Condensation polymers, equilibrium

Howardt describes a model system used to test the molecular weight distribution of a condensation polymer The polymer sample was an acetic acid-stabilized equilibrium nylon-6,6. Analysis showed it to have the following end group composition (in equivalents per 10 g) acetyl = 28.9,... 

Equilibrium between Monomer and Polymer. A monomer-with-polymer equilibrium is quite different from the polymer-with-condensation-product equilibrium discussed in Section 13.1.1. If the condensation product is removed from the reaction mixture, a condensation polymer increases in molecular weight. If the monomer is removed when it is in equilibrium with the polymer, the polymer depolymerizes to re-form the monomer. At temperatures suitable for long-term use, the equihbrium will be shifted toward stable polymer. However, at fabrication temperatures and at the high temperatures common in devolatilization, the production of monomer and low-molecular-weight ohgomers can be significant. 

A general theory of the equilibrium polycondensation of an arbitrary mixture of monomers, described by the FSSE model, has been developed [75]. Proceeding from rigorous thermodynamic considerations a branching process has been indicated which describes the chemical structure of condensation polymers and expressions have been derived which relate the probability parameters of this stochastic process to the thermodynamic parameters of the FSSE model. 

Although we will not be discussing the mechanism of each type of step growth polymer because these reactions are very similar to the corresponding monomer chemistry, we should be aware of this analogy. For instance, an acid reacts with an alcohol under acid-catalyzed conditions by a certain well-studied and proven mechanism. This same mechanism is followed each time an ester linkage of a polyester is formed. One such transformation is outlined in Fig. 14.8. The equilibrium is shifted in the direction of the product by distillation of the water from the reaction mixture (and condensing it in a separate container—hence the name condensation polymers for this type). 

If the equilibrium constant K has a value between 1 and 10, less than a thousandth of the total amount of water formed in the reaction mixture is sufficient to prevent the formation of really high-molecular-weight condensation polymers. Hence it follows that it is extremely important to remove as completely as possible the low-molecular-weight reaction products, for example, water, eliminated during a polycondensation. In principle, these equilibriums are also known in stepwise addition polymerizations (polyaddition) like the back-reactions of urethane groups. Since they mostly occur at higher temperatures only, they can be neglected. 

It has been established that random interchange reactions occur with polyesters and polyamides at the higli temperatures at which these thermoplastics are normally polymerized and subsequently extruded or molded. Other condensation polymers probably undergo similar shufflings of repeating units under the proper conditions. This implies that the equilibrium constant for condensation reactions like that in reaction (a) of Fig. 5-2 is independent of the sizes of the molecules which are reacting. 

There are several areas of interest in chemical processing that should be reviewed in this chapter. These areas include retrograde condensation, polymers, and electrolytes. In each case, the material is more extensive than can be covered in any detail in this chapter. Reference should be made to Prausnitz, Lichtenthaler, and Gomes de Acevedo. Furthermore, the principles covered in this chapter form the basis for viewing and calculating the phase equilibrium that applies. The subjects of polymers and electrolytes are so complex that they require a chapter by themselves. However, a brief statement follows describing retrograde condensation. 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

There are several product quality reasons for favoring flow reactors. If the life of a growing chain is small, as in free-radical polymerizations, a perfectly mixed CSTR will give the lowest polydispersity and the narrowest composition distribution for copolymers. Heat and mass transfer are best accomplished in flow systems. Thus the continuous mode is preferred for vinyl addition polymers where there is a large exotherm. It is also preferred for condensation polymers where the by-product must be removed to overcome an equilibrium limitation and for situations in general where a small molecule, typically solvent or unreacted monomer, must be removed as part of a clean-up operation. 

Polymer molecular weights can also be increased during processing. This situation occurs in condensation polymers where, effectively, a chemical equilibrium exists as shown in Eq. (12-1) ... 

High humidity is a significant cause of reliability problems because many corrosion mechanisms require water to operate. A humid environment is an excellent source of water, even when it is not condensing. Polymers commonly used in PCBs are hygroscopic that is, they absorb moisture readily from the environment. This phenomenon is reversible the moisture can be driven out of the PCB by baking it. The amount of moisture absorbed and the time to reach equilibrium with a humid environment depend on the laminate material, its thickness, the type of solder mask or other surface coating, and the conductor pattern. 

Condensation polymers are the result of a condensation reaction between monomers, with or without the formation of a condensation by-product (Chapter 3). Examples of polymers produced by condensation are polyamide[6.6], (Nylon 6,6) the result of the intermolecular condensation of hexamethylenediamine and adipic acid, and polyamide[6], (Nylon 6) which is the product of intramolecular condensation of a-caprolactam. This type of reaction is generally sensitive to thermodynamic equilibrium and requires the removal of the by-product, which is often volatile. 

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