Dehydration, Micellization, and Phase Separation of Thermosensitive Polyoxazoline Star Block Copolymers in Aqueous Solution

Suitably end-functionalized diblock copolymers (2-isopropyl-2oxazoline)-b-(2-ethyl-2-oxazoline) (PIPOZ-b-PEOZ) were linked to a tetrafunctional core to synthesize two isomeric thermosensitive 4-arm star block polymers which have the PIPOZ block near the core, core-(PIPOZ-bPEOZ)(4), or near the outer surface the star polymer, core-(PEOZ-b-PIPOZ)(4) The solution properties of the star copolymers in water were monitored by turbidimetry, microcalorimetry, and small-angle X-ray scattering (SAXS). The dehydration and cloud-point temperatures of both core-(PIPOZ-b-PEOZ)(4) and core-(PEOZ-b-PIPOZ)(4) in water are in the vicinity of 50 degrees C. Above this temperature, core-(PIPOZ-b-PEOZ)(4) forms starlike aggregates or star micelle, whereas core-(PEOZ-b-PIPOZ)(4) remains isolated, with no sign of aggregation. These results demonstrate the importance of chain architecture on the association of thermosensitive tetra-arm star block copolymers in water above the dehydration temperature.