Combined Optical Techniques for Estimating the Particle Size Distribution and Optical Properties of a Polymer Latex

GUGLIOTTA, LUIS M.; VEGA, JORGE R.; GONZALEZ, VERÓNICA D.G.; MEIRA, GREGORIO R.; FRONTINI, GLORIA R.; ELICABE, GUILLERMO E.

Foz de Iguazu, Paraná

Conferencia; 11th International Conference on Surface and Colloids Science; 2003

International Association of Colloid and Interface Scientists

The particle size distribution (PSD) of a polymer latex is related with end-use quality properties, and for this reason its estimation has an important technological interest. Electron microscopy (EM) is probably the most important technique for determining the PSD, but presents recognized disadvantages, because it is expensive, time-consuming, and in some cases requires special sample preparation. Several optical techniques can be alternatively used for estimating the PSD, such as turbidimetry (T), elastic light scattering (ELS), and dynamic light scattering (DLS). In these techniques, the PSD estimate is obtained after inverting a first-order Fredholm equation, whose kernel is calculated through the Mie light scattering theory. Irrespectively of the used technique, the estimated PSD is inexact due to: a) the low information content in the measurements; b) the unavoidable measurement noise; c) the ill-conditioned nature of the inversion problem; and d) the uncertainties in some optical parameters (typically, the particle refractive index). An improved PSD estimate can be obtained by combining: i) the measurements obtained from different optical techniques, or ii) the DLS measurements taken at different detection angles. To these effects, a joint mathematical model obtained by combining the individual models must be solved, which also requires a set of, a priori unknown, normalization parameters. The measurement combination allows to increase the information content on the PSD thus improving its estimation. Moreover, by combining two or more measurements it is also possible to estimate the particle refractive-index function. Simulation and experimental results are presented, showing that: 1) an improve PSD estimate is obtained by combining DLS measurements at several angles (multiangle DLS); 2) the particle refractive index as a function of the wavelength  can be estimated from ELS + T measurements; and 3) a small fraction of contaminants can be detected by multiangle DLS + T.