Fracture behavior in nylon 6 fibers

A reaction rate model of fracture in polymer fibers is described. This model assumes that bond rupture is governed by absolute reaction rate theory with a stress-aided activation energy. It is demonstrated that the key in obtaining good agreement between the model and experiment lies in taking proper account of the variation of stress on the tie-chain molecules. The more taut chains rupture first, and the load is redistributed among the remaining unruptured tie chains. The effect of varying the temperature both in the model and in experiments on fracture in fibers is explored. Good agreement between predictions of the model and experiment is possible only with an undeterstanding of the distribution in stress on the tie chains. The distribution in stress on the chains was experimentally determined by monitoring the kinetics of bond rupture with electron paramagnetic resonance (EPR) spectroscopy. Temperature is found to have two effects on macroscopic strength. (1) The thermal energy aids the atomic stress in breaking the atomic bonds; as a consequence the rate of bond rupture of a family of bonds under a given molecular stress is increased. In this respect temperature might be viewed as decreasing the “strength” of a bond. (2) Temperature also serves to “loosen” the molecular structure and in this way modify the distribution in stress on the tie chains. To explain bond rupture and macroscopic fracture behavior quantitatively, account must be taken of both effects.     

Drawing of nylon 6 fibers (bristles)

By means of electron microscopy of surface replicas and both small-angle and wide-angle x-ray scattering, nylon 6 fibers were investigated in the as-spun state, after drawing at 180°C to a draw ratio up to 4.95, and after subsequent annealing. As spun, the fiber exhibits a small fraction of row-nucleated cylindrites and a great many spherulites (with an average diameter of a few microns) side by side. Drawing deforms the spherulites into spindle-shaped structures (λ = 2) and subsequently produces well-aligned microfibrils. Small-angle x-ray scattering yields a two-point diagram at small λ and a fourpoint diagram at high λ. The long period seems to decrease slightly with draw ratio. Annealing at temperatures above the temperature of drawing increases the long period to a greater extent with samples of lower λ. The crystal lattice orientation is nearly complete at λ = 4.95.     

Creep failure in highly oriented nylon 6 fibers

Creep failure in oriented nylon 6 fibers has been studied. The results suggest that the variations in the lifetime under various loading histories are inherent, but statistical, characteristics of the material itself. The treatment of experimental data by a stochastic theory shows that the creep failure can be regarded as a nucleation process. An interpretative analysis of the structural changes during creep indicates that the nucleation is brought about by bond rupture in the amorphous regions of the fiber structures.     

The fatigue process in highly oriented nylon 6 fibers

The micromechanism of the fatigue process in highly oriented nylon 6 fibers is discussed on the basis of changes in mechanical and structural properties during fatiguing. Experimental results show that the fatigue process can be divided into two stages. The characteristic features in the initial period are increases in breaking strength, long period, and molecular orientation, and a reduction in dye penetration. In the second period, after about 500 cycles, breaking strength and orientation decrease slightly, and the long period, permanent strain, and dye penetration increase with duration of fatiguing. It is demonstrated that the structural changes mainly occur in the amorphous regions of the fiber structure. The structural and mechanical changes in the initial period lead to the conclusion that the initial cyclic strain causes strain hardening caused by extended tie chains which do not rupture. A combination of load bearing by tie chains and sliding motion of the fibrillar elements can explain the progressive degradation of the fiber during the second stage of fatiguing.     

Production and properties of fibers spun from nylon 6/lithium chloride mixtures

The possibility of producing high-modulus nylon 6 fibers by incorporation of lithium chloride (LiCl) in the polymer prior to spinning and drawing has been examined. Samples containing 2% and 4% LiCl (w/w) together with an unsalted control were studied. Particular attention was given to optimizing the spinning process by varying the melt temperature and the draw-down. The spun fibers were subsequently drawn in a tensile testing machine at 135°C, preliminary studies having established that this was desirable for the production of high-modulus material. The influence of annealing after drawing was also examined. Drawn fiber moduli in the range 8–9 GPa were obtained, compared with ca. 5–6 GPa for unsalted material. Limited structural studies (birefringence and wide-angle x-ray diffraction) suggest that the enhancement of modulus is due to an increase in the stiffening effect of extended molecules in the noncrystalline regions. Dynamic mechanical measurements show that there is reduced chain mobility in the disordered regions of the polymer, suggesting strong polymer-ion interactions. The salt can be readily removed by washing the fibers in boiling water, with significant reduction in moduli. This militates against commercial application of the salted fibers.     

