SYNTHESIS AND LINEAR VISCOELASTICITY OF FLUORINATED HYDROPHOBICALLY-MODIFIED ETHOXYLATED URETHANES (F-HEUR)

The influence of the terminal segment of hydrophobically modified etho xylated 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 st ructure has been synthesized by reacting POE with a large excess of is ophorone diisocyanate (IDPI) to produce an isocyanato functional precu rsor, followed by the reaction of the terminal isocyanato group with a perfluoroalkyl alcohol (C8F17(CH2)(11)-OH). The linear viscoelasticit y of aqueous solutions has been investigated as a function of the poly mer concentration (noted c(pol), in weight percent), temperature, and intermediate chain length. For the M-n = 10 000 chains as c(pol) is ra ised from 0.1% to 5%, an increase of more than 6 decades is observed i n the static viscosity around c(pol) similar to 1.5%. This increase c oincides very precisely with the onset of viscoelasticity of the solut ions. Moreover, the Linear mechanical responses exhibit striking featu res: 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 netw ork. Compared with already published data [Annable et al., J. Rheol. 1 993, 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 opportun ity to probe the dynamics of the transient network in the time scale o f minutes.