Depolarization current in phenol-formaldehyde resin below the glass transition

A thermally stimulated depolarization (t.s.d.) current study of a novolac phenol-formaldehyde resin performed using electric poling at temperatures T ? T_g (T_g being the glass transition temperature which equals 323 ± 2 K) shows a continuous distribution of polarizability in the range from 295 to 323 K. The efficiency of poling is found to be dependent upon the physical ageing of resin. In addition to the known current peaks P₁ and P₂ which appear at T_m1 = (320 ± 2)K and T_m2 = (334 ± 6)K respectively, a new peak, P₀, is detected at T_m0 = (296 ± 1)K. P₀ appears reversibly with the same magnitude regardless of the electrical history of the resin or the physical ageing. The activation energy determined by the initial rise method and the whole curve integration method is found to be 500 and 570 kJ mol^?1, respectively. It is assumed that P₀ is caused by a discrete phase transition in the resin.