Segmental dynamics in poly(vinylethylene)/polyisoprene miscible blends revisited. A neutron scattering and broad-band dielectric spectroscopy investigation

Despite the great experimental effort carried out over the past years by me ans of different techniques, the segmental dynamics in the model blend syst em poly(vinylethylene)/polyisoprene (PVE/PI) is still a topic of strong deb ate. One of the most controversial questions is the possibility of a bimoda l distribution of segmental relaxation times for polyisoprene in the blend. In this work, we have combined a microscopic technique, quasielastic neutr on scattering (QENS), with molecular isotopic labeling and broad-band diele ctric spectroscopy. The measurements have been carried out on the same samp les and at the same temperature (270 K). By means of this combination, we h ave shown that all the PI in the miscible blend sample PVE/PI(50 wt % PVE-5 0 wt % PI) dynamically behaves very close to pure PI. In contrast, the alph a-relaxation of PVE component in the blend is drastically "plasticized". In addition, it has been shown for the first time by QENS that the secondary beta-like processes seem not to be modified by blending, at least for the b lend component displaying the higher T-g. These experimental facts contradi ct many of the results previously published about the dynamics of the PI/PV E system, and as far as our knowledge, they cannot be explained in the fram e of the models presently available for polymer blends dynamics.