Andrey V. Dobrynin

Extraction of a hydrophilic compound from water into liquid CO2 using dendritic surfactants

Dendrimers are well defined, highly branched polymers that adopt a roughly spherical, globular shape in solution. Their cores are relatively loosely packed and can trap guest molecules, and by appropriate functionalization of the branch tips the macromolecules can act as unimolecular micelle-like entities. Here we show that dendrimers with a fluorinated shell are soluble in liquid carbon dioxide and can transport CO2-insoluble molecules into this solvent within their cores. Specifically, we demonstrate the extraction of a polar ionic dye, methyl orange, from water into CO2 using these fluorinated dendrimers. This observation suggests possible uses of such macromolecules for the remediation of contaminated water, the extraction of pharmaceutical products from fermentation vessels, the selective encapsulation of drugs for targeted delivery and the transport of reagents for chemical reactions (such as polymerization) in liquid and supercritical CO2 solvents.

Conductive Thin Films of Pristine Graphene by Solvent Interface Trapping

Graphites insolubility in conventional solvents is a major obstacle to its utilization. This challenge is typically addressed by chemical modification such as oxidation, followed by reduction. However, pristine graphene possesses superior properties as oxidation and reduction lead to degradation of the graphene. Here we demonstrate the use of an interfacial trapping technique to assemble laterally macroscopic films of pristine graphene that are up to 95% transparent. This is accomplished by modest sonication of natural flake graphite in a water/heptane mixture to form continuous films at the interface between two immiscible liquids. Furthermore, the graphene sheets readily climb hydrophilic solid substrates, forming a homogeneous thin film one to four layers thick. These films are composed of a network of overlapping graphene sheets and shown to have long-range structure with conductivities on the order of 400 S/cm.

Electrophoresis of polyampholytes

We consider the motion and the deformation of Gaussian polyampholytes in free flow electrophoresis, i.e., in an applied external electric field. The electrophoretic mobility and the deformation of the chains are calculated in the linear regime, as functions of the charge distribution along the backbone and of the salt concentration. The results in salt-free solutions differ from those in solutions with a high concentration of salt even at the level of scaling laws. It is shown that in solutions with a high salt concentration, the electrophoretic mobility of a given polyampholyte strongly depends not only on its total charge but also on the details of the charge distribution along the chain. The very direction of motion can also depend on it. Indeed, even “neutral” polyampholytes, i.e., chains with equal number of positive and negative charges can move in an applied electric field. To demonstrate further these effects, we systematically compare the behavior of the linear and circular chains.

Solvent-free, supersoft and superelastic bottlebrush melts and networks.

Polymer gels are the only viable class of synthetic materials with a Youngs modulus below 100 kPa conforming to biological applications, yet those gel properties require a solvent fraction. The presence of a solvent can lead to phase separation, evaporation and leakage on deformation, diminishing gel elasticity and eliciting inflammatory responses in any surrounding tissues. Here, we report solvent-free, supersoft and superelastic polymer melts and networks prepared from bottlebrush macromolecules. The brush-like architecture expands the diameter of the polymer chains, diluting their entanglements without markedly increasing stiffness. This adjustable interplay between chain diameter and stiffness makes it possible to tailor the networks elastic modulus and extensibility without the complications associated with a swollen gel. The bottlebrush melts and elastomers exhibit an unprecedented combination of low modulus (∼100 Pa), high strain at break (∼1,000%), and extraordinary elasticity, properties that are on par with those of designer gels.

Stabilization of graphene sheets by a structured benzene/hexafluorobenzene mixed solvent.

Applications requiring pristine graphene derived from graphite demand a solution stabilization method that utilizes an easily removable media. Using a combination of molecular dynamics simulations and experimental techniques, we investigate the solublization/suspension of pristine graphene sheets by an equimolar mixture of benzene and hexafluorobenzene (C(6)H(6)/C(6)F(6)) that is known to form an ordered structure solidifying at 23.7 °C. Our simulations show that the graphene surface templates the self-assembly of the mixture into periodic layers extending up to 30 Å from both sides of the graphene sheet. The solvent structuring is driven by quadrupolar interactions and consists of stacks of alternating C(6)H(6)/C(6)F(6) molecules rising from the surface of the graphene. These stacks result in density oscillations with a period of about 3.4 Å. The high affinity of the 1:1 C(6)H(6)/C(6)F(6) mixture with graphene is consistent with observed hysteresis in Wilhelmy plate measurements using highly ordered pyrolytic graphite (HOPG). AFM, SEM, and TEM techniques verify the state of the suspended material after sonication. As an example of the utility of this mixture, graphene suspensions are freeze-dried at room temperature to produce a sponge-like morphology that reflects the structure of the graphene sheets in solution.

Adhesion of nanoparticles.

