Polyamide chains

Primary structure refers to the sequence of amino acids in the polyamide chain. 

There are approximately 20 common naturally occurring amino acids, hence 20 different R groups that appear as pendents on the polyamide chain. Many other amino acids have been isolated or prepared, each representing a variation in R. The number of isomeric stmctures is myriad. Protein biosynthesis is mediated by other biopolymers, the nucleic acids. 

FIGURE 19.13 The strength of nylon fibers is an indication of the strength of the hydrogen bonds between neighboring polyamide chains. 

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 molecular weight increases with increasing conversion. Regulation of the molecular weight can be achieved by adding small amounts of substances (e.g., benzoic acid) that can react with the polyamide chains by transamidation. Because of the transamidation reaction and hydrolysis of amide bonds, an equilibrium molecular-weight distribution is finally attained (see Sect. 4.1). 

Further calculations on aliphatic polyester and polyamide chains ... 

Several ester monomers have also been studied. In methanol solution, methyl acrylate (16,21), ethyl acrylate (21,54), 2-ethyl hexyl acrylate (55), allyl acrylate (44), glycidyl acrylate (56,57) usually in methanol solution, have been extensively studied. Methyl methacrylate (19, 21, 28, 30, 36, 39, 58) in diversified media composition, has been successfully grafted on polyamides. Vinyl acetate is reported also for grafting on polyamide backbone (19, 28, 35, 58), as well as n-butyl maleate in methanol solution (55). Diallyl maleate as cross-linking agent for polyamide chains is reported (44). Other less usual ester monomers have been mentioned in the literature to form grafts (41,59,60). 

Fig. 3.23 Simple representation of a polyamide chain (both NH2 groups do not belong to the basic structure).

Nylon 6 is another polyamide, which is made by heating an aqueous solution of e-caprolactam. The seven-membered ring of the lactam is opened to form 6-aminohexanoic acid, the monomer that reacts with more lactam to form the polyamide chain. This step-growth polymerization thus begins with a single difunctional monomer that has two different functional groups, NH2 and COOH. 

F. 42. Conformation of the pani-aromatic polyamide chain with different angle a and 0 at the carbon and nitrogen atom of the amide group... 

The weakest bond in the aliphatic polyamide chain is said to be that between the carbon atom of the carbonyl and the nitrogen atom of the —NH— function [84], Cleavage of such bonds anywhere within the molecular chain of nylon-6,6 would produce a frE ment that could cyclize to form carbon monoxide and cyclopentanone... 

We see that all are alpha-am ino carboxylic acids in two cases (proline and hydroxyproline) the amino group forms part of a pyrrolidine ring. This common feature gives the amino acids a common set of chemical properties, one of which is the ability to form the long polyamide chains that make up proteins. It is on these common chemical properties that we shall concentrate. 

Where the backbone of the protein molecule is a polyamide chain (a polypeptide chain), the backbone of the nucleic acid molecule is a polyester chain (called a polynucleotide chain). The ester is derived from phosphoric acid (the acid portion) and a sugar (the alcohol portion). 

The incidence of the amide linkage in the polyamide chain is considerably lower than in Nylon 6 and 6-6, and this increased hydrocarbonlike character of the material is shown in a lower moisture-absorbing capacity as well as a little lower dyeing affinity. 

Also a relatively recent development is the use of aromatic diamines and diacids to form polyamide chains of inherently increased stiffness and order... 

Hydrophobic interaction chromatography Mode of chromatography using as a retention mechanism the molecular interactions between the hydrophobic or nonpolar regions of the external surface of protein molecules and chains bonded to a siUca support. Although C18 alkyl chains would qualify, the name is reserved to polyether or polyamide chains which cause little degradation of the purified proteins. 

The chain extension modifier reacts completely already in the first processing step based on optimised additive properties. The melt viscosity increases to a high level after the first processing step and decreases just parallel to the standard polyamide later on (Fig. 7). For practical applications most additives could never be used without problems in reproducibility and uncontrollable cross-linking reactions because these compounds have a too high activity and the reaction rate is faster than the proper additive distribution in the melt. Therefore, the practical value of these commercial additives was limited and still is. With a lowering of the reaction rate and improvement of the distribution properties during reactive extmsion a controlled polyamide chain extension could be realised. The attained levels of melt viscosity are quite stable also under multi-extmsion conditions. 

Titzschkau K, BeckP (2001) Maack business services. Polyamide 2001,2nd WOTldCraigiess, The polyamide chain, resins, products, developments, technologies, markets. Session IB, Diisseldorf, 11-13 June 2001... 

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