Molecular weights of polystyrenes in toluene solutions by light scattering. Comparison between classical values and values deduced from a new method

Light scattering measurements in toluene solutions are performed for a series of monodisperse polystyrenes with a molecular weight M-w range from 4 x 10(3) to 8 x 10(6). The scattered polarized intensities I-v and the natural depolarization ratios rho(n) are registered with different apparatus at lambda = 633 or 488 nn and the M-w values are deduced through different formulae. The complete Carr and Zimm formula (CLa), from I-v and rho(n), and the usual simplified formula (Cl-b), from I-v, are considered for the classical method. An already demonstrated formula is considered for the new method (New). Values of M-w and related parameters do not depend on the experimental systems used but deviations appear when using different formulae. The deviations are generally low (about 10%) but often systematic: M-w(CLa) < M-w(CLb) < M-w(New). The most important difference concerns the effect of destructive interferences for M-w > 5 x 10(5): the new formula leads to a lower increase from theta = 90 degrees to theta --> 0 for M-w values (theta is the observation angle). For instance, in the 8 x 106 sample, M-w(theta - 0)/M-w(theta = 90 degrees) = 3.6 instead of 6.1, which implies a revision of the usual determination of the radius of gyration, R-g. (C) 1998 Elsevier Science B.V. All rights reserved.