Structures for monodisperse oligoamides. A novel structure for unfolded three-amide nylon 6 and relationship with a three-amide nylon 6 6

Three-amide oligomers of nylon 6 and nylon 6 6 have been investigated using electron microscopy (imaging and diffraction), X-ray diffraction, and computational modeling. A new crystal structure has been discovered for the three-amide oligomer of nylon 6. This material crystallizes from chloroform/dodecane solutions into an unfolded crystal form that has progressively sheared hydrogen bonding in two directions between polar (unidirectional) chains. This structure is quite different from the usual room temperature α-phase structure of chain-folded nylon 6 crystals, in which alternatingly sheared hydrogen bonding occurs between chains of opposite polarity in only one direction. The occurrence of this new structure illustrates the extent to which progressively sheared hydrogen bonding is preferred over alternatingly sheared hydrogen bonding. Indeed, the progressive hydrogen bonding scheme occurs in the three-amide nylon 6 material even though it requires a disruption to the lowest potential energy all-trans conformation of the chain backbone, and requires all the chains in each hydrogen-bonded layer to be aligned in the same direction. We believe the presence of chain folding, which necessarily incorporates adjacent chains of opposite polarity into the crystal structure, prevents the formation of this new crystal structure in the nylon 6 polymer. In contrast, the three-amide nylon 6 6 crystal structure is analogous to the polymeric nylon 6 6 α-phase structure, found in both fibers and chain-folded crystals, and consists of progressive hydrogen-bonded sheets which stack with a progressive shear. In both structures, the molecules (≈ 3 nm in length) form smectic C-like layers with well-orchestrated stacking of 2.2 nm to form a three-dimensional crystal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2849–2863, 1998     

High-temperature annealing of drawn nylon 66 fibers

The structure and morphology of highly oriented fibers were modified by thermal treatments at varying tensions. The structural changes were characterized by wide- and small-angle x-ray diffraction, broadline nuclear magnetic resonance, and electron microscopy of surface replicas. Increasing temperatures caused increases in local ordering, number of regular chain folds, mobile fraction or amount of fluidlike mobility, and surface recrystallization in localized areas. Each of these changes was also a function of the amount of tension on the fiber during the annealing; each change was maximized when the fiber was free to shrink but minimized when the fiber was stretched.     

Annealing experiments on drawn nylon 66 fibers

Drawn, nylon 66 yarns have been annealed in oil under relaxed conditions. Shrinkage and tensile modulus were measured and the yarns were examined by wide- and small- angle x-ray diffraction. The results are similar to those obtained by Dismore and Statton, but there are significant differences. The data indicate a model for a drawn nylon 66 fiber in which substantial amounts of folds remain.     

d.c. conductivity and structure of nylon 6-LiCl mixtures

d.c. Conductivity experiments have been performed for a wide range of temperature under a constant voltage application for nylon 6 and its mixture with 4% w/w LiCl. The data confirm previous indications of decrease of crystallinity and increase of glass transition due to lithium halides. Support is also given to previous interpretations of the conduction mechanism for polyamides, i.e. essentially electronic below 80° but essentially ionic above 100°.     

Effect of processing consitions on the development of morphology in clay nanoparticle filled nylon 6 fibers

The effect of melt temperature on the phase behavior and preferential orientation development in Nylon 6/montmorillonite nanocomposites were investigated at melt spinning temperatures ranging from 230° to 250°C. The fibers were found to exhibit mostly γ crystalline form that is typical of Nylon 6 filled with montmorillonite nanoparticles. At higher take-up speeds α-crystals begin to appear in the crystalline phase. The presence of nanoparticles was found to impart substantial chain orientation levels even at low to moderate take up speeds reaching a plateau at moderate take up speeds. This was attributed to the increased spin line stress in the presence of nanoparticles that increase the overall viscosity due to their large contact areas with the polymer chains. This increased spinline tension was found to cause fiber breakup at moderate speeds. Increasing melt temperature from 230° to 250°C alleviated this problem.     

