Understanding the UCST-type transition of P(AAm-co-AN) in H2O and D2O: dramatic effects of solvent isotopes

The UCST-type transition of poly(acrylamide-co-acrylonitrile) (P(AAm-co-AN)) (molar fraction of AN: 13.3%; PDI = 3.2) in H2O and D2O is explored and compared by applying turbidity, DLS as well as FTIR measurements. The transition temperature of P(AAm-co-AN) in D2O is observed to be almost 10 °C higher than that in H2O at the same concentration, demonstrating a dramatic solvent isotope effect. Such a phenomenon could be rooted from a stronger interaction among polymer chains in D2O than in H2O, as indicated from DLS results. It is also observed in second-derivative analysis of FTIR spectra in the ν(CO) region, where all CO groups participate in the formation of inter-/intra-chain hydrogen bonds (CO⋯H–N) in D2O while there is still part of relatively “free” CO groups in H2O. Moreover, we find in the temperature-dependent FTIR spectra that CN groups exhibit hydrating behavior while CO groups present increased inter-/intra-molecular hydrogen bonding interaction (CO⋯H–N) upon cooling, revealing the later process to be the driving force of the UCST-type transition.