Analog Computer Plus LITERATURE CITED( 1 ) Crout, P. D., Trans. A m . I m t .

The unimodal molecular weight distributions found in many synthetic polymers can be fairly well characterized by the measurement of three different average molecular weights: the number‐, weight‐, and Z‐averages. The first two are easily measured; rhe third is more difficult to determine. This paper describes a light‐scattering method for the estimation of the Z‐average molecular weight which is applicable to random coil polymers in the molecular weight range between 0.2 × 106 and 2 × 106 and for which a suitable solvent is available. The solvent may be either a theta solvent or a good solvent for which the relation between intrinsic viscosity and molecular weight is known. The method depends on the fact, that, when angular dissymmetry of scattered light is observed, the Z‐average mean‐square radius of gyration of the polymer molecules, r2 can be determined. In a thela solvent, r2 is proportional to M, so that measurement of rz2 is equivalent to the determination of M̄z. In good solvents, r2 = kMb, and the corresponding molecular weight, called M̄d, is slightly different. The parameters k and b are derived from the relation between intrinsic viscosity and molecular weight in the test solvent. Provided that the dispersion of molecular weight is not too great, M̄d does not differ widely from M̄z, as b cannot exceed 1.2 for random coil molecules. Experimental tests in toluene on narrow molecular weight fractions of poly‐a‐methyl‐styrene and binary combinations of them gave results in accord with theory.

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