Gelation of aqueous solutions of diblock copolymers of ethylene oxide and D,L‐lactide

The thermally‐reversible gelation of aqueous solutions of diblock copolymers of ethylene oxide (E) and D , L ‐lactide (L), E 42 L 12 , E 38 L 16 , E 39 L 20 , E 41 L 26 , E 78 L 14 and E 77 L 26 , has been studied using gelation temperature measurements, polarised light microscopy and NMR spectroscopy. The solutions showed a high temperature gel‐sol boundary but no low temperature gel‐sol boundary. Polarised light microscopy showed that the gel phase was isotropic, and the dependence of sol‐gel temperature on concentration suggested that the micelles were prolate ellipsoidal, rather then spherical. NMR spectroscopy showed that below ca. 60°C, the L block in the micelle core become highly restricted. It is tentatively suggested that this is due to the L blocks forming an ordered structure, thus explaining the absence of a low temperature gel‐sol boundary.