Polymer Probe Diffusion in Globular Protein Gels and Aggregate Suspensions

Transport properties of macromolecules in dense aggregate suspensions and gels of proteins are important for usage of these biomaterials in areas such as pharmaceutics, food, and cosmetics. The mobility of polymers in protein gels has received some attention in the past, but the mobility in dense aggregate suspensions has not yet been investigated. In this study, self-diffusion of probe dextran chains was studied in suspensions of aggregates with different size and morphology and in gels using fluorescence recovery after photobleaching over a wide range of concentrations. Brownian diffusion of the probes was observed in aggregate suspensions as well as in weak gels formed just beyond the critical gel concentration. Diffusion of polymers in dense suspensions of protein aggregates depends not only on the concentration but also on the size and morphology of the aggregates. It is not directly related to the viscosity or the dynamic correlation length. Diffusion of polymers in protein gels is anomalous and occurs on logarithmic time scales. The recovery of the fluorescence for densely cross-linked gels was logarithmic with time, suggesting an exponential distribution of diffusion coefficients

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

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

Progressive Freezing-in of the Junctions in Self-Assembled Triblock Copolymer Hydrogels during Aging

The evolution with time was investigated for self-assembled networks formed by triblock copolymers in aqueous solution. The polymers consisted of a central hydrophilic poly­(acrylic acid) block and two hydrophobic end-blocks formed by random copolymers of 50% acrylic acid and 50% <i>n</i>-butyl acrylate units. The rheological properties of the systems at steady state were strongly influenced by the degree of ionization (α) and thus by the pH. This allows one to obtain systems ranging from low viscosity solutions to hydrogels just by varying α. However, steady state was not reached instantaneously when α was changed, but proceeded through a slow progressive increase of the viscosity. The rate at which the systems aged was independent of α and of the polymer concentration and is attributed to slow reorganization of the cores formed by the self-assembled hydrophobic blocks

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

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

Stabilization of Water-in-Water Emulsions by Polysaccharide-Coated Protein Particles

The phase diagram of mixtures of xyloglucan (XG) and amylopectin (AMP) in aqueous solution is presented. Water-in-water emulsions prepared from mixtures in the two-phase regime were studied in detail, and the interfacial tension was determined. It is shown that the emulsions can be stabilized by addition of β-lactoglobulin microgels (βLGm), but only at pH ≤ 5.0. Excess βLGm preferentially entered the AMP phase at pH > 5.0 and the XG phase at lower pH. The inversion was caused by adsorption of XG onto βLGm that started below pH 5.5. It is shown that modification of the surface of particles by coating with polysaccharides is a potential lever to control stabilization of water-in-water emulsions

Stabilization of Water-in-Water Emulsions by Polysaccharide-Coated Protein Particles

The phase diagram of mixtures of xyloglucan (XG) and amylopectin (AMP) in aqueous solution is presented. Water-in-water emulsions prepared from mixtures in the two-phase regime were studied in detail, and the interfacial tension was determined. It is shown that the emulsions can be stabilized by addition of β-lactoglobulin microgels (βLGm), but only at pH ≤ 5.0. Excess βLGm preferentially entered the AMP phase at pH > 5.0 and the XG phase at lower pH. The inversion was caused by adsorption of XG onto βLGm that started below pH 5.5. It is shown that modification of the surface of particles by coating with polysaccharides is a potential lever to control stabilization of water-in-water emulsions

Ionization Of Amphiphilic Acidic Block Copolymers

The ionization behavior of an amphiphilic diblock copolymer poly­(<i>n</i>-butyl acrylate_50%-<i>stat</i>-acrylic acid_50%)₁₀₀-<i>block</i>-poly­(acrylic acid)₁₀₀ (P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀-<i>b</i>-PAA₁₀₀, DH50) and of its equivalent triblock copolymer P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀-<i>b</i>-PAA₂₀₀-<i>b</i>-P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀ (TH50) were studied by potentiometric titration either in pure water or in 0.5 M NaCl. These polymers consist of a hydrophilic acidic block (PAA) connected to a hydrophobic block, P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀, whose hydrophobic character has been mitigated by copolymerization with hydrophilic units. We show that all AA units, even those in the hydrophobic block could be ionized. However, the AA units within the hydrophobic block were less acidic than those in the hydrophilic block, resulting in the preferential ionization of the latter block. The preferential ionization of PAA over that of P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀ was stronger at higher ionic strength. Remarkably, the covalent bonds between the PAA and P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀ blocks in the diblock or the triblock did not affect the ionization of each block, although the self-association of the block copolymers into spherical aggregates modified the environment of the PAA blocks compared to when PAA was molecularly dispersed

Chain Stopper-Assisted Characterization of Supramolecular Polymers

Supramolecular polymers are dynamic materials; consequently, their molar mass is concentration dependent. However, the present experimental results show that an efficient chain stopper (i.e., a monofunctional monomer) can be used to block the concentration dependence of the molar mass of a hydrogen-bonded supramolecular polymer, over a realistic concentration range. This fact was used to derive the molecular weight and radius of gyration of the stopped supramolecular chains (by light scattering) as well as the intrinsic viscosity. In a second step, the molecular weight of the bis-urea-based supramolecular polymer (EHUT) was determined in the absence of a chain stopper