ABSTRACT
The final power of any process, including chemical degradation, is the change offree energy, ..1 G. The proceeding reaction becomes possible if:
LlG = G~ - G;n < 0 (I) where the free energies of end (G~) and initial (G;n) systems determine the formation. The condition of thermodynamic possibility of the reaction initiation, with performance defined by kinetic parameters, is set by the equation:
LlG = 0 (2)
Frequently conditions occur at which, with regularity, Eq. (I) is fulfilled at storage and exploitation of polymeric materials [ l-9] as a result of the change in external parameters. Consider the conditions fulfilling Eq. (2) parameters for polymers, existing in different phases, at which degradation process proceeding can be initiated in the system. We can restrict through thermodynamics (formation free energy) the initial and end states, in which definite chemical substances are contained, because the end state at the defined initial one does not depend on how it is reached. That is, it is possible not to take into account the detailed mechanism of the process to clear up thermodynamic possibility. Consider the proceeding possibilities of the following reactions:
Scheme 1
Reactions (a) and (c) describe the process of a chemical bond break in the main chain, and (b) and (d) in the side chain. Reactions (a) and (b) are characteristic for thermodegradation, and (c) and (d) for thermooxidation
degradation. It is evident that reactions (a} and (b) differ sufficiently from (c) and (d) by the concrete mechanism of the proceeding reaction. In particular, for polymers under thermodynamic consideration, it is necessary to
take into account the process of the first attachment of oxygen (or other chemical agents) only. Further on it will proceed the chemical transformation of groups, neighbor to attached oxygen, followed by a chain fracture. That is why we first consider the conditions of thermodynamic possibility of the initiation of(a) and (b) type reactions. Depending on the place of its proceeding, reaction (a) will occur with a sufficient decrease of molecular weight at the breaking of the chain middle. If the chain break occurs by the end, reaction (a) will be similar to that of the (b) type. We will consider that the components of free energy additive, which in some cases can lead to different parameters which differ from parameters at which the process occurs. The probability of such a consideration is connected with independence of the free energy of the formation of the end and defined initial system states, i.e., in this case as though it is composed the way oftransition for each stage, during which only one component of free energy changes. First, consider the simplest case-thermodynamic opportunity of degradation reaction initiation in diluted solution.
