Polyamides structural variation

Further variation is possible in the polymer structure and properties by using mixtures of the appropriate reactants such that the polymer chain can have different R and R groups. Thus polyamide structures of types XXXV and XXXVI are possible variations on structures XXXIII and XXXIV, respectively. A polymer such as XXXV or XXXVI has two different repeat units and is referred to as a copolymer the process by which it is synthesized is... 

The glassy polymers such as the aromatic polyamides and polycarbonates have significant hindrances to intramolecular mobility. The data for these materials appear to be correlated fairly well in terms of the "specific free volume" discussed by Lee(52). Structural variations that suppress the ability to pack tend to reduce the quality of the barrier while those that improve the ability to pack produce better barriers. The free volume in this case is defined as the difference between the actual polymer molar volume at the temperature of the system and at 0°K. This latter parameter is determined by group contribution methods. 

Figure 4.2 Structural variations in textiles, yams, and fibers (a) woven denim fabric 60 x, (b) knitted jersey 60 x, (c) chenille yam 57 x, (d) high-twist yam 56 x, (e) wool fiber 210 x, and (f) polyamide monofilament 210 x.

The family of polymers that we refer to as nylons consists of molecules composed of amide groups alternating with short runs of methylene units. These molecules are also known as polyamides, which may be shortened to PA. The generic chemical structure of a nylon molecule is shown in Fig. 23.1. Variations on this basic structure include the length of the polymethylene sequences and the orientation of the amide groups relative to their neighbors. Figure 23.2 shows the chemical structures of nylon 6 and nylon 66, which are the two most common types of nylon. 

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 structures is myriad. Protein biosynthesis is mediated by other biopolymers, the nucleic acids. 

Many combinations of diacids—diamines and amino acids are recognized as isomorphic pairs (184), for example, adipic acid and terephthalic acid or 6-aminohexanoic acid and 4-aminocyclohexylacetic acid. In the type AABB copolymers the effect is dependent on the structure of the other comonomer forming the polyamide that is, adipic and terephthalic acids form an isomorphic pair with any of the linear, aliphatic C-6—C-12 diamines but not with -xylylenediamine (185). It is also possible to form nonrandom combinations of two polymers, eg, physical mixtures or blends (Fig. 10), block copolymers, and strictly alternating (187—188) or sequentially ordered copolymers (189), which show a variation in properties with composition differing from those of the random copolymer. Such combinations require care in their preparation and processing to maintain their nonrandom structure, because transamidation introduces significant randomization in a short time above the melting point. 

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. 

Thus muscle, collagen (in bone), keratin (in hair, nails and beaks) and albumin are all copolymers of very similar amino acids but have quite different physical properties. In deoxyribonucleic acid (DN A), the genetic template, the sequence of monomers is precise and variations are the cause of genetic mutations. Although the polypeptides are of ultimate importance in life processes they are not important in the context of materials and will not be considered further in this book. However, they have had a significant impact on modern polymer science since the synthesis of the first man-made polyamide fibre. Nylon, by Carothers was modelled on the structure of a silk, a naturally occurring polypeptide. 

It is important to note that the chemical structures of the membranes are proprietary and local variations can occur due to the nature of membrane manufacturing processes. In order to avoid possible misinterpretation of membrane chemieal structure due to local variations, ESCA analysis was repeated for selected membranes. The XPS analysis revealed that CALP was a cellulose acetate membrane represented for the most part by tri-acetate species (acetate (O—C=0)be 289.7 eV) (Fig. 27). The LFCl membrane showed a polyamide active layer apparently crosslinked to a polysulfone (S (2p)be 170-168 eV) infrastructure as suggested by the XPS surface scan. 

Information on physical parameters of the molecular structure of polyamide fibers are usually obtained by x-ray diffraction methods, electron and light microscopies, infrared spectroscopy, thermal analyses such as differential thermal analysis, differential scanning calorimetry, and thermomechanical analysis, electron spin resonance, and nuclear magnetic resonance (NMR) spectroscopy. X-ray diffraction provides detailed information on the molecular and fine structures of polyamide fibers. Although the diffraction patterns of polyamide fibers show wide variation, they exhibit usually three distinct regions ... 

Variations on the diamine-diacid chloride theme have provided oth rigid linear aromatic polyamides which are ultra-high modulus fibre formers, structures Il-V being notable examples and numerous comonomers have been described. Polymer II is said only to give good fibre properties if its structure is deliberately disordered slightly by inclusion of small quantities of co-reactants, the effect being attributed to better solubility and consequent improved spinnability. However, this... 

