Changes in nanoassembly of oligocaprolactone end-capped Pluronic F127 and the abnormal hydrophobicity trend of phase transition

Abstract

Compared with unmodified F127, the concentration range exhibiting sol-gel transition increased for the CL4-F127-CL4 (F-CL4); however, it decreased for the CL12-F127-CL12 (F-CL12), even though both F-CL4 and F-CL12 were hydrophobically modified by the oligocaprolactone (OCL). To understand the abnormal behavior of the OCL end-capped F127, the difference in basic nanoassemblies among the F127, F-CL4, and F-CL12 were investigated at a low concentration of 0.10 wt % as well as at high concentrations exhibiting sol-gel transitions. Dynamic mechanical analysis, 13C NMR spectroscopy, hydrophobic dye solubilization, dynamic light scattering, microcalorimetry, and transmission electron microscopy suggested that F127's undergo a unimer-to-micelle transition, whereas F-CL4's undergo unimer-to-vesicle transition as the temperature increased. On the other hand, F-CL12's with relatively large end-capped OCL formed aggregated micelle structures that undergo temperature-sensitive conformation changes. In addition, as the temperature increased, sol-to-gel-to-syneresis and gel-to-sol-to-gel-to- syneresis transitions were observed for F-CL4 and F-CL12 aqueous solutions, respectively, whereas a sol-to-gel-to-sol transition was observed for Pluronic F127 aqueous solution. The findings suggest that the end-capping of F127 by OCL induces changes in nanoassemblies, which play a key role in different physicochemical properties leading to the abnormal phase behavior. © 2013 American Chemical Society.