Crystalline state fibers

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Polyarylates prepared from cyclohexyl-HQ (Ch-HQ) and PEC (Ch-HQ/PEC) did not show liquid crystallinity due to the more bulky substituent on the HQ unit compared to those on f Bu-HQ and Ph-HQ. As-spun fibers of Ch-HQ/PEC exhibited lower moduli than those of fBu-HQ/PEC and Ph-HQ/PEC. Therefore, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state (7j — 7j,) is an influential factor, as shown in Table 19.1. 

Because as-spun fibers of Me-HQ/Cl-PEC showed a higher modulus than those of Ph-HQ/Cl-PEC, the flexural modulus of injection molded specimens of Me-HQ/Cl-PEC exhibited a higher flexural modulus than those of Ph-HQ/Cl-PEC due to the liquid crystalline state. Although flexural-fractured injection molded specimens of Me-HQ/Cl-PEC exhibited highly oriented fibrils, Ph-HQ/ Cl-PEC no longer displayed fibrils because of the lack of liquid crystallinity. 

The moduli of as-spun fibers of polyarylates depend highly upon the stability of the liquid crystalline state, the rigidity of the polymer chain, and the degree of elongational flow orientation (F-value). On the other hand, the moduli of injection molded specimens of polyarylates depend upon the stability of the liquid crystalline state, and the rigidity and packing density of the polymer chain. 

Finally, because the amylose molecule is a polyalcohol, chemical derivatives of it can be easily prepared. Included in this are the acetate, methyl, ethyl and similar derivatives, all of which belong to a class of polymers with completely different properties from the parent substance. Many of these derivatives demonstrate useful film and fiber properties, but they have not reached significant commercial utilization. All any lose derivatives crystallize easily, and many show interesting features in their crystalline state. 

Models for the dyeing of polyester fibers with disperse dyes have been developed [8], When the dye is applied from aqueous medium, it is adsorbed from the molecularly dispersed aqueous solution onto the fiber surface and then diffuses into the interior of the fiber. The following parameters determine the rate of dyeing and, to some extent, the leveling properties (1) the dissolution rate during the transition from the dispersed crystalline state of the dye into the molecularly dispersed phase, and (2) the diffusion rate at the fiber surface and, especially, in the interior of the fiber. The rates of both processes vary with temperature. 

Proceeding from the aforesaid, it is possible to admit, that modified hexsaazocyclanes PETP - fibres possess the greater degree crystalline state. It once again confirms the conclusion made earlier, that entered molecules hexsaazocyclanes become the additional centers of crystallization, thus, increasing a degree crystallization modified PETP - fibers. 

The response of the cotton fiber to heat is a function of temperature, time of heating, moisture content of the fiber and the relative humidity of the ambient atmosphere, presence or absence of oxygen in the ambient atmosphere, and presence or absence of any finish or other material that may catalyze or retard the degradative processes. Crystalline state and DP of the cotton cellulose also affect the course of thermal degradation, as does the physical condition of the fibers and method of heating (radiant heating, convection, or heated surface). Time, temperature, and content of additive catalytic materials are the major factors that affect the rate of degradation or pyrolysis. 

These have a rigid-rod molecular chain structure. The liquid crystalline state, as we shall presently see, has played a very significant role in providing highly ordered, extended chain fibers. 

Liquid crystalline compounds are remarkable because of their ability to show spontaneous anisotropy and readily induced orientation in the liquid crystalline state. When polymers are processed in the liquid crystalline state, this anisotropy may be maintained in the solid state and can readily lead to the formation of materials of great strength in the direction of orientation. A particularly important example of the use of this property for polymers is in the formation of fibers from aromatic polyamides which are spun from shear oriented liquid crystalline solutions Solutions of poly(benzyl glutamate) also show characteristics of liquid crystalline mesophases, and both of these types of polymers are examples of the lyotropic solution behaviour of rigid rod polymers which was predicted by Flory... 

The transition from crystalline to melt state, which is normal for crystalline polymers, is not observed with cellulose under normal conditions. It appears that the secondary bonds giving rise to the crystalline state are too strong and too numerous to be broken by a rise in temperature. Thermal degradation (beginning at ca. 180 °C) precedes melting under atmospheric pressure conditions. Nevertheless, a plastic deformation interpreted as melting has recently been reported for cellulose fibers exposed to laser radiation in a highly confined (pressurized) space [43]. The fracture surface of a thermoplastically deformed cellulose disc is shown in e Fig. 10. 

The spinning of fibers from the nematic liquid crystalline state may at least in principle result in fibrous structures exhibiting nearly perfect molecular orientation. Imperfections such as chain ends should then be randomly distributed. A large amount of work has been performed in recent years on semicommercialized LCP s and also on more research based LCP s (13-161. 

---