Inhomogeneous-collapse driven micelle–vesicle transition of amphiphilic block copolymers

Abstract

Understanding the morphological transition dynamics related to the hydrophilic–hydrophobic interface has been a challenge due to the lack of an effective evaluation method. Herein, nuclear magnetic resonance spectroscopy was employed to study the morphological transition related chain collapse of poly(N,N′-diethylaminoethylmethacrylate)-b-poly(N-isopropylacrylamide) (PDEAEMA₁₃₃-b-PNIPA₃₂₂) and poly(N,N′-dimethylaminoethylmethacrylate)-b-poly(N-isopropylacrylamide) (PDMAEMA₉₅-b-PNIPA₂₂₈) and was proved to be a powerful technique in morphological transition mechanism studies once combined with dynamic light scattering and transmission electron microscopy. Unlike the cooperative coil collapse of two blocks in the PDMAEMA₉₅-b-PNIPA₂₂₈ alkaline solution upon heating which induces the assembly of a nanostructure (∼200 nm) with a hydrophobic core containing both collapsed PDMAEMA and PNIPA segments and a hydrophilic surface part consisting of un-shrunk PDMAEMA and PNIPA segments, PDEAEMA₁₃₃-b-PNIPA₃₂₂ with a low-temperature core–shell micelle structure showed a micelle–vesicle transition due to temperature-induced inhomogeneous-collapse of PNIPA. The PNIPA segments in the shell sequentially collapse outside (starting at the core–shell interface), accompanied by a gradual decrease in micelle size. Above the critical temperature, the residual hydrophilic PNIPA segments become too short to stabilize the micelle structure, the micelles then transform into vesicles of a slightly larger size, instead of micelle aggregation and precipitation as normally expected.