Core-shell brush

Keywords Block copolymers ABC triblock copolymers Janus micelles Cylinder brushes Core-shell nanoparticles Graft copolymers Micelles Vesicles Copolyampholytes Polyelectrolyte block copolymers Aggregation... 

Kim et al. used the exchange reaction to synthesize cross-linked AuNP-PNIPAM core-shell hybrid structures, as well as a brush-type AuNP/PNIPAM hybrid through surface-initiated ATRP in an aqueous medium. The disulfide initiators, [BrC (CH3)2COO(CH2)iiS]2, were bound to AuNPs synthesized by citrate reduction. They have studied the effect of cross-linking on the thermo-responsiveness of the AuN / PNIPAM hybrids for potential use as a stimuli responsive optical device, such as surface plasmon resonance-based sensing materials [91]. 

As with normal hydrocarbon-based surfactants, polymeric micelles have a core-shell structure in aqueous systems (Jones and Leroux, 1999). The shell is responsible for micelle stabilization and interactions with plasma proteins and cell membranes. It usually consists of chains of hydrophilic nonbiodegradable, biocompatible polymers such as PEO. The biodistribution of the carrier is mainly dictated by the nature of the hydrophilic shell (Yokoyama, 1998). PEO forms a dense brush around the micelle core preventing interaction between the micelle and proteins, for example, opsonins, which promote rapid circulatory clearance by the mononuclear phagocyte system (MPS) (Papisov, 1995). Other polymers such as pdty(sopropylacrylamide) (PNIPA) (Cammas etal., 1997 Chung etal., 1999) and poly(alkylacrylicacid) (Chen etal., 1995 Kwon and Kataoka, 1995 Kohorietal., 1998) can impart additional temperature or pH-sensitivity to the micelles, and may eventually be used to confer bioadhesive properties (Inoue et al., 1998). 

Abstract Polyelectrolyte block copolymers form micelles and vesicles in aqueous solutions. Micelle formation and micellar structure depends on various parameters like block lengths, salt concentration, pH, and solvent quality. The synthesis and properties of more complicated block and micellar architectures such as triblock- and graft copolymers, Janus micelles, and core-shell cylinder brushes are reviewed as well. Investigations reveal details of the interactions of polyelectrolyte layers and electro-steric stabilization forces. 

Important parameters that control the size of micelles are the degree of polymerization of the polymer blocks, NA and NB, and the Flory-Huggins interaction parameter %. The micellar structure is characterized by the core radius Rc, the overall radius Rm, and the distance b between adjacent blocks at the core/shell-interface as shown in Fig. 1. b is often called grafting distance for comparisons to polymer brush models, b2 is the area per chain which compares to the area per head group in case of surfactant micelles. In the case of spherical micelles, the core radius Rc and the area per chain b2 are directly related to the number of polymers per micelles, i.e., the aggregation number Z=4nR2clb2. 

Starblock (or radial star) copolymers form another kind of amphiphilic nanoparticles which can be regarded as unimolecular micelles. Alternatively, cylindrical core-shell brushes can be regarded as unimolecular cylinder micelles. Due to the covalent attachment of the block copolymers at one end, frustrated micellar structures can be made which would never form spontaneously. The cylindrical systems will be reviewed in Sect. 4.2. 

The core-shell cylinder brushes can be visualized by scanning force microscopy (SFM). Figure 26 shows cylinders with 1500 side chains with 31 AA and 48 nBA units each, which have a very uniform size distribution [125]. 

The ability of the hydrophilic PAA core of the amphiphilic core-shell brushes to coordinate with different metal cations can be used for the synthesis of novel nanosized organic/inorganic hybrids or for the generation of gold clusters or cobalt nanowire [126]. 

Recently, core-shell type microgels, which contain a hydrophobic core and a hydrophilic thermosensitive shell, have become attractive for scientists because such systems can combine the properties characteristic of both the core and the shell [53], We have prepared core-shell microgel particles consisting of a poly(styrene) core onto which a shell of polyCA-isopropylacrylamide) (PS-PNIPA) has been affixed in a seeded emulsion polymerization [54-56], In this case, the ends of the crosslinked PNIPA chains are fixed to a solid core, which defines a solid boundary of the network. In this respect, these core-shell latex particles present crosslinked polymer brushes on defined spherical surfaces. The solvent quality can be changed from good solvent conditions at room temperature to poor solvent conditions at a temperature... 

