesophase molecules in the disclination loop.The density of dots is much higher in the skin,which may reflect the shear experienced briefly at the capillary wall as the stream exits the spinneret. In 1990,Bourrat et al.(1990a)published observations by high-resolution electron microscopy(HREM)to identify +m and-m wedge disclinations on transverse sections of mesophase fiber heat-treated to 1600C.Later these authors (Bourrat et al.,1990b,c) demonstrated the presence of +2 and-2m as well as +m and-m disclinations,along with bend,splay,and folding,in mesophase filaments in the as-spun condition.The presence of these liquid-crystalline structural features in finished fiber was confirmed by Pennock et al.(1993).Figure 1.4 is a lattice-fringe image of an Amoco P25 mesophase carbon filament (Bourrat et al.,1990c);the structural diagram locates the +m,-m,and -2 disclinations in the micrograph.From these observations,mesophase carbon fibers may be viewed as carbonized fossils of highly oriented mesophase streams with non-equilibrium microstructures frozen in place as each stream is swiftly drawn to a filament. Although an extensive patent literature has come to exist for mesophase carbon fiber, little information was published on the formation of microstructure within the spinneret until Hamada and co-workers at Nippon Steel undertook their comprehensive micrographic (a)】 (b) 10nm Figure 1.4(a)A high resolution lattice-fringe of an Amoco P25 mesophase carbon filament.(b)The structure diagram locates +m disclinations by U and-T disclinations by Y.From Bourrat et al.(1990c). ©2003 Taylor&Francis

The zigzag bands, only a fraction of a micron in width, consist of well-aligned layers within each band, and the boundaries are sharply defined. The zigzag angle, referred to later as the ripple angle, appears not to be fixed, but tends to 90 in the outer bands. Note the presence of many dots and short dashes appearing in reverse contrast to the bands in which they occur. These appear to be / disclination loops (Zimmer and White, 1982) inherited from the mesophase pitch as it enters the spinneret; the contrast is due to the local rotation of mesophase molecules in the disclination loop. The density of dots is much higher in the skin, which may reflect the shear experienced briefly at the capillary wall as the stream exits the spinneret. In 1990, Bourrat et al. (1990a) published observations by high-resolution electron microscopy (HREM) to identify and wedge disclinations on transverse sections of mesophase fiber heat-treated to 1600 C. Later these authors (Bourrat et al., 1990b,c) demonstrated the presence of 2 and 2 as well as and disclinations, along with bend, splay, and folding, in mesophase filaments in the as-spun condition. The presence of these liquid-crystalline structural features in finished fiber was confirmed by Pennock et al. (1993). Figure 1.4 is a lattice-fringe image of an Amoco P25 mesophase carbon filament (Bourrat et al., 1990c); the structural diagram locates the , , and 2 disclinations in the micrograph. From these observations, mesophase carbon fibers may be viewed as carbonized fossils of highly oriented mesophase streams with non-equilibrium microstructures frozen in place as each stream is swiftly drawn to a filament. Although an extensive patent literature has come to exist for mesophase carbon fiber, little information was published on the formation of microstructure within the spinneret until Hamada and co-workers at Nippon Steel undertook their comprehensive micrographic (a) (b) 10 nm Figure 1.4 (a) A high resolution lattice-fringe of an Amoco P25 mesophase carbon filament. (b) The structure diagram locates disclinations by U and disclinations by Y. From Bourrat et al. (1990c). © 2003 Taylor & Francis

studies,commencing with publication in 1987.The investigations included optical and elec- tron micrography,as well as x-ray and electron diffraction,applied to monofilaments spun from a spinneret as outlined in Fig.1.5a.The transverse microstructure,as-received from the pitch reservoir,or as modified by stirring before entrance to the capillary,was maintained with little loss of detail through extrusion and draw-down.The microstructural scale,as measured by the spacing of extinction contours,was found to be proportional to the diame- ter of the rod or filament,thus establishing the strong quantitative memory of viscous mesophase.When the stirrer was not in place,transverse sections of both extruded rods and spun filaments exhibited radial preferred orientation(PO),which was ascribed to conver- gent flow in the precapillary cone.Then the rapid