D. Cavallo1, F. Barbieri1, L. Sangroniz2, A. Santamaria2, R.G. Alamo3 and A.J. Müller2
1Department of Chemistry and Industrial Chemistry, Genova (Italy)
2POLYMAT/Polymer Science and Technology Dept., San Sebastián, (Spain)
3Department of Chemical and Biomedical Engineering, Tallahassee, (USA)
Contrary to low molar mass molecules, semicrystalline polymers exhibit a strong influence of the melt-annealing conditions on the re-crystallization. An enhanced concentration of nuclei is found after a mild melting treatment, in comparison to annealing at high temperatures and long times. [1,2] The intimate nature of these “self-nuclei” is still under debate, since they are elusive to the most commonly adopted characterization techniques. [2,3]
In this work, the self-nucleation process is studied by probing the effect of self- nuclei on the rheological properties of the non-equilibrium melt. Three poly(ε- caprolactone)s with different molar mass were investigated by shear rheometry. The viscoelastic functions of the melt were determined at the same temperatures, reached either from the solid state or from the melt, i.e., with or without the presence of self-nuclei.
Differences in the rheological behavior of these melts are striking. The Newtonian viscosity, plateau modulus and flow activation energy show a drastic increase with respect to the isotropic melt. The results indicate the existence of an attractive intermolecular interaction between chain segments within the self- nuclei, which is able to provide “extra” physical entanglements, different from the topological ones. As such, the flow behavior of self-nucleated melts display analogies with that of “associative” polymers possessing specific inter-chain interactions, e.g., hydrogen or ionic bonds.