Molecular weight scattering

The total polarisability per unit volume of solute plus solvent has been invoked in Eq. (22) so that the dimensions of the Rayleigh ratio R0 in Eq. (23) are not cm2 as it appears, but actually cm 1. For pure liquids and polymers of small-moderate molecular weight, scattering at an angle of 90° is used and the Rayleigh ratio in such instances is given simply as... 

Neutron scattering Weight average molecular weight Scattering intensity is proportional to nuclear cross ... 

High molecular weight Scatter factor/hepatocyte growth factor... 

A graphical method, proposed by Zimm (thus tenned the Zinnn plot), can be used to perfomi this double extrapolation to detemiine the molecular weight, the radius of gyration and the second virial coefficient. An example of a Zinnn plot is shown in figure Bl.9.6 where the light scattering data from a solution of poly... 

One of the most important fiinctions in the application of light scattering is the ability to estimate the object dimensions. As we have discussed earlier for dilute solutions containing large molecules, equation (B 1.9.38) can be used to calculate tire radius of gyration , R, which is defined as the mean square distance from the centre of gravity [12]. The combined use of equation (B 1.9.3 8) equation (B 1.9.39) and equation (B 1.9.40) (tlie Zimm plot) will yield infonnation on R, A2 and molecular weight. 

As should be expected, both (fg ) and r show the same dependence on the degree of polymerization or molecular weight. Since the radius of gyration can be determined experimentally through the measurement of viscosity or light scattering, it is through this quantity that we shall approach the evaluation of 1. 

Fox and Floryf used experimental molecular weights, intrinsic viscosities, and rms end-to-end distances from light scattering to evaluate the constant in Eq. (9.55). For polystyrene in the solvents and at the temperatures noted, the following results were assembled ... 

Next let us consider the light scattered by liquids of low molecular weight compounds. We are actually not directly interested in this quantity per se, but in scattering by solutions-polymer solutions eventually, but for now solutions of small solute molecules. The solvent in such a solution does scatter, but, in practice, the intensity of light scattered by pure solvent is measured and subtracted as a blank correction from the scattering by the solution. 

Thus we have finally established how light scattering can be used to measure the molecular weight of a solute. The concentration dependence of r enters Eq. (10.54) through an expression for osmotic pressure, and this surprising connection deserves some additional comments ... 

This result uses the already established fact that M = when the molecular weight is determined by light scattering for a polydisperse system. 

In Example 10.5 we extracted both the molecular weight and the radius of gyration from Ught-scattering data. There may be circumstances, however, when nothing more than the dimensions of the molecule are sought. In this case a simple alternative to the analysis discussed above can be followed. This technique is called the dissymmetry method and involves measuring the ratio of intensities scattered at 45° and 135°. The ratio of these intensities is called the dissymmetry ratio z ... 

Benoit et al.f prepared a mixture of two different fractions of cellulose nitrate and determined the molecular weight of the mixture by light scattering. The mixture was 25.8% by weight fraction A and 74.2% fraction B, where the individual fractions have the following properties ... 

The molecular weight and its distribution have been determined by laser light scattering, employing a new apparatus for ETFE dissolution and solution clarification at high temperature diisobutyl adipate is the solvent at 240°C. The molecular weight of molten ETEE is determined by high temperature rheometry (21). 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

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