In this work, samples of amorphous and semicrystalline PEN (polyethylene-2,6-naphthalene dicarboxylate) were subjected to DC electric fields for different durations up to their dielectric breakdown, with or without depolarization. The field-induced mechanical strains in the samples were studied by means of a nondestructive optical technique without resorting to any physical contact. The results showed that amorphous samples were more vulnerable to the field-induced mechanical deformation when compared with similar semicrystalline samples. However, both morphologies showed in some samples contradictory behavior at prebreakdown field strength, some caused yielding of the polymer while the others seemed to have the opposite effect. In the latter category, yet some showed the maximum yield point at a particular critical field starting from vhich the deformation began to diminish significantly to reach a total cancellation at the breakdown field. This critical field quantified at 260 kV/mm is probably a pronouncement of critical ageing and breakdown in PEN. The deformation seems to be an evolution very similar to the evolution of DC current at high fields. Furthermore, the varying response of the samples implies further work to be undertaken to validate a clear mechanism. (c) 2007 Wiley Periodicals, Inc.