Research Article Contribution of Raman Spectroscopy to In Situ Monitoring of a High-Impact Polystyrene Process A high-impact polystyrene (HIPS) process has been studied using Raman spec- troscopy as a monitoring technique for the first time. Thermal polymerization of styrene was carried out in the presence of polybutadiene. A Raman probe with a short focal distance, immersed into the reaction medium, allowed in situ moni- toring of styrene consumption over the whole reaction progress. A Raman probe with a long focal distance set outside the glass reactor was applied to detect phase separation and phase inversion. Finally, coupling Raman spectroscopy and flow rheometry into a single experimental setup allowed in situ monitoring of both phase behavior and viscosity variation during the HIPS process. Results obtained from Raman spectroscopy were correlated to those from laser granulometry and gravimetric analysis. Keywords: High-impact polystyrene, Monitoring, Phase inversion, Raman spectroscopy Received: June 20, 2013; revised: September 06, 2013; accepted: November 13, 2013 DOI: 10.1002/ceat.201300421 1 Introduction Free radical polymerization of styrene in the presence of poly- butadiene (PB) is the basic principle of the synthesis of high- impact polystyrene (HIPS). Because of the limited miscibility of polystyrene (PS) and PB, phase separation occurs at rather low styrene conversions leading to a biphasic reaction medi- um, with a disperse phase containing the major part of styrene and PS. With reaction progress, the volume of the styrene + PS phase increases and phase inversion takes place when mono- mer conversion is high enough [1–3]. The liquid phase con- taining styrene and PS then becomes the continuous one. Simultaneously, the disperse phase acquires a complex struc- ture with PS inclusions within the PB-rich nodules, producing the so-called salami morphology [4, 5]. In addition, chain transfer reactions between growing radicals and PB chains form graft copolymers which have a tendency to accumulate at the interface between the two phases. This phenomenon strongly influences the morphology variation while reaction proceeds. Thus, the reaction medium of the HIPS manufactur- ing process involves simultaneous chemical reactions and mor- phological evolutions, both being closely related to each other. In addition, end-use properties of the final material are di- rectly related to previous aspects. Literature provides detailed data about partition of a reac- tant between phases [6, 7], kinetic models [8–13], structural characterization of formed macromolecules, transient and final morphology as well as resulting mechanical properties of a material [14–17]. General methodology consisted in the com- bination of various characterization methods followed by the analysis of correlations between these data. Up to now, no multi-scale in situ monitoring by coupled techniques has been attempted for HIPS synthesis. On the one hand, this strategy provides direct and instantaneous correlations between macro- scopic and molecular data. On the other hand, thanks to tech- nology advances, some techniques may be used under experi- mental conditions closer to manufacturing processes and even directly in the industrial process. The design of in situ monitoring devices for polymerization reactors is a current research topic which finds applications in both industry and academia. In industry, quality and process control in real time is a great challenge in order to improve efficiency and reliability of chemical processes. For academic research, in situ monitoring provides useful data about chemi- cal and physical phenomena taking place in polymerization reactors, and thus contributes to experimental investigation and modeling. Raman spectroscopy has been applied to monitor monomer consumption during polymerization reactions as well as to characterize polymer materials [18–21]. To the best of our knowledge, in the case of HIPS, Raman spectroscopy has been used for characterizing the polymer microstructure or compo- sition [22–25] but not to monitor the polymerization process. Chem. Eng. Technol. 2014, 37, No. 2, 275–282 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Nadège Brun 1,2,3 Marie-Claire Chevrel 4,5 Laurent Falk 4,5 Sandrine Hoppe 4,5 Alain Durand 6,7 David Chapron 1,2 Patrice Bourson 1,2 1 Université de Lorraine, LMOPS, Metz, France. 2 Supélec, LMOPS, Metz, France. 3 Total Petrochemicals France, Paris La Défense, France. 4 CNRS, LRGP, Nancy, France. 5 Université de Lorraine, LRGP, Nancy, France. 6 CNRS, LCPM, Nancy, France. 7 Université de Lorraine, LCPM, Nancy, France. – Correspondence: Nadège Brun (nadege.brun@supelec.fr), Université de Lorraine, LMOPS, EA 4423, 2 Rue E. Belin, 57070 Metz, France. High-impact polystyrene 275