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

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

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