Thermal Degradation of Polymer:
Thermal degradation of polymer may follow either unzipping or the random route. The unzipping mechanism gives the pure monomer while random degradation leads to the formation of a host of a products, depending on the structure of the polymer since many polymer have a C-C chain as the backbone, their thermal stability is dependent on the stability of the C - C bond.
Factors affecting the C - C bond stability/ thermal stability:
1/ If the number of substitute group increase of the backbone chain of polymer, the thermal stability of the polymer backbone chain decrease.
Here as we proceed from left to right, the number of substituents increase and the stability of the polymer decrease. The result is that while poly-ethylene is thermally stable, poly-propylene & poly-iso-butylene is less stable.
2/ All the substituents don't always reduce the thermal stability of the polymeric system. For instance, Poly-tetrafluro ethylene (Teflon) has all the hydrogen atom of ethylene substituted by fluorine, but it is one of the most stable polymers.
Teflon can withstand temperature, as high as 400°c without undergoing degradation. Its thermal stability is attributed to the very high dissociation energy of the C - F bonds. Besides, the highly electro-negative nature of the fluorine protects the C - C bond in the teflon from an external attack.
3/ Generally the aromatic groups in a polymer backbone increase the thermal stability. Polycarbonate is a case in point.
4/ A substance like poly-phenylene, where the chain backbone is formed entirely of aromatic groups, is by the same taken thermally more stable than poly-carbonates.
5/ The high thermal stability of poly-tetrafluoro-ethylene & that of poly-phenylene are combined in poly-tetra-fluoro-phenylene which is relatively new polymer, capable of withstanding temperature upto 500°c.
6/ The branching & the presence of oxygen atom in the polymer chain are other important factors which make polymer susceptible to thermal degradation.
Mechanism of thermal degradation of PVC:
There are PVC which degrades around 200°c & giving HCl with thermal degradation.
PVC degradation is highly complex in nature. A free radical path seems to play a major role. The free radical (R°) produced from impurities, which attacks methylene group & abstract H from it.
The liable chlorine atom, which is at a beta position with respect to the free radical carbon atom is now released as a free stabilize the structure.
The chlorine can now attack a methylene group & abstract a hydrogen (H) & produce yet another liable chlorine atom.
The chain reaction proceeds in a similar way, leading to the production of polymeric chain having conjugated double bonds:
PVC is basically colorless, thus turns deep yellow, orange, brown or black depending on the concentration of the conjugated double bonds. Which depends on the degree of degradation.