The potential for high performance fiber from nylon 6

Abstract A review has been attempted in this article of the multidisciplinary character of the development of high performance fibrous materials. The effort has focused on both the theoretical recognition of the possibility of such materials and a recounting of significant aspects of the art for producing fiber with high performance characteristics. A knowledge of the primary valence bonds in a basic molecule, the atom configuration along the backbone of the molecule and the valence angles permits one to utilize spectroscopic inputs to compute the theoretical maximum modulus of elasticity in a given ideal fiber structure. An infinite replication of such molecules, all parallel one with the other is envisioned as this ideal structure. In order to obtain the ultimate modulus values, it is necessary to know the three-dimensional arrangements between chains, knowledge which is usually derived from X-ray diffraction studies. The ultimate tenacity, on the other hand, is dependent on some approximation of the intermolecular potential between polymer chains. The Morse potential, 61 a strictly empirical relation describing the force of separation of polymer segments, has proved to be one of the simpler methods for obtaining an estimate of ultimate tenacity. Other intermolecular potentials may or may not provide similar estimates. The Morse potential provides an explicit elongation at the point of break if this is based on maximum force of separation. It varies with the value of U 0 , the dissociation energy, and a parameter a , characteristic of an inverse length which is determined in turn by spectroscopic data. Based on the initial work of Flory, lattice techniques may be utilized for understanding and describing the development of order in infinite molecules when the ordering process is subjected to an external biasing probability imposed by the presence of some type of an ordering field. One could envision the macroscopic picture of extensional and stretching flows and similar macroscopic processes as resulting in the presence of this microscopic biasing probability. This probability influences the conditions for the formation of mesophases as a function of polymer concentration. Polymer nucleation has been described as being due to the development of a sequence of nuclei along a predetermined aligned structure. This structure is perhaps brought about by a particularly long molecular component accidentally extended in a particular direction due to some aspect of the microscopic processes. These processes are responsible for initiating and maintaining phase separation in the polymer solution or melt. Phase transformation of the ordered structures is carried out by the empirical art, leading to practical processes for the development of extended chain structures within the solidfying polymeric system. Quiescent crystallization from solutions of nylon 6 and similar polymers is often found to produce “lath-like” crystals. These are distinguished from the dynamically induced fibrous crystals known as shish kebabs. The latter had been found earlier in simpler polymeric systems that have led to high performance fibers, notably polyethylene. A casual examination of the crystal structures of nylon 6 formed by various extrusion and other proceses may suggest to the reader that the fibrous crystals of nylon 6 involve a structure where the hydrogen bonds between molecules tend to be aligned with the axis of the fibrillar structure. The molecules are oriented at right angles to the fibril axis. However, closer reading of this literature and review of the work by at least one author 131 suggest that another backbone habitat can be formed, at least in the case of nylon 4. The work of Pennings 98 involves a strong solvent which permits, in quiescent crystallization, the formation of lath-like crystals which have the hydrogen bonding coinciding with the axis of the lath, which was also reported by Kyotani. 124 This is to be contrasted with the work of the present authors, 157 involving the shearing of a highly concentrated polymer solution in meta -cresol, which suggests that fibrillar crystals may be obtained quickly in a threadline, producing a reasonably good fiber or filament which will have the proper orientation. This should involve an orientation of the primary backbone of the molecule parallel to the fibrillar axis. If this possibility exists, as one would expect from the theoretical work of Vasilenko, 73 we are led to believe that fibrils with the molecules oriented parallel to the fibril axis will be the dominant features in such fibers. Given that this is feasible, then one can lean on the results obtained in the commercial products known as Kevlar ® and Spectra ® , and conclude that a nylon 6 filament containing fibrils with microstructure of the kind suggested will, in fact, have a good chance to exhibit high performance characteristics. This is further supported by the Toyobo patent 19 and the more recent results of Pennings 21 which involved a different solvent from the ones used earlier, 98 suggesting that there is a possibility of producing a filament which has a tenacity of up to 1.16 GPa and modulus of up to 19 GPa. It is clear that a preferred process for the production of a high performance fiber may well involve a substantial departure from any traditional extrusion process, as evidenced by the work of Pennings on polyethylene. Even that early work on polyethylene has been further modified by Penning in an attempt to produce high performance properties in nylon 6. As further evidence of the use of nontraditional processes in forming high performance fibers we cite the work of Allied in producing high performance Spectra ® and of du Pont in producing high performance Kevlar ® . It, therefore, seems necessary that one should attempt to modify either a melt process or a solution process in trying to produce high performance fibers from nylon 6. Recently George 162,163 and Lim 164 have suggested that all high performance fibers involve at least a two-element sequence in which an initial fibril is converted into a chain-extended, close-packed form near the end of the total process of formation by a load induced phase change. Pennings 165 has shown this as well as Sawyer. 84 Very early work by Kauffman and George 166 deduced a change in the fiber diameter in the later stages of deformation of a drawn nylon 6,6 filament. In our opinion, the area that presents the most promise is the extrusion of plasticized melt. A similar sort of preliminary probe was reported by Sawyer in his Ph.D. thesis in 1984 involving polyethylene terephthalate, wherein he heavily plasticized the polymer and then melt extruded the result. The resulting fibers, while amorphous to X-ray diffraction, produced, when stretched and annealed in a restrained condition, extremely high birefringence. Small sections of this fiber also showed sonic moduli of 10 to 20 GPa. The principal result of this work seems to be that given an adequate molecular weight and a properly modified extrusion process it should be possible to obtain a high performance fiber having microfibrillar structure from the nylon 6 polymer system.