Low-Frequency Internal Friction Study on the Structural Changes in Polymer Melts

With the help of the low-frequency internal friction method, we investigate the structural properties of polymer melts, such as amorphous polystyrene (PS), poly(methyl methacrylate) (PMMA), and semi-crystalline poly(ethylene oxide) (PEO). An obvious peak of relaxation type is found in each of the
internal friction curves. The peak temperature T_p follows the relation T_p≈(1.15-1.18)T_g for PS and PMMA melts, while it follows T≈1.22T_m for PEO melt, with T_g being the glass transition temperature and T_m the melting temperature. Based on the analysis of the features of this peak, it is found that this peak is related to the liquid-liquid transition temperature T_ll of
polymer melts. Mechanism of the liquid-liquid transition is suggested to be thermally-activated collective relaxation through cooperation. This finding may be helpful to understand the structural changes in polymer melts. In addition, the internal friction technique proves to be effective in studying dynamics in polymer melts.