In the years following the World War I, a number of chemists recognized the need for developing a basic knowledge of polymer chemistry. In the early 1930’s, Wallace M. Carothers and his associates at E. I. DuPont de Nemours & Company began fundamental research of dicarboxylic acids and diamines. This research led to the synthesis of the first purely synthetic fiber, a polyamide-- Nylon 66.
The term polymer means "many parts" and refers to a molecule formed from many smaller molecules, called monomers, which are linked together into large molecules or macromolecules. Hence, nylon 66 is so named because it is synthesized from two different organic compounds, each containing six carbon atoms.
Nylon 66 is usually synthesized by reacting adipic acid with hexamethylene diamine:
This movie (over 4MB ... could be slow) shows the synthesis of nylon 66. Adipic acid and hexamethylence diamine are dissolved in an aqueous ethanol and they react to form a one-to-one salt called nyon salt.
Nylons are one of the most common polymers used as a fiber. Nylon is found in clothing all the time, but also in other places, in the form of a thermoplastic. Nylon's first real success came with it's use in women's stockings, in about 1940. They were a big hit, but they became hard to get, because the next year the United States entered World War II, and nylon was needed to make war materials, like parachutes and ropes. It may be surprising to learn that before stockings or parachutes, the very first nylon product was a toothbrush with nylon bristles.
Nylons are also called polyamides, because of the characteristic amide groups in the backbone chain. These amide groups are very polar, and can hydrogen bond with each other. Because of this, and because the nylon backbone is symmetrical, nylons are often crystalline, and make very good fibers.
Nylons are polymers of intermediate crystallinity. Unlike other polymers, the percent crystallinity can be varied over a relatively wide range. The crystalline portion consists of crystalites of different size and perfection. These can be detected using x-ray diffraction because the differences influence the intensities and widths of wide-angle diffraction peaks. The amorphous content adds a diffuse scattering halo. The molecules fold to form thin ribbon- or layer-shaped crystals termed lamellae. The lamellae, which may be observed with the electron microscope, aggregate into spherical stacks or clusters called spherulites or into less well developed aggregates, which can be observed in the optical microscope. Orientation results when operations such as drawing or rolling induce the partial breakup of lamellae and spherulites and form new structures. Stress orientation can also be found in the amorphous chains for semicrystalline and amorphous nylons. Many commercial nylon samples, such as fiber and filaments have a high degree of orientation. Molded nylons have a much lower orientation and are predominately spherulitic. Orientation is usually restricted to a surface layer as a result of the high shear rates near the surface during processing. An improved understanding of physical and mechanical properties requires a knowledge of crystal structure and morphology.
The primary chemical structure of alphatic nylons consists of amide groups separated by methylene sequences (outlined in orange below). The amide group is essentially planar due to the partial double-bond character of the C-N bond.
The ability of the NH group to form strong hydrogen bonds (H-bonds) with the CO group is the determining factor for the crystal structures of nylon. The chains are oriented in such a way as to maximize hydrogen bonding. The intermolecular H-bonds connect neighboring chain segments and form extended planar sheet that contains the H-bonds. The formation of the extended sheets dominates the structure.
THE ALPHA STRUCTURE
Nylons are divided into two main types of stable crystal structures: alpha and gamma. Except for under a few controlled conditions, nylon 66 contains the alpha structure. The alpha structure describes the even-even nylons. The term "even-even" stems from the sequence of the methylene positions and on which side of the chain they are located. The chains in the alpha structure are in a fully extended zigzag formation and form planar sheets of H-bonded molecules that are stacked upon one another. The crystal symmetry is triclinic with one chemical repeat unit per unit cell which differs in the c-axis dimension by a length proportional to the addition of methylene groups in the repeat unit. The only symmetry element is a center symmetry in both the diamine and the diacid portions and is preserved in the crystal structure. The even-even chains have no directionality so that parallel and antiparallel chains are equivalent.
The figure to follow represents the crystallographic planes in the unit cell of Nylon 66. Note that although four chains run along the c-axis, there is only one chemical repeat [i.e., on –NH-(CH2)6-NH-CO-(CH2)4-CO- group] per unit cell because the four chains on the edges are shared by four unit cells. The molecules are H-bonded (indicated by the dotted lines) in the a-c faced of the cell. The H-bonded sheets are developed by an extension of the a-c faces and correspond to the (010) planes.
From left to right, numbered 1-5:
1) Perspective drawing of the triclinic unit cell of nylon 66 in the alpha-structure
2) (010) Plane parallel to H-bonded sheets
3) (100) Plane along molecular chain but cutting through H-bonded sheets
4) (001) Repeat planes cutting through molecular chain.
5) (110) Diagonal planes.
