Looped Structure of Flowerlike Micelles Revealed by 1H NMR Relaxometry and Light Scattering

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• 作者：Albert J. de Graaf ; Kristel W. M. Boere ; Johan Kemmink ; Remco G. Fokkink ; Cornelus F. van Nostrum ; Dirk T. S. Rijkers ; Jasper van der Gucht ; Hans Wienk ; Marc Baldus ; Enrico Mastrobattista ; Tina Vermonden ; Wim E. Hennink
• 刊名：Langmuir
• 出版年：2011
• 出版时间：August 16, 2011
• 年：2011
• 卷：27
• 期：16
• 页码：9843-9848
• 全文大小：804K
• 年卷期：v.27,no.16(August 16, 2011)
• ISSN：1520-5827

文摘

We present experimental proof that so-called 鈥渇lowerlike micelles鈥?exist and that they have some distinctly different properties compared to their 鈥渟tarlike鈥?counterparts. Amphiphilic AB diblock and BAB triblock copolymers consisting of poly(ethylene glycol) (PEG) as hydrophilic A block and thermosensitive poly(N-isopropylacrylamide) (pNIPAm) B block(s) were synthesized via atom transfer radical polymerization (ATRP). In aqueous solutions, both block copolymer types form micelles above the cloud point of pNIPAm. Static and dynamic light scattering measurements in combination with NMR relaxation experiments proved the existence of flowerlike micelles based on pNIPAm_16kDa-PEG_4kDa-pNIPAm_16kDa which had a smaller radius and lower mass and aggregation number than starlike micelles based on mPEG_2kDa-pNIPAm_16kDa. Furthermore, the PEG surface density was much lower for the flowerlike micelles, which we attribute to the looped configuration of the hydrophilic PEG block. ¹H NMR relaxation measurements showed biphasic T₂ relaxation for PEG, indicating rigid PEG segments close to the micelle core and more flexible distal segments. Even the flexible distal segments were shown to have a lower mobility in the flowerlike micelles compared to the starlike micelles, indicating strain due to loop formation. Taken together, it is demonstrated that self-assemblies of BAB triblock copolymers have their hydrophilic block in a looped conformation and thus indeed adopt a flowerlike conformation.