抄録
For a series of aqueous solutions of perfluorooctyl sulfonate (C8F17SO3−; abbreviated as FOS) micelles having a mixture of tetraethylammonium (N+(C2H5)4; TEA) and tetramethylammonium (N+(CH 3)4; TMA) ions as the counter cations, linear viscoelastic behavior was examined at 20°C. The solutions had the same FOS concentration (0.045 mol L−1) and various TEA fraction in the counter cations, φTEA = 0 − 1, and the spherical FOS micelles therein were connected into threads and further organized into dendritic networks. For φ TEA ≥ 0.5, the FOS threads/networks exhibited the Maxwell-type terminal relaxation reflecting their thermal scission. In this range of φTEA, the terminal relaxation time τ increased with decreasing φTEA while the steady state compliance Je was insensitive to φTEA. On a further decrease of φTEA below 0.3, τ became insensitive to φTEA and Je gradually increased possibly because the motion of the threads in the unscissored form became faster than the thermal scission to govern the terminal relaxation.These rheological features were discussed in relation to the effects of TEA and TMA on the thermal scission of the FOS threads/networks: Since the charge was the same for TMA and TEA but the bare radius was smaller for TMA, the TMA cations should be preferentially bound on the FOS thread. The thermal scission of the FOS thread, occurring through an exchange of the bound and non-bound TEA cations (Watanabe et al., Rheol Acta, 28, 110 (2000)), appeared to be strongly suppressed by the preferentially bound TMA cations that effectively blocked the exchanging sites. Indeed, in a range of φTEA ≥ 0.5 where the thermal scission governed the relaxation of the system, the rheological data were well described by a simple model considering this blocking effect.
