Effect of Anions on the Cloud Point Temperature of Aqueous Poly(2-ethyl-2-oxazoline) Solutions

Poly(2-alkyl-2-oxazoline)s have recently gained attention in especially biological applications due to their lower critical solution temperature being close to the body temperature and their biocompatibility. The understanding of how cloud point temperature (T c) depends on the salt concentration and the molecular mechanisms responsible for such behavior are important to tune T c as desired by the applications. In this paper, we report the effect of a series of sodium salts on T c of aqueous poly(2-ethyl-2-oxazoline) (PEOX) solutions by dynamic light scattering. PEOX samples having four different molecular weights were investigated, and the results were compared with those of poly(N-isopropylacrylamide) (PNIPAM), the mostly investigated and used thermoresponsive polymer. Kosmotropic anions decreased T c linearly while chaotropic anions increased T c nonlinearly with salt concentration. The contributions of different mechanisms to T c change have been discussed. Our results indicate that the dominant mechanism is the dehydration of PEOX for divalent kosmotropic anions (CO3 2–, SO4 2–, S2O3 2–) and direct binding for chaotropic anions (NO3 –, I–, ClO4 –, SCN–). For the remaining monovalent kosmotropic anions (H2PO4 –, F–, Cl–, Br–), a combination of dehydration and surface tension mechanisms was in effect. The additional contribution of the surface tension mechanism for the monovalent kosmotropic anions was inferred for different molecular weight PEOX samples and also for PNIPAM. With PEOX molecular weight decreasing from 500 000 to 5000 g/mol, T c decreased less with salt concentration which was attributed to the contribution of the surface tension mechanism. For PEOX samples, the decrease of T c with kosmotropic anion concentration was faster compared to PNIPAM due to differences in their chemical structure. Our results show that the molecular mechanisms of interactions between PEOX chains and specific anions can simply be inferred from determination of T c by a common techniquedynamic light scattering.