The names of many polymers are based on the monomers from which they were prepared. There is, however, frequent variation in the format. A nomenclature of polymers was reconunended by lUPAC and is used in some publications. Strict adherence to the recommendation, however, is mainly found in reference wor. Also, problems are often encountered with complex polymeric structures that are crosslinked or have branches. In addition some polymers derive their names from trade names. For instance, a large family of polyamides is known as nylons. When more than one functional group is present in the structure, the material may be called according to all functional groups in the structure. An example is a polyesteramide. A thermoset polymer prepared from two different materials may be called by both names. For instance, a condensation product of melamine and formaldehyde is called melamine-formaldehyde polymer. 

Kwak and Ihm [7] used AFM and solid state NMR spectroscopy to characterize structure-property-performance correlations in high-flux RO membranes. The membranes were thin film composites, whose thin active layers were based on aromatic polyamide formed by the interfacial polymerization of MPD and trimesoyl chloride (TMC). These membranes, each coded as SH-I, SH-II, and SH-III, were provided by Saechan (Yongin-city, Korea). The variations among these commercial membranes are difficult to know. Most likely, they vary by the amount of catalyst or surfactant added to the aqueous MPD solution. Table 8.2 shows water flux, salt rejection, and the roughness parameter of those membranes, together with the data for another membrane, MPD/TMC, which was prepared at the laboratory of Kwak and Ihm [7]. 

IS measurements were performed to determine the membrane variations associated with (i) Dense and porous layers of a commercial RO membrane (ii) Different PEG concentrations in the top dense layer of a polyamide/polysulfone experimental membrane (iii) Hydrophobic character of one layer in a composite or multilayer structure (iv) Membrane matrix material modification and (v) Protein (BSA) fouling of a porous commercial membrane. The results obtained with other characterization techniques, such as morphological, chemical, and adsorption analyses, have validated the information obtained from the IS results. 

The greatest increase in synthetic fiber materials has been an increase in the polyesters, which now account for almost two-thirds of total synthetic fiber production. It must be remembered that terms such as polyester and polyamide refer to broad classes of compounds and not to specific materials. Each class contains untold thousands of possible variations in molecular structure, both from the chemical identity of the monomers used and from the order in which they react during polymerization. 

Cone calorimetry is used to evaluate the flammability imder flre-like conditions. Relevant parameters such as the rate of heat release (HRR) and its peak value, heat of combustion (He), smoke yield (specific extension area, SEA), and carbon monoxide 5ueld are obtained. Table 2 shows some t5q)ical data for layered silicate nanocomposites based on organically treated montmorillonite, with polyamide 6, poly(propylene-gra -maleic anhydride), and polystyrene as the host matrix. Nanocomposites imder investigation have either delaminated (PAG) or intercalated-delaminated structures. In all cases there is a substantial reduction in peak HRR value (50-75%), whereas He and CO formation show little variation. 

Aromatic polyamide fibers are produced by spinning liquid crystalline polymer solutions of PPTA-sulfuric acid dopes into a water coagulation bath [414], resulting in the formation of a crystalline fiber with a surface skin. Variations in the structure produced by annealing at elevated temperature are known to increase the fiber modulus due to a more perfect alignment of the molecules [472]. The chemistry and physics of the aromatic polyamide fibers have been reviewed [419]. 

The PI in Eq. (6) has provided excellent adhesive strengths from 25 to 232 C. This polymer evolved from a study on structure/property relationships and came to be known as LARC-2 (Langley /Research Center) and then LARC-TPI. The of the PI has been reported as 256-275°C. The variation in T s is due to different methods of determination and different molecular weights. Early adhesive work found few(2-methoxyethyl)ether (dig-lyme) to be a good solvent for the polyamide acid and when these solutions were used to prepare adhesive tapes, Ti TSS were fabricated... 

As indicated in the literature [4,6], although the (3 peak shows only little variation with the PA/PE ratio, the a peak decreases in temperature for lower PA/PE ratios and a shorter polyamide block length as observed in Figure 10. Such a lowering of the a peak is attributed to an internal plasticization coming from oligoether miscible components inside the polyamide-rich regions. Another possible interpretation could be that, since the immiscible polyamide and polyether components are linked by chemical bonds, the structure and mobility of each block affect the others [15]. 

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