Self-Association of a Thermosensitive Poly(alkyl-2-oxazoline) Block Copolymer in Aqueous Solution

We have investigated the heat-induced self-association in water of a block copolymer (PIPOZ-b-PEOZ) comprising two thermosensitive blocks: a poly(2-isopropyl-2-oxazoline) block (degree of polymerization 71) and a poly(2-ethyl-2-oxazoline) block (degree of polymerization 38) using differential scanning calorimetry (DSC), static light scattering (SLS), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) together with visual observation on the macroscopic and microscopic scales. The dehydration temperatures of the PIPOZ and PEOZ blocks in water are 43 and 54 °C, respectively. When heated abruptly to 50 °C where PIPOZ-b-PEOZ is amphiphilic, the copolymer first forms star micelles that further aggregate to form large concentrated phase droplets, and finally the droplets coalesce into a bulk liquid phase, having a copolymer concentration as high as 0.8 g/cm3. When heated abruptly to 70 °C, where both blocks are hydrophobic, the copolymer solution also separates into two liquid phases, consisting of phase-separated polymer-rich micrometer or submicrometer size droplets dispersed in a polymer-poor liquid phase, but the droplets do not coalesce into a liquid bulk phase. We discuss the role of the more hydrophilic PEOZ block in the macroscopic phase separation behavior. No microphase separation takes place in the concentrated phase of the aqueous PIPOZ-b-PEOZ block copolymer solution.