Different Particle Size Information Obtained From Static and Dynamic Laser Light Scattering

Static light scattering (SLS) and dynamic light scattering (DLS) techniques are used to measure characteristics of particles in dispersion. SLS measures the dependence of the average scattered intensity I, on the scattering angle and is sensitive to spatial variations in the dielectric constant E. DLS measures the time dependence of the scattered light. One fundamental application of light scattering techniques is the accurate measurement of the size distribution of particles in dispersion. Detailed experimental investigation of SLS and DLS has been attempted in this work, with experimental work based on dilute water dispersions of two different spherical particles, polystyrene latexes and poly(N-isopropylacrylamide) (PNIPAM) microgels. Size information is obtained from SLS measurements in the form of a particle size distribution G (R,) where R, is the static radius. Size information is obtained from DLS measurements in the form of a decay rate distribution G (I?) which depends on the decay rate I?. Although both SLS and DLS can be used to obtain size information from the scattered light, the information obtained using SLS and DLS is different. For the three polystyrene latex sphere samples studied in this thesis, the mean static radii obtained are consistent with that of the mean radii provided by the supplier. For all three samples, the apparent hydrodynamic radius is larger than the mean static radius by about 12%. For PNIPAM microgel spheres, the apparent hydrodynamic radius is also larger than the mean static radius. The size of the PNIPAM microgel particles is extremely temperature sensitive; the radius decreases by a factor of three as the temperature is raised from 15OC to 50?? Both the hydrodynamic and static radii show this behavior. The effect of the chemical crosslinker (N, N'-methylenebisacrylamide) content on the temperature sensitivity of the PNIPAM microgels and the temperature dependence of the ratio of the apparent hydrodynamic radius to the mean static radius are also discussed.