Light Scattering by Oligostyrene Solutions

Abstract

Abstract Light-scattering photometers were partially modified in order to measure light scattering by low molecular-weight molecules. Corrections to the molecular weights were then theoretically estimated, when the light scattering due to fluid density fluctuations were approximated by that due to the density fluctuations of a pure solvent. The molecular weight of benzene was determined, in an ethyl methyl ketone solvent, using the light-scattering method prior to the determination of the molecular weights of oligostyrene molecules. Measurements of light scattering by oligostyrene molecules with molecular weights from 600 to 20000 were then carried out. The weight-average molecular weight observed by the light-scattering method was compared with the number-average molecular weights which were determined by the vapor-phase osmometric and NMR methods. The molecular-weight heterogeneity of the specimen is discussed. The second virial coefficient, in the ethyl methyl ketone solvent system, was found to change from negative to positive with increasing molecular weight. It is also shown that the molecular optical anisotropy changes markedly in the range of molecular weights between 2000 and 3000, the change being considered to reflect the conformational changes of oligostyrene molecules.