Dielectric and viscoelastic relaxation behaviours of polyethylene 2,6 naphthacene dicarboxylate (PEN)

PEN films of different morphologies, amorphous (300 μm thick), semi-crystalline (300 μm thick, obtained by thermally treating the amorphous samples at temperatures of 240°C and 260°C) and semi-crystalline biaxially stretched oriented films (25 μm thick) have been studied by dielectrical (DEA) and mechanical dynamic (DMA) analysis. By both methods three relaxation phenomena are observed which are due to different molecular motions: the β-, β*- and α-relaxation in order of increasing temperature. The temperature dependence of the relaxation rate of the β-relaxation can be described by an Arrhenius law with an activation energy E_A which is quite similar to values found for the β-relaxation of PET. So it is concluded that the β-process is due to local fluctuations of the ester linkage. The temperature dependence of the relaxation rate of the β*-relaxation can be also described by an Arrhenius law but the activation energy is rather high. So probably the β*-process is due to cooperative motions of the naphthalene groups or aggregates of it. Moreover dielectric and mechanical measurements gave different values for E_A. This is discussed together with the morphology dependence of the β*-process. For the a-relaxation the temperature dependence of the relaxation rate agrees with glass transition temperature measured by DSC. It is concluded that the α-process is due to the glass transition which is discussed also in dependence of the structure