## KbHbDf xlxj

where bH and bD are the scattering lengths of the protonated and deuterated monomers and bs' is the scattering length of a solvent normalized via the ratio of the specific volumes of the monomer and the solvent molecule.

For isotopic mixtures of poly(dimethyl siloxane) (PDMS) in carbon dioxide (CO), bH=0.086 10-12cm, bD =6.33 10-12cm, and b'= 1.47 10-12to 3.76 10-12cm in the range between the critical (pco2 =0.469 g/cm3) and the liquid state densities (pco2=1.2 g/cm3). For a given combination (bH < bs' c bD), it is possible to set L=0 (Eq.4) at any density of SC CO2 (Fig.2) by adjusting the concentration (x) of hydrogenated PDMS (e.g. L= 0 at x=0.512 and pco2 =0.95 g/cm) [9].Similarly, for isotopic mixtures of polystyrene (PS) in deutero acetone (AC-d) (bH -2.33 10-12 cm, bD = 10.66 10-12cm,andbs- 8.88 10-cm) or PDMS in deutero bromobenzene (BrBz-d) (bs' -5.8 10-12 cm) the pre-factor L becomes zero at x= 0.214 and 0.085, respectively. As follows from Eqs. (1-4), the condition L=0 completely eliminates the contribution of "total scattering" and Eq. 1 gives the single chain (intramolecular) scattering directly. For some solutions (e.g. PS in deutero cyclohexane, CH-d), there is no isotopic ratio, 0<x< 1, which satisfies the condition L(x)=0, and I(Q,x) always contains a contribution from the total (intermolecular) scattering which may be subtracted via [2]:

The value of the prefactor of the second term in Eq(5) is on the order of several percent and thus the effect of this correction to I(Q,x) is negligible for T >> Tc [2]. However, the correction can become finite near TC due to the divergence of It(0) ~ x -1.24 [10]. As soon as deuteration of the part of the polymer leads to a shift of Tc of several degrees [ 11], the subtraction in Eq.5 should be performed at the same values of x rather than at the same values of the absolute temperature.

The radius of gyration Rg may be obtained by fitting Ss with the Debye function :

For solutions of non-overlapping polymer chains under © conditions, Eq.(6) in the limit of small 0 [12,13] yields:

and the value of the correlation length (© conditions) is:

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