Dielectric properties of thermooxidized poly(vinyl formal)

An investigation was made of the correlation between changes in dielectric properties (110 Hz–4 MHz, 213–443 K) of poly(vinyl formal) and chemical structure during thermooxidation. As thermooxidation proceeds, the loss peak of the αa relaxation is displaced to higher temperature and cannot be detected in this temperature range. On the other hand, two loss peaks are observed in the region of the β relaxation and can be assigned to local motion of the main chain (βlocal) and end groups (βend). These two β relaxations are closely correlated with changes in chemical structure: the former is attributed to cleavage of the formal ring, formation of unsaturated bonds and cross‐linking; and the latter is assigned to aldehyde end groups. A good correlation exists between the total dielectric relaxation strength of the β relaxation and changes in carbonyl concentration. The carbonyl dipole moment is calculated as 2.68D by the Frölich‐Onsager equation. Thus, it is clear that carbonyl groups formed by thermooxidation have a strong influence on the dielectric properties.