The interaction of xenon with cryptophane-A in 1,1,2,2-tetrachloroethane-d 2 is investigated by 129Xe and 1H NMR spectroscopy. Xenon is reversibly trapped into the cavity of this host to form a 1 to 1 host−guest complex with an apparent association constant K of the order of at least 3 × 103 M-1 at 278 K. The exchange between the free and bound xenon is slow on the 129Xe NMR time scale, and the bound xenon resonance is shifted by approximately 160 ppm to lower frequencies with respect to the free xenon resonance. The xenon complex is at least 4 and 20 times more stable, respectively, than the corresponding chloroform and methane complexes under the same conditions. The stability of this xenon complex appears to be much greater than that of the previously described xenon complex of α-cyclodextrin in water. This is probably due to the combination of three favorable effects:  (i) good size matching between the guest and the cryptophane cavity in its most relaxed conformation, resulting in the optimization of the London forces between the highly polarizable guest and the electron rich aromatic rings of the host (enthalpic stabilization); (ii) no rotational or vibrational entropy loss of the monatomic guest in the cryptophane cavity; and (iii) no (or little) entropy loss due to reduction of the conformational freedom of the host. Analysis of the line widths of the signals corresponding to the free and bound xenon as a function of the relative xenon/cryptophane ratio suggests that the incoming xenon atom must displace the departing one to enter the cryptophane cavity, and that the empty cryptophane is not involved in the complexation equilibrium.

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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)11OH). 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.