The picture above is more detailed than the first, showing the triclinic unit cell of nylon 66. The lattice constants are a=0.49 nm, c=1.72 nm (fiber axis), alpha=48.5°, beta=77°, gamma=63.5°. Projection along the b direction showing the a-c faces; the H-bonds are indicated by dotted lines.
Nylons exhibit a more varied polymerphism than most other classes of polymers. This is due to the H-bonding, amide group formation, and the tight fold that require optimum structural energetics. Even nylons can be converted from the alpha form to the gamma form by treatment with aqueous iodine-potassium iodide. The relative amounts of alpha and gamma obtained on crystallization from solution, the melt and the glassy state, can be measured by x-ray diffraction. In all cases, the weight fraction of the gamma form decreases with increasing temperature and time of crystallization, 430K being the crossover temperature. In the case of nylon 66, only alpha crystals can be obtained, but in two modifications: alpha-I from higher temperature treatment and alpha-II from lower temperatures or quenching. The alpha-II crystals have a pseudohexagonal structure. This pseudohexagonal structure is found when the irregularity of the H-bond formation is maximized. The fraction of the various components can be quantified using thermal, infrared, and diffraction data.
After high-temperature annealing under vacuum, nylon 66 shows another structure, which has a crystal density close to that of the amorphous phase. The structure is labeled the beta structure and is triclinic and similar to the alpha structure. Due to the structural variations in reports, the beta structure does not constitute a separate class of nylon structures.
For all semicrystalline polymers, the growth rate is constant at a constant temperature but changes with undercooling. Typical nylons begin to crystallize on cooling 20-30K below their melting point. At temperatures close to the melting temperature, the crystallization rate is low, and large crystal sizes can be obtained. At lower temperatures, far below melting, the crystallization is faster, but smaller crystals are formed.
Nucleation rates can be measured by counting nuclei using an optical microscope. Two distinct morphologies are found in nylon 66: single crystals from dilute solutions and spherulites from concentrated solutions and melts. Single crystals grow from primary nucleation sites until sample is depleted. Spherulites nucleate similarly but terminate by impingement on other spherulites. Crystallization is faster by decreasing the amide concentration because of the decrease in flexibility and also, a lower molecular mass nylon will crystallize faster. Symmetry also effects crystallization rate as the highest degree of symmetry leads to the fastest rate. Nylon 66, with a few repeat units and even-even symmetry, crystallizes fast compared to other nylons. The highest spherulitic growth rate is noted at about 430K.
In the above graph, the two lines with triangles represent the spherulitic growth rate for two samples of nylon 66 with different molecular masses. The line with the black dotes represents spherulitic growth for nylon 6. It is obvious that the highest growth rate of nylon 6 does not compare to the higher growth rates of nylon 66 samples. Although the max growth rate for nylon 66 is not shown, it is expected to be at higher temperatures.
DYNAMICS IN CRYSTALS
Due to the sequences of methylene groups being separated by H-bonded amine groups, the samples do not melt at temperatures where other polymers like polyethylene would already be molten. This aspect of the nylon structure confers an increased mobility to the methylene groups in the solid state. The onset of mobility occurs well below the melting point. This observation was found by Brill in 1942 when he saw the merging of the two strongest x-ray diffraction peaks of nylon 66, the (100) and (010) reflections, into a single peak at 435K. This is now referred to as the Brill transition and characterizes the change from a triclinic crystal structure to the pseudohexagonal one.
The dynamics present above the Brill transition temperature were indirectly probed by annealing studies of highly drawn nylon 66 fibers. A discontinuous set of properties were found when the annealing temperature was higher than the Brill temperature. The higher annealing temperatures caused an increase in crystallinity, a long period of the crystallites and a scattering intensity as well as a fluid-like mobility. These changes were explained by the extensive rearrangement of lamellar folds.
An endothermic transition is associated with the Brill transition for solution-crystallized samples. Another study focused on the determination of the crystallographic parameters at different temperatures provided the temperature dependence of the crystalline density. A large decrease from room temperature up to about 470K was followed by a constancy of the specific volume of the pseudohexagonal crystals until just below melting, where an abrupt decrease was observed.
The above graph shows the crystalline density of nylon 66 (also called PA-66) as a function of temperature, calculated from the lattice parameters. The lines serve to show the initial decrease, followed by no change and then a second, sharp decrease.
The remarkable decrease of about 14% can be explained by the volume requirements due to the large-amplitude motion of the methylene groups within the nylon 66 crystals. At the Brill transition temperature the pseudohexagonal structure is obtained wherein the methylene motion occurs in a cylindrical region about the chain axis and no expansion is needed. This explains the constant density and volume found for the pseudohexagonal phase.