extension and quench experienced in the draw-down cone were seen as critical factors in determining the final degree of radial orientation in the spun filaments (Hamada et al.,1990). Figure 1.5 illustrates schematically three types of monofilament spinnerets that have been used in exploring the formation of microstructure in mesophase fiber.Although some designs might be difficult to incorporate in an industrial multi-filament spinneret,their prin- cipal use at this point has been to demonstrate the wide range of microstructures that are accessible in spinning mesophase. Figure 1.6 illustrates four such microstructures(Fathollahi,1996)extruded from a low- viscosity mesophase pitch produced by alkylbenzene polymerization and pyrolysis(Sakanishi et al.,1992).Figure 1.6a is a polarized-light micrograph of an extruded mesophase rod,at a stage just prior to draw-down to filament;in this case no special manipulation was applied to flow in the spinneret.The microstructure is that expected of a nematic liquid crystal,but the scale is very fine,e.g.,disclinations can just be resolved.Sensitive-tint observations indicate a radial PO that strengthens with increasing radius.There are concentric markings in the rim (not shown here)that correspond in location to the zigzag bands of finished fiber. In recent years rheologists have turned their attention to modeling the flow of a discotic nematic liquid crystal through a spinneret.The flow of an anisotropic liquid comprised of disk-like aromatic molecules was found to be inherently unstable,and rippled and zigzag structures are to be expected when the liquid enters a shear field (Didwania et al.,1998; Singh and Rey,1998).In 1D extension,the molecules will align with their largest dimension parallel to the extension,and 2D extension(as in the wall of an expanding bubble)will effect stronger alignment than 1D extension (Singh and Rey,1995).The formation of +2m (a) (b】 (c) Pitch Pitch reservoir rrer Stirrer Screen Screen Quenching UD=1.33 tube U/D=2 D=150um D=140um L/D=1 Relaxation tube D=300μm Figure 1.5 Schematic designs of three monofilament spinnerets used in laboratory-scale spinning: (a)stirring within the spinneret,from Hamada et al.(1988);(b)screened flow,from Matsumoto et al.(1993);(c)stirring with screened flow and quenching capability,from Fathollahi et al.(1999a). 2003 Taylor Francis

studies, commencing with publication in 1987. The investigations included optical and electron micrography, as well as x-ray and electron diffraction, applied to monofilaments spun from a spinneret as outlined in Fig. 1.5a. The transverse microstructure, as-received from the pitch reservoir, or as modified by stirring before entrance to the capillary, was maintained with little loss of detail through extrusion and draw-down. The microstructural scale, as measured by the spacing of extinction contours, was found to be proportional to the diameter of the rod or filament, thus establishing the strong quantitative memory of viscous mesophase. When the stirrer was not in place, transverse sections of both extruded rods and spun filaments exhibited radial preferred orientation (PO), which was ascribed to convergent flow in the precapillary cone. Then the rapid extension and quench experienced in the draw-down cone were seen as critical factors in determining the final degree of radial orientation in the spun filaments (Hamada et al., 1990). Figure 1.5 illustrates schematically three types of monofilament spinnerets that have been used in exploring the formation of microstructure in mesophase fiber. Although some designs might be difficult to incorporate in an industrial multi-filament spinneret, their principal use at this point has been to demonstrate the wide range of microstructures that are accessible in spinning mesophase. Figure 1.6 illustrates four such microstructures (Fathollahi, 1996) extruded from a lowviscosity mesophase pitch produced by alkylbenzene polymerization and pyrolysis (Sakanishi et al., 1992). Figure 1.6a is a polarized-light micrograph of an extruded mesophase rod, at a stage just prior to draw-down to filament; in this case no special manipulation was applied to flow in the spinneret. The microstructure is that expected of a nematic liquid crystal, but the scale is very fine, e.g., disclinations can just be resolved. Sensitive-tint observations indicate a radial PO that strengthens with increasing radius. There are concentric markings in the rim (not shown here) that correspond in location to the zigzag bands of finished fiber. In recent years rheologists have turned their attention to modeling the flow of a