Estimation of Z‐average molecular weight by light scattering in good solvents

Abstract

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.

Light Scattering

“…The approximations and limitations of the determination of radius of gyration of chain molecules in good solvents from the angular curves were discussed by Prud'homme and Sicotte (261). Badley (9) indicated a procedure for estimating the 2-average molecular weight from light scattering of polymers in theta solvents and in good solvents for which the relation between the intrinsic viscosity and molecular weight is known. Kratochvil (190) presented an analysis of the angular dependence of scattering by polymer solutions containing large microgel particles.…”

Molecular Weight Characterization of Conjugated Polymers Through Gel Permeation Chromatography and Static Light Scattering

“…The values reported in this work agree with previous reports for similar polymers under different conditions. 14,15,20,34,35 With a known dn/dc, we can calculate the percent recovery of the polymer that is detected with the in-line differential refractive index detector in the GPC. The values for P3HT in chlorobenzene at 40 °C are shown in Figure 2b as values next to the corresponding data points.…”

Kinetics of thermal polymerization of shellac. V. Turbidimetric and fractionation studies

“…A knowledge of molecular weight and size is essential for an understanding of the physical behavior of high po1ymers.l Generally, the polymers consist of molecules with a more or less broad range of sizes. 2 It should be pointed out that the size disparity arises from the random nature of chemical reaction and equal reactivity of the same class. 3 In heterogenous polymers, the molecular size is not sufficiently characterized by an average value.4 Therefore, it seems indispensible to determine the distribution of molecular weights or sizes about the mean value for correlating with reaction kinetics5 and also for tailoring polymers so as to have desired physical properties6 In fact, there are several methods for the determination of the molecular weight distribution function, but the turbidimetric titration is commonly used for a variety of reason^.^ On this account, an effort has been made to determine the molecular weight distribution function turbidimetrically.…”