Branched Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes

The structure of the self-assemblies formed by amphiphilic comblike copolyelectrolytes dispersed in water has been investigated by scattering techniques (light and neutron) and by transmission electronic microscopy. The comblike polymers consisted of a polystyrene backbone grafted with a fixed amount of pendant <i>N</i>,<i>N</i>-dimethyl quaternary ammonium alkyl groups of various lengths ranging from C12 up to C18. In aqueous solution, the polymers self-assembled into small spherical aggregates at low concentrations and into cylindrical aggregates above a critical concentration with a diameter that increased with the length of the alkyl side chains. The length of the cylindrical aggregates increased with increasing concentration, and branching occurred at higher concentration, which induced gelation above a critical percolation concentration. Growth and branching were favored by increasing the ionic strength of the solution. The dynamics slowed down with decreasing temperature and increasing alkyl length, and the assemblies of polymers with C16 and C18 pendant chains were kinetically frozen at 20 °C

Highlighting the Role of the Random Associating Block in the Self-Assembly of Amphiphilic Block–Random Copolymers

pH-sensitive random P­(<i>n</i>BA_1–<i>x</i>-<i>stat</i>-AA_<i>x</i>)₁₀₀ (MHx) and block–random P­(<i>n</i>BA_1–<i>x</i>-<i>stat</i>-AA_<i>x</i>)₁₀₀-<i>b</i>-PAA₁₀₀ (DHx) amphiphilic copolymers have been synthesized, where x stands for the molar ratios of pH-sensitive hydrophilic acrylic acid (AA) units statistically distributed with hydrophobic <i>n</i>-butyl acrylate (<i>n</i>BA) ones within the random block. Static and dynamic light scattering revealed that self-assembly of the random associating block (MHx) and block–random (DHx) copolymers is strongly affected by the pH and ionic strength of the solution and also by the amount of AA units within the MHx blocks. Below a characteristic pH, MHx self-assembles into finite size spherical particles that grow in size with decreasing pH until they eventually become insoluble. DHx self-assembles into similar spherical particles, but the hydrophilic PAA₁₀₀ corona surrounding the MHx core prevents insolubility at low pH. Self-assembly of DHx at higher pH is fully correlated to that of the neat MHx blocks, indicating that it is possible to control precisely the extent of self-assembly of diblock copolymers by tuning the hydrophobic character of their associating block. Here this was done by controlling the fraction of charged units within the random associating block

Particles Trapped at the Droplet Interface in Water-in-Water Emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly­(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10^–6 N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation

Photo-Cross-Linked Self-Assembled Poly(ethylene oxide)-Based Hydrogels Containing Hybrid Junctions with Dynamic and Permanent Cross-Links

Homogeneous hydrogels were formed by self-assembly of triblock copolymers via association of small hydrophobic end blocks into micelles bridged by large poly­(ethylene oxide) central blocks. A fraction of the end blocks were photo-cross-linkable and could be rapidly cross-linked covalently by in situ UV irradiation. In this manner networks were formed with well-defined chain lengths between homogeneously distributed hybrid micelles that contained both permanent and dynamically cross-linked end blocks. Linear rheology showed a single relaxation mode before in situ irradiation intermediate between those of the individual networks. The presence of transient cross-links decreased the percolation threshold of the network rendered permanent by irradiation and caused a strong increase of the elastic modulus at lower polymer concentrations. Large amplitude oscillation and tensile tests showed significant increase of the fracture strain caused by the dynamic cross-links

Dynamic Mechanical Properties of Networks of Wormlike Micelles Formed by Self-Assembled Comblike Amphiphilic Copolyelectrolytes

