Gérard Riess

Micellization of block copolymers

This article deals with recent progress including the author’s work in the field of block copolymer self-assembly in solution and on solid surfaces. The synthesis methods for producing block copolymers with well-defined structures, molecular weights and composition are outlined with emphasis on ionic and controlled free radical polymerization techniques. A general overview of the preparation, characterization and theories of block copolymer micellar systems is presented. Selected examples of micelle formation in aqueous and organic medium are given for di- and triblock copolymers, as well as for block copolymers with more complex architectures. Current and potential application possibilities of block copolymer colloidal assemblies as stabilizers, flocculants, nanoreservoir in, among others, controlled delivery of bioactive agents, catalysis, latex agglomeration and stabilization of non-aqueous emulsion are also discussed. q 2003 Elsevier Science Ltd. All rights reserved.

New morphologies in rubber-modified polymers

Abstract New morphologies, especially “core-shell” and multilayered structures (“onion structure”) have been obtained for polymer blends containing block copolymers. For triblock copolymers Cop A-B-C, having three sequences of different types, in this case, poly(styrene-b-isoprene-b-methyl methacrylate), the conditions are given for the formation of core-shell structures. A scheme is presented showing the structures of the mesomorphic phases, as a function of the composition of the copolymer. Multilayered and core-shell structures have also been obtained by a suitable combination of two diblock copolymers, Cop A-B, i.e., poly(styrene-b-isoprene), with a corresponding homopolymer, i. e., polystyrene. One of the copolymers can act as an emulsifier for the second one forming the dispersed phase of the blend. An extension of the classical mesomorphic structures is presented, and it is shown that the emulsifying effect of block copolymers is quite general.

Photochemical stabilization of block copolymer micelles

Abstract The crosslinking or stabilization of the polybutadiene cores of polystyrene-b-polybutadiene block copolymer micelles in selective solvents was studied for a range of block copolymers differing in type, composition and molecular weight. Observations pertaining to the efficiency of the process and to the existence, size and stability of the crosslinked species were made with respect to the different characteristics of the parent copolymer. Information regarding the fractionation of parent copolymer on stabilization was obtained.

Interaction studies of styrene—ethylene oxide block copolymers with ionic surfactants in aqueous solution

Abstract The interaction of styrene—ethylene oxide block copolymers with four anionic surfactants (sodium dodecyl sulfate, sodium dodecanoate, sodium dodecylbenzenesulfonate, and sodium dodecanoyl sarcosinate), and two cationic surfactants (tetradecyl- and hexadecyl-trimethylammonium bromide), was studied and each surfactant showed a distinct interaction with the copolymer in aqueous solution. Usually two transitions, one below and one above the critical micelle concentration, CMC, of the surfactants, were observed from conductance, surface tension, and dye solubilization measurements. These transitions indicate the beginning and completion of polymer—surfactant interaction. The viscometric results showed the formation of a polyelectrolyte complex. The interaction between copolymer and sodium dodecyl sulfate was also examined by 1 H NMR. The influence of the molecular characteristics of the block copolymers, the nature and type of surfactants, temperature and added salt on the interaction is described. A possible mechanism for such an interaction is proposed.

Stabilization of dispersion polymerization by poly(styrene-b-ethylene oxide) copolymers

Abstract A series of poly(styrene-b-ethylene oxide) copolymers were investigated as stabilizers for the dispersion polymerization of styrene in methanol. The effects of stabilizer molecular weight, stabilizer/anchor ratio and stabilizer concentration on particle diameter and polydispersity were examined. Large (2 μm) monodisperse polystyrene particles were formed when a low molecular weight stabilizer was used, while broad bimodal distributions were found for polystyrene particles stabilized by higher molecular weight copolymeric stabilizers.

Adsorption at the solid—solution interface and micelle formation in water of the poly(oxyethylene—styrene—oxyethylene) triblock copolymer

Abstract ABA-type copolymers composed of polystyrene (PS) as the B block and polyoxyethylene (POE) as the A block, having weight fractions (nps) of PS and Mn values in the ranges 16–35% and 12000–27000 respectively, were synthesized by sequential anionic polymerization. Micelle formation in water by triblock copolymers in the concentration range 10−5–1 g dm−3 was investigated by fluorescence probing (FP) and small-angle X-ray scattering (SAXS) methods. The data obtained by the FP method indicate that the CMC is extremely small and not observable over the concentration range studied. The micelle size determined by SAXS was found to increase with the total copolymer chain length. The results are in good agreement with the Star model, and a power-law dependence is found between the radius of gyration R of the micelles and the Mn values of the copolymers investigated. Adsorption measurements from water onto titanium dioxide particles were made with copolymer concentrations ranging from 10−1 to 1 g dm−3. The maximum amount adsorbed and the surface density were found to decrease with increasing nps and molecular weight of the PS block respectively.

