Lifeng Zhang

Bio: Lifeng Zhang is an academic researcher from McGill University. The author has contributed to research in topics: Crew cut & Acrylic acid. The author has an hindex of 14, co-authored 18 publications receiving 5789 citations.

Multiple Morphologies of "Crew-Cut" Aggregates of Polystyrene-b-poly(acrylic acid) Block Copolymers.

Abstract: The observation by transmission electron microscopy of six different stable aggregate morphologies is reported for the same family of highly asymmetric polystyrene-poly-(acrylic acid) block copolymers prepared in a low molecular weight solvent system. Four of the morphologies consist of spheres, rods, lamellae, and vesicles in aqueous solution, whereas the fifth consists of simple reverse micelle-like aggregates. The sixth consists of up to micrometer-size spheres in aqueous solution that have hydrophilic surfaces and are filled with the reverse micelle-like aggregates. In addition, a needle-like solid, which is highly birefringent, is obtained on drying of aqueous solutions of the spherical micelles. This range of morphologies is believed to be unprecedented for a block copolymer system.

Ion-Induced Morphological Changes in “Crew-Cut” Aggregates of Amphiphilic Block Copolymers

Abstract: The addition of ions in micromolar (CaCl2 or HCl) or millimolar (NaCl) concentrations can change the morphology of "crew-cut" aggregates of amphiphilic block copolymers in dilute solutions. In addition to spherical, rodlike, and univesicular or lamellar aggregates, an unusual large compound vesicle morphology can be obtained from a single block copolymer. Some features of the spontaneously formed large compound vesicles may make them especially useful as vehicles for delivering drugs and as models of biological cells. Gelation of a dilute spherical micelle solution can also be induced by ions as the result of the formation of a cross-linked "pearl necklace" morphology.

Formation of crew‐cut aggregates of various morphologies from amphiphilic block copolymers in solution

Abstract: Crew-cut micelle-like aggregates represent a new type of aggregate formed in solution by the self-assembly of highly asymmetric amphiphilic block copolymers. The aggregates are termed crew-cut because the dimensions of the core are bigger than those of the corona. They are prepared by first dissolving copolymers, such as polystyrene-b-poly(acrylic acid), in a common solvent, e.g. dimethylformamide (DMF), and then adding water to the solution to induce aggregation of the polystyrene segments. Since the first description of an experimental study of spherical crew-cut micelles in 1994, over 20 papers have appeared in the literature. One of the most noteworthy phenomena seen in the crew-cut aggregate systems is the existence of multiple morphologies. These include spheres, rods, vesicles, lamellae, large compound vesicles, large compound micelles and several other structures. This paper presents an overview of the phenomenon, and a description of the qualitative principles underlying the formation of crew-cut aggregates of various morphologies in solution. The main topics of this paper are the preparation of crew-cut aggregates, the critical micellization concentration, the aggregation number, the common solvent content in the core, the mobility of the core-forming blocks in the core region, and the relative importance of thermodynamics versus kinetics during micellization as a function of added water content. The formation and morphology of the crew-cuts are also compared with those in other self-assembled systems, i.e. small molecule amphiphiles in solution, block copolymers in bulk and blends of block copolymers and homopolymers. It has been found that the most important factors influencing the morphology of the aggregates are the copolymer composition, the initial copolymer concentration in solution, the added water content, the nature of the common solvent and the type and concentration of added ions (salt, acid or base). The morphology is mainly controlled by a force balance involving three factors, namely the stretching of the polystyrene (PS) blocks in the core, the surface tension between the core and the outside solvent, and the repulsion among the corona chains. The morphogenic effects of the above-mentioned parameters can, in general, be ascribed to their influence on this force balance during the formation of the aggregates. In particular, the length of corona-forming blocks or the addition of ions mainly affects the repulsive interactions of the corona chains; the effect of the water content in solution is related to the change of the surface tension between the PS core and the solvent; and the effect of changing the common solvent from DMF to tetrahydrofuran or to dioxane on the morphology is due mainly to the increased dimensions of the core-forming blocks resulting from the changes of the solvent contents in the core and the aggregation numbers. © 1998 John Wiley & Sons, Ltd.

