Comparative vibrational study of two N-isopropylacrylamide-based co-polymers: Influence of the polymer hydrophobicity on the phase transition

In this study, the temperature induced phase transition in solutions (10 wt%) of two random co-polymers based on N-isopropylacrylamide (NIPA) was investigated by FTIR spectroscopy in ATR geometry. Acrylamide (AAm) and dimethylacrylamide (DMA) were chosen as co-monomers because they are hydrophilic and are differentiated by the presence of amide hydrogen atoms, which limits the possibility of the DMA monomeric unit to form inter/intra-chain hydrogen bonds. The study was performed in de-ionized water and in a standard phosphate buffer (NaH2PO4 + Na2HPO4, pH = 7.4, I = 0.26 mol/l). The characteristic absorption bands associated with marker groups of NIPA, AAm and DMA were utilized to monitor the microenvironmental changes around the chemical groups, under the impact of chain microstructure and of the Hofmeister effect given by the ionic species from the phosphate buffer system. The dynamics of water residing in the first hydration shell of polar groups has been evaluated from the OH stretching vibration. Here, the changing in water configuration upon heating has been analyzed by using a simplified two-state model of water. The objective of the study was to get a closer picture at the molecular level of the water expelling out of the polymer coils and of the functional groups’ dynamics.