Differences between Isotropic and Self-Nucleated PCL Melts Detected by Dielectric Experiments

Melt memory effects on polymer crystallization are commonly reported in the literature, even when they are not completely understood. In particular, the exact nature of the melt heterogeneities that cause an enhanced nucleation (i.e., the "self-nuclei") is unknown. This is partly due to sensitivity limitations of the experimental techniques employed to study melt memory. In this work, the melt memory effect of semicrystalline polymers is studied for the first time by dielectric measurements. Polycaprolactones of two different molecular weights have been investigated. Isotropic or self-nucleated melt states are obtained, at a given experimental temperature, by cooling from the isotropic melt or heating from the semicrystalline solid, respectively. A detectable decrease in electrical permittivity is obtained for a self-nucleated melt, consistent with the presence of molecular dipoles with restricted mobility in the case of samples displaying crystalline memory. The volume fraction of repeating units involved in the formation of self-nuclei is estimated to be lower than 0.4%. The relative difference in dielectric permittivity between self-nucleated and isotropic melt state shows excellent correlation with rheological measurements that detect an increase in Newtonian viscosity and with the enhancement of nucleation density, measured by DSC. Each of these measured parameters showed a different sensitivity to the presence of self-nuclei, which is linked both to their nature and to the features of the specific measurements. It is suggested that the relatively strong memory effect displayed by PCL, which can be evidenced by different techniques, is related to the presence of weak intermolecular hydrogen-bonding interactions.