Polyelectrolyte Block Copolymer Research Articles

Spherical polyelectrolyte block copolymer micelles: Structural change in presence of monovalent salt

Spherical polyelectrolyte block copolymer micelles were investigated as a function of added NaCl salt concentration using Small-Angle Neutron Scattering (SANS) and Light Scattering (LS). The micelles are formed by the self-association of charged-neutral copolymers made of a long deuterated polyelectrolyte moiety (NaPSS(d))251 and a short hydrophobic moiety (PEP)52. In presence of salt, the core shape and the aggregation number of the micelles are not affected. The hydrodynamic radius of the micelle is found to be identical to the radius of the whole micelle deduced from neutron scattering and thus the hydrodynamic radius is a valid measure of the corona thickness. At the lowest salt concentrations investigated the thickness of the corona, R(s), remains essentially constant and a contraction is observed above an added-salt concentration c(s) of 2 x 10(-2) M where this crossover concentration corresponds to the average ionic strength of the free counterions in the corona. The contraction takes place while maintaining a rod-like behavior of the chains at short scale and obeys to: R(s) approximately c(s)(-0.18). The exponent 0.18 suggests an electrostatic persistence length proportional to the Debye screening length.

Synthesis, Characterization, and Properties of Polyelectrolyte Block Copolymer Brushes Prepared by Atom Transfer Radical Polymerization and Their Use in the Synthesis of Metal Nanoparticles

Diblock copolymer polyelectrolyte brushes of either polystyrene (PS) or poly(methyl acrylate) (PMA) and poly(acrylic acid) (PAA) were synthesized using sequential monomer addition from a tethered 2-bromoisobutyrate initiator. Si/SiO2//PS-b-poly(tert-butyl acrylate) (P(t-BA)) and Si/SiO2//PMA-b-P(t-BA) were prepared by the “grafting from” method using atom transfer radical polymerization. The PAA block was prepared by hydrolysis of poly(tert-butyl acrylate) (P(t-BA)) and the polyelectrolyte was formed by subsequent treatment with either aqueous silver acetate or sodium tetrapalladate. Conversion of P(t-BA) to PAA and attachment of either silver or palladium ions to the PAA block was confirmed using ATR-FTIR spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy. Solvent switching of the diblock copolymer brush using DMF and anisole was incomplete both before and after treatment with silver acetate, although the degree of switching was larger before treatment. The polyelectrolyte diblock copolymer brushes were used for the synthesis of inorganic nanoparticles by reduction of the treated PAA. These samples were characterized using AFM, XPS, and ATR−FTIR.

Structure of pH-Dependent Block Copolymer Micelles: Charge and Ionic Strength Dependence

We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aqueous solution as a function of pH and ionic strength. The block copolymers carry a common core block, 2-(diethylamino)ethyl methacrylate (DEAEMA), and one of three coronal blocks, 2-(dimethylamino)ethyl methacrylate (DMAEMA), poly(ethylene oxide) (PEO), and DMAEMA, whose side chain amine groups are selectively quaternized with benzyl chloride (Q-DMAEMA). The PEO−DEAEMA, DMAEMA−DEAEMA, and Q-DMAEMA−DEAEMA copolymers form micelles with electrostatically neutral, weakly charged, and highly charged coronae, respectively. We adjust the fractional charge α on the DEAEMA and DMAEMA blocks by adjusting the solution pH. For DMAEMA−DEAEMA micelles increasing the fractional charge α swells the micelle corona while decreasing the aggregation number due to electrostatic repulsions. The decrease in aggregation number is also observed with increasing α for the PEO−DEAEMA and Q-DMAEMA−DEAEMA micelles, due to electrostatic repulsions between the hydrophobic DEAEMA blocks. Increasing the ionic strength causes the DMAEMA−DEAEMA micelle corona to shrink as the salt screens electrostatic repulsions within the corona. In all three copolymers increases in the ionic strength cause the micelle aggregation number to increase by screening the electrostatic repulsions between chains. Trends in the corona thickness with varying fractional charge and ionic strength are compared with a number of theoretical models providing additional insight into the micelle structure.

Fusion of Charged Block Copolymer Micelles into Toroid Networks

The association behavior of polyelectrolyte block copolymers in aqueous solution as a function of polymer and salt concentration is investigated for poly(ethylethylene-b-styrenesulfonic acid) (PEE-PSSH). This copolymer combines a soft hydrophobic block with a highly charged polyelectrolyte block that allows direct dissolution in aqueous solutions to investigate micellization under equilibrium conditions. Increasing polymer and salt concentration leads to the fusion of polyelectrolyte micelles into vesicles and fractal toroid-micronetworks. Association structures were characterized by static light scattering (SLS), small-angle neutron scattering (SANS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). At high salt concentrations the solution phase separates into a dilute micellar phase and a concentrated gel phase.

Synthesis of amphiphilic polyelectrolyte block copolymers using "living" radical polymerization. Application as stabilizers in emulsion polymerization

This paper describes the synthesis of amphiphilic block copolymers composed of an ionic poly(styrenesulfonate) first segment and a hydrophobic polystyrene second one, using TEMPO-mediated “living” radical polymerization. These copolymers proved to be efficient stabilizers in the emulsion polymerization of styrene.

Synthesis of amphiphilic polyelectrolyte block copolymers using “living” radical polymerization. Application as stabilizers in emulsion polymerization

Synthesis of amphiphilic polyelectrolyte block copolymers using “living” radical polymerization. Application as stabilizers in emulsion polymerization

Polyelectrolyte Block Copolymers as Effective Stabilizers in Emulsion Polymerization

Amphiphilic block copolymers consisting of hydrophobic block of hydrogenated polybutadiene and a partly sulfonated polyelectrolyte block of poly(styrenesulfonate) are effective stabilizers in emulsion polymerization. At low relative amounts of block copolymer (0.4 wt % as a fraction of monomer weight) and salt-free conditions, well-defined and stable latices with particle diameters of ca. 100 nm and solid contents of 20 wt % are obtained. This high stabilization efficiency of optimized block copolymer systems enables the formulation of latex systems with a relatively low remaining polarity in solid films and offers new interesting model systems with exclusively electrosteric stabilization. A comparison of polymerization in high or low ionic strength solution and the variation of the degree of sulfonation of the poly(styrenesulfonate) block shows an optimum stabilization of the latices at low ionic strength during polymerization. Fully sulfonated polymers systems presumably show a molecular orientation perpendicular to the particle surface, whereas 50% sulfonated species take on a traillike conformation along the surface, which is explained by remaining hydrophobic interactions. Due to this multiple surface particle contacts, the application of partly sulfonated polymers leads to more effective stabilization. At higher stabilizer concentrations, aggregates are found, which can be redispersed by ultrasonification or addition of low molecular weight surfactant solution.