Mei Y, Lu Y, Polzer F, Ballauff M, Drechsler M (2007) Catalytic activity of palladium nanoparticles encapsulated in spherical polyelectrolyte brushes and core-shell microgels. Chem Mater 19 1062-1069... 

Similarly, core-shell cylindrical brushes were prepared via block copolymerization [308,313]. They consist of the soft pnBA cores and hard pSt shells [308]. The high resolution AFM micrographs of the block copolymer pBPEM-g-(pnBA-fc-pSt) brushes shows a necklace morphology. The synthesis of well-defined brush block copolymers demonstrates the synthetic power of ATRP. It was used to create a well-defined backbone with a degree of polymerization of 500, which was followed by a transesterification and the subsequent grafting of pnBA chains using ATRP. A final chain extension with St produced the block copolymers. 

So far, the grafting from strategy has proven to be the most successful one for the preparation of cylindrical brushes. Both anionic [30, 31, 142, 143] and cationic [29] polyelectrolyte cylindrical brushes with well-defined backbones and side chains were prepared. We have also prepared core-shell brushes with PAA as the core or shell, which will be discussed later [30, 31], Recently we have been able to prepare strong anionic brushes with sulfonate groups by direct ATRP of ionic monomers in organic solvent [144],... 

Amphiphilic and Double-Hydrophilic Core-Shell Cylindrical Brushes... 

Core-shell cylindrical brushes have gained increasing interest due to their unique properties and applications as nano-templates. Thus, Matyjaszewski et al. and our group first prepared core-shell brushes [30, 134], We have been studying core-shell brushes containing ionic blocks [30, 31], Amphiphilic and double hydrophilic coreshell brushes were prepared and studied for their solution properties. 

Fig. 18 H-NMR spectra of an amphiphilic core-shell polymer brush with PS-b-PAA side chains ([S23 - b- AAi86]310) in (a) CD30D, and (b) CD30D/CDC13 (v/v = 1/1) [30], Reprinted by permission of ACS...

We also prepared double hydrophilic core-shell brushes with PMAA as the core and poly(oligoethyleneglycol methacrylate) (POEGMA) as the shell. Due to the weak polyelectrolyte nature of the PMAA, at low pH the brushes also showed typical pearl-necklace conformations, whereas at high pH the core adopts more extended structures [158], The polychelates of the brush and Fe3+ behave in a similar way to the amphiphilic polychelates. 

One-Dimensional Magnetic Nanohybrid Templated by Core-Shell Brushes... 

The core-shell polyelectrolyte brushes can serve as the one-dimensional templates for different nanomaterials since different metal ions or small molecules can be... 

Scheme 12 Schematic illustration for the synthesis of a wire-like assembly of magnetic/ semiconducting NPs inside a core-shell cylindrical polymer brush...

We have used core-shell cylindrical polymer brushes with PAA core and PnBA shell as templates for the growth of different inorganic NWs and nanohybrids, such as CdS [160], CdSe [164], and Fe203 [161] (see Scheme 12). 

Superparamagnetic maghemite (Fe2C>3) NPs were generated in situ in the core of the PAA-b-PnBA core-shell brushes [31, 161] as described in Scheme 12. The carboxylate groups coordinate with Fe3+ ions and form the polychelate. AFM and TEM measurements show distinct pearl-necklace conformations for the polychelates of the brushes with Fe3+. After the alkalization and oxidation reaction small maghemite NPs formed within the core of the brushes distributed in a wire-like manner and the brushes returned to the extended worm-like morphologies. 

Polyelectrolyte cylindrical brushes behave in similar ways as the polyelectrolyte stars in many aspects. Due to their anisotropic architecture, their morphologies can be tuned between worms, helices, and spheres. Polyelectrolyte core-shell cylindrical brushes have been used for the fabrication of inorganic NPs or NWs. In particular, superparamagnetic hybrid cylinders with magnetic NPs in the core of the brushes were prepared. They can be aligned on the substrate in magnetic field. This provides another way for the directed assembly of hybrid materials in a controlled manner. 

---