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 means of different techniques, the segmental dynamics in the model blend system poly(vinylethylene)/polyisoprene (PVE/PI) is still a topic of strong debate. One of the most controversial questions is the possibility of a bimodal distribution of segmental relaxation times for polyisoprene in the blend. In this work, we have combined a microscopic technique, quasielastic neutron scattering (QENS), with molecular isotopic labeling and broad-band dielectric spectroscopy. The measurements have been carried out on the same samples and at the same temperature (270 K). By means of this combination, we have shown that all the PI in the miscible blend sample PVE/PI(50 wt % PVE-50 wt % PI) dynamically behaves very close to pure PI. In contrast, the alpha-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 blend component displaying the higher T-g. These experimental facts contradict many of the results previously published about the dynamics of the PI/PVE system, and as far as our knowledge, they cannot be explained in the frame of the models presently available for polymer blends dynamics.