Relations between dynamic mechanical properties and melting behavior of nylon 66 and poly(ethylene terephthalate)

Nylon 66 films exhibiting form I melting behavior show the γ mechanical relaxation at ?140°C. Samples which have form II melting behavior do not show this relaxation. The γ relaxation disappears when material having form I behavior is converted to material having form II behavior by annealing or by cold drawing. The form I and form II types of melting behavior are also found in poly(ethylene terephthalate); the interconversions and thermal behavior of the forms are analogous to the nylon 66 case. In poly(ethylene terephthalate), the β relaxation at ?40 to ?60°C is present only when form I melting behavior is found. Conversion to form II melting behavior by annealing or drawing (80°C) again causes the relaxation to disappear. No β relaxation was found in amorphous polymer. The γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) can therefore be associated with the crystalline structure responsible for form I melting behavior. Form I melting behavior has been associated with foldedchain crystals based on previous work. It is therefore postulated that the γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) are associated with motions in the chain folds. This assignment is not inconsistent with the change in the γ dispersion of nylon 66 with the number of backbone CH₂ units, since these will affect the fold structure.     

Dynamic mechanical properties of high molecular weight nylon 66 fiber

Dynamic mechanical properties of the ultrahigh molecular weight nylon 66 film and fiber produced by thermally induced solid-state polycondensation are presented. The α peak temperature of tan δ of these treated films and fibers is shifted 8–32°C higher than that of the appropriate control nylon 66 (film and fiber) while the maximum height of the tan δ peaks is decreased. The treated fibers have higher moduli at all temperatures (20–145°C) and humidities (30% RH) than do their control counterparts. The moduli of the treated fibers at 30% RH compare favorably with control yarn at 0% RH. These yarns also have a greater per cent of the 25°C modulus retention as temperature increases.     

Morphology of cold-drawn nylon 66

A nylon 66 composed of uniformly sized spherulites approximately 50 μ in diameter was examined before and after cold drawing by light and electron microscopy of thin sections and by low-angle x-ray diffraction. Spherulites retained their identity through drawing, but the spherulites elongated less than the bulk specimen indicating that relative motion of spherulities must have occurred. The observation of dilations (0.3 μ long) at interspherulitic boundaries support this contention. The thin-section electron micrographs indicated that the spherulites were composed of radiating lamellae approximately 95 A. thick. After drawing, the lamellae were preferentially oriented both parallel and perpendicular to the draw direction. Lamellae parallel to draw had thinned to approximately 70 A. While lamellae perpendicular to the draw had apparently thickened to 150 A. Three low-angle x-ray diffraction patterns yielded quantitative agreement with the electron-micrograph data. The pattern form the undrawn nylon was a diffuse ring corresponding to a 95 A. spacing. On the drawn specimen, with the beam parallel to draw, a ring corresponding to the 150 A. spacing was obtained, while with the beam perpendicular to draw two arcs were recorded at spacings of 70 and 150 A. The drawing was done at room temperature and proceeded by neck formation and propagation, yielding a 4:1 draw ratio.     

Thermal degradation of nylon 66

The rate of gel formation and color formation in poly(hexamethylene adipamide), nylon 66, is found to be dependent upon the rate of removal of the volatile products of degradation. If a sample of nylon is heated above its melting point in a sealed tube, the material will remain soluble for extended periods of time as the intrinsic viscosity first passes through a maximum, then a minimum, followed by the abrupt formation of insoluble material. The color remains reasonably white. On the other hand, if the volatile material is permitted to escape, rapid gelation and color formation will occur, even in the complete absence of oxygen. Intermediate rates of gelation and color formation can be obtained by control of the rate of volatile material distillation. The decrease in molecular weight evidenced in the sealed tubes is probably due to hydrolysis and ammonolysis of the amide groups which occur simultaneously with the formation of multifunctional crosslinking agents. The volatile material contains an intense absorption in the 290 mμ region. Analysis of the volatile material shows that it contains inter alia, water, carbon dioxide, ammonia, cyclic monomer, hexylamine, hexamethyl-eneimine, hexamethylenediamine, cyclopentanone, 2-cyclopentylcyclopentanone, 2-cyclopentylidinecyclopentanone, and 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridine, which has an intense absorption at 287 mμ, ε = 8.87 × 10⁴l./mole-cm, (methanol).     

Multiple melting in nylon 66

Either of the two endothermic melting peaks found by differential thermal analysis of nylon 66 may be converted to the other by appropriate choice of annealing conditions. The two peaks are considered due to the melting of two morphological species, forms I and II. Form I is relatively fixed in melting temperature, while the form II melting temperature varies with annealing conditions and can be either above or below form I. The two forms can be distinguished by whether or not the conversion I → II takes place; if the sample is in form II no change in the thermogram is observed under suitable conversion conditions. The conversion of form I to form II also takes place during cold drawing. It has been previously shown that form I results from rapid cooling from the melt, and form II results from slow cooling. Form I appears to be kinetically favored, while form II is thermodynamically preferred. The variability in the form II melting point is attributed to variable crystal size and/or perfection.     

Mechanical anisotropy in rolled nylon 66

The nine independent stiffness constants C_pq for rolled nylon 66 at thickness reduction ratios λt = 2.4, 4.9, 7.6 have been measured from ?40 to 50°C by an ultrasonic method at 10 MHz. Analysis of x-ray pole figures indicates that at high λt the material has a uniplanar-axial texture, with hydrogen-bonded (010) planes parallel to the roll plane and the molecular chains along the roll direction. The mechanical behavior is determined not only by the alignment of molecular chains and hydrogen bonds but also by the microfibrillar morphology. On the basis of this idea the magnitudes of the tensile moduli along the three principal axes can be understood in terms of the Takayanagi model. The effects of water absorption have also been investigated.     

