Poly(N-isopropylacrylamide) (PNIPAM) is a thermo-responsive polymer with numerous applications in the different areas science and engineering. While the mechanism behind the phase transition behavior of aqueous PNIPAM has been extensively studied, the structure of PNIPAM in solution is still remains a matter of debate. Here, we present our effort on elucidating the structure of PNIPAM in water using a bottom-up approach. involving the monomer, dimer, and trimer units as models and a combination of IR spectroscopies (including FTIR and 2DIR spectroscopies) along with some computational studies (molecular dynamics simulations, MD). Our IR studies reveal the presence of two peaks in the amide I region of the spectrum of the oligomers and the polymer, which are assigned to different backbone conformations. These conformers present distinct IR signatures because of the strong vibrational coupling between neighboring units. MD simulations and vibrational theory confirm that the oligomers have distinct backbone conformation in solution, each with specific vibrational signatures. The bottom-up approach allowed us to infer that such backbone conformations also exist in PNIPAM as it reveals the same IR signatures. Finally, the proposed molecular model is successfully used to describe the PNIPAM amide I band changes with temperature in terms of its molecular structure. Overall, our studies strongly suggest that PNIPAM does not have a completely random backbone structure, but have distinct backbone arrangements between neighboring amides.

Using a bottom-up approach to investigate the molecular structure of aqueous poly(N-Isopropylacrylamide) at room temperature via IR spectroscopies

ACS Spring Meeting 2021

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