Micellization of pH-stimulable poly(2-vinylpyridine)-b-poly(ethylene oxide) copolymers and their complexation with anionic surfactants.

The micellization behavior was examined for a series of 3 pH-stimulable poly(2-vinylpyridine)-b-poly(ethylene oxide) (P2VP-b-PEO) copolymers, with a constant composition of 67.5±1.5wt% PEO and increasing molecular weight. The micellar characteristics were determined by dynamic, static and electrophoretic light scattering, fluorescence spectroscopy, as well as by (1)H NMR in the pH range of 2-7 and in the presence or in the absence of the anionic surfactant sodium dodecylsulfate (SDS). In the absence of SDS, two pH regimes were investigated. For the pH range below 5, the micellar characteristics, such as C.M.C., particle size and zeta potential, were determined. For the first time, it could be shown that the micellization range could be extended to low pH values for relatively high copolymer concentrations. Above pH 5, correlations between the molecular characteristics and the aggregation number as well as the hydrodynamic diameter were established. A fair agreement could be demonstrated with the theoretical predictions for star-like micelles having a P2VP core and a PEO corona. Another original aspect of these double hydrophilic copolymers is their complex formation by electrostatic interaction between the protonated P2VP and SDS. At low pH and low copolymer concentrations, where only protonated unimers are present, the SDS induces the micellization at a given neutralization degree (DN). The process of complex formation was analyzed as a function of DN and pH by DLS, fluorescence spectroscopy, zeta potential measurements and by (1)H NMR, this last technique providing information on the mobility of the P2VP/SDS micellar core. A model for the micelle formation mechanism was suggested.

Synthesis and purification of a poly(ethylene oxide)-poly(γ-benzyl-l-glutamate) diblock copolymer bearing tyrosine units at the block junction

An AB-type diblock copolymer, composed of poly(ethylene oxide) as the A block and poly(γ-benzyl-l-glutamate) as the B block, was prepared by the polymerization of (γ-benzyl-l-glutamate) N-carboxyanhydride (BLG-NCA) using α-(2-(aminoethyl)-ω-methoxypolyoxyethylene as a macroinitiator. o-Benzyl-l-tyrosineN-carboxyanhydride (BT-NCA) was used to introduce a tyrosine function at the block junction.

An improved technique of the cumylpotassium preparation: application in the synthesis of spectroscopically pure polystyrene-poly(ethylene oxide) diblock copolymers

Abstract A series of AB-type diblock copolymers composed of polystyrene (PS) as the A block and poly(ethylene oxide) (PEO) as the B block was synthesized by sequential anionic polymerization. The prepared copolymers had fixed molecular weights for the PS blocks and varying lengths for the PEO chains. Problems connected with the preparation of the initiator, cumylpotassium, were analysed and a new synthetic approach for its preparation proposed. Our synthetic procedure enables spectroscopically pure PS-PEO diblock copolymers free of high molecular weight impurities to be prepared.

Block copolymer stabilized nonaqueous biocompatible sub-micron emulsions for topical applications

Polyethylene glycol (PEG) 400/Miglyol 812 non-aqueous sub-micron emulsions were developed due to the fact that they are of interest for the design of drug-loaded biocompatible topical formulations. These types of emulsions were favourably stabilized by poly (2-vinylpyridine)-b-poly (butadiene) (P2VP-b-PBut) copolymer with DPBut>DP2VP, each of these sequences being well-adapted to the solubility parameters of PEG 400 and Miglyol 812, respectively. This type of block copolymers, which might limit the Ostwald ripening, appeared to be more efficient stabilizers than low molecular weight non-ionic surfactants. The emulsion characteristics, such as particle size, stability and viscosity at different shear rates were determined as a function of the phase ratio, the copolymer concentration and storage time. It was further shown that Acyclovir, as a model drug of low water solubility, could be incorporated into the PEG 400 dispersed phase, with no significant modification of the initial emulsion characteristics.