When does a diblock copolymer probe the interfacial rheological effect?

Abstract

Lateral diffusion of diblock copolymer residing on the interfaces between two immiscible liquids is investigated at single molecular level. Fluorescence correlation spectroscopy was used to study the diffusion of fluorescence-labeled diblock copolymer, polystyrene-b-polyisoprene, at the interface formed between two immiscible liquids. The interfaces are formed between N,N-dimethylformamide (DMF) and a few immiscible liquids, n-alkane and polyisoprene. Interfacial diffusion coefficient of the diblock copolymer probe is found to decrease monotonously with the increase of the molecular length of the interface constituting liquids. The decrease of diffusion coefficient follows the prediction by Einstein relation using the viscosity of the constituting liquids as the variables only for interfaces between DMF and very small n-alkanes. For interfaces formed between DMF and bigger alkanes and especially between DMF and polyisoprene, the diffusion coefficient is much higher than the calculated value, indicating that the probe molecule starts to probe the much less viscous interfacial region because the interfacial width gets larger, whose thickness is comparable to the molecule size of the liquids.