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

Abstract

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(C=O) region, where all C=O groups participate in the formation of inter-/intra-chain hydrogen bonds (C=O···H-N) in D2O while there is still part of relatively "free" C=O groups in H2O. Moreover, we find in the temperature-dependent FTIR spectra that C≡N groups exhibit hydrating behavior while C=O groups present increased inter-/intra-molecular hydrogen bonding interaction (C=O···H-N) upon cooling, revealing the later process to be the driving force of the UCST-type transition.

Modulating the solubility of zwitterionic poly((3-methacrylamidopropyl)ammonioalkane sulfonate)s in water and aqueous salt solutions via the spacer group separating the cationic and the anionic moieties

“…Interestingly, similarly marked hydrogen-deuterium isotope effects were reported for blends of poly(acrylamide) and poly(acrylic acid), or copolymers of acrylamide and acrylonitrile also, both showing UCST-type behaviour in aqueous solution. 42,43 Cooperative complementary hydrogen bonding between the different polymer segments has been invoked as an explanation, which obviously cannot apply in our case. In any case, a major role of hydrogen bonding to provoke the strong isotope effects is also consistent with the finding that the cloud points of the polySHPP series (Table 3) are the same or even slightly higher than the ones of the polySPP series, 14 although a priori, the additional hydrophilic hydroxyl group would have been expected to lower the cloud points somewhat.…”

pH dependent thermoresponsive behavior of acrylamide–acrylonitrile UCST-type copolymers in aqueous media

“…However, stability studies under different pH conditions were not carried out. 27 Whereas the phase transition temperature of poly(AAm-co-AN) was not significantly affected by pH after dissolution, the ions that follow the Hofmeister series do influence hydrogen bonding of macromolecules. The effect of ions has been thoroughly investigated on thermoresponsive polymers with LCST like PNIPAAm 28 as well as for non-ionic UCST-type polymers.…”

Tunable, concentration‐independent, sharp, hysteresis‐free UCST phase transition from poly(N‐acryloyl glycinamide‐acrylonitrile) system

“…In the last few years more and more attention has been given to the nonionic polymers with phase transitions dominated by hydrogen (H)‐bonding . Acrylamide and acrylonitrile random copolymers (poly(AAm‐co‐AN)) have emerged out as one of the promising systems with sharp and reversible phase transitions of UCST type and tunable cloud points (temperature at which phase separation occurs depending on polymer concentration) Acrylamide, small hysteresis, and tolerance toward electrolytes . Moreover, cross‐linked poly(AAm‐co‐AN) could provide thermophilic hydrogels .…”