Depolarization current of a novolac p-fluorophenol-formaldehyde resin

A Novolac p-fluorophenol-formaldehyde (NFF) resin was prepared by condensation of p-fluorophenol with formaldehyde. DSC showed the glass transition effect or coinciding endothermal peak depending upon the thermal history of samples. It is supposed that the peaks are caused by breaking of the intermolecular bondings in the resin during the glass transition. The bondings are formed in the resin during the storage at room temperature. Thermally stimulated depolarization current (TSDC) measurements were carried out in the temperature range of 290 to 350 K, with the samples having an average number molecular weight M?_n of 375 and 434. TSDC curves mainly showed the dipolar relaxation α peaks. The influence of poling temperatures, the influence of M?_n, the activation energy E_a, and the physical ageing of the samples were investigated. Physical ageing was determined as the reciprocal polarizability R_p vs. time of ageing. Samples. with higher M?_n showed a higher glass transition temperature, a lower E_a, and a higher increase in R_p than the sample with lower M?_n. The increase in M?_n increased the rigidness of NFF samples. The effects are attributed to the strong hydrogen bonding. The comparison with analogous results in novolac phenol-formaldehyde resin without fluorine is given.