Chemical oxidative degradation 3-scission

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

Alder reaction. But, at higher temperature or in the presence of a stronger base, it also creates degradation such as oxidation or scissions that can be evidenced by the measurements of the decrease in the intrinsic viscosity, caused by the decrease in molecular weight [19]. In order to avoid any degradation, the created double bonds can become the site of the addition of several agents like diamines, bisphenols or peroxides, that can increase the mechanical and chemical properties. The formation of the scissions in the network are explained in Sect. 3.1.4. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

When two surfaees start to slide against one another, steady state is not immediately achieved. Both the surface of the polymer and the eounterfaee may be modified over a period of time. Abrasion removes the original polymer surface with its inherent properties, and the new face may be annealed by frictional heating or suffer thermal or oxidative degradation, resulting in chain scission or chemical modification. The eounterfaee may also suffer abrasive wear (especially when the polymer is filled), or it may become coated with a film abraded from the polymer surface. Thus it takes some time before a steady rate of wear is achieved. 

In reality, both of these mechanisms are usually present, but one will predominate over the other and this is determined by the chemical structure of the polymer backbone and the availability of hydrogen (or other reactive species) for the free radicals to react with. Once a free radical has formed on the polymer molecule by hydrogen abstraction (a typical route in oxidative degradation), it becomes stable either by a rearrangement reaction (often leading to chain scission) or by reacting with another radical (cross-link formation). 

---