| Popis: |
Several authors have investigated the Rayleigh linewidth of light scattered from thegels of cross—linked flexible polymers such as polyacrylamide14. The width of thespectral broadening varies as sinL(O/2), and diffusion coefficients may becalculated in the same way as for a macromolecular solution5. According to thetheory of Tanaka et al.1, this corresponds to freely diffusing fluctuations inpolymer segment density and is equal to the ratio of the longitudinal elasticmodulus to the force per unit volume required to maintain unit relative velocitybetween the polymer network and solvent. The degree of spectral broadening has, ingeneral, been found to be small enough (ca. 10%) for the optical beating to beregarded as largely heterodyne, most of the scattered light arising from astationary component in the density fluctuations.Gels formed from stiff polymer chains of biological origin, however, have a randomlybranched fibrous structure, the fibres consisting of a side—by—side association ofpolymer chains. In this case the degree of spectral broadening is much smaller (e.g.less than O.O1 for a 4% agarose gel), and the spectral linewidth does not vary assin 2(0/2), much less variation being observed6,7. These gels are regarded as beingalmost completely stationary at a molecular level. The very low degree of spectralbroadening facilitates the measurement of well defined diffusion coefficients ofcompact macromolecules such as globular proteins or dextran fractions within thesegels, thus enabling the interstitial spaces to be investigated and estimates made ofthe mean mass per unit length of the fibrous network6,7. To date calcium alginate6and agarose7 gels have been investigated in this way. Similar measurements havebeen made with polyacrylamide gels4 but here there is a complex interaction betweenthe diffusion coefficients of the gel and the diffusing macromolecule — |
