Polyamides crystalline structure

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

Aliphatic polyamides are extensively studied by natural abundance 15NNMR spectroscopy in solution. However, characterization of polyamides in solution is limited by the insolubility of many (particularly aromatic) polyamides. On the other hand, chemical shifts of amide nitrogens are strongly dependent on the nature of a solvent, and for a particular polyamide, could cover approximately 20 ppm, as in the case of fluorosulfonic acid and trifluoroethanol (see Fig. 2). Since the important properties of solid polyamides such as crystalline structure and hydrogen bonding cannot be studied by solution spectra, the various solid state 15N NMR techniques have been used for structural and dynamical characterization of these polymeric materials. 

Any nucleating agent conventionally used in the production of polyamides having a fine crystalline structure is suitable for polyphthalamide. ... 

Dencheva N, Nunes T, Oliveira M J and Denchev Z (2005) Crystalline structure of polyamide 12 as revealed by solid-state C NMR and synchrotron WAXS and SAXS, J Polym Sci Part B Polym Phys 43 3720-3733. 

A typical shish-kebab crystalline structure has been foimd by Maiti and Okamoto (2003) and Kim et al. (2001) in polyamide/organoclay nanocomposite and by Choi and Kim (2004) in PP/EPR/talc nanocomposite where a preferential orientation of polymer lamellae perpendicular to the surface of organoclay layers was inspected by TEM measurements. The unique observation of lamellar orientation (Ml the clay layers was ascribed to nucleation and epitaxial crystallization at the interface between layered silicate and polymer matrix especially the surfaces of clay platelets acted as heterogeneous nucleation sites. Orientation of iPP crystals was also enhanced in rPP/PP-MA/o-MMT injection-moulded parts, especially manufactured by dynamic packing injection moulding (Wang et al. 2005). MMT... 

Dencheva, N., Denchev, Z., Oliveira, M.J., Nunes, T.G., and Funari, S.S. (2008) Relationship between the crystalline structure and mechanical behavior in isotropic and oriented polyamide 12. Journal of Applied Polymer Science, 109, 288-302. 

On the nanometer scale the structure of the samples is studied by SAXS. Figure 6.1 sketches structural features that can be probed by SEM on the micrometer scale and by SAXS on the nanometer scale, respectively. The SAXS of the samples arises mainly from the semi-crystalline structure of the HDPE matrix, as has previously been shown [14]. The reasons are, firstly, that the major component is HDPE. Secondly, polyamide has a low electron density contrast between its crystalline and amorphous regions compared to the corresponding contrast in HDPE. 

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. 

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