Textures of cellulosic

SPAN module. It was mentioned at the beginning that the special polyacrylonitrile fibers of SPAN have a wall thickness of 30 gm, which is considerably thicker than the 8 gm wall thickness of the SMC modules [19]. As a consequence, the presence of stronger capillary effects from the special porous fiber material of the SPAN module would be a reasonable conclusion. Furthermore, the texture of the special polyacrylonitrile fibers is expected to have better surface properties, supporting the permeation of molecules as compared with synthetically modified cellulose. In conclusion, both convection and diffusion effectively contribute to the filtration efficiency in a SPAN module, whereas for the SMC membrane, diffusion is the driving force for molecular exchange, the efficiency of which is also considerable and benefits from the large surface-to-volume ratio. 

Figure 81 is an electron micrograph of section of cellulose fibre, enlarged 39,000 times. The micellar texture of the fibre is clearly visible. 

At the present moment it is difficult to decide which of the two hypotheses concerning the structure of cellulose is correct the idea of an amorpho-crystalline structure, or that postulating solely an amorphous texture. Nikitin assumes that the first hypothesis is the more probable, more especially as it is well in line with the most recent work of Zaydes and Sinitskaya [45] who conclude on the basis of electron diffraction investigations that in the natural cellulose fibre of Chinese nettle, there exist phases having a distinct microcrystalline structure. This suggests that structures shown in Figs. 78, 79 and 80 are the most probable. 

Further evidence in favour of the amorpho-crystalline texture was recently provided by Ranby [46]. In a series of his papers several questions connected with the microstructure of cellulose are made clear. Cellulose to be examined in an electron microscope was initially dispersed by means of ultrasonic waves. In this way Ranby has isolated elementary thread-like micelles of about 70 A dia. Any dimension characterizing the length of the micelle is however missing. 

The plant cell wall is a composite of cellulose (the main fibre) plus shorter lengths (hemicellulose) that help bind the fibre, plus pectin (the main matrix adhesive) and some proteins. There are fruits that have a soft melting texture when ripe (e.g., avocado and blackberry) in which the cell wall swells noticeably. This swelling is related to the degree of solubilisation of the pectin (Redgwell et al. 1997) which can be removed in vitro using enzymes or other chemicals. 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

The relaxation behavior, or the transient behavior of cellulosic liquid crystalline solutions upon the cessation of steady flow, is unique for LCPs. There are two kinds of relaxation. The bulk stresses relax quickly while the structures relax over a much longer time. Mewis and Moldenaers suggested that two levels of structures exist.Stress relaxation reflects fast relaxation at the molecule level and is independent of the previous shear rate. Structure relaxation reflects the gradual change of the textures. This slow process is unique in... 

For a cellulosic material such as wood chips or crop waste, cellulase enzymes cannot penetrate the structure and make glucose. A pretreatment is required to destroy the fiber structure and allow cellulase access to the substrate. Pretreatment is typically carried out at 180-250 °C for a few seconds to a few minutes in 0.5% to 2% sulfuric acid. The resulting material is of a muddy texture. An aqueous slurry of pretreated cellulose is made at 5% to 15% sohds. Cellulase enzymes are added at a concentration of 5 to 25 filter paper units per gram of cellulose. The slurry is stirred, and the enzymatic hydrolysis is carried out for 4 to 7 days. At this point, most of the cellulose is converted to glucose, and the unhydrolyzed residue consists primarily of lignin. 

Fig. 12.—Electron Micrograph of Mandibular Tendon of Lobster Dispersed with a Waring Blendor. (Fibrillar texture is similar to that of cellulose, with fibrils down to 150 A. visible.)...

Plant cell walls are of vital importance to the activity of cells and are also of significance to the texture of foods originating from plant sources. This type of cell wall is mostly composed of cellulose, pectin and hemicellulose, proteins,... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The strengths of the resulting dried sheets were tested by applying mechanical compression forces to determine the relative effects of the bacterial strains. As shown (Table 2), there were no significant differences between the textures of all bacterial cellulose strength levels derived from the three strains. Both coconut and pineapple juices yielded the same strength rating. The mechanical properties of bacterial cellulose, both air-dried and hot-pressed... 

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