Overview of Chemistry and Analytical Techniques

If high impact resistance is required, the PA can be modified with rubber particles.11,15 The blends are usually made by reactive compounding from maleic-anhydride-modified rubbers, such as, EPDM, EPR, polybutadiene, or SEBS. Partial amorphous PA with a high Tg combines to give a high dimensional stability and good solvent resistance with transparency. 

PA belongs to the group of condensation polymers about which many excellent reviews have been written about.4 13 The PA (AB) can be synthesized from 

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29  

The equilibrium constant K (also called in the synthesis of PA-6) has also been found to be independent of the water concentration. Giori and Hayes30 found that with up to 0.5 mol water per kilogram of reaction mass (0.9 wt %), K increases linearly with water content (Fig. 3.14). At water concentrations higher than 1 mol kg-1 (1.8 wt %), K decreases again. At very high water concentrations, K seems to be independent of water content. 

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