Synthesis and Linear Viscoelasticity of Fluorinated Hydrophobically Modified Ethoxylated Urethanes (F-HEUR)

The influence of the terminal segment of hydrophobically modified ethoxylated urethane (HEUR) on the linear mechanical behavior is examined through the synthesis of perfluoroalkyl telechelic HEUR (defined as F-HEUR). A series of perfluoroalkyl-modified poly(oxyethylene) (POE) of molecular weight M n = 6000, 10 000, and 20 000 with a well-defined structure has been synthesized by reacting POE with a large excess of isophorone diisocyanate (IDPI) to produce an isocyanato functional precursor, followed by the reaction of the terminal isocyanato group with a perfluoroalkyl alcohol (C 8 F 17 (CH 2 ) 11 OH). The linear viscoelasticity of aqueous solutions has been investigated as a function of the polymer concentration (noted c pol, in weight percent), temperature, and intermediate chain length. For the M n = 10 000 chains as c pol is raised from 0.1% to 5%, an increase of more than 6 decades is observed in the static viscosity around ∼ 1.5%. This increase coincides very precisely with the onset of viscoelasticity of the solutions. Moreover, the linear mechanical responses exhibit striking features: In all experiments performed, the stress relaxation function G ( t ) decreases as a stretched exponential of the form G ( t ) = G 0 exp[−( t /τ) α ] with α = 0.8 ± 0.05. Here, G 0 denotes the plateau modulus and τ the macroscopic relaxation time of the transient network. Compared with already published data [Annable et al., J. Rheol. 1993, 37, 695−726], τ is found to be nearly 3 decades larger than for fully hydrogenated end caps, and the associated activation energy is twice the one previously reported (53 k B T against ∼25 k B T ). These data are actually remarkable since they provide the opportunity to probe the dynamics of the transient network in the time scale of minutes.