Understanding the role of water in the aggregation of poly(N,N-dimethylaminoethyl methacrylate) in aqueous solution using temperature-dependent near-infrared spectroscopy

Abstract

For understanding the role of water in the aggregation of polymers, the variation of water structures with the structural change of polymers in the process of aggregation was studied by temperature-dependent near-infrared (NIR) spectroscopy. The NIR spectra of the aqueous poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) solutions of different concentrations were measured at different temperatures. The spectral changes of the polymer and water with temperature were analyzed by N-way principal component analysis (NPCA). It was found that, at low concentration, the chains of the polymer tend to form a loose hydrophobic structure below 36 °C and then aggregate into a micelle at a lower critical solution temperature (LCST) of around 39 °C. In the process of the aggregation, the water species with two hydrogen bonds (S₂) increases gradually before 36 °C and then a sudden decrease occurs after that temperature. The results clearly indicate that water species S₂ plays an important role in the formation of the intermediate, i.e., the loose hydrophobic structure of the polymer chains linked by the two hydrogen bonds of S₂ water. When the temperature increases, the dissociation of the hydrogen bonds enables the intermediate to be destroyed to form a micelle structure. For the high concentration solution, however, the spectral information of S₂ was not found in the aggregation, suggesting direct formation of the micelle from the dehydrated chains.