Miscible polyisoprene/polystyrene blends: Distinct segmental dynamics but homogeneous terminal dynamics

The segmental and terminal dynamics of the individual components in miscible blends of polyisoprene (PI) and polystyrene (d ₃PS) were characterized over a wide temperature range. Though the system has a large positive Flory-Huggins interaction parameter Χ ∼ 0.1, it is miscible in the temperature range of study due to low molecular weight. ¹³C and ²H NMR relaxation measurements were performed to extract the segmental relaxation times. Pulse-gradient spin-echo NMR was used to determine the center-of-mass diffusion coefficients. Though the segmental dynamics of PI and PS components differ by more than 2 decades at T _g + 50 K in the blends, their terminal dynamics (in terms of monomeric friction coefficients) are essentially identical. We know of no other system with this behavior. The distinct component segmental dynamics can be reasonably interpreted by the Lodge/McLeish model. The unusual homogeneous terminal dynamics may be due to a large thermodynamic barrier to diffusion in this system. The monomeric friction coefficients are well reproduced by the Gordon-Taylor mixing rule.