Polymer Communication Evidences of correlation between polymer particle size and Raman scattering Marlon M. Reis, Pedro H.H. Arau ´jo, Claudia Sayer, Reinaldo Giudici * Departamento de Engenharia Quı ´mica, Escola Polite ´cnica, Universidade de Sa ˜o Paulo, Av. Prof. Luciano Gualberto, trav. 3, No 380 CEP 05508900 Sa ˜o Paulo, SP, Brazil Received 20 March 2003; received in revised form 15 July 2003; accepted 18 July 2003 Abstract This work describes evidences of correlation between polymer particle size and Raman scattering and shows that it is possible to use Raman scattering to monitor the evolution of average particle size during emulsion polymerization reactions. The main focus is the estimation of the average polymer particle diameter from spectra collected in a short acquisition time and consequently low signal-to-noise ratio. Finally, a multivariate linear model, (Partial Least Square-PLS), is fitted from the reaction data and a good linearity between spectra and average polymer particle diameter is found. It is shown that despite varying monomer and polymer concentrations it is possible to monitor average particle sizes during emulsion polymerization reactions using Raman spectroscopy. q 2003 Elsevier Ltd. All rights reserved. Keywords: Raman spectroscopy; Average polymer particle diameter; Emulsion polymerization 1. Introduction Emulsion polymerization is a free radical polymerization performed in a heterogeneous reaction system, yielding submicron solid polymer particles dispersed in an aqueous medium. Emulsion polymerization is a widely used industrial process for the production of synthetic polymer colloids or latexes of several different types of polymers, which are used in a wide variety of applications: synthetic rubber, coatings, paints, adhesives and binders. In emulsion polymerization, particle size distribution (PSD) is one of the most important characteristics of the latex, as it may affect the reaction rate and the final properties of the latex, such as film formation, latex stability, etc. The final PSD is determined by the particle nucleation and the growth of polymer particles. There is still a great deal of controversy regarding the dominant mode of particle formation in emulsion polymerization (micellar, homogeneous, coagulative and droplet nucleation). How- ever, it is commonly accepted that in conventional emulsion polymerization one mechanism generally dominates par- ticle formation depending on, for example, the surfactant concentration or the monomer solubility in the aqueous phase. The size and the number of resulting particles depend on the operative mechanism of particle formation and affect the rate of reaction directly. The growth of particle size is due to radical propagation inside polymer particles. However, in several systems the particle aggregation may also affect the size and number of particles during the reaction process. Despite the large volume of research in the field of emulsion polymerization some aspects of the process, such as the mechanisms that affect the PSD, are still not fully understood. The on-line monitoring of particle size during the polymerization reaction, especially in industry, would bring very important information about particle formation and growth rate leading to a better control of the final PSD [1–3]. Real time monitoring and control of average particle diameter during emulsion polymerization reactions remains a challenge since most available measurement techniques are either quite time consuming (e.g. Transmission Electronic Microscopy—TEM) and/or require sample dilution (e.g. Capillary Hydrodynamic Fractionation— CHDF, Dynamic Light Scattering—DLS). Also, all these techniques require sampling from the reactor what may not be feasible in an industrial reactor during the reaction. With 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00669-4 Polymer 44 (2003) 6123–6128 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 55-1130912254; fax: þ 55-1138132380. E-mail address: rgiudici@usp.br (R. Giudici).