Dilute polymer blends: Are the segmental dynamics of isolated polyisoprene chains slaved to the dynamics of the host polymer?

C-13 NMR has been used to investigate the segmental dynamics of isolated polyisoprene (PI) chains in host matrices of polybutadiene (PB), poly(vinylethylene) (PVE), and polystyrene (PS). In the dilute regime, where intermolecular concentration fluctuations are minimal, the segmental dynamics of isolated PI chains do not become slaved to the segmental dynamics of the host matrix but are significantly biased toward the dynamics of pure PI. Using the framework of the Lodge-McLeish model, it is shown that self-concentration effects can account for the dilute PI segmental dynamics. The value of self-concentration was determined to be 0.41 for PI segmental dynamics in PVE blends, a value which agrees with findings at higher compositions and which is also in reasonable agreement with the LodgeMcLeish prediction (0.45 for PI). In contrast, the observed self-concentration values for dilute PI blends with PS (similar to0.20) and PB (similar to0.85) do not agree with the model prediction of 0.45. These results indicate the importance of self-concentration in understanding the component dynamics and the rheology of miscible polymer blends.