Hybrid two-dimensional correlation infrared study on the temperature- and pressure-induced concentration dynamics and structural variations of poly(N-isopropylacrylamide)

The changes in hydration of poly(N-isopropylacrylamide) in aqueous solution have been monitored by FTIR spectroscopy as a function of temperature and pressure. Heating the sample above the LCST induces a phase separation that reflects a dehydration of the polymer chain on the molecular level, whereas above the LCSP the phase separation is associated with increased chain hydration. Using these data, we demonstrate the principle of hybrid 2D correlation analysis for the case of two independent data sets obtained under two different physical perturbations. For comparison we also have examined the individual processes by sample-sample (SS) two-dimensional (2D) correlation spectroscopy (COS). SS hybrid 2D correlation analysis enables one to explore the correlation between the concentration dynamics of PNiPA species induced by pressure and temperature, whereas homo SS 2D COS investigates the concentration dynamics of the PNiPA species induced by either one of these physical perturbations. The synchronous SS hybrid-2D correlation spectrum shows a strong negative correlation in the high pressure-temperature region, indicating the pressure- and temperature-induced species are different. Variable-variable (VV) hybrid-2D spectrum is applied to obtain information pertaining to the correlation of conformational variations of PNiPA during the transitions. A critical examination of the method and its applicability is given.