A Photoelectric Instrument for Light Scattering Measurements and a Differential Refractometer

The method of determining particle sizes and molecular weights by light scattering measurements on solutions is finding increasing fields of application. In order to obtain the necessary data quickly and conveniently, an instrument has been developed which allows the required light scattering measurements to be taken on a routine basis. This apparatus is described. It measures transmission and the scattering of light at 90°; two different schemes can be employed to obtain the angular intensity distribution of the scattered light. A description is also given of a compensating refractometer which is used to determine the difference in refractive index of solvent and solution. The instrument, which is simple in design, gives a direct reading of the refractive index difference and has an accuracy of better than 10−5. The light scattering instrument has been tested using polystyrene of known molecular weight, and has been applied to study the change of apparent molecular weight (or particle size) of a series of polyvinyl ethers as a function of concentration in some cases, also a number of solvents. For a series of polyvinyl n-butyl ether samples which was studied, the relation between the intrinsic viscosity [ηi] of the benzene solutions and the weight average molecular weight M determined by light scattering is ηi = .365×10−7M1.45.