Effect of Blending on the Chain Dynamics of the "Low-Tg" Component in Nonentangled and Dynamically Asymmetric Polymer Blends

We have studied polyisoprene (PI) component chain dynamics in poly(tert-butylstyrene) (PtBS) miscible blends of low molecular weight over the entire composition range by broadband dielectric spectroscopy (DS). Blends with two different molecular weight of PtBS, (M-n) over bar = 1300 Da and (M-n) over bar = 2300 Da, having notably different glass transition temperatures, T-g = 330 K and T-g = 373 K, respectively, have been investigated. The molecular weight for PI was (M-n) over bar = 2700 Da, and T-g = 204 K, i.e., well below the T, of PtBS. The molecular weights of all the polymers were chosen in order to be well below the entanglement limit for PI. In addition to the slowing down of the dielectric response of PI as the high-T-g component PtBS increases, there is a gradual broadening of both low- and high-frequency tails of the normal mode relaxation as PtBS% increases and for a given composition as the temperature decreases. The magnitude of the broadening depends on (i) the concentration, (ii) the temperature, and (iii) the dynamic asymmetry, i.e., the difference between the pure component's T-g. The onset of the mentioned broadening correlates well with the freezing of the segmental relaxation of the high-T-g PtBS component. Finally, the terminal dynamics of PI component shows a stronger T-dependence than its segmental dynamics, and the effect is more pronounced the higher the PtBS content. As a result, the separation between the maxima of both relaxations ranges from 3 decades in pure PI and PI >= 50% content blends to similar to 5 decades for 20% PI blends. The results are discussed in terms of the dynamic asymmetry between the different blend components and compared with simulation results of dynamically asymmetric bead-spring polymer blends showing anomalous scaling behavior of the Rouse modes.