Homopolymer induced aggregation of poly(ethylene oxide)n-b-poly(butylene oxide)m polymersomes

(Homo)polymer-mediated colloidal stability of micellar solutions

Despite their wide range of applications, there is a remarkable lack of fundamental understanding about how micelles respond to other components in solution. The colloidal stability of micellar solutions in presence of (homo)polymers is investigated here following a theoretical bottom-up approach. A polymer-mediated micelle-micelle interaction is extracted from changes in the micelle-unimer equilibrium as a function of the inter-micelle distance. The homopolymer-mediated diblock copolymer micelle-micelle interaction is studied both for depletion and adsorption of the homopolymer. The fluffy nature of the solvophilic domain (corona) of the micelle weakens the depletion-induced destabilization. Accumulation of polymers into the corona induces bridging attraction between micelles. In fact, both depletion and adsorption phenomena are regulated by the coronal thickness relative to the size of the added polymer. Penetration of guest compounds into the coronal domain of crew-cut micelles, with a narrower yet denser corona, is less pronounced as for starlike micelles (with a more diffuse corona). Therefore, crew-cut micelles are less sensitive to the effect of added compounds, and hence more suitable for applications in multicomponent systems, such as industrial formulations or biological fluids. The trends observed for the colloidal stability of crew-cut micelles qualitatively match with our experimental observations on aqueous dispersions of polycaprolactone-polyethylene glycol (PCL-PEO) micellar suspensions with added PEO chains.

Depletion driven self-assembly of block copolymer solutions by homopolymers

The addition of a non-adsorbing homopolymer to a block copolymer solution provides a convenient strategy for regulating its self-assembly. We systematically investigate the depletion effect from a homopolymer on the morphologies of AB diblock and BAB triblock copolymers in selective solvents. Increasing the homopolymer content results in larger spherical micelles, and the curvature of micelles is proportional to the square of homopolymer concentrations. A high enough homopolymer concentration may transfer micelles into vesicles. A deep analysis shows that the depletion effect produces attractive interaction between hydrophilic B blocks as well as their contraction on the micellar surface. The size of triblock copolymer micelles is not affected by homopolymers significantly, and spherical-to-wormlike micelle transition occurs at high homopolymer contents. These results have important applications for the precise design of self-assembled nanostructures of copolymer systems.

Polymeric lipid vesicles with pH-responsive turning on-off membrane for programed delivery of insulin in GI tract

A kind of polymeric lipid vesicles (PLVs) with pH-responsive turning on-off membrane for programed delivery of insulin in gastrointestinal (GI) tract was developed, which was self-assembled from the grafted amphipathic polymer of N-tocopheryl-N'-succinyl-ɛ-poly-l-lysine (TP/SC-g-PLL). By controlling the grafting ratio of hydrophobic alkane and ionizable carboxyl branches, the permeability of membrane was adjustable and thus allowing insulin release in a GI-pH dependent manner. The effects of grafting degree of substitution (DS) on the pH-responsive behavior of the formed vesicles were confirmed by critical aggregation concentration determination, morphology and size characterization. Their transepithelial permeability across the GI tract was proved by both confocal visualization in vitro model of Caco-2 cellular monolayer and in vivo hypoglycemic study in diabetic rats. Accordingly, the work described here indicated that the self-assembled PLVs could be a promising candidate for improving the GI delivery of hydrophilic biomacromolecule agents.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

Bilayers connected by threadlike micelles in amphiphilic mixtures: a self-consistent field theory study

Binary mixtures of amphiphiles in solution can self-assemble into a wide range of structures when the two species individually form aggregates of different curvatures. A specific example of this is seen in solutions of lipid mixtures where the two species form lamellar structures and spherical micelles, respectively. Here, vesicles connected by threadlike micelles can form in a narrow concentration range of the sphere-forming lipid. We present a study of these structures based on self-consistent field theory (SCFT), a coarse-grained model of amphiphiles. First, we show that the addition of sphere-forming lipid to a solution of lamella-former can lower the free energy of cylindrical, threadlike micelles and hence encourage their formation. Next, we demonstrate the coupling between composition and curvature; specifically, that increasing the concentration of sphere-former in a system of two bilayers connected by a thread leads to a transfer of amphiphile to the thread. We further show that the two species are segregated within the structure, with the concentration of sphere-former being significantly higher in the thread. Finally, the addition of larger amounts of sphere-former is found to destabilize the junctions linking the bilayers to the cylindrical micelle, leading to a breakdown of the connected structures. The degree of segregation of the amphiphiles and the amount of sphere-former required to destabilize the junctions is shown to be sensitive to the length of the hydrophilic block of the sphere-forming amphiphiles.