Polymer degradation chain length

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

While the reduced enzymatic susceptibility of the aromatic ester bonds is caused by the interaction of the long chains in the polymer, the chain length itself has no direct influence on the biodegradability above a minimum molar mass. Tests with copolyesters from terephthalic acid, adipic acid and 1,4-butanediol, which were chain extended with hexamethylene-diisocyanate, did not result in a decrease in the biological degradation rate (in a compost simulation test), although the molar mass of the prepolyesters of about 48,000 D was increased to 232,000 D by the chain extension [85]. 

Improving the mechanical and thermal strengths of membrane is critical to solve the physical degradation problems, and this has been mainly achieved by reducing the polymer side chain length of PFSA membranes or by using the reinforced/composite membranes. 

Chain scission results from degradation of the polymer skeleton to smaller fragments, typically oligomers with different chain lengths. 

Some degradation reactions occur without an increase in cross-linking or a lessening in chain length. Thus, with minute amounts of HCl, water, ester, etc. elimination can occur with vinyl polymers giving localized sites of double bond formation. Because such sites are less flexible and more susceptible to further degradation, these reactions are generally considered as unwanted. 

Polymer degradation typically occurs via random chain scission, depolymerization, or both, resulting in a loss of chain length and properties associated with polymer length. 

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