Dynamic behavior of crosslinked amphiphilic block copolymer nanofibers dispersed in liquid poly(ethylene oxide) below and above their glass transition temperature

Abstract

Crosslinked polystyrene nanofibers were synthesized via RAFT-mediated aqueous emulsion polymerization and showed good shape stability in both the presence of solvent and at high temperature (typically above the T_g of polystyrene). The viscoelastic properties of their suspension in a low molar mass poly(ethylene glycol) matrix (PEG400) was studied as a function of the temperature, i.e., below the T_g of polystyrene at 25 °C, and above at 130 °C. Below T_g, the critical concentration ϕ* between the dilute and the semi-dilute regimes determined from the crossover of the scaling law on the zero shear viscosity for the dilute regime and the semi-dilute regime showed that a Brownian motion of the nanofibers was the dominant mechanism of relaxation according to the Doi–Edwards theory. Above T_g, the nanofibers are flexible and their Brownian dynamics did not obey the Doi–Edwards theory anymore. From the concentration dependence of the zero shear viscosity, it appeared that their dynamics obey the power laws for polymer chains in solution. Moreover, the flow activation energy at T > T_g was drastically dependent on the nanofiber concentrations whereas it was observed to be constant at T < T_g as expected from the Doi–Edwards theory. Finally, the flow activation energy became closer to the flow activation energy of polystyrene at concentrations above ϕ = 15%. Such qualitative agreement with the dynamics of flexible chains has never been observed before and might be fortuitous; it would therefore require further theoretical investigations.