Nylons typically have crystallinities of around 50%, there fore it is useful to describe the amorphous structure in addition to the structure of the crystals. It is very difficult to quench nylons to the 100% amorphous state. Some crystallinity is retained in quenched samples and is seen in x-ray diffraction patterns. The pattern can be distinguished from that of the high-temperature pseudohexagonal form by the absence of the (002) reflection with a broader diffraction peak. This peak indicates that the H-bonded sheets are not well formed. Crystallinity may be reduced by randomly introducing occasional defects in the chain. Studies have shown that the process of drawing results in a sharper amorphous peak, indicating that drawing not only increases the crystallite size but also improves the alignment of the amorphous chains. Amorphous chains orient along the drawing direction and are packed at about 20-30% more densely compared to unoriented amorphous chains. The amorphous chains prefer an extended conformation about the amide group.
The structure of polymer crystals above the dimensions of the unit cell has been firmly established from investigations using optical and electron microscopes in conjunction with x-ray scattering. The basic structural elements have been found to be lamellae, thin-layer crystals formed by folding of the chains during crystallization, and spherulites that are spherically shaped aggregates of lamellae.
A practical aspect of morphology is its effect on mechanical properties. It has been shown that dry nylon 66 samples with small spherulites show higher flexural modulus, higher upper yield stress, and lower ultimate elongation than do samples with large spherulites, both have similar amount of crystallinity (50%). Property differences associated with spherulite size are most pronounced in dry samples at low temperatures and decrease with added water or temperature. The yield points of spherulitic films are substantially higher than those without visible spherulites independent of crystallinity.
The lamellae in nylon are of a flat, ribbon-like shape. There are crystals that usually are lath-shaped and often aggregate into sheaves. Usually, regular folding occurs during crystallization on specific planes determined by a balance of forces between the minimization of surface free energy at the plane and the specific interactions between the chains of the lamellae. Nylon 66 has the surface of the lamella containing folds parallel to the (001) planes. The c-axis has an inclination of 46 degrees with respect to the fold surface, and the a-axis is the direction of greatest lamellar elongation, therefore has the fastest crystal growth. It is along this direction that H-bonds are formed.
The spherulite structure is the characteristic of crystallization in an environment free from stress including both mechanical stress from agitation and thermal stress from strong temperature gradients. Processing such as extrusion or molding usually causes considerable stress and yields non-spherulitic oriented structures.
The picture above shows a compression-molded sample of nylon 66. The transcrystalline spherulitic brushes at the surface and row structures can be seen.
Nylons, under processing conditions, are different from most polymers in that they result in a more spherulitic structure. The reason is that the melt tends to supercool to a much greater degree. By the time crystallization begins, the stresses have been reduced greatly from the initial values.
The picture above shows a typical spherulite of nyon 66, showing the characteristic "Maltese cross" appearance.
Spherulites are characterized by their birefringence which is the difference between the refractive indices and is detected by an optical microscope. Birefringence results when the index of refraction for light traveling through a crystal is different for light vibrating in different directions. A crystal of nylon is inherently birefringent due to the anisotropy resulting from the alignment of the H-bonds. If a spherulite itself grows with a certain axis of the crystals parallel to its radius the spherulite will be birefringent.
It has been discovered that the sperulite structure can be that of rolled up lammela
The concept of orientation refers to the change in a polymer obtained by controlled thermomechanical treatment of a sample in order to improve its physical properties. Fibers and filaments can be uniaxially oriented by extrusion or can achieve a planar orientation by molding or casting. Alignment of polymer chains occurs during drawing which increases the tensile strength and modulus in the drawing direction. Annealing nylon 66 increases the number of folds and results in an increase in shrinkage and crystallinity and a decrease in modulus, elongation-to-break, and breaking strength. It is possible to obtain a multiaxial orientation through a process involving elevated temperatures, rolling and drawing. In this case, the H-bonded sheets prevent splitting in the longitudinal direction without a sacrifice in tensile strength.
In the first stage of fiber production, crude nylon 66 is melted, spun into fibers, and cooled. Next, the melt-spun fibers are drawn at room temperature (cold-drawn) to about four times its original length. As the fibers are drawn, individual polymer molecules become oriented in the direction of the fiber axis, and hydrogen bonds form between carbonyl oxygens of one chain and amide hydrogens of another chain.
The effects of orientation of polyamide molecules on the physical properties of the fiber are dramatic; both tensile strength and stiffness are increased markedly. The described cold drawing process described is an important step in the production of all synthetic fibers.
Properties of nylon 66.
The carbon sources for the production of nylon 66 are derived entirely from petroleum, which is a non-renewable resource.
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