Two-Dimensional Raman Correlation Spectroscopy Study of an Emulsion Copolymerization Reaction Process

Abstract

The emulsion copolymerization of styrene and 1,3-butadiene using an oligomeric nonionic surfactant as an emulsifier to make a styrene–butadiene rubber (SBR) copolymer latex was monitored by real-time in situ Raman spectroscopy. Time-resolved Raman spectra collected during the early stage of the polymerization reaction were subjected to a series of data analysis techniques, including two-dimensional (2D) correlation spectroscopy, multivariate self-modeling curve resolution (SMCR), and kernel analysis, to elucidate the fine details of the complex reaction process. Generalized 2D correlation analysis of time-resolved Raman spectra readily identified the characteristic Raman scattering bands for the monomers and copolymer. Cross-peaks appearing in 2D Raman correlation spectra showed that the decrease in the spectral intensity of Raman bands assignable to 1,3-butadiene occurs before the band intensity changes for styrene or SBR copolymer. The positions of asynchronous cross-peaks were used to identify a spectral region with the most distinct pattern of intensity variations, which in turn could be used as the starting point for the alternating least squares iteration of the SMCR analysis. SMCR analysis of the time-resolved Raman spectra generated a set of estimated pure component spectra and concentration profiles of styrene, 1,3-butadiene, and SBR copolymer without requiring independently measured calibration data. The estimated concentration profiles of monomers and copolymer indicated that the reaction of 1,3-butadiene started before the consumption of styrene and production of SBR copolymer. Kernel analysis of the estimated concentration profiles provided a succinct measure of the similarity and dissimilarity of the concentration changes of monomers and copolymer.