Effect of the concentration on sol–gel transition of telechelic polyelectrolytes

Simulation Study of the Conformational Properties of Diblock Polyelectrolytes in Salt Solutions

Coarse-grained molecular dynamics simulations are performed to understand the behavior of diblock polyelectrolytes in solutions of divalent salt by studying the conformations of chains over a wide range of salt concentrations. The polymer molecules are modeled as bead spring chains with different charged fractions and the counterions and salt ions are incorporated explicitly. Upon addition of a divalent salt, the salt cations replace the monovalent counterions, and the condensation of divalent salt cations onto the polyelectrolyte increases, and the chains favor to collapse. The condensation of ions changes with the salt concentration and depends on the charged fraction. Also, the degree of collapse at a given salt concentration changes with the increasing valency of the counterion due to the bridging effect. As a quantitative measure of the distribution of counterions around the polyelectrolyte chain, we study the radial distribution function between monomers on different polyelectrolytes and the counterions inside the counterion worm surrounding a polymer chain at different concentrations of the divalent salt. Our simulation results show a strong dependence of salt concentration on the conformational properties of diblock copolymers and indicate that it can tune the self-assembly behaviors of such charged polyelectrolyte block copolymers.

Self-assembly of polyelectrolyte diblock copolymers at monovalent and multivalent counterions

In this work, the self-assembly behaviors of diblock copolymers consisting of one hydrophobic block and one ionizable polyelectrolyte (PE) block in the presence of monovalent and multivalent counterions are systematically discussed through molecular dynamics simulation. Copolymers are molded as bead-spring chains and the ions are explicitly considered. First, the self-assembled structures of symmetrical block copolymers at different charge fractions are analyzed in detail. Spherical hydrophobic cores are favored by all of the micelles. The effect of counterion valence is much more noticeable at high values of charge fraction. When the PE blocks are fully charged, the presence of multivalent counterions preferably provokes the formation of macroscopic structures. A precipitant spherical micelle is generated in the presence of divalent counterions. Special shapes of coronas are created in the presence of trivalent ions, and a remarkable one dimensional macroscopic cylindrical aggregation of micelles forms; the whole assembly is not typical core-shell micelles, but rather a cylinder with alternating spherical micelles arranged perpendicular to the cylinder axis. The self-assemblies with different lengths of fully charged PE blocks are also discussed. Surprisingly, in the presence of divalent counterions, two dimensional in-plane macroscopic aggregation of micelles is realized when the proportion of PE blocks is larger than 1/2; the self-assembled spherical micelles locate approximately in the same plane to form an inter-linked network. One dimensional aggregation of micelles in the presence of trivalent counterions is maintained with an increased proportion of the PE block. Owing to the dominant intra- and inter-condensation of divalent and trivalent counterions, respectively, two and one dimensional macroscopic aggregation of the micelles is achieved. Our findings indicate that varying the counterion valence is a powerful mechanism to tune the properties of self-assemblies, and the bridging effect introduced by multivalent counterions is the key parameter for the aggregation of the micelles.

Self-organization of hydrophobic-capped triblock copolymers with a polyelectrolyte midblock: a coarse-grained molecular dynamics simulation study

We present the results of a Langevin dynamics simulation study of micellar organization and hydrogel formation in the solutions of coarse-grained ABA copolymer chains. Polymer chains are modeled as bead-spring chains of Lennard-Jones particles by explicit treatment of ionic species in implicit solvent. The studied copolymer is composed of a polyelectrolyte midblock flanked by two hydrophobic endblocks. We explore the self-assembly of copolymer solutions at a fixed polymer concentration and temperature upon systematic variation of the midblock charge fraction, valency of neutralizing counterions, and the stiffness and length of hydrophobic endblocks. Minimization of the surface energy, conformational entropy of the midblock chains, electrostatic repulsion of midblock charges, and the translational entropy of counterions are found to play central roles in controlling the self-organization features of copolymer solutions. Flower-like micelles with A-blocks forming the core of spherical aggregates and B-blocks constituting the micelle corona are established for the neutral midblocks. Increasing the charge content of B chains lowers the fraction of loop conformations and yields a spanning hydrogel network with midblocks bridging the hydrophobic clusters. Counterion valence is shown to exert a strong effect on the micelle size and network structure. The increase in the rigidity of terminal A-blocks increases the fraction of bridging chains and results in the formation of a hydrogel network with bundle-like hydrophobic domains. Longer endblocks are shown to increase the hydrophobic cluster size and enhance the bridged midblock fraction. The qualitative agreement between the experimental and theoretical studies is also discussed. The comprehensive molecular picture provides a framework for the future studies of stimuli-responsive copolymer systems.

Recent trends in pH/thermo-responsive self-assembling hydrogels: from polyions to peptide-based polymeric gelators

In this article, we highlight some recent developments in "smart" physical hydrogels achieved by self-assembling of block type macromolecules. More precisely we focus on two interesting types of gelators namely conventional ionic (or ionogenic) block copolymers and peptide-based polymers having as a common feature their responsiveness to pH and/or temperature which are the main triggers used for potential biomedical applications. Taking advantage of the immense skills of conventional block copolymer hydrogelators, namely macromolecular design, self-assembling mechanism, gel rheological properties, responsiveness to various triggers and innovative applications, the development of novel self-assembling gelators, integrating the new knowledge emerging from the peptide-based systems, opens new horizons towards bio-inspired technologies.