Dynamics of polymers in concentrated solutions: A weaker self-concentration effect

We examine the validity of the Lodge-McLeish (LM) self-concentration model on polymer dynamics in concentrated polymer/small molecule solutions. Three solutions, 1,2-polybutadiene/2-ethylhexyl benzoate, 1,4-polyisoprene/ phenyloctane, and polystyrene/2-ethylhexyl benzoate, are investigated by rheological technique. The temperature dependence of their normalized terminal relaxation times is fitted by the WLF equation to extract the effective glass transition temperature. The calculated self-concentration is found lower than the LM prediction, in contrast with the significant values in miscible polymer blends. According to the molecular dynamic simulation, this weak self-concentration effect is explained by the overestimation of the polymer's interchain concentration, which is postulated as the global average concentration in the original LM model. The lower interchain concentration in solutions is attributed to the higher probabilities for small molecules to fill the cooperative volume. Both experimental and simulation results may motivate the further theoretical modification on the interchain concentration in order to establish a self-consistent model predicting both miscible blends and polymer solutions.