discotic nematic liquid crystal through a spinneret. The flow of an anisotropic liquid comprised of disk-like aromatic molecules was found to be inherently unstable, and rippled and zigzag structures are to be expected when the liquid enters a shear field (Didwania et al., 1998; Singh and Rey, 1998). In 1D extension, the molecules will align with their largest dimension parallel to the extension, and 2D extension (as in the wall of an expanding bubble) will effect stronger alignment than 1D extension (Singh and Rey, 1995). The formation of 2 L/D=2 D = 140 µm Screen Stirrer L/D = 1.33 D = 150 µm Pitch reservoir .................. Quenching tube L/D=1 Relaxation tube D = 300 µm Screen Pitch reservoir Stirrer (a) (b) (c) Figure 1.5 Schematic designs of three monofilament spinnerets used in laboratory-scale spinning: (a) stirring within the spinneret, from Hamada et al. (1988); (b) screened flow, from Matsumoto et al. (1993); (c) stirring with screened flow and quenching capability, from Fathollahi et al. (1999a). © 2003 Taylor & Francis

disclination arrays in screened flow (described later) has been modeled using EricksenLeslie continuum equations (Didwania et al., 1999a). The analysis reveals a class of spatially periodic solutions to these equations for specific values of Leslie viscosities. An array of 2 disclinations, oriented along the flow direction is observed in the regions of negligible shear in the transverse plane. 3 Manipulation of mesophase flow in a spinneret Hamada’s observations (1988) of the proportional reduction of microstructure by spinning lead directly to a concept of microstructural miniaturization, in which flow is manipulated to produce a desired microstructure at a workable scale in the upper part of the spinneret, then this is reduced by a thousand-fold to a nearly identical nanostructure by convergent flow in the capillary and draw-down to the filament. Flow manipulation must be limited by the need for simple design because industrial spin-packs use multiple spinnerets to spin fiber tow with as many filaments as practical. Even simple stirring may be difficult if the stirrer must extend into each spinneret as in Fig. 1.5a. Spinneret design should also avoid 180º entry geometry, as in Fig. 1.5b, where the corners can create a vortex or weak secondary flow which can produce pyrolysis bubbles into the mainstream (Fathollahi, 1996). Some practical flow manipulations include the use of screens, perforated plates, or even just a single transverse bar or slot (Ross and Jennings, 1992). A Figure 1.6 Some effects of manipulating mesophase flow within the spinneret, as observed on transverse sections of extruded rods: (a) direct flow from pitch reservoir without manipulation, some radial PO is present; (b) flow with strong stirring, concentric PO can be produced; (c) flow through a single 200-mesh screen, with some relaxation after passing the screen; (d) flow through two screens of 400- and 50-mesh, oriented at 45 each other. Crossed polarizers. (a) (b) (c) (d) 10 µm 10 µm 10 µm 10 µm © 2003 Taylor & Francis

relaxation zone below the region of flow disruption may be useful to allow the decay of short- lived structures generated by flow instabilities(Didwania et al.,1999b). Stirring in the pitch reservoir may be desirable to maintain thermal and chemical homo- geneity,but this can induce concentric PO in the feed to the spinneret.Stirring also refines the scale of fibrous or lamellar microstructures entering the spinneret(Hamada et al.,1988). Strong concentric stirring at the spinneret entrance can introduce a concentric PO sufficient to outweigh the radial PO induced by convergent flow later in the spinneret,thus producing a concentric microstructure in the extruded rod(see Fig.1.6b). Wire screens are readily incorporated in a spinneret and,if the mesh is sufficiently fine, can profoundly alter the microstructure to the grid pattern seen in Fig.1.6c.Fine screens may need support by a coarse screen to withstand the stress involved in spinning at high lev- els of viscosity;this can produce the grid-within-a-grid microstructure seen in Fig.1.6d. Hara et al.(1990)appear to have been first to publish the use of screens to benefit the microstructure and properties of mesophase carbon fiber;their patent emphasizes the need for timely passage from screen to capillary in order to produce a clear reduced grid in the spun filament.The studies of screened flow by Matsumoto et al.