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 (C8F17(CH2)(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 c(pol)* similar to 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(o) exp[-(t/tau)(alpha)] with alpha = 0.8 +/- 0.05. Here, G(o) denotes the plateau modulus and tau the macroscopic relaxation time of the transient network. Compared with already published data [Annable et al., J. Rheol. 1993, 37, 695-726], tau 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 similar to 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.