The effect of hydration in corona layer on temperature responsiveness of polymer micelles consisting of poly(N-vinyl pyrrolidone)-block-poly(n-octadecyl acrylate) (PVP-b-PODA) was investigated. Small-angle X-ray scattering and dynamic light scattering showed two-step shape change of PVP-b-PODA micelles around 45 and 65 °C with elevating temperature, although only one-step shape change was observed at 45 °C in cooling process. In the first step, shape of PVP-b-PODA micelles was changed from disk to ellipsoidal oblate at the melting temperature (Tm) of PODA, although similar micelles consisting of another amphiphilic block copolymers containing PODA simply changed from disk to sphere at the Tm with elevating temperature. PVP-b-PODA micelles changed to spherical shape above 65 °C. Two-dimensional (2D) ¹H-NMR showed the PVP chains were perfectly dehydrated above 65 °C. Therefore, it was suggested that the appearance of ellipsoidal shape between Tm of PODA and 65 °C was caused owing to shape memory effect of pseudo network of corona layer due to robust hydration of PVP chains.
Keywords: hydration; polymer micelle; small-angle X-ray scattering; temperature responsiveness.