The rapid photodegradation of polyether-polyester block copolymers was studied under ultraviolet (UV) irradiation in air for a short duration under which the effect of oxygen was neglected, and the degradation mechanism is discussed in relation to the photochemical reaction.
(1) The intrinsic viscosity of block copolymers decreased markedly as a result of the degradation. The number of carboxyl groups resulted from the chain scission increased with irradiation time.
(2) From the results obtained, it seems resonable to assume that n-π
* transition caused by UV-irradiation may promote the chain scission at the sites of ester linkages formed between polyether block chains and polyester ones.
(3) The rate of increase of carboxyl groups with the degradation was proportional to the concentration of ester linkages between two types of block chains, and the process of chain scission obeyed the first order kinetics.
(4) When the acid or ether component at the site of ester linkages between two block chains was replaced with other ones, respectively, the rate of chain scission varied, and the variation agreed with that of the photoreaction between the carbonyl group and the hydrogen atom of γ-position.
(5) The activation energy of this degradation process was approximately 14 Kcal/mol.
It was concluded from the results that the degradation is principally governed by the chain scission at the site of ester linkages fromed between the polyether and the polyester block chains.
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