(1993)confirm Hamada's rule that the scale of microstructure remains proportional to the diameter of the stream. Taylor and Cross(1993)used optical and electron microscopy to examine screened-flow microstructures and found the orthogonal arrays outlined in Fig.1.7,where the lighter lines represent traces of mesophase layers on the transverse section.Even in extruded rods,the extinction contours lie near the limit of optical resolution,but the v2-effect sketched into the diagram is helpful in recognizing,on a microscope with rotating stage,the orthogonal array of mesophase cylinders,each of which comprises a concentric +2m disclination.Thus the microstructures in Figs.1.6c-d indicate potential precursors for filaments consisting of a composite of nanotubes. Non-circular spinnerets have been used to produce particular shapes,such as ribbons, where the goal is not a particular transverse microstructure but highly oriented mesophase in convenient form for good thermal conductivity (Robinson and Edie,1996;Edie,1998; Lu et al.,2000).Matsumoto et al.(1993)have shown how a square or rectangular spinneret can be used to modify screened flow to give elegant Moire-like patterns of extinction contours. The regular array of +2m and-m disclinations shown in Fig.1.8 was observed in the ini- tial screened flow experiments (Fathollahi,1996).The balanced array of disclinations was produced by gentle flow through a 200-mesh screen.The potential feasibility of such detailed control of microstructure in the spinneret motivated the studies described next. 2 国 d2=2d1 Figure 1.7 The orthogonal grid sketched by Taylor and Cross(1993)to represent the microstruc- ture of a mesophase filament spun from a spinneret with a fine screen and observed on transverse section by crossed polarizers.On rotating the microscope stage by 45, the spacing of extinction contours changes by v2(Fathollahi,1996). ©2003 Taylor&Francis

relaxation zone below the region of flow disruption may be useful to allow the decay of shortlived structures generated by flow instabilities (Didwania et al., 1999b). Stirring in the pitch reservoir may be desirable to maintain thermal and chemical homogeneity, but this can induce concentric PO in the feed to the spinneret. Stirring also refines the scale of fibrous or lamellar microstructures entering the spinneret (Hamada et al., 1988). Strong concentric stirring at the spinneret entrance can introduce a concentric PO sufficient to outweigh the radial PO induced by convergent flow later in the spinneret, thus producing a concentric microstructure in the extruded rod (see Fig. 1.6b). Wire screens are readily incorporated in a spinneret and, if the mesh is sufficiently fine, can profoundly alter the microstructure to the grid pattern seen in Fig. 1.6c. Fine screens may need support by a coarse screen to withstand the stress involved in spinning at high levels of viscosity; this can produce the grid-within-a-grid microstructure seen in Fig. 1.6d. Hara et al. (1990) appear to have been first to publish the use of screens to benefit the microstructure and properties of mesophase carbon fiber; their patent emphasizes the need for timely passage from screen to capillary in order to produce a clear reduced grid in the spun filament. The studies of screened flow by Matsumoto et al. (1993) confirm Hamada’s rule that the scale of microstructure remains proportional to the diameter of the stream. Taylor and Cross (1993) used optical and electron microscopy to examine screened-flow microstructures and found the orthogonal arrays outlined in Fig. 1.7, where the lighter lines represent traces of mesophase layers on the transverse section. Even in extruded rods, the extinction contours lie near the limit of optical resolution, but the √2-effect sketched into the diagram is helpful in recognizing, on a microscope with rotating stage, the orthogonal array of mesophase cylinders, each of which comprises a concentric 2 disclination. Thus the microstructures in Figs. 1.6c–d indicate potential precursors for filaments consisting of a composite of nanotubes. Non-circular spinnerets have been used to produce particular shapes, such as ribbons, where the goal is not a particular transverse microstructure but highly oriented mesophase in convenient form for good thermal conductivity (Robinson and Edie, 1996; Edie, 1998; Lu et al., 2000). Matsumoto et al. (1993) have shown how a square or rectangular spinneret can be used to modify screened flow to give elegant Moire-like patterns of extinction contours. The regular array of 2 and disclinations shown in Fig. 1.8 was observed in the initial screened flow experiments (Fathollahi, 1996). The balanced array of disclinations was produced by gentle flow through a 200-mesh screen. The potential feasibility of such detailed control of microstructure in the spinneret motivated the studies described next. d1 d2 d2 = √2 d1 Figure 1.7 The orthogonal grid sketched by Taylor and Cross (1993) to represent the microstructure of a mesophase filament spun from a spinneret with a fine screen and observed on transverse section by crossed polarizers. On rotating the microscope stage by 45, the spacing of extinction contours changes by √2 (Fathollahi, 1996). © 2003 Taylor & Francis