Morphogenic Effect of Solvent on Crew-Cut Aggregates of Apmphiphilic Diblock Copolymers

Abstract: It is shown that the morphologies and other characteristics of crew-cut aggregates of polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymers are related to the nature of the initial common solvent in which the micelle-like aggregates are prepared. Polymer−solvent interactions determine the dimensions of both the core and the corona of the aggregates. Solubility parameters and dielectric constants of the solvents can be used to estimate the strength of the PS−solvent interaction (which influences the solvent content in the core) and the strength of PAA−solvent interaction (which influences the repulsion among corona chains). The closer the match between the solubility parameter of the solvent and that of the core forming block, the higher the solvent content of the core and the higher the degree of stretching of the core chains. The lower the polarity of the solvent, the weaker the PAA−solvent interaction and the weaker the repulsive interactions among the corona chains; this increases the aggregat...

Thermodynamic vs kinetic aspects in the formation and morphological transitions of crew-cut aggregates produced by self-assembly of polystyrene-b-poly(acrylic acid) block copolymers in dilute solution

Abstract: Thermodynamic vs kinetic aspects of the formation of crew-cut aggregates of various structures prepared from polystyrene-b-poly(acrylic acid) diblock copolymers in DMF/water mixtures are explored. In particular, the reversibility of the transitions between the crew-cut aggregates of various morphologies is studied by “jumps” in the polymer or ion concentration at various water contents. The aggregates are prepared by two different methods: micellization is induced either by the addition of water to polymer/DMF solutions or by the direct dissolution of the polymer in DMF/water mixtures. It is shown that as the polymer concentration increases, the morphology of the aggregates changes from spheres to rodlike micelles, to interconnected rods, and then to bilayers. As the water content increases, the boundaries for the formation of the different morphologies move toward lower polymer concentrations. The thermodynamic vs kinetic control of the morphology depends largely on the water content and the method of a...

Self-assembly of block copolymers

Abstract: Block copolymer (BCP) self-assembly has attracted considerable attention for many decades because it can yield ordered structures in a wide range of morphologies, including spheres, cylinders, bicontinuous structures, lamellae, vesicles, and many other complex or hierarchical assemblies. These aggregates provide potential or practical applications in many fields. The present tutorial review introduces the primary principles of BCP self-assembly in bulk and in solution, by describing experiments, theories, accessible morphologies and morphological transitions, factors affecting the morphology, thermodynamics and kinetics, among others. As one specific example at a more advanced level, BCP vesicles (polymersomes) and their potential applications are discussed in some detail.

Surface and Colloid Science

Abstract: 1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Micellar nanocarriers: pharmaceutical perspectives.

Abstract: Micelles, self-assembling nanosized colloidal particles with a hydrophobic core and hydrophilic shell are currently successfully used as pharmaceutical carriers for water-insoluble drugs and demonstrate a series of attractive properties as drug carriers. Among the micelle-forming compounds, amphiphilic copolymers, i.e., polymers consisting of hydrophobic block and hydrophilic block, are gaining an increasing attention. Polymeric micelles possess high stability both in vitro and in vivo and good biocompatibility, and can solubilize a broad variety of poorly soluble pharmaceuticals many of these drug-loaded micelles are currently at different stages of preclinical and clinical trials. Among polymeric micelles, a special group is formed by lipid-core micelles, i.e., micelles formed by conjugates of soluble copolymers with lipids (such as polyethylene glycol-phosphatidyl ethanolamine conjugate, PEG-PE). Polymeric micelles, including lipid-core micelles, carrying various reporter (contrast) groups may become the imaging agents of choice in different imaging modalities. All these micelles can also be used as targeted drug delivery systems. The targeting can be achieved via the enhanced permeability and retention (EPR) effect (into the areas with the compromised vasculature), by making micelles of stimuli-responsive amphiphilic block-copolymers, or by attaching specific targeting ligand molecules to the micelle surface. Immunomicelles prepared by coupling monoclonal antibody molecules to p-nitrophenylcarbonyl groups on the water-exposed termini of the micelle corona-forming blocks demonstrate high binding specificity and targetability. This review will discuss some recent trends in using micelles as pharmaceutical carriers.