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

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

pH- and Thermoresponsive Self-Assembly of Cationic Triblock Copolymers with Controlled Dynamics

Transient hydrogels formed by cationic BAB triblock copolymers consisting of a hydrophilic poly­(dimethyl­amino­ethyl methacrylate) (P­(DMAEMA)) A block and amphiphilic B blocks composed of randomly distributed DMAEMA and <i>n</i>-butyl methacrylate (<i>n</i>BMA) units were investigated. Oscillatory shear measurements revealed formation of dynamic networks with terminal relaxation times that can be controlled by tuning the ionization degree (α) of the DMAEMA units or the temperature up until 50 °C. A sol–gel transition could be induced by increasing the pH. Above 50 °C irreversible aggregation was observed. The behavior of these pH-sensitive cationic copolymers is compared with that of pH-sensitive anionic copolymers, revealing that incorporating stimuli-responsive hydrophilic units within the hydrophobic blocks of amphiphilic block copolymers may be a general way to control the exchange dynamics of the latter

Synthesis of Amphiphilic Poly(acrylic acid)‑<i>b</i>‑poly(<i>n</i>‑butyl acrylate-<i>co</i>-acrylic acid) Block Copolymers with Various Microstructures via RAFT Polymerization in Water/Ethanol Heterogeneous Media

The batch copolymerization of <i>n</i>-butyl acrylate (50 mol %) and acrylic acid (50 mol %)two monomers with close reactivity but with very different water solubilitywas performed under RAFT (reversible addition–fragmentation chain transfer) control, in heterogeneous conditions in water or water/ethanol mixtures. The polymerizations were conducted in the presence of poly­(acrylic acid) end-capped with a trithiocarbonate reactive group, serving as a control agent, a precursor for the in situ synthesis of amphiphilic block copolymers and a stabilizer for the formed particles. Good control over the polymerization and stable colloidal suspensions were achieved under such conditions. The kinetic study demonstrated that the polymerization rates of each monomer varied upon a change of the water/ethanol volumetric ratios and of the overall comonomer concentrations. The compositional microstructure of the copolymers was characterized by NMR analysis and by potentiometric titration. This work demonstrates that the distribution of the comonomer units in a copolymer formed under heterogeneous conditions can be controlled by the solubility of the monomers in the medium, hence leading to various types of compositional gradient structures

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

Viscoelastic Properties of Hydrogels Based on Self-Assembled Multisticker Polymers Grafted with pH-Responsive Grafts

Graft copolymers consisting of a poly­(acrylic acid) (PAA) backbone and random grafts of <i>n</i>-butyl acrylate and acrylic acid, P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀, were synthesized by free radical polymerization of the backbone followed by ATRP polymerization of the grafts via a grafting-from approach. The rheological properties of their aqueous solutions were measured by oscillatory shear measurements at different temperatures, pHs, and concentrations. All graft copolymers formed transient networks above their percolation concentrations with pH-dependent relaxation times. These results implied that incorporation of hydrophilic AA units within the hydrophobic grafts allowed controlling their exchange dynamics in a pH-dependent way, leading to viscoelastic fluids with a pH-tunable terminal relaxation time. Provided that the grafting density remained low, the rheological properties of the graft copolymers were very similar to those of model BAB triblock copolymers consisting of a PAA central block and P­(<i>n</i>BA_50%-<i>stat</i>-AA_50%)₁₀₀ lateral blocks

Patchy Supramolecular Bottle-Brushes Formed by Solution Self-Assembly of Bis(urea)s and Tris(urea)s Decorated by Two Incompatible Polymer Arms

In an attempt to design urea-based Janus nanocylinders through a supramolecular approach, nonsymmetrical bis­(urea)­s and tris­(urea)­s decorated by two incompatible polymer arms, namely, poly­(styrene) (PS) and poly­(isobutylene) (PIB), were synthesized using rather straightforward organic and polymer chemistry techniques. Light scattering experiments revealed that these molecules self-assembled in cyclohexane by cooperative hydrogen bonds. The extent of self-assembly was limited for the bis­(urea)­s. On the contrary, reasonably anisotropic 1D structures (small nanocylinders) could be obtained with the tris­(urea)­s (<i>N</i>_agg ∼ 50) which developed six cooperative hydrogen bonds per molecule. ¹H transverse relaxation measurements and NOESY NMR experiments in cyclohexane revealed that perfect Janus nanocylinders with one face consisting of only PS and the other of PIB were not obtained. Nevertheless, phase segregation between the PS and PIB chains occurred to a large extent, resulting in patchy cylinders containing well separated domains of PIB and PS chains. Reasons for this behavior were proposed, paving the way to improve the proposed strategy toward true urea-based supramolecular Janus nanocylinders

Self-Assembly and Critical Solubility Temperature of Supramolecular Polystyrene Bottle-Brushes in Cyclohexane

The formation of polystyrene (PS) supramolecular bottle-brushes by self-assembly in cyclohexane of hydrogen-bonding tris­(urea) units decorated by PS chains was investigated using light and neutron scattering. Atom transfer radical polymerization (ATRP) was used to control the length of the PS side-chains and allowed the straightforward synthesis of the targeted tris­(urea)­s. It was shown that their extent of self-assembly strongly depended on the degree of polymerization and chemical nature of the polymer side chains, in contrast with what was previously observed with cyclic oligopeptides, another type of self-assembling units. With sufficiently short PS side-chains, anisotropic supramolecular bottle-brushes could be obtained. Their critical solubility temperature, <i>T</i>_c, was measured in cyclohexane, proving experimentally for the first time that densely grafted PS bottle-brushes exhibit a much lower <i>T</i>_c than linear PS or even star-shaped PS of similar molecular weight

Competition Between Steric Hindrance and Hydrogen Bonding in the Formation of Supramolecular Bottle Brush Polymers

The formation of supramolecular bottle-brush polymers consisting of a noncovalent backbone assembled through directional hydrogen bonds and of poly­(isobutylene) (PIB) side-chains was investigated in cyclohexane by light scattering. Two limiting cases were observed depending on the balance between the favorable formation of hydrogen bonds and the unfavorable stretching of the PIB chains within the supramolecular bottle-brushes, in agreement with a theoretical model developed by Wang et al. On one hand, a bisurea self-assembling unit able to form four cooperative hydrogen bonds per molecule led to relatively short supramolecular bottle-brushes, the length of which could be varied by modifying steric hindrance or by using solvent mixtures. On the other hand, supramolecular bottle-brush polymers exhibiting persistent lengths of more than 300 nm could be obtained by using trisureas that are able to form six hydrogen bonds per molecule. Their easy synthesis and the fact that it is possible to control their self-assembly into long supramolecular bottle-brush polymers make polymer-decorated bisureas and trisureas an attractive alternative to cyclopeptides and shape-persistent rings for the creation of supramolecular nanostructures