Conclusions

The low temperature studies of the model compounds clearly point out the importance of intramolecular mechanisms for the photodegradation of the polymer. In addition intermolecular mechanisms can be observed but require the existence of free OH groups. This is in accordance with previous studies [9].

The results of the semiempirical study show that both intramolecular mechanisms are reasonable pathways for the photodegradation of the polymer PPE. After absorption of light, the primarily formed S1 state must undergo intersystem crossing to the triplet manifold and after internal conversion the system is in the lowest triplet T1 state. In this state ether bond cleavage as well as 1,5-hydrogen transfer might occur with a small or even no activation barrier.

These results have several important consequences for the attempts to stabilize PPE and to suppress its photoyellowing.

• The photodegradation is an inherent property of the geometrical arrangement of the monomeric unit. This also explains our observation that photodegradation occurs in samples of high purity or with no structural defects [8].
• The relative orientation of the monomeric units and the orthogonality of the aromatic systems suggest that energy migration along the chain should be slow. Therefore, the concentration of (triplet) quenchers must be very high in order to be effective.
• While removing free OH groups in the polymer by protecting the endgroups will reduce the rate of photodegradation, this will not avoid photoyellowing due to the importance of intramolecular reaction mechanisms.

Acknowledgement

Contents

Computational Study

Experimental Study