Tuning the Fast Dynamics of PNIPAM-Based Systems with Bio-Cosolvents ^†

Smart polymers as poly-N-isopropylacrylamide (PNIPAM) are well known due to their fast response to variations of parameters like temperature, pH, and pressure. PNIPAM phase behavior results from the competition between hydrophobicity of methyl and methylene groups and the ability of amide groups to make strong hydrogen bonds. When the temperature increases above a lower critical solution temperature (LCST) ~32°C, molecular agitation disrupts hydrogen bonds and leads to a coil-to-globule transition which is reminiscent of the folding transition of proteins. Different environments can impact on this delicate balance between hydrophilic and hydrophobic interactions and strongly affect the LCST [1], in the same way that the addition of cosolvents can lead to stabilization or denaturation in protein folding. Interestingly, these are not the only thermodynamic conditions in which there are analogies between protein and PNIPAM behaviors. The onset of anharmonic dynamics on the picosecond scale observed at ~250K in concentrated PNIPAM samples has been recognized as an analogue of the protein dynamical transition [2]. As in proteins [3], it is interesting to observe how different solvents can change the features of this intriguing phenomenon. We use a multi-technique approach to study how different solvent mixtures can affect the dynamics of PNIPAM-based systems under different thermodynamic conditions. The use of Raman and IR spectroscopy is well suited to investigate the role of protic and aprotic cosolvents in the changes in hydration state of PNPAM across the LCST, while neutron scattering techniques give access to information on the effect of protein stabilizers on the PNIPAM “dynamical transition”.