Fiber rigid chain

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. 

At present, it is known that the structures of the ECC type (Figs 3 and 21) can be obtained in principle for all linear crystallizable polymers. However, in practice, ECC does not occur although, as follows from the preceding considerations, the formation of linear single crystals of macroscopic size (100% ECC) is not forbidden for any fundamental thermodynamic or thermokinetic reasons60,65). It should be noted that the attained tenacities of rigid- and flexible-chain polymer fibers are almost identical. The reasons for a relatively low tenacity of fibers from rigid-chain polymers and for the adequacy of the model in Fig. 21 a have been analyzed in detail in Ref. 65. 

Schaefgen JP, Bair TI, Ballou JW, Kwolek SL, Morgan PW, Panar M, Zimmermann J (1979) In Cifferti A, Ward IM (eds) Rigid chain polymers, properties of solution and fibers. Banking, England, p. 173... 

It is important to be able to regulate the degree of chain stiffness, as rigid chains are preferred for fiber formation whereas flexible chains make better elastom. The flexibility of a polymer depends on the ease with which the backbone chain bonds can rotate. Highly flexible chains will be able to rotate easily into the various available conformations, whereas the internal rotations of bonds in a stiff chain arc hindered and impeded. 

Objects for investigation are highly oriented flexible- and rigid-chain polymers in the form of films and fibers. In such samples, macromolecules are packed quite perfectly along the draw axis, and the most extended conformations are realized. 

Consequently, molecular segments, or, better still, whole molecular chains, must be oriented in some way and then fixed in position to form fibers or filaments. In principle, two procedures are suitable for this purpose spinning fluid systems and splitting oriented films. The spin technologies and final properties of the fibers or filaments depend on whether flexible chain molecules, rigid chains, or emulsions are to be spun. Whether spinning can be carried out with melts or solutions is another consideration. 

Liquid Crystaiiine Soiutions. Cellulose esters, when dissolved in the appropriate solvents at the proper concentration, show liquid crystalline characteristics similar to those of other rigid chain polymers (9) because of an ordered arrangement of the polymer molecules in solution. Cellulose triacetate dissolved at 30-40 wt% in trifluoroacetic acid, dichloroacetic acid, and mixtures of trifluoroacetic acid and dichloromethane exhibits brilliant iridescence, high optical rotation, and viscosity-temperature profiles characteristic of typical aniostropic phase-containing liquid crystalline solutions (10). Similar observations have been made for cellulose acetate butyrate (11), cellulose diacetate (12), and other cellulose derivatives (13,14). Wet spinning of these liquid crystalline solutions yields fibers... 

Figure 77 demonstrates different creep behavior in the narrow deformation intervals for ultimately drawn fiexible-chain and rigid-chain polymers, melt-crystallized and gel-cast UHMWPE films, and poly(paraphenylene terephthalamide) (PPTA) fibers (Kevlar 49, DuPont). Tensile stresses equal to a half of fracture stress CTf were applied. Total deformation to break was equal to 25%, 13%, and 1.32%, respectively, in these samples. To see better a difference in the development of creep process in the investigated polymers, the inserts in Fig. 77 are presented for very narrow deformation intervals. 

---