An application of concatenated 2D correlation spectroscopy: Exploration of the reversibility of the temperature-induced hydration variation of poly(N-isopropylmethacrylamide) in aqueous solution

We have demonstrated an application of “concatenated” 2D correlation analysis in quantitative or semi-quantitative examination of the reversibility of the temperature-induced hydration variation of PNiPMA in water solution. Hydration reversibility of hydrophilic amide group and hydrophobic CH group are compared in different temperature ranges. The appearance of significant cross-peaks in the asynchronous spectrum calculated from the roundtrip data matrix indicates the existence of certain inter-asynchronicity between heating and cooling cycle. The contribution of different factors to the 2D asynchronous spectrum could be separated by selecting the range of the spectral dataset for concatenation. The irreversibility of the temperature-induced hydration variation in PNiPMA is not constant during the whole round trip: it is most obvious in the phase separation process, while in the range far away from that period it is nearly neglectable. The hydration variation of PNiPMA manifests significant irreversibility in the phase separation stage. The asynchronous peaks of hydrophobic CH groups are much weaker than that of hydrophilic amide group during any temperature range in the temperature round trip. The irreversibility of CH groups is not as strong as that of amide group during the round trip. It further supports the conclusion that the hydration variation of hydrophilic amide group is dominated in the temperature-induced phase separation of the polymer in water solution.