Investigations on the thermoresponsive behavior of copoly(2-oxazoline)s in water

Polymers with a lower critical point solution temperature (LCST) are widely investigated responsive materials and are already in use for smart materials applications, e.g., for medicinal purposes. Hydrogels prepared from these polymers usually exhibit a shrinking behavior over a broad temperature range, rather than an expected sharp phase transition. One reason for this is their cloud point temperature (T-cp ) dependence on the concentration in water. Here, we investigate this behavior on the example of copolymers prepared by copolymerization of 2-ethyl-2-oxazoline (EtOx) with the respective heptyl (HepOx), butyl (BuOx), and isopropyl (iPrOx) derivatives, respectively. The resulting copolymers show greatly different dependencies of their T-cp. on concentration. While P(EtOx-stat-HepOx)s show a strong linear increase of T-cp, on polymer concentration, the T-cp of P(EtOx-stat-BuOx)s is only slightly raising at higher concentration. P(EtOx-stat-iPrOx)s have two plateau regions in their water/polymer phase diagram. In general, copolymers with less strong differences in their hydrophobicity might be best suited for discretely thermo-switchable hydrogels, because they show the lowest dependence of T-cp, on polymer concentration.