Chain scission is a term used in polymer chemistry describing the degradation of a polymer main chain It is often caused by thermal stress (heat) or ionizing radiation (e.g. light, UV radiation or gamma radiation), often involving oxygen. During chain cleavage, the polymer chain is broken at a random point in the backbone to form two - mostly still highly molecular – fragments.The classic model that describes chain scission in elastomers was proposed many years ago by Lake and Thomas. The aim of the model is to calculate the energy dissipated in breaking all the polymer strands that have adjacent cross-links on either side of the crack plane. The basic assumption of this model is that all the main chain bonds in any strand that breaks must be strained to the dissociation energy of a main chain bond. It is assumed that this energy in the strand is dissipated when the chain fractures The Lake-Thomas model is specifically for the threshold toughness, that is to say the toughness as the crack speed tends to zero, as there are expected to be viscoelastic contributions to the toughness at finite crack growth rates. However, Ghatak et al. has argued that the model cannot be correct because chain scission is a thermally activated process and so, as the crack speed tends to zero, strands at the crack tip will undergo scission when the average energy per main chain bond is much less than the dissociation energy. In principle, this argument is correct, but in reality the Lake-Thomas model works rather well. Probably there is a range of slow crack rates where the viscoelastic dissipation is small but in thermally activated terms, scission is fairly fast so a constant chain breakage force (or energy per main chain bond) is a reasonable approximation