Aromatic polyamide Structure

The most extensive research on furanic polyamides is recent (33) and deals essentially with furanic-aromatic structures, although an important effort was also devoted to all-fiiranic compositions. The reaction of the diacid 11a with various aromatic diamines leads to high-molecular weight polymers with good thermal stability and ciystallinity. Structure 23, obtained with p-phenylenediamine, exhibited features resembling closely those of polyaramides ... 

The reverse reaction is an intramolecular acidolysis of amide group by the (9-carboxylic acid to reform anhydride and amine. This unique feature is the result of an ortho neighboring effect. In contrast, the acylation of an amine with benzoic anhydride is an irreversible reaction under the same reaction conditions. The poly(amic acid) structure (8) can be considered as a class of polyamides. Aromatic polyamides that lack ortho carboxylic groups are very... 

Structure Level I. Structure Level I variations for aromatic polyamides are broad. The wide range of segmental structures possible with these polymers is what makes them so interesting for membrane science. The discussion of Structure Level I will be limited to some representative segmental units in polyamides, polyhydrazides and polyamide-hydrazides. Structures and abbreviations for some typical diamines that are condensed with mixtures of isophthaloyl chloride (l) and terephthaloyl chloride (T) to give the aromatic polyamides discussed in this paper are shown in Table III. 

Aliphatic or aromatic structure, as weU as liner or branched structure of the reactants, can give the microcapsule shell different porosity and permeability, which can greatly inflnence the release performances. Multifunctional reactants can help to achieve more thermal mechanical stable microcapsules since the wall is a three-dimensional cross-linked polymer network. Experiments have shown that dichlorides with less than eight carbon atoms do not prodnce qnahty polyamide microcapsules. The reason behind this is the competition between interfacial condensation and the hydrolysis reaction of dichlorides. More hydrophobic dichlorides can favor the polymerization and slow the hydrolysis. Similarly, for polyurethane and polyurea type microcapsules, polymeric isocy-nates are preferred because they might favor the formation of less permeable miCTocapsnles for the hydrolysis of isocynate groups are limited, which consequently reduced the COj release that contribute to the porosity increase of the polymer wall." ... 

Aliphatic polyamides are macromolecules whose structural units are characteristically interlinked by the amide linkage —NHCO—. The nature of the structural unit constitutes a basis for classification. Aliphatic polyamides with structural units derived predominantly from aliphatic monomers are members of the generic class of nylons, whereas aromatic polyamides in which at least 85% of the amide linkages are directly adjacent to aromatic structures have been designated aramids. This chapter is concerned with nylons, especially those of commercial importance. Aramids are discussed in a separate chapter. 

Hydrocarbons such as dibenzyl toluene and products with an aliphatic-aromatic structure are used as extenders for immersion and rotation molding pastes. Polyamide flexibilizing with the help of plasticizers with comparable polar structures (e.g., Cetamoll in the sulfamide group), has receded into the background ever since the development of PA copolymers with elastomeric qualities that are for the most part resistant to low temperatures. 

Figure 6.2 Aromatic polyamide polymer structure. Source [1],...

In the general formula of polyamides, as given by — (NHRCO) — or — (NHRiNHCOR2CO) —, the substituent R, Ri or R2 can also be an aromatic ring. Such aromatic polyamide (aromatic amide = aramid) are commercially available, a well-known example being Kevlar. The basic form of the structure of Kevlar is provided below. It should be noted that the aromatic rings are substituted in the para position. Other aromatic... 

The statements made for polyamide 6 and 66 can be applied to PA 6/6T, although PA 6/6T degrades and discolors somewhat more rapidly under UV light because of its aromatic structures [83]. 

Potyimides obtained by reacting pyromellitic dianhydride with aromatic amines can have ladder-like structures, and commercial materials are available which may be used to temperatures in excess of 300°C. They are, however, somewhat difficult to process and modified polymers such as the polyamide-imides are slightly more processable, but with some loss of heat resistance. One disadvantage of polyimides is their limited resistance to hydrolysis, and they may crack in aqueous environments above 100°C. 

Aromatic polyamides, 5 Aromatic polyester chains, disruption of structural regularity of, 50-52 Aromatic polyesters... 

Polyamides are macromolecules with acidamide units —CONH—, where the chemical structure of the other parts of the monomers can be aliphatic and/or aromatic. Similar structures are found in nature, for example, polypeptides. Although in principle a large number of potential polyamide structures can be produced, only a few polyamides are produced in industrial scale. 

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. 

Fiber spinning, 11 174, 175, 170-171 carbon-nanotube, 13 385-386 methods of, 16 8 models of, 11 171-172 of polyester fibers, 20 12-15 Fiber structure, of aromatic polyamides, 19 727... 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

The PET polymer structure can also be generated from the reaction of ethylene glycol and dimethyl terephthalate, with methyl alcohol as the byproduct. A few producers still use this route. The aromatic rings coupled with short aliphatic chains are responsible for a relatively stiff polymer molecule, as compared with more aliphatic structures such as polyolefin or polyamide. The lack of segment mobility in the polymer chains results in relatively high thermal stability, as will be discussed later. 

The use of HMF or the corresponding dialdehyde precursors obviously applies to the synthesis of monomers for polycondensation reactions as shown by the examples given in Scheme 2. ITiese difimctional structures again mimic the corresponding well-known aliphatic and aromatic counterparts used in the preparation of polyesters, polyamides, polyurethanes, etc. 

Note 1 In most cases (e.g., in vinyl polymers, polyamides) degradation is accompanied by a decrease in molar mass. In some cases (e.g., in polymers with aromatic rings in the main chain), degradation means changes in chemical structure. It can also be accompanied by cross-linking. 

Polymers are real and all around us. We can look at giant molecules on a micro or atomic level or on a macroscopic level. The PET bottles we have may be composed of long chains of poly(ethylene terephthate) (PET) chains. The aramid tire cord is composed of aromatic polyamide chains. Our hair is made up of complex bundles of fibrous proteins, again polyamides. The polymers you study are related to the real world in which we live. We experience these large molecules at the macroscopic level everyday of our lives and this macroscopic behavior is a direct consequence of the atomic-level structure and behavior. Make pictures in your mind that allow you to relate to the atomic and macroscopic worlds. 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

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