(a (c) T Concentnc Helical Radial +2 +2 +2 2元 40um Figure 1.8 A regular array of wedge disclinations formed by mesophase after flow through a 200- mesh screen:(a)transverse section observed by crossed polarizers;(b)map of wedge disclinations defined by A=-m,D=+m,.=+2m;(c)wedge disclinations in a dis- cotic liquid crystal(Zimmer and White,1982).For a material in which the layers are parallel everywhere except at disclination cores,the total disclination strength over any appreciable field tends to zero. 3.1 Screened flow within the spinneret Here we summarize studies at University of California at San Diego to understand how microstructure forms within a spinneret,utilizing stirring,screening,and relaxation to manipulate flow.Spinnerets of various designs were used;that shown in Fig.1.5c is typical. All were machined from aluminum,for good thermal properties,and all incorporated quenching tubes to freeze structures in place when a desired spinning condition had been attained.Many spinnerets included one or two screens to establish cellular microstructures, and most included a relaxation tube to allow some decay of transient structures.The work described here used an alkylbenzene-based pitch(Sakanishi et al.,1992),fully transformed to mesophase,and supplied by the Mitsubishi Oil Co.The softening point is 285C,and flow temperatures ranged from 290 to 315C,corresponding to a viscosity range of 325 to 20 Pa.s. Upon passing through a plain square screen,a mesophase stream splits into a set of ministreams that rejoin below the screen to form an array of square cells(Fathollahi and White,1994).Below each screen wire,a weld-zone forms,as seen in Fig.1.9.The weld zone is narrow relative to the wire diameter,and initially the new and strongly oriented microstructure is finer than can be resolved by polarized light.The strong planar orientation in the weld zone result from 2D extension in passing the aperture (Singh and Rey,1995). Mesophase more centrally located in each cell develops "ripples"in the shear fields of ©2003 Taylor&Francis

3.1 Screened flow within the spinneret Here we summarize studies at University of California at San Diego to understand how microstructure forms within a spinneret, utilizing stirring, screening, and relaxation to manipulate flow. Spinnerets of various designs were used; that shown in Fig. 1.5c is typical. All were machined from aluminum, for good thermal properties, and all incorporated quenching tubes to freeze structures in place when a desired spinning condition had been attained. Many spinnerets included one or two screens to establish cellular microstructures, and most included a relaxation tube to allow some decay of transient structures. The work described here used an alkylbenzene-based pitch (Sakanishi et al., 1992), fully transformed to mesophase, and supplied by the Mitsubishi Oil Co. The softening point is 285 C, and flow temperatures ranged from 290 to 315 C, corresponding to a viscosity range of 325 to 20Pa.s. Upon passing through a plain square screen, a mesophase stream splits into a set of ministreams that rejoin below the screen to form an array of square cells (Fathollahi and White, 1994). Below each screen wire, a weld-zone forms, as seen in Fig. 1.9. The weld zone is narrow relative to the wire diameter, and initially the new and strongly oriented microstructure is finer than can be resolved by polarized light. The strong planar orientation in the weld zone result from 2D extension in passing the aperture (Singh and Rey, 1995). Mesophase more centrally located in each cell develops “ripples” in the shear fields of + + – –2 +2 Concentric +2 Radial +2 Helical +2 – 40 µm + +2 – (a) (c) (b) Figure 1.8 A regular array of wedge disclinations formed by mesophase after flow through a 200- mesh screen: (a) transverse section observed by crossed polarizers; (b) map of wedge disclinations defined by , D , ● 2; (c) wedge disclinations in a discotic liquid crystal (Zimmer and White, 1982). For a material in which the layers are parallel everywhere except at disclination cores, the total disclination strength over any appreciable field tends to zero. © 2003 Taylor & Francis