Gel-like modes. of polymer solutions in « θ » solventsReport as inadecuate

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Abstract : A polymer solution forms a transient network of lifetime Tr. In good solvents, the mesh size l is identical to the correlation length ξ for the fluctuations of the monomer concentration c. But in « θ » solvents where the pair interaction vanishes - is much smaller than ξ. The dynamics involve then two quantitatively very different elastic moduli E : :1 at low frequencies, E = E0 = kT-ξ3 N c3 is small; 2 at higher frequencies, E = E gel kTc2 and Egel > E0. This leads to two modes in photon-beat experiments at wave vectors q larger than a certain value qg, the high frequency mode being the gel mode. Although this gives only a small fraction of the total scattered light, it still dominates the slope of the dynamical structure factor Sq, t at small times. We investigate the corresponding diffusion coefficient D g in the three regimes : a « many chain » regime qg < q < 1-ξ, Dg ≃ kT-6 πη s a is very high a is a monomer size and ηs the solvent viscosity ; b entangled « single chain » regime ξ-1 < q < l-1 , Dgq ∼ 1-q; c disentangled « single chain » regime ql > 1, Dg ∼q. The regimes b and c could be observed only in neutron experiments. But the cross-over from a to b could explain certain photon-beat results on PS in cyclohexane.

Mots-clés : diffusion in liquids elastic moduli light scattering liquid structure polymer solutions

Author: F. Brochard



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