Light scattering dissymmetry ratio

Figure 18 Plot of light-scattering dissymmetry ratio [7(45°)//(135°) against concentration c for a polystyrene-i)/oc/c-poly(ethylene/propylene) (37000 59 700 M ) copolymer in decane at 30 °C. The experimental points are indicated by circles ( ). The full curve and squares ( ) indicates the behaviour predicted theoretically using a hard-sphere model (reproduced...

Apart from their utility in determining the correction factor 1/P( ), light-scattering dissymmetry measurements afford a measure of the dimensions of the randomly coiled polymer molecule in dilute solution. Thus the above analysis of measurements made at different angles yields the important ratio from which the root-mean-square... 

Fig. 46.—Dissymmetry ratio for light scattered at 45° and 135° as a function of -x/r /X for random coil polymer chains. ...

In the determination of M , the intensity of scattered light is measured at different concentrations and at different angles 6). The incident light sends out a scattering envelope that has four equal quadrants (Figure 3.11a) for small particles. The ratio of scattering at 45° compared with that at 135° is called the dissymmetry factor or dissymmetry ratio Z. The reduced dissymmetry factor Z is the intercept of the plot of Z as a function of concentration extrapolated to zero concentration. 

If the intensity of light scattered by a colloidal dispersion is measured as a function of c and 0, the Zimm method enables us to convert this information into several parameters that characterize the colloid M, B, and Rg. In some situations this is more information than is actually needed. If spatial extension is the only information sought, a simpler method for evaluating it employs the so-called dissymmetry ratio. 

Tn the critical region of mixtures of two or more components some physical properties such as light scattering, ultrasonic absorption, heat capacity, and viscosity show anomalous behavior. At the critical concentration of a binary system the sound absorption (13, 26), dissymmetry ratio of scattered light (2, 4-7, II, 12, 23), temperature coefficient of the viscosity (8,14,15,18), and the heat capacity (15) show a maximum at the critical temperature, whereas the diffusion coefficient (27, 28) tends to a minimum. Starting from the fluctuation theory and the basic considerations of Omstein and Zemike (25), Debye (3) made the assumption that near the critical point, the work which is necessary to establish a composition fluctuation depends not only on the average square of the amplitude but also on the average square of the local... 

Particles which are too small to show a series of maxima and minima in the angular variation of scattered light are frequently studied by measuring the dissymmetry of scattering (usually defined as the ratio of the light scattered at 45° to that scattered at 135°). The dissymmetry of scattering is a measure of the extent of the particles compared with A. If the molecular or particle size is known, it can be related to the axial ratio of rod-like particles or the coiling of flexible linear macromolecules. 

Doty and Zimm (1947). These workers measured the turbidity by transmission and characterized the angular dependence by dissymmetry measurements. The dissymmetry is the ratio of the intensities scattered at two angles equidistant from 90°, the intensity at the smaller value of being made the numerator. By applying equations (63) and (55) to the dissymmetry and turbidity, both L and M can be found. The light scattering results (M — 40-io6 L = 2700 A.) were compared with those from electron microscopy and viscosity on the same sample all were in excellent agreement. 

The intensity of light scattered at 45° and 135° was measured in a Brice-Phoenix light scattering photometer, and the dissymmetry was calculated from the ratio of these intensities. The latex sample was diluted to such a concentration that the dissymmetry was independent of polymer concentration. The dissymmetries were measured at four different wavelengths of light, and the average particle size was estimated using tables calculated for this purpose (8). 

Figure 9-8. Dependence of the dissymmetry coefficient z of scattered light at angles of 45° and 135° on the ratio of >/X for spheres (sph), unimolecular random coils cm)y polymolecular (Mn) = 2) random coils Cp), and rods (r). Here Xo is the wavelength of light in the medium of refractive index n, and D corresponds to the diameter of the spheres, the length of the rods, and the chain end-to-end distance of coiled macromolecules.

---