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
(a) (b) 10um 10μm 10μm Figure 1.6 Some effects of manipulating mesophase flow within the spinneret,as observed on trans- verse 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. disclination arrays in screened flow (described later)has been modeled using Ericksen- Leslie continuum equations (Didwania et al.,1999a).The analysis reveals a class of spa- tially periodic solutions to these equations for specific values of Leslie viscosities.An array of+2m disclinations,oriented along the flow direction is observed in the regions of negli- gible 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 180 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 ©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