Physical review letters 2009, 103 (11), 118301

PRL 103, 118301 (2009)
PHYSICAL REVIEW LETTERS
week ending 11 SEPTEMBER 2009
pH-Triggered Block Copolymer Micelle-to-Micelle Phase Transition
Li Xu,1 Zhichen Zhu,1 Oleg V. Borisov,2,* Ekaterina B. Zhulina,3,* and Svetlana A. Sukhishvili1,* 1Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology,
We synthesized poly[2-(dimethylamino)ethyl methacrylate-block-(N-isopropylacrylamide)] (PDMAb-PNIPAM) by atom transfer radical polymerization using ethyl 2-bromoisobutyrate as an initiator and copper(I) bromide/1,1,4,7,10,10-hexamethyltriethylene-tetramine as a catalyst-ligand pair. The synthetic procedure for the PDMA-Br macroinitiator and its use for preparation of PDMA-b-PNIPAM have been described elsewhere [10,11]. The PDMA and PNIPAM content in the resulting diblock copolymers was determined by 1H NMR and gel permeation chromatography (GPC). Data presented here were obtained from experiments involving two specific block copolymers: PDMA42-b-PNIPAM52 and PDMA57-b-PNIPAM97, where the subscript denotes the number of monomer units in copolymer blocks. Weight average molecular weights Mw were 15 and 24 kDa, respectively, and polydispersity indices were 1.08–1.10 for PDMA blocks and 1.25 for both copolymers, as determined by GPC in tetrahydrofuran.
Copolymer micellization as a function of pH and temperature was monitored by dynamic light scattering (DLS) using a Zetasizer Nano-ZS (Malvern, Inc.), as well as by atomic force microscopy (AFM) using an NSCRIPTORTM dip pen nanolithography system (Nanoink, USA). In DLS, cumulants and CONTIN analyses of autocorrelation data gave results consistent within 12%. Aggregation numbers of micelles (Nagg) were determined by static light scattering (SLS) using a laser light scattering system (Brookhaven, Inc.) equipped with a BI-200SM goniometer.
We present a first experimental observation and provide a theoretical interpretation of a pH-induced micelle-to-micelle phase transition in aqueous solutions of spherical block copolymer micelles with polybasic coronas. Dynamic light scattering, static light scattering, and atomic force microscopy confirm sharp changes in micellar hydrodynamic size and aggregation number occurring in a narrow pH range, ÁpH < 0:1. In agreement with theory, potential measurements indicated an abrupt change in ionization of polymer chains in the micellar corona at the transition pH.
Stimuli-responsive self-assembly of amphiphilic copolymers was pioneered by Eisenberg and co-workers [3]. In their systems, morphological transitions were triggered by the addition of organic solvent, and did not occur in water because of glassy polystyrene cores. Armes and co-workers introduced a new type of ‘‘schizophrenic’’ diblock copolymers with WPE blocks whose charge and solubility could be altered by pH variations giving micellar ‘‘flip-flop’’ [4]. Studies of pH- and temperature-triggered self-assembly of amphiphilic copolymers into micelles have flourished since that time [5–8]. Often, self-assembly of micelles composed of pH- and/or temperatureresponsive block copolymers is reversible both with temperature and pH [8,9], likely due to rapid equilibration of micellar cores composed of polar hydrated polymer blocks. Previous studies, however, did not report differences from the well-established behavior of WPE in solution, where pH changes result in continuous variations in WPE charge density. Here we present the first experimental evidence that spherical block copolymer micelles with
DOI: 10.1103/PhysRevLett.103.118301
PACS numbers: 82.35.Rs, 64f-assembly of amphiphilic ionic macromolecules in aqueous media exemplifies hierarchical structural organization in biological systems and is extensively exploited by nature. Self-assembled aggregates of synthetic ionic copolymers mimic certain features of bio-nanoassemblies (e.g., proteoglycans [1]). Association of amphiphilic copolymers is a powerful tool for constructing innovative soft functional materials. These structures can be easily dispersed in aqueous media and can have the ability to trap, transport, and release poorly soluble compounds such as proteins, fragrances, and drugs [2]. Of specific interest are stimuli-responsive polymer materials which respond to variations in pH or temperature by restructuring the polymer matrix. This function is usually achieved by inclusion of weak polyelectrolyte (WPE) blocks whose charge density can be altered by changing environmental pH, or by using polymers that are able to phase separate with temperature variation.