Light intramolecular interference

Note from Eq. (8.2.4) that 5(0) = 1, as we expect, since there is no destructive intramolecular interference between light waves scattered from different parts of the molecule when q — 0 they all travel the same distance to the point of observation and hence arrive with the same phase. At high values of q, however, there may be destructive interference between light waves scattered from different parts of the molecule, reducing S(q) from its zero argument value. 

If, however, intramolecular interference is important, intramolecular motions may, in some circumstances, affect the spectral distribution of the scattered light. The general condition for such contributions is that the terms containing the MO and by(0) must contribute a time-dependence to S(q, /). Three cases should be distinguished. 

For flexible polymers the structural change due to intramolecular motions must be large enough for the light wave to detect the difference between the various molecular shapes. Only under these circumstances will intramolecular interference affect the lightscattering spectral distributions. An extreme example of this case, the Rouse-Zimm dynamic model of the Gaussian coil, is discussed in detail in Section 8.8. 

Assuming that the particles are small (no intramolecular interference, see Section 9.5.5), that they are independent of each other (ideal gas or infinitely dilute solution), and that there is no loss of light intensity due to absorption. Equations (9-28) and (9-29) may be combined ... 

Extrapolation of KC/hconcentration allows the determination of I/M2 and 2A2, with the two latter facts corresponding to the ordinate at the origin and the slope of this variation, respectively. For the reasons indicated earlier, the radii of gyration of macromolecules of size lower than X/20 cannot be determined. With macromolecules of the same size as the light wavelength, intramolecular interference, which is a source of attenuation of the scattered intensity, have to be taken into account the relation between the scattered intensity and the molar mass of the macromolecules thus requires correction, and P(6) is introduced in the above expression to this end as shown previously. At infinite dilution (c 0), this relation reduces to... 

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