pH-triggered reversible sol–gel transition in aqueous solutions of amphiphilic gradient copolymers

Abstract

We demonstrate the possibility of reversible pH-controlled sol-gel transition in aqueous solution of associating amphiphilic triblock copolymer poly(styrene-grad-acrylic acid)-b-poly(acrylic acid)-b-poly (styrene-grad-acrylic acid), (PS-grad-PAA)-b-PAA-b-(PS-grad-PAA), synthesized via nitroxidemediated (NM) radical copolymerization. The presence of pH-sensitive co-monomer units of the acrylic acid in the terminal blocks ensures the dynamic nature of the styrene-rich hydrophobic nanodomains which are formed at low pH. At small polymer concentrations the association triggered by lowering the pH gives rise to flower-like micelles stabilized by partially ionized PAA coronae. The pH-controlled association was monitored by DLS-titration and manifested in the evolution of a correlation peak in the SANS spectra. The resulting copolymer aggregates were visualized by TEM, which confirmed the spherical shape of the dense styrene-rich domains. Above the micelle overlap concentration a decrease in pH provokes macroscopic gelation. Here the styrene-reach domains perform as cross-links in the transient network. The pH-triggered sol-to-gel transition is manifested in an abrupt and strong (up to 3 orders of magnitude) increase in the zero-shear viscosity and in a characteristic change in the frequency dependence of the storage and loss moduli. The discovered effect can be used for efficient pH-control of the rheological properties of aqueous solutions.

Stimuli-Responsive Spherical Brushes Based onD-Galactopyranose and 2-(Dimethylamino)ethyl Methacrylate

“…On heating the PDMAEMA blocks contract somewhat, forcing a detachment of the nanoparticles sticking together, thereby decreasing the distribution width. A similar influence of heterogeneity in block copolymer composition on pH‐driven micellization has been observed previously …”

Synthesis of amphiphilic block-gradient copolymers of styrene and acrylic acid by Nitroxide Mediated Polymerization

“…The method of synthesis of di and triblock gradi ent copolymers of S and AA was developed in refs [11][12][13][14][15] and is schematically presented in Schemes 1 and 2. The reaction was performed in dioxane at 120°C in the presence of alkoxyamine, 2 methylaminoxypropi onic SG1 (BlocBuilder ® or MAMA, 99%) or dialkoxyamine (DIAMS, 99%) as an initiator and slight excess of N tert butyl (1 diethyl phosphono 2,2 dimethylpropyl) nitroxide SG1 (9%) as counter radical in order to avoid side reactions and to ensure a good control of the chains growth.…”

“…The increase in the apparent activity of AA monomers is explained by the boot strap effect, that is, by enrichment of the vicinity of the reaction centre by the AA monomers preferentially adsorbed by the PAA chain. The obtained amphiphilic ionic gradi ent copolymers are capable of self assembly in aque ous solutions and at air water interfaces giving rise to stimuli responsive nanostructures and mesophases [14,15,39,40]. While diblock gradient copolymers demonstrate reversible pH and salinity controlled micellization [15], the presence of the second terminal gradient block leads to qualitatively new features in self assembly of triblock gradient copolymers [14]: a semi dilute solution of such copolymers undergoes reversible, pH triggered sol to gel transition.…”

“…The obtained amphiphilic ionic gradi ent copolymers are capable of self assembly in aque ous solutions and at air water interfaces giving rise to stimuli responsive nanostructures and mesophases [14,15,39,40]. While diblock gradient copolymers demonstrate reversible pH and salinity controlled micellization [15], the presence of the second terminal gradient block leads to qualitatively new features in self assembly of triblock gradient copolymers [14]: a semi dilute solution of such copolymers undergoes reversible, pH triggered sol to gel transition. On the basis of these findings we anticipate that novel amphiphilic block gradient copolymers can be used as building blocks for novel functional smart materials with a wide range of potential applications.…”

Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers

“…One way to render these systems dynamic is to incorporate hydrophilic units within the hydrophobic blocks [28][29][30][31][32][33][34][35]. We have studied the effect of incorporating acrylic acid (AA) within the poly(n-butyl acrylate) B block of the triblock P(nBA-stat-AA)-b-PAA-b-P(nBA-stat-AA) [36][37][38][39].…”