Nylons crystal structures

Rubber incorporation into the nylon also affects crystallinity. Although the overall extent of PA 66 crystallinity is not greatly altered (15), the rate of crystallisation and final nylon spherullite size is affected by the rubber particles. Presumably due to smaller rubber particles, functionalised EPR also leads to a more refined nylon crystal structure than does unmodified EPR in the blends studied here. 

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

The molecular arrangement within the crystal units cells of nylon is governed by the need to maximize hydrogen bonding between adjacent chains. Hydrogen bonding within crystallites is facilitated by the fact that nylon chains adopt planar zig-zag conformations with dipoles perpendicular to the chain axis to thin the plane of the molecule. Examples of nylon crystallite structures are shown in Figs. 23.8 and 23.9 for nylon 6 and nylon 66, respectively. In the... 

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

The thin films of the nylon samples that were melt-pressed for FTIR examination gave similar results. Linear samples pressed out easily to make thin films that were excellent for IR evaluation. Star branched materials would not press thin enough to give good spectra even at maximum press pressure. It was originally hoped that star and linear nylons would have different crystal structure forms (JL4) unfortunately, FTIR has shown little differences other than those that occur from differences in sample preparation (Figure 5). 

Based on high-resolution solid-state NMR and TEM observation, the mechanism for the crystallisation of nylon 46 is summarised in Figure 27.69 When the temperature of the melt sample decreased to the crystallisation temperature, hydrogen bond is formed with the nearby chains. At this stage, the size of the crystalline phase is small and the crystal structure is not energetically stable. The amorphous phase is mobile and the conformation of the amorphous phase contains the gauche conformation around the bond. As the crystallisation proceeds, the crystal... 

Compare the chain conformations and types of crystal structures found in nylon 6 and silk. Include in your answer a discussion of the role of hydrogen bonding and other interactions. 

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

This kind of topological representation of crystal structures may be extended to more complex compounds if we focus our attention on the limited number of stronger bonds that hold the structure together. Nylon is formed by condensing... 

A new crystal structure with tilted chains has recently been found in pentamer of nylon-6 and nylon-8, termed A-phase.108 Molecules are in the all-trans conformation and hydrogen bond to antiparallel neighbors to form the usual nylon 6 hydrogen-bonded sheets. However, in this structure, the sheets stack with progressive c-axis shear, and consequently, the molecular layer thickness is noticeably reduced. 

Side chains must be absent or comparatively simple because, if they are bulky, they keep the main chains apart and prevent the formation of compact crystal structures. The effect of this is to separate the main chains from each other by a distance which is too great for the binding forces to come into play. This is well illustrated by the progressive methoxymethylation of nylon, as demonstrated in Table 2.1. 

Figure 2.11 (a) Hydrogen bonds between neighboring chains of polyamide, (b) Arrangement of chains in hydrogen-bonded sheets in the crystal structure of nylon-6,6. 

It is known that nylon 6 takes two types of a- and y-forms as polymorphs. Weeding et al. [12] and Okada et al. [13] observed the CP/MAS NMR spectra of nylon 6 samples with the a- and y-forms as shown in Fig. 12.7. Table 12.4 shows that there is a significant chemical shift difference between these forms. It is demonstrated that solid-state NMR provides useful information about the crystal structure of polyamides. This leads to solid-state NMR studies on nylon 7 [14] and 11 [15] with polymorphs. 

The crystal structure of the nanocomposites was studied with XRD and DSC. The XRD spectra in Figure 5 shows the effect of alkyl chain length on the different crystal structure formation in the skin (Figure 5a) and at the core (Figure 5b) of the nanocomposites. By observing the diffraction pattern of the injection moulded test bars core (Figure 5b) between 10 and 40°, the relative content of amorphous material and a and y crystals in the polymer matrix can be determined. The peak at 24.6° corresponds to the y crystal structure and the peaks at 23.7° and 27.3° to the ai and a2 crystal in nylon 6 respectively. As reported in previous literature (16-18) the peak at 21.4° corresponds to the amorphous content in the matrix, but it is not prominent in Figure 5b Contrary to this literature however, the a and y peaks are not located at the same 20 values... 

The DSC scans support findings from the XRD data, showing two distinct crystal structures arising in the polymer. Some contribution to this effect is expected from chain segmentation during the extrusion process (79, 20), however the effect on nylon 6 polymorphism due to the presence of clay is undeniable. 

A few polymers have lower resistivities. Polyamides, with hydrogen bonds that lie in parallel planes in the crystal structure, have resistivities a factor of 100 smaller than non-H-bonded polymers. At temperatures above 120 °C, at least half of the conduction in nylon 6,6 is due to protonic carriers, with hydrogen being liberated at one electrode. Nevertheless, the resistivity is still high. 

Reprinted by permission of John Wiley Sons, Inc. from Holmes, D. R., Bunn, C. W. and Smith, D. J. The crystal structure of polycaproamide nylon 6 , J. Polymer Sci. 17, 159 (1955). Copyright 1955 John Wiley Sons, Inc. 

---