We have developed a new model of nanoparticle adhesion which explicitly takes into account the change in the nanoparticle surface energy. Using combination of the molecular dynamics simulations and theoretical calculations we have showed that the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter beta proportional, variant gamma(p)(GR(p))(-2/3)W(-1/3), where G is the particle shear modulus, R(p) is the initial particle radius, gamma(p) is the polymer interfacial energy, and W is the particle work of adhesion. In the case of small values of the parameter beta < 0.1, which is usually the case for strongly cross-linked large nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. However, we observed a significant deviation from the classical JKR theory in the case of the weakly cross-linked nanoparticles that experience large shape deformations upon particle adhesion. In this case the interfacial energy of the nanoparticle plays an important role controlling nanoparticle deformation. Our model of the nanoparticle adhesion is in a very good agreement with the simulation results and provides a new universal scaling relationship for nanoparticle deformation as a function of the system parameters.

Morphologies of Planar Polyelectrolyte Brushes in a Poor Solvent: Molecular Dynamics Simulations and Scaling Analysis

Using molecular dynamics simulations and scaling analysis, we study the effect of the solvent quality for the polymer backbone, the strength of the electrostatic interactions, the chain degree of polymerization, and the brush grafting density on conformations of the planar polyelectrolyte brushes in salt-free solutions. Polyelectrolyte brush forms: (1) vertically oriented cylindrical aggregates (bundles of chains), (2) maze-like aggregate structures, or (3) thin polymeric layer covering a substrate. These different brush morphologies appear as a result of the fine interplay between electrostatic and short-range monomer-monomer interactions. The brush thickness shows nonmonotonic dependence on the value of the Bjerrum length. It first increases with the increasing value of the Bjerrum length, and then it begins to decrease. This behavior is a result of counterion condensation within a brush volume.

Detailed Molecular Dynamics Simulations of a Model NaPSS in Water

Hydrophobic polyelectrolytes are known to form necklace-like structures of dense beads connected by strings of monomers. This structure appears as a result of optimization of the electrostatic and short-range interactions. To elucidate the effect of counterion condensation and solvent on polyelectrolyte conformations, we performed two sets of molecular dynamics simulations of model poly(styrene)-co-styrene sodium sulfonate (NaPSS) chains with the degree of polymerization N = 16-64 and fraction of charged monomers f = 0.25-1 in aqueous solutions: (1) water molecules were considered explicitly using the TIP3P-PME model and (2) water molecules were modeled as a dielectric continuum with the dielectric constant 77.73. Our simulations showed that with increasing fraction of sulfonated groups f a polystyrene sulfonate chain adopts an elongated conformation. There is a transition between collapsed and elongated states which is manifested in the change of the scaling dependence of the chain size on the degree of polymerization. The effect of the water-ion interactions on counterion condensation was analyzed by comparing the radial distribution functions between the sulfonated groups and counterions for chains with different f values. In the case of the collapsed NaPSS chains, it was found that ionized groups are located at the globular surface.

Nucleation-Controlled Polymerization of Nanoparticles into Supramolecular Structures

Controlled assembly of inorganic nanoparticles (NPs) into structurally defined supramolecular polymers will create nanomaterials with new collective properties. However, supramolecular polymerization of isotropic NPs remains a challenge because of the lack of anisotropic interactions in these monomers to undergo directional associations for the cooperative growth of supramolecular chains. Herein we report self-assembly behavior of poly(l-glutamic acid)-grafted gold NPs in solution and describe how combined attractive and repulsive interactions influence the shape and size of the resulting supramolecular assemblies. The study shows that the chain growth of supramolecular polymers can be achieved from the NP monomers and the process occurs in two distinct stages, with a slow nucleation step followed by a faster chain propagation step. The resulting supramolecular structures depend on both the grafting density of the poly(l-glutamic acid) on the NPs and the size of the NPs.

Perfect mixing of immiscible macromolecules at fluid interfaces

The difficulty of mixing chemically incompatible substances--in particular macromolecules and colloidal particles--is a canonical problem limiting advances in fields ranging from health care to materials engineering. Although the self-assembly of chemically different moieties has been demonstrated in coordination complexes, supramolecular structures, and colloidal lattices among other systems, the mechanisms of mixing largely rely on specific interfacing of chemically, physically or geometrically complementary objects. Here, by taking advantage of the steric repulsion between brush-like polymers tethered to surface-active species, we obtained long-range arrays of perfectly mixed macromolecules with a variety of polymer architectures and a wide range of chemistries without the need of encoding specific complementarity. The net repulsion arises from the significant increase in the conformational entropy of the brush-like polymers with increasing distance between adjacent macromolecules at fluid interfaces. This entropic-templating assembly strategy enables long-range patterning of thin films on sub-100 nm length scales.