Fiber flexible chains

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

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. 

The elastomeric polymer is obtained by extending the prepolymer through its reaction with short-chain diols such as butanediol or diamines such as ethylene diamine, thus completing the formation of hard groups between soft, flexible chains. When amines are used, the final step is typically done in a polar solvent such as dimethyl acetamide. The conversion of these polymers into usable fibers may be accomplished by wet-, dry-, or melt-spinning operations, depending on the polymer. Additives to impart whiteness or improve resistance to ultraviolet radiation and... 

In conclusion then, we have synthesized a series of extended-chain, aromatic polyazomethlnes under on-degradatlve conditions. Fusible, tractable polymers were obtained by use of unsymmetrlcally placed substituents, copolymerization, and/or limited proportions of flexible chain units. Many of the polymers yield liquid crystalline melts which were spun Into oriented, high tenacity, high modulus fibers. The fibers were further strengthened by heat treatment. The ease of preparation of the aromatic polyazomethlnes and the outstanding tenacity and modulus range of the fibers make these products excellent candidates for use as reinforcing fibers In resins and rubber. 

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. 

The fiber-like structure of polypeptide block copolymers is essential in organogels that are formed by the side-by-side packing of polypeptide rods. Similarly, such side-by-side packing of polypeptide rods can also be found in cylindrical micelles self-assembled from polypeptide block copolymers. In both the fiber-like gels and cylindrical micelles, polypeptide rods assemble into ordered structures, while the flexible chains are spread out into the surroundings to stabilize the structures. 

There are now numerous compositions of liquid crystalline polymers under consideration as fiber spinning and injection molding materials. However, the problems involved in processing these systems are similar. In particular, how can one process these polymers to yield desirable isotropic properties or at least have biaxial orientation how can one achieve the optimum properties from a given composition and how does the chemical composition and structure affect the properties In flexible chain systems one must quench in orientation in a time scale which is faster than the relaxation process of the molecules. Typically there is a distribution of relaxation times in which the longest relaxation time is a matter of a few seconds. This longest relaxation time also governs a number of other flow characteristics. 

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. 

Spin Processes and Fiber Structure 38.3.1. Flexible Chain Molecules... 

The mobility of flexible chains in gels is well described by the biased reptation model [1], which is indicated schematically in Fig. 4. In the model, the fibers of the gel are coarse grained into a reptation tube that confines the chain. The chain thus slithers along the tube contour (the reptation part) under the influence of the electric field, which provides a tendency for the slithering motion to be in the direction of the electric field (the biased part). 

The cholesterol organogelator 53 contains two benzene chromophores linked by a flexible chain combining both amide and ethylene glycol sequences [77]. Interestingly, both hydrogen-bond formation and tt-tt interaction play a key role in explaining fiber formation. Electronic absorption spectroscopy studies showed the formation of H-aggregates in which the ben-... 

The highest values of crystallinity achieved after annealing of fibers of CPE-1 and CPE-2 are 35 and 25%, respectively. Both copolymers under study appear to be semicrystalline materials. In contrast to the majority of flexible-chain polymers, the non-crystalline phase of both stiff-backbone copolyesters is not an amorphous but a mesomorphous one. It was shown that this structure may be identified as an ordered LC smectic state. However, the main difference between the non-crystalline structures of CPE-1 and CPE-2 consists in periodic or aperiodic packing of layers within the LC smectic phase, respectively. 

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