The rheological properties of viscoelastic aqueous solutions of wormlike micelles formed by the self-assembly of comblike copolyelectrolytes have been investigated by flow and dynamic measurements. The comblike polymers consisted of a polystyrene backbone grafted with a fixed amount of pendant <i>N</i>,<i>N</i>-dimethyl quaternary ammonium alkyl groups of various lengths ranging from C12 up to C18. Upon increasing concentration, the increase in size of the wormlike micelles and their branching results in the formation of a system spanning network through a percolation process at a critical concentration that decreases when salt is added or when the temperature is decreased. In this manner transient gels are formed with a viscoelastic relaxation time that does not depend on the polymer concentration or on the ionic strength, but their elastic modulus increases with increasing polymer or salt concentration. When the size of the alkyl groups is increased from C12 to C16, the relaxation time increases very strongly, but the temperature dependence remains characterized by the same activation energy. For C18, the systems are frozen at least up to 80 °C

Particles Trapped at the Droplet Interface in Water-in-Water Emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly­(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10^–6 N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation

Effect of Arm Exchange on the Liquid–Solid Transition of Dense Suspensions of Star Polymers

Star polymers with dynamic arm exchange are formed in water by self-assembly of amphiphilic diblock copolymers based on poly­(ethylene oxide) end capped with a small hydrophobic block. The arm exchange was arrested <i>in situ</i> by photo-cross-linking of the core. The effect of dynamic arm exchange on the osmotic compressibility and viscosity was investigated systematically as a function of the concentration and temperature. The discontinuous liquid–solid transition reported for dense polymeric micelle suspensions was found to be preserved after dynamic arm exchange was arrested <i>in situ</i>. The effect of cross-linking and aggregation number on the liquid–solid transition was investigated

Particles Trapped at the Droplet Interface in Water-in-Water Emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly­(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10^–6 N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation

Exploiting Salt Induced Microphase Separation To Form Soy Protein Microcapsules or Microgels in Aqueous Solution

Self-assembly of native glycinin at room temperature was investigated as a function of the pH and the NaCl concentration. Microphase separation leading to the formation of dense protein microdomains was observed by confocal laser scanning microscopy. Depending on the conditions, the microdomains coalesced into a continuous protein rich phase or associated into large clusters. Addition of β-conglycinin inhibited phase separation and reduced the pH range in which it occurred. Microdomains of glycinin that were formed in the presence of 0.1 M NaCl transformed into hollow stable cross-linked microcapsules when heated above 60 °C with diameters between 3 and 30 μm depending on the protein concentration and a shell thickness between 1.0 and 1.4 μm. The microcapsules were stable to dilution in salt free water, whereas microdomains formed at room temperature redispersed. Microdomains formed in mixtures with β-conglycinin did not transform into microcapsules, but they became stable cross-linked homogeneous microgels

pH-Controlled Rheological Properties of Mixed Amphiphilic Triblock Copolymers

Aqueous mixtures of pH-sensitive block random BAB triblock copolymers with different hydrophobic B blocks connected to the same hydrophilic A block were studied in order to investigate comicellization and the impact on the dynamic mechanical properties. The B blocks were statistical copolymers of acrylic acid (AA) and <i>n</i>-butyl acrylate (<i>n</i>BA) with varying AA contents, whereas the A block was a pure PAA. Neat triblocks self-assembled into transient networks for which the mechanical relaxation time depended both on the AA content within the B blocks and on the pH, which affected the ionization of the AA units. Static and dynamic light scattering measurements were done on mixtures of equivalent AB diblock copolymers that showed that comicellization occurred only at conditions at which both copolymers considered separately self-assemble. When comicellization occurred, the characteristic escape time of both types of B blocks from the mixed hydrophobic cores impacted the rheological properties of the binary triblock mixture. Using binary mixtures of BAB triblock copolymers exhibiting pH-controlled dynamics thus allows control and fine-tuning of the viscoelastic properties at constant pH by formulation without the need to synthesize a large number of different polymers. Moreover, the more dynamic B blocks were slowed down in the presence of the less dynamic ones, and vice versa, so that a frozen network could be transformed into a transient one by coassembly with very dynamic chains