Transient 2D IR Correlation Spectroscopy of the Photopolymerization of Acrylic and Epoxy Monomers TATSUHIKO NAKANO,* SHIGERU SHIMADA, RIEKO SAITOH, and ISAO NODA* Nippon Bio-Rad Laboratories, Analytical Instruments Division, 3-6 Kachidoki 5-chome, Chuo-ku, Tokyo 104, Japan (T.N., S.S., R.S.); and The Procter and Gamble Company, Miami Valley Laboratories, P.O. Box 398707, Cincinnati, Ohio 45239-8707, U.S.A. (I.N.) Transient two-dimensional infrared (2D IR) correlation spectroscopy coupled with real-time Fourier transform infrared (RT FT-IR) mea- surement was used to analyze the reaction processes of photopolymer- ization systems. Unlike the previously developed 2D IR methods based on sinusoidaily varying IR signals, a newly developed 2D correlation formalism applicable to transient spectroscopic signals having an arbi- trary waveform was used. By this method, features associated with spec- tral intensity changes and peak shifts arising from polymerization re- actions were clearly observed. Index Headings: Correlation spectra; Infrared; Spectroscopic technique; Photopolymerization; Real-time FT-IR; Time-resolved spectroscopy; 2D IR spectroscopy. INTRODUCTION A two-dimensional infrared (2D IR) correlation tech- nique was developed to analyze the dynamic fluctuation of spectral intensity2 ,2 With the application of a corre- lation analysis, IR spectra defined by two independent wavenumbers were obtained to provide detailed chemical information about the system. Until now, however, 2D IR correlation analysis has been applied mainly to simple sinusoidally varying IR signals) Application of this tech- nique has, therefore, been limited to the analysis of sys- tems such as elastic polymers,~ 5liquid crystals,6 or phase- modulated photoacoustic spectroscopy. 7 Recently, a new correlation formalisms has been developed to analyze spectral signals obtained as an arbitrary fluctuation of time, or any other physical variable. The 2D IR corre- lation spectra obtained by this method can effectively accentuate useful information often obscured in the orig- inal time-resolved IR spectra. This development opens up the possibility of introducing the versatile capability of 2D correlation analysis to much wider ranges of ap- plications. In this report, this new correlation technique was applied to transient IR spectra of photopolymeriza- tion systems. Monomers such as vinylesters and epoxides can be rapidly polymerized by UV irradiation in the presence of photosensitive initiators. The exposure time to initiate the polymerization is quite short, usually less than a second, which makes it difficult to analyze the kinetics of such reaction processes. Several methods, such as IR spectroscopy and photodifferential scanning calorime- tory (PDSC), 9 have been employed to follow the kinetics of photopolymerization. IR spectroscopy is especially Received 26 March 1993. * Authors to whomcorrespondenceshouldbe sent. useful, because it can monitor the change of functional groups of reacting molecules. In earlier work, a real-time infrared (RT IR) method developed by Decker and Moussa was used to follow the kinetics of photopolymerization processes2 °,u However, in their study, a spectrometer which monitored a fixed wavelength was used. For the simultaneous recording of both monomer consumption and polymer formation, the instrument made use of two detectors set at the relevant wavelengths. A modern rapid-scan FT-IR spectrometer can measure fast transient phenomena, since it records the full infrared spectrum rather than simply monitoring the IR signal, a single wavenumber at a time. The major advantage of rapid, full spectral data collection is that a reaction process can be followed by monitoring the sev- eral functional groups simultaneously. The use of profile curves for specific functional groups calculated from the peak intensities of transient (time-resolved) IR spectra can be used to monitor the reaction mechanism. Tran- sient 2D FT-IR correlation analysis can also be used to effectively enhance the pertinent features of spectral changes in the transient IR data without selecting spe- cific peak positions. Since the 2D correlation provides enhanced spectral resolution, simplification of complex IR spectra consisting of overlapping peaks can be achieved. BACKGROUND Correlation Analysis. The two-dimensional correlation of transient spectra is carried out as follows,s Given a spectral intensity variation y(v, t) observed during a pe- riod T, the synchronous and asynchronous correlation intensities, ,I~(vl,//2) and xI,@~,//2), become 1 £~(oo). £2"(~) doo (1) cI~(//1, //2) -Jc i~I'(//,,//2) = where Yl(OO)is the time-domain Fourier transform of :~(//1, t), and Y2*(w) is the conjugate of the Fourier trans- form of Y@2, t). Thus, the transient 2D IR correlation spectra can be directly obtained from any time-depen- dent IR spectra having an arbitrary waveform, as long as the Fourier transform of the time dependence can be calculated for the intensity changes at each wavenumber. Properties of Transient 2D IR Spectra. The basic char- acteristics of transient 2D IR correlation spectra are sim- ilar to those of previously developed 2D IR spectra de- rived from sinusoidally varying signals. 1,2 A complex IR spectrum consisting of overlapping bands can be sim- plified by spreading peaks along the second spectral di- Volume 47, Number 9, 1993 ooo3-7o2s/93/47o9-133nzoo/o APPLIED SPECTROSCOPY 1337 © 1993 Society for Applied Spectroscopy