Solution crystallization of nylon 6

Electron microscopy and x-ray diffraction data have been obtained on nylon 6 which has been crystallized from solutions in 1,6-hexanediol and 1,2,6-hexanetriol. Lamellar single crystals and spherulites of the γ form are obtained by crystallization from 1,2,6-hexanetriol. The morphology of the single crystals is different from that obtained from glycerine solutions. The spherulites of the γ form are composed of larger lamellae. Sheaflike crystals of the α form are obtained from both solvents. α-form and γ-form crystals both grow from 1,2,6-hexanetriol at appropriate crystallization temperatures. α-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 56 to 66 Å. The melting behavior of the solution-grown crystals is examined and discussed. Effects of solvent on growth of the two crystalline forms from solution are investigated.     

Electron paramagnetic resonance study of mechanically degraded poly(ethylene terephthalate) and nylon 6 fibers

Peroxide radical concentrations were measured from both PET and nylon 6 fibers mechanically stretched in air at room temperature and quickly quenched into liquid nitrogen. The radical concentrations depend on degree of stretching as well as conditions under which the fibers were made, i.e., morphology. Drawn fibers of PET and nylon 6 produced peroxide radical concentrations of the same order of magnitude at the breaking points. These results indicate that chain scissions occur both in PET and nylon 6 under mechanical stretching.     

Annealing of melt-crystallized nylon 6

Effect of annealing on thermal behaviour and crystalline structure of meltcrystallized nylon 6 has been investigated.The annealing process is found to be characterized by an incubation period followed by a more or less doubling of the SAXS long spacing and of the crystallinity.The extrapolated heat of melting of the crystalline phase of nylon 6 in the-modification is 188 Jg^–1 and its extrapolated equilibrium melting temperature is 260 °C.Presented in part at 28th IUPAC Symposium on Macromolecules, Strasbourg, July, 1981.     

Crystal structure of nylon 12

The crystal structure of nylon 12 prepared by polymerization of dodecalactam has been determined by x-ray diffraction. Nylon 12 fiber exhibits only the γ form as its stable crystal structure. The unit cell of nylon 12 was determined with the aid of the x-ray diffraction pattern of a doubly oriented specimen. The unit cell is monoclinic with a = 9.38 Å, b = 32.2 Å (fiber axis), c = 4.87 Å and β = 121.5° and contains four repeating monomer units. The chain is planar zigzag for the most part but is twisted at the position of amide groups, forming hydrogen bonds between neighboring parallel chains. The chain conformation is similar to that of the γ form of nylon 6 proposed by Arimoto. It was deduced from the calculations that there are two chain conformations statistically coexistent according to the direction of twisting. In each conformation, hydrogen bonds are formed between parallel chains to make pleated sheetlike structures. The sheets are nearly parallel to (200) and in the sheet the directions of the neighboring chains are antiparallel, as is the case with nylon 6.     

Reverse osmosis and fine structure of decrystallized acrylic acid grafted nylon 6 membranes

Nylon 6 film and acrylic acid grafted nylon 6 (GN) membrances were reacted with paraformaldehyde and lactic acid in the presence of acid catalysts. Decrystallized nylon 6 (DN) and decrystallized grafted nylon 6 (DGN) membranes were thus obtained. The cross sections of GN and DGN membranes were observed by a transmission electron microscope. Branch poly(acrylic acid) penetrated toward the center of the membrane and deposited homogeneously within the membrane as the extent of grafting exceeded about 100%. The reverse osmosis of DN and DGN membranes was investigated. The water permeability through the membranes was improved by the decrystallization reaction. DGN membranes with more than 100% grafting show high values of the salt rejection (Rs) as compared with DN and DGN membranes with about 50% grafting, especially at the region of the high hydraulic permeability coefficient of water (K). The relationships among Rs, K, and the volume fraction of water (H) are discussed by considering the results of the decrystallization reaction and electron microscopy.     

Application of the high-tension multiannealing to nylon 46 fibers

Nylon 46 fibers produced by the high-temperature zone-drawing treatment were treated by repeating high-tension annealing treatments, that is, a high-tension multiannealing (HTMA) treatment to improve their tensile properties. The HTMA treatment was carried out at a repetition time of 10 times and treating temperature of 110°C under high tension (538.2 MPa) close to the tensile strength at break. Although the HTMA treatment was carried out at 110°C, which is much lower than the crystallization temperature of 265°C for nylon 46, the degree of crystallinity increased up to 59%. The orientation factor of crystallites increased dramatically up to 0.949 by the first high-temperature zone-drawing treatment and slightly during the subsequent treatments. This observation indicated that the orientation of crystallites due to slippage among molecular chains did not occur during the HTMA treatment. The treatments shifted the melting peak to slightly higher temperatures, and the HTMA fiber has a melting endotherm peaking at 285°C. The fiber obtained finally had a storage modulus of 12.5 GPa at 25°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2737–2743, 1998     

