Molecular weight from light scattering

The force-distance profiles Al, A2 appear to show the relaxed, or quasi-equilibrium limit for the interaction between the mica plates bearing the PEO in the good solvent conditions of the present study. The adsorbed layer thicknesses 6 are then about half the value of D at which onset of repulsion (A curves) is first noted. 6 thus corresponds to some 3Rg for both polymers in the present investigation, a value comparable to that obtained for hydrodynamic layer thickness of PEO absorbed on latex particles in water, for similar molecular weights, from light scattering studies. 

Equation 3.27 forms the basis for determination of Molecular weight from light scattering data. Like Osmotic pressure measurements, it is essential to consider the non-ideality of solutions and the concentration dependence. Following Debye, eq. 3.27 gets modified to... 

A previous investigation ( ) found that the presence of aggregates had no measurable effect on [tj] for PVB in THF. The same value of [tj] was obtained regardless of whether or not solutions were heated prior to measurement. The same heat treatment reduced the apparent molecular weight (from light scattering measurements) by 30% and increased A2 up to 50% ( ). We made measurements on heated and unheated solutions of PVB in pure MeOH and 9 1 MIBK/MeOH and also found no effect on [tj]. The lack of any effect of heat treatment on [17] again supports the conclusion that... 

For a polydisperse polymer, the average molecular weight from light scattering is A/w, but the radius of gyration which is estimated is the z average. 

These authors have also shown that for a homologous series of polypeptides, the intrinsic viscosity varies with the molecular weight (from light scattering) to the power of 1.7, which is in perfect accord with Simha s theoretical value (see Fig. 3). 

In studies of C(2.45)A in a methylene chloride/methanol mixture (50 50 v/v). Tanner and Berry found equal to -0,2 (with (Sn/ac3)c2 p,T 0.0982 ml/g), e.g., that methylene chloride preferentially solvated the polymer. Data of Patel and coworkers on C(2.96)A for the same solvent mixture gave X -0.3, also indicating preferential solvation of the polymer by methylene chloride. Data of Shakhparonov al. (4) for C(2.45)A in methylene chloride/methanol mixtures containing 0 to 30% methanol indicate positive, or preferential solvation by methanol rather than methylene chloride. Their conclusions are based on apparent values of the molecular weight from light scattering, and are subject to error from the effects of the association discussed by Tanner and Berry. 

Table 3. Molecular Weights from Light Scattering Photometry ... 

Chemically uniform products can be distinguished quantitatively from those that are chemically heterogeneous by the dependence of the apparent molecular weight from light scattering measurements on the refractive index of the solvent (Section 9.5.3). 

Figure 3> The molecular weight (from light scattering) versus elution volume for fractions of BBL, showing severe effects of association for some of the data which deviate markedly from the solid curve. ...

A/iP-weight average molecular weight from light scattering. 

---