MCR–ALS for sequential estimation of FTIR–ATR spectra to resolve a curing process using global phase angle convergence criterion Original Research Article

Curing reactions include side and consecutive reactions while the polymer is growing. In this paper, we used multivariate curve resolution–alternating least squares (MCR–ALS) to obtain quantitative information about the concentration of the chemical species involved in these reactions. The cationic curing reaction between diglycidyl ether of bisphenol A (DGEBA) and γ-valerolactone (γ-VL) was monitored by infrared spectroscopy (FTIR–ATR). If the MCR–ALS method is to be used with recorded spectral data, the rank deficiency usually present in the data matrix needs to be overcome and the goodness of the results depends on the initial estimates of the chemical species involved in the reaction. Our strategy was to sequentially apply MCR–ALS in the time intervals where there is selectivity for some reactions and to use the error criterion based on the global phase angle to identify the optimal number of iterations in ALS. The estimated spectra were sequentially incorporated into the data matrix to overcome the rank deficiency. The MCR–ALS results are evaluated by the residuals and parameters such as lack of fit, the percentage of explained variance and the coefficient of dissimilarity between the recovered spectra and the spectra of the pure species, when it is possible. For intermediate species, the correspondence between the ALS spectra solution and the chemical knowledge of this species was also qualitatively evaluated. The goodness of the estimated concentration profile of the two reagents γ-VL and DGEBA, was evaluated by the correlation coefficient value between the estimated profile and the profile obtained when the specific absorption bands were monitored at 1776 and 910 cm−1, respectively for each reagent. The correlation coefficient values were 0.9954 and 0.9885, respectively.