Kinetic Processes in Polymers

The previous sections dealt primarily with phase transformations and corrosion in materials. Polymers also undergo phase transformations. For example, there are many polymers that utilize nucleation and growth kinetics to transform from amorphous to crystalline polymers. The kinetics of these transformations are very similar, in principle, to the preceding descriptions for glasses, so it is not necessary to duplicate that material here. Polymers also are susceptible to corrosion, but the term degradation is more 

1 Kinetics of Stepwise Poiymerization. Consider the reaction of hexam-ethylene diamine and adipic acid  

Note first of all that there is the characteristic loss of a small molecule for stepwise reactions, which in this case is water, and that the loss of water results in an amide (HN-CO) linkage hence the resulting polymer is called a polyamide. This particular polyamide is very common and is better known by its trade name nylon. There are many types of nylons, depending on the exact formula of the diamine and diacid used to form the polyamide, but in this case, there are six carbon units between the nitrogens in the diamine and there are six carbon units (including the carbons which contain the double-bonded oxygens) in the diacid, so this polyamide is called Nylon 66. 

The kinetics of this reaction are analyzed by monitoring the disappearance of one of the reactants, or the formation of the product. If we choose to follow the disappearance of the adipic acid, the rate expression for a catalyzed reaction is 

If we start with equal amounts of diamine and diacid, then for the duration of the reaction we have [COOH] = [NH] = c, and we have a second-order differential equation  

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Mortimer, R. J. Dynamic processes in polymer modified electrodes. In Research in Chemical Kinetics, Compton, R. G., Hancock G., Eds. Elsevier Amsterdam, 1994 Vol. 2, pp 261-311. 

The desorption (exit) of free radicals from polymer particles into the aqueous phase is an important kinetic process in emulsion polymerization. Smith and Ewart [4] included the desorption rate terms into the balance equation for N particles, defining the rate of radical desorption from the polymer particles containing n free radicals in Eq. 3 as kftiN . However, they did not give any... 

Ermakov, G. V. (2002) Thermodynamic Properties and Boiling-Up Kinetics of Superheated Liquids, UrO RAN, (Ekaterinburg,), in Russian. Skripov, P. V. and Puchinskis, S. E. (1996) Spontaneous Boiling-Up as a Specific Relaxation Process in Polymer-Solvent Systems, J. Appl. Polym. Sci. 59, 1659-1665. 

Kinetics of Deformation, Relaxation, and Fracture Processes in Polymers in the Temperature Range Between... 

It is essential to understand the precise nature of these two very different kinetic processes in order to design and control appropriate polymerization reactors. However, in order to understand more fully the correlations between polymer molecular structure and properties, it is sufficient to examine the nature of the repeat units which are introduced into a linear polymer chain by each mechanism. The dynamic and thermodynamic characteristics of chain... 

T.A. Walker, D.J. Frankowski, R.J. Spontak, Thermodynamics and kinetic processes of polymer blends and block copolymers in the presence of pressurized carbon dioxide. Advanced Materials 20 (5) (2008) 879- 98. 

The problem of non-equivalent kinetics is inherent to polymer reactions in solids [2], In this case particles existing in different surroundings react with different rate constants. As a result, the most active particles will be removed from the reaction, and the overall rate constant will decrease with time. On the other hand, relaxation processes in polymers restore the initial distribution of particles and so their reactivity. Thus the kinetics will depend on the relation between the rate of the chemical reaction and the rate of the relaxation processes [3], This fact also makes it necessary to reconsider critically the validity of extending the results of accelerated tests for polymer ageing. 

A certain period in polymer publications has been studied, when such characterizations as technological , operational , mechanic , forced , segmental etc. were added to the term compatibility to describe properties of thermodynamically incompatible polymers or polymers with a limited compatibility. In our opinion this differentiation doesn t have a reasonable physical meaning. This problem has more than once been referred to in publications (see, for example [109,163]). Application of these amendments could he stipulated hy kinetic aspects ignored during analysis of mixing and phase separation processes in polymer hlends and failed attempts to provide scientific base under practical preparation methods of polymer blends with satisfactory ( ) properties . 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

The Permeation Process Barrier polymers limit movement of substances, hereafter called permeants. The movement can be through the polymer or, ia some cases, merely iato the polymer. The overall movement of permeants through a polymer is called permeation, which is a multistep process. First, the permeant molecule coUides with the polymer. Then, it must adsorb to the polymer surface and dissolve iato the polymer bulk. In the polymer, the permeant "hops" or diffuses randomly as its own thermal kinetic energy keeps it moving from vacancy to vacancy while the polymer chains move. The random diffusion yields a net movement from the side of the barrier polymer that is ia contact with a high concentration or partial pressure of the permeant to the side that is ia contact with a low concentration of permeant. After crossing the barrier polymer, the permeant moves to the polymer surface, desorbs, and moves away. 

In contrast to statics, the relaxational kinetics of living polymers and of giant wormlike micelles is unique (and different in both cases). It is entirely determined by the processes of scission/recombination and results in a nonlinear approach to equilibrium. A comparison of simulational results and laboratory observations in this respect is still missing and would be highly desirable. 

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