Fiber crystallinity

The tendency of the strong, highly crystalline fibers to fibnUate, ie, to develop a hairy surface on wet-abrasion has, for the textile appUcations, been minimized by process changes both in fiber production and fabric manufacture. However, for nonwoven or speciaUty paper appUcations, this property can aUow potential users to develop ceUulosic microfibers during processing. 

The viscosity range of CN products can be adjusted in advance by choosing the starting cellulose with an appropriate degree of polymerization (DP). A study of the different celluloses examined the impact of various cellulose properties, such as morphological factors (percent crystallinity, fiber length, and distribution), chemical composition (DP, ash content), and hemiceUulose and lignin content, on the nitration behaviors of cellulose (55). 

Water-Holding Capacity (WHC). AU polysaccharides are hydrophilic and hydrogen bond to variable amounts of water. HydratabUity is a function of the three-dimensional stmcture of the polymer (11) and is kifluenced by other components ki the solvent. Fibrous polymers and porous fiber preparations also absorb water by entrapment. The more highly crystalline fiber components are more difficult to hydrate and have less tendency to sweU. Stmctural features and other factors, including grinding, that decrease crystallinity or alter stmcture, may iacrease hydratioa capacity and solubUity. 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

Tubercles consisted of hard, hlack oxide shells overlaid with friable carbonate-containing deposits. In places, several laminate black magnetite shells existed. The outer crust could be crushed by gentle pressure with a finger. Tubercles were riddled with white crystalline fibers. Other detritus was incorporated into the tubercle core and crust. Metal loss was less than 0.030 in. (0.076 cm) below each tubercle. Wall thickness was almost 0.25 in. (0.64 cm). 

Hard, crystalline fibers may be cold drawn above M.W. 12,000 excellent strengtfii ... 

Alternatively, a fiber can be one member of a group or aggregate, as a fibril is one part of a fiber. Spherulites (Fig. 1. IH), fan-shaped fibrous crusts or rosettes, and the veins depicted in Fig. 1.1A are examples of aggregates created when many crystalline fibers grow in close proximity. During fibrous growth, lateral interference may produce planar but irregular bound-... 

Whiskers are synthetic crystalline fibers of variable size, but with diameters of usually less than 25 microns. An upper limit in diameter exists because the physical and chemical properties approach those of the bulk material as the diameter of fibrous sample increases. Since the purpose of synthesis is to take advantage of some characteristic property of the material in fibrous form, such as enhanced strength for small volume, the optimum material has a small diameter. For similar reasons the aspect ratio of useful whiskers is often well over 100. Whiskers can be single crystals, but many are polycrystalline aggregates of fibrils with preferred orientations. The compositions and crystal structures of the compounds synthesized as crystalline fibers also have the broadest possible variety (Brenner, 1958). 

A self-replicating system based on the catalytic action of reversed micelles has been presented in Chapter 1. Other cases of micellar catalysis have been discussed in Ref. 119. The use of semi crystalline fibers to immobilize catalysts and substrates was also proposed [120]. Another approach to the enhancement... 

Hoeve, C. A. J.., and A. Ciferri Limitations of the application to semi-crystalline fibers of thermoelastic relations for high elastic materials A reply to W. Prins. J. Polymer Sci. 60, 68 (1962). 

These changes are generally considered to be first-order crystalline transitions. The molecular conformation below and a-bove the 19° C transition are shown in drawings of molecular models in Fig.3 and the changes in the j I diffraction patterns of crystalline fibers at these transitions are illustrated in Fig. 4. Below 19° C the... 

One continuing and unresolved issue is whether or not a crystalline fiber is more desirable than an amorphous fiber. Thus, efforts to make amorphous, SiC-based ceramics are still an area of interest as suggested by the following section. 

In dermatan sulfate a vacuum UV circular dichroism study showed that the iduro-nate ring is present only in the C4 conformation [220]. This applies to the solution as well as to the dried films. The evidence obtained from X-ray diffraction on crystalline fibers is just the opposite. According to this study, iduronate should adopt a 4Cj conformation in the solid state [221]. 

Therefore, 80-pm cut-lengths were used for PET studies. For other more crystalline fibers, experimental conditions seem less critical. Table I shows the crystallinity indices for several cellu-losic fibers cut at 30pm and at 80 pm. Those indices do not seem sensibly different in this range of cut-lengths. 

The optical behavior of asbestos fibers viewed with crossed polars has been described. Crystalline fibers have positions of extinction 90° apart. The fact that crystalline fibers have retardation has also been mentioned. With crossed polars and a first order red plate in place, asbestos fibers will go from yellow to extinction to blue, back to yellow, etc., upon rotation of the stage. If the fiber bends, this is equivalent to a rotation of the stage and the color will change. If the fiber... 

It seems that the major effect at small X is to orient entire crystalline fibers along the stretching direction the void volume is then decreased and the film is more dense. Something else must occur for X > 2. Apparently, fibers begin to slide past one another without much internal change beyond X 6, cracks appear [88]. Orientation would then be limited by... 

The ultra-high molecular weight polyethylene fiber is a highly crystalline fiber with very high stiffness and strength. All of this results from some innovative processing and control of structure of polyethylene. 

X-ray diffraction measurements may be made on the fibrous material. Although the method is limited in application, it does enable definite statements to be made about the minimal length of the crystalline fiber component. Thus the minimal length of the anhydro-n-glucose polymer chain in ramie was found to be about 1000 A, and in wood, about 600 A. From these data it may be calculated that the degree of polymerization is greater than 200,and indeed may be found to be the approximate length of the basic molecular unit. ... 

Whiskers are single crystalline fibers with a diameter of several tenths of a im and lengths of 10 to 200 pm. They are characterized by maximum achievable mechanical properties. Thus, for example, tensile strengths of over 10 GPa have been measured with SiC-whiskers. 

Tipersul [Du Pont]. TM for fibrous potassium titanate, crystalline fibers 1 micron in diameter melting at 1371C, useful to 1204C. 

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

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