Analysis of small-angle X-ray scattering from fibers: Structural changes in nylon 6 upon drawing and annealing

The changes in the fibrillar and the lamellar structure in nylon 6 fibers resulting from drawing and annealing were studied by a detailed analysis of their two-dimensional small-angle scattering patterns. The scattering object that gives to rise the diffuse equatorial scattering in the angular range of Q = 0.02 to 0.3 Å⁻¹ is assumed to be a fibril. There are two distinct regimes in the equatorial diffuse scattering. The scattering at Q < 0.1 Å⁻¹ is dominated by scattering due to the longitudinal dimension of the fibril, and that at Q > 0.1 Å⁻¹ to the lateral dimensions/organization of the fibril. The interfibrillar regions, unlike the interlamellar regions that are essentially made of amorphous chain segments, may have microvoids in addition to amorphous chain segments. The intensity distribution within the lamellar reflections was used to obtain the lamellar spacings and the dimension of the lamellar stacks. The length of the fibrils is between 1000 and 3000 Å, the higher values being more prevalent at lower draw ratios. The fibril length is larger than the length of the lamellar stack, and approaches the latter at higher draw ratios. Annealing does not change the lengths of the fibrils, but the length of the lamellar stack increases. The fibrils form crystalline aggregates with a coherence length of ∼200 Å at higher draw ratios. The diameter of the fibrils (50–100 Å) determined from the lamellar reflection using both the Scherrer equation and the Guinier law are consistent with the lateral size of the crystallites derived from wide-angle x-ray diffraction. The longitudinal correlation of the lamellae between the neighboring fibrils improves upon drawing and decreases upon annealing. The degree of fibrillar and lamellar orientation is about the same as the crystalline orientation. Lamellar spacing increases upon drawing (from ∼60 to 95 Å) and annealing (from ∼85 to 100 Å). This is accompanied by an increase in the width of the amorphous domains from 30 to 50 Å in drawn fibers, and from 45 to 55 Å in annealed fibers. The diameter of the fibrils decreases slightly upon drawing and increases considerably upon annealing. © 1996 John Wiley & Sons, Inc.     

Mechanical relaxations and moduli of oriented nylon 66 and nylon 6

The mechanical relaxations of dry and wet nylon 66 and nylon 6 with draw ratios λ = 1–3 have been studied from ?180 to 160°C and in the frequency range of 1 Hz to 10 MHz. The five independent elastic moduli C_11, C_12, C_13, C_33, and C₄₄ have also been determined by an ultrasonic method at 10 MHz. Wide-angle x-ray diffraction and birefringence measurements reveal that the crystalline orientation rises sharply at low λ and becomes saturated near λ = 3; the amorphous orientation function increases continuously, reaching values of 0.3–0.5 at λ = 3. The alignment of molecular chains and the presence of taut tie molecules in the amorphous regions lead to a lowering of segmental mobility, thereby reducing the magnitude and increasing the peak temperature and activation energy of the α relaxation. Water absorption weakens the interchain bonding and so gives rise to effects opposite to those of drawing. At low temperature, the development of mechanical anisotropy is largely determined by the overall chain orientation, with the c-shear mechanism contributing a small additional effect. However, above the α relaxation, where the amorphous region is rubbery, the stiffening effect of taut tie molecules becomes dominant and leads to increases in all moduli.     

Crystalline texture of injection-molded nylon 6

Injection-molded specimens of nylon 6 were examined by x-ray diffraction as a function of depth in three characteristic directions. A skin and a core were always found to be present which differed in the degree of crystalline perfection and crystal modification. While the core of pure nylon 6 was found to be not oriented, the core of specimens containing nucleating agents was found to contain a typical texture of the monoclinic modification of nylon 6 in which the distribution probability of the a^* and c^* axes resembles ellipsoids with three unequal axes. A model explaining this texture as due to degenerated (deformed) spherulites is proposed. With a transcrystalline nylon 6 specimen the direction of the fastest growth in the unit cell (which forms the radius of spherulites) is found to be close to the a axis.