Effects of Side-Chain Deuteration of Poly(N-isopropylacrylamide) on the Thermal Transition Behaviors in Water

Polymers with lower critical solution temperature (LCST) in water are promising building blocks in designing temperature-responsive materials. The thermal transition of polymer and proteins in water is known to depend on deuteration (either of solvent or of polymer/protein), but a full understanding of this phenomenon remains elusive. In this study, we focus on the fact that although partially deuterated poly(N-isopropylacrylamide) (PNIPAm) has been widely used in many studies due to its facile synthesis, its thermal transition behavior has not been independently characterized. We investigate the effect of side-chain deuteration of PNIPAm on its electronic structure, hydrogen bonding, and hydration in water. The thermal transition of PNIPAm with its isopropyl side chain deuterated (d₇-PNIPAm) is broader and approximately 2 °C higher than that of h-PNIPAm in dilute solution when measured by microcalorimetry. While the underlying mechanisms are complex, our results currently point to weaker hydrogen bonding of d₇-PNIPAm due to a small destabilization of the negative dipole in the amide nitrogen, which results in a weaker cooperativity in hydration.