Quantitative comparison of reversibility in thermal-induced hydration of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) in aqueous solutions by “concatenated” 2D correlation analysis

“Concatenated” two-dimensional (2D) correlation analysis is a newly proposed method. By extracting more detailed information, such as the comparison of effects of two different perturbations or different systems, it helps one to clearly reveal the subtle difference between two dynamic changes resembling to each other. We demonstrate the new applications of “concatenated” 2D correlation analysis in quantitative comparison of the reversibility in the thermally induced hydration and conformational changes of poly(N-isopropylacrylamide) (PNiPA) and poly(N-isopropylmethacrylamide) (PNiPMA) in aqueous solution. The reversibility of the two polymers will be compared both by hydrophilic amide group and hydrophobic CH group in different temperature ranges. By the pretreatment of an intensity normalization on the original spectra, the intensities of cross-peaks in the asynchronous spectrum calculated from the roundtrip data matrix are quantitatively compared. The contribution of different factors to the 2D asynchronous spectrum can be separated by selecting different spectral section for concatenation. The irreversibility of the thermally induced hydration in PNiPA is not constant during the entire round trip process: it is most obvious in the narrow temperature interval of phase separation process, while in the temperature ranges far away from the interval it is negligible. The irreversibility of the behavior of CH groups in both PNiPA and PNiPMA is not as strong as that of amide group during the round trip. It further supports the conclusion that the hydration of hydrophilic amide group has the dominant effect in the temperature-induced phase separation of the polymer in aqueous solution.