Polyelectrolyte Stars

Effects of Concentration and Interactions in Star Polyelectrolyte Solutions... 

In comparison with linear-linear PEMs, multilayers formed by star polyelectrolytes usually exhibit some unique properties [30]. Such unique properties of star-star PEMs should be attributed to the topological structure of star polyelectrolytes and the resulted distinct behavior of chain interpenetration between the layers. Star-shaped polyelectrolytes usually have a more compact structure, giving rise to a more limited interpenetration in comparison with that of linear chains [30]. Thus, it is anticipated that the behavior of chain interpenetration in the growth of star-star PEMs should be different from that in the growth of linear-linear PEMs. As the arm number of star polyelectrolytes changes, the resulted steric effect can also influence the chain interpenetration. Actually, QCM-D not only can tell the extent of chain interpenetration, but also can provide which kind of polyelectrolyte will penetrate into the oppositely charged layer. 

Choi 1, Suntivich R, Pamper FA, Synatschke CV, Muller AHE, Tsukruk VV (2011) pH-controlled exponential and linear growing modes of layer-by-layer assemblies of star polyelectrolytes. J Am Chem Soc 133 9592-9606... 

Jusufi A, Likos CN, Lowen H. Counterion-induced entropic interactions in solutions of strongly stretched, osmotic polyelectrolyte stars. J Chem Phys 2002b 116 11011-11027. 

A variational theory which includes all these different contributions was recently proposed and applied for completely stretched polyelectrolyte stars (so-called porcupines ) [203, 204]. As a result, the effective interaction V(r) was very soft, mainly dominated by the entropy of the counterions inside the coronae of the stars supporting on old idea of Pincus [205]. If this pair potential is used as an input in a calculation of a solution of many stars, a freezing transition was found with a variety of different stable crystal lattices including exotic open lattices [206]. The method of effective interactions has the advantage to be generalizable to more complicated complexes which are discussed in this contribution-such as oppositely charged polyelectrolytes and polyelectrolyte-surfactant complexes-but this has still to be worked out in detail. 

Likos CN, Hoffmann N, Jusufi A, Lowen H (2002) Interactions and Phase Behaviour of Polyelectrolyte Star Solutions (2002) J Phys Condensed Matter (in press)... 

For quite some time, there have been indications for a phase-separation in the shell of polyelectrolyte block copolymer micelles. Electrophoretic mobility measurements on PS-PMAc [50] indicated that a part of the shell exhibits a considerable higher ionic strength than the surrounding medium. This had been corroborated by fluorescence studies on PS-PMAc [51-53] and PS-P2VP-heteroarm star polymers [54]. According to the steady-state fluorescence and anisotropy decays of fluorophores attached to the ends of the PMAc-blocks, a certain fraction of the fluorophores (probably those on the blocks that were folded back to the core/shell interface) monitored a lower polarity of the environment. Their mobility was substantially restricted. It thus seemed as if the polyelectrolyte corona was phase separated into a dense interior part and a dilute outer part. Further experimental evidence for the existence of a dense interior corona domain has been found in an NMR/SANS-study on poly(methylmethacrylate-fr-acrylic acid) (PMMA-PAAc) micelles [55]. 

Scheme 12 Synthesis of star-shaped polyelectrolytes by the a arm first and b core first methods...

Figure 3. Star-block copolymer comprised of elastic inner blocks and polyelectrolyte outer blocks.

The synthesis of polyelectrolytes with well-defined architectures, however, has imposed many challenges to the polymer chemists. Many polymerization techniques are not tolerable to the ionic functional groups. In most cases, preparation of polyelectrolytes involves the protection and deprotection of the ionic groups in the monomer. For polyelectrolytes with different architectures, various synthetic strategies are required. Recently, we have synthesized various complex architectures containing polyelectrolytes with different nonlinear topologies, such as combshaped [22], hyperbranched [23-25], Janus-type [26], stars [27, 28] and brushes [29-31],... 

There are only a few cases in which polyelectrolyte stars have been prepared by the arm-first strategy. Qiao et al. prepared pH responsive poly(acrylic acid) stars by the MI method using atom transfer radical polymerization (ATRP), which was used to form layer-by-layer (LBL) polyelectrolyte multilayers with linear cationic polyelectrolytes [54], Matyjaszewski et al. obtained cationic poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) stars and anionic PAA stars also using the MI method, which formed all-star LBL layers [55], Ishizu et al. obtained... 

The PDMAEMA stars are weak cationic polyelectrolytes. But they can be easily transformed into strong cationic polyelectrolytes poly [2-(methacryloyloxy)ethyl] trimethylammonium iodide] (PMETAI) stars by quaternization with methyl iodide. Asymmetric field flow fractionation (AFFF) measurements showed that the silsesquioxane core remained intact. The PMETAI brushes were also subjected to... 

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