title: Looped structure of flowerlike micelles revealed by 1H NMR relaxometry and light scattering.
abstract: We present experimental proof that so-called "flowerlike micelles" exist and that they have some distinctly different properties compared to their "starlike" 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. (1)H NMR relaxation measurements showed biphasic T(2) 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.
authors.affiliation: Utrecht Institute for Pharmaceutical Sciences, Pharmaceutics, Utrecht University, P.O. Box 80.082, 3508TB Utrecht, The Netherlands.
authors.forename: Albert J
authors.initials: AJ
authors.lastname: de Graaf
authors.forename: Kristel W M
authors.initials: KW
authors.lastname: Boere
authors.forename: Johan
authors.initials: J
authors.lastname: Kemmink
authors.forename: Remco G
authors.initials: RG
authors.lastname: Fokkink
authors.forename: Cornelus F
authors.initials: CF
authors.lastname: van Nostrum
authors.forename: Dirk T S
authors.initials: DT
authors.lastname: Rijkers
authors.forename: Jasper
authors.initials: J
authors.lastname: van der Gucht
authors.forename: Hans
authors.initials: H
authors.lastname: Wienk
authors.forename: Marc
authors.initials: M
authors.lastname: Baldus
authors.forename: Enrico
authors.initials: E
authors.lastname: Mastrobattista
authors.forename: Tina
authors.initials: T
authors.lastname: Vermonden
authors.forename: Wim E
authors.initials: WE
authors.lastname: Hennink
mesh_terms: D008027:Light; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008823:Micelles; D012542:Scattering, Radiation
chemical_list: D008823:Micelles