Study of mechanical and tribological properties of nylon 66‐titanium dioxide microcomposite

Nylon 66 microcomposites with various weight percentage of titanium dioxide (TiO₂) were prepared by a twin screw extruder and investigated for mechanical and tribological properties. Mechanical properties of the composite such as tensile strength/modulus, flexural strength/modulus, impact, and compressive strength first showed an increase up to 6 wt% TiO₂ followed by a decrease at higher filler loading. The value of heat deflection temperature increased with the increase in wt% of TiO₂. Sliding wear tests were performed on pin‐on‐disk equipment under different loads, sliding velocity, and sliding distance combinations. It was found that micro‐TiO₂‐Nylon 66 composite exhibited reduced wear and coefficient of friction up to 6 wt% TiO₂. Micro‐TiO₂ at 2 wt% was most effective in improving the tribological properties of plain nylon 66. The worn surfaces were examined by scanning electron microscopy to understand the wear mechanism. The optimal combination from 2 wt% to 6 wt% micro‐TiO₂‐Nylon 66 can be used depending upon the application requiring improvement in tribological or mechanical properties, respectively.     

Elastic–inelastic behavior of PET and nylon 66 monofilaments under lateral compression

Results of experiments on short lengths of nylon 66 and PET monofilament specimens, subjected to lateral compressive force between two parallel platens, are described. In the experimental procedure the change in the compressed diameter and the area of contact were determined as a function of the applied force. From these data elastic moduli in lateral compression were estimated. The load–deformation behavior beyond the elastic range exhibited a yield-like phenomenon. SEM examination of the deformed specimen suggest the possible modes of deformation in the post-yield behavior. It is inferred that the yield occurs due to concentration of deformation along the maximum shear planes. The deformed specimens also show specific forms of fracture initiation and growth.     

Copolymers of nylon 266 and nylon 66: Synthesis and characterization

Copolymers of nylon 266 and nylon 66 were prepared by interfacial polymerization of N-glycyl hexanediamine and hexanediamine with adipoyl chloride. According to the results of intrinsic viscosity measurements and GPC analysis, the molecular weights of the copolymers were relatively high. The structure of the copolymers was confirmed by FTIR, and the compositions were determined by ¹H-NMR spectroscopy. The copolymers have similar solubility features as nylon 66. Both melting and glass transition temperatures showed a minimum at around 20–30% nylon 66 content. The copolymers are semicrystalline. Copolymers with lower T_m could be melt-spun into fibers without appreciable thermal degradation. © 1993 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.     

Wideline NMR studies of oriented nylon 66

Wideline NMR techniques have been applied to both singly and doubly oriented specimens of nylon 66. Variations in the spectra obtained are observed for different orientations of the specimens relative to the applied field. These variations demonstrate that the zigzag chain axis is essentially parallel to the draw direction and that motion occurs in all of the sample. The motion is of two types: random in the mobile regions and rotation of segments about the chain axis in the rigid regions.     

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.     

The heat of fusion of 66 nylon

The heat of fusion ΔH_f of 66 nylon has been determined by use of the Clapeyron equation. Measurements of ΔH_f and the unit-cell parameters on molding pellets show that this material contains the α₂ crystal phase, which is less dense than the α₁ phase obtained by crystallization from solution. The value of ΔH_f-45–46 cal/g, is in good agreement with earlier reports.     

An oxyluminescence investigation of the auto-oxidation of nylon 66

The kinetics of the thermal oxidation of stabilised and unstabilised nylon 66 fibres and films have been studied by photon counting oxyluminescence methods from 50°C to 150°C. The activation energies for initiation (E₁), propagation (E₃) and termination (E₅) over this temperature range are: E₁ = 16 kcal mol^?1, E₃ = 17·5 kcal mol^?1 and E₅ ≈ 12 kcal mol^?1. The extent of orientation of the polymer does not change the nature of the oxyluminescence curve or E₃ and E₅ above 110°C.Significant losses of critical mechanical properties of the fibres occur in the induction period at 100°C and non-stationary kinetics are described to enable this region to be studied by oxyluminescence. The oxidation rate in the induction period and the limiting rate region in air is one-third the rate in oxygen at atmospheric pressure. Non-stationary methods show that alkyl radical reactions are competitive with alkyl peroxy radical formation in air over the temperature range 100°C to 140°C. This affects the course of the oxidation reaction and the stabiliser efficiency and explains the observation of unsaturated oxidation products by phosphorescence spectroscopy.     

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.     

Electronic and ionic conductivity in nylon 66

The conductivity of dry poly(hexamethylene adipamide) (nylon 66) was measured as a function of time and temperature. Three temperature ranges were observed in which the time dependence of conductivity differed: (a) below 80°C. the conductivity decreased continuously with time; (b) between 80°C. and 110°C. the conductivity remained constant over long periods; (c) above 120°C. a continuous decrease in conductivity was again observed. In other experiments the volume of gas evolved from the nylon film was measured under continuous potential and compared with the total current passed through the sample. It was observed that above 120°C. the gas evolved corresponded to about one-half the volume calculated if the conduction process involved only protons. Below 120°C. the gas evolved corresponded to an increasingly small fraction of the total current until below 90°C. no evolution of gas was observed. This suggests that at temperatures above 120°C. conduction involves the transport of both protons and electrons, whereas at lower temperatures it is electronic. Mechanisms of conduction are discussed.