The dielectric and dynamic mechanical behaviour of bi-stretched non-treated and annealed sernicrystalline poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) films are studied as a function of different morphologies obtained by thermal treatments at temperatures close to the melting temperature of a semicrystalline film. Differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment for bi-stretched films. However, the melting temperatures and the degree of crystallinity increase with the value of annealing temperature. Both dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA) display three relaxation processes. In order of decreasing temperature, can be observed: the alpha-relaxation due to the glass transition, the beta*-process assigned to cooperative molecular motions of the naphthalene groups which aggregate and the beta-relaxation due to local fluctuations of the carbonyl groups. The alpha-relaxation process shifts to higher temperatures for the 250 and 260 degreesC treated bi-stretched semicrystalline samples compared to the sample thermally treated at 240 degreesC according to DRS data but shifts to lower temperatures according to the DMA measurements for the three annealed samples. This discrepency results from the different sensitivity of each methods with regards to the release of orientation. At a fixed frequency the temperature associated to beta*-relaxation is lower for the non-treated bi-stretched semicrystalline samples than for the treated ones using DMA but no difference can be seen in DRS. The associated apparent activation energies are rather high which suggest cooperative motions. It is assumed that the orientation of the samples prevents coupling between the naphthalene groups due to the stretched chain configuration in the amorphous phase. The activation energy for the beta-process given by DRS is independent of the thermal treatment and the value agrees with those found for poly(ethylene terephthalate) (PET) and amorphous PEN. Evidence of the decrease of orientation in the sample with thermal treatment can be seen via the onset of mobility, both by DRS and DMA. Thus, the orientation induces a greater change of properties compared to the crystalline samples obtained from the thermal treatment of an amorphous sample. Finally, a three phase model is proposed since there is evidence of a rigid amorphous phase present in PEN biaxially stretched samples which was favoured by the dependence of dielectric relaxation strengths on the degree of crystallinity for the beta*- and alpha-relaxation. (C) 2003 Elsevier Science Ltd. All rights reserved.