Structure and interaction of flexible dendrimers in concentrated solution

Dendrimer solutions: a Monte Carlo study

We study the conformational properties of dendrimers with flexible spacers in solutions over a wide range of concentrations from dilute solutions to melts. By combining large scale computer simulations using the bond fluctuation model with scaling arguments we identify the semi-dilute regime of dendrimers which is controlled by the concentration behavior of the linear spacers. Associated with this observation we find that the decrease in the size of flexible dendrimers is accompanied by increasing interpenetration between the molecules with increasing concentration of the solution. In the melt state we show that the size of individual dendrimers follows the scaling prediction for isolated dendrimers at the θ-point rather than that of collapsed dendrimers. The pair correlation functions between the centers of dendrimers indicate that for short spacers dendrimer solutions retain the morphological characteristics of simple liquids. For long spacers the functions reveal high penetration of neighboring dendrimers in the melt state. Our studies show that flexible dendrimers in solution can be understood with arguments similar to those of linear polymers. The role of generation is to influence the particular form of the crossover-function.

Dendrimer nanofluids in the concentrated regime: from polymer melts to soft spheres

Understanding dendrimer structures and their interactions in concentrated solutions is important to a wide range of applications, such as drug delivery and lubrication. However, controversy has persisted concerning whether, when confined to proximity, dendrimers would entangle as observed for polymer systems, or act as deformable spheres. Furthermore, how such behavior may be related to their size-dependent molecular architecture remains unclear. Using small-angle X-ray scattering (SAXS), the intermolecular interactions and structures in aqueous nanofluids containing three generations of carboxyl-terminated poly(amidoamine) (PAMAM) dendrimers (G0.5, Rg = 9.3 Å; G3.5, Rg = 22.6 Å; G5.5, Rg = 39.9 Å, where Rg is the radius of gyration) over a mass fraction range 0.005 ≤ x ≤ 0.316 have been studied. In the highly concentrated regime (x ≥ 0.157), we observe that the solution properties depend on the dendrimer generation. Our results suggest that the smaller G0.5 dendrimers form a highly entangled polymer melt, while the larger dendrimers, G3.5 and G5.5, form densely packed and ordered structures, in which the individual dendrimers exhibit some degree of mutual overlap or deformation. Our results demonstrate the tunability of interdendrimer interactions via their molecular architecture, which in turn may be harnessed to control and tailor the physical properties of dendrimer nanofluids.

Patchy worm-like micelles: solution structure studied by small-angle neutron scattering

Triblock terpolymers exhibit a rich self-organization behavior including the formation of fascinating cylindrical core-shell structures with a phase separated corona. After crystallization-induced self-assembly of polystyrene-block-polyethylene-block-poly(methyl methacrylate) triblock terpolymers (abbreviated as SEMs = Styrene-Ethylene-Methacrylates) from solution, worm-like core-shell micelles with a patchy corona of polystyrene and poly(methyl methacrylate) were observed by transmission electron microscopy. However, the solution structure is still a matter of debate. Here, we present a method to distinguish in situ between a Janus-type (two faced) and a patchy (multiple compartments) configuration of the corona. To discriminate between both models the scattering intensity must be determined mainly by one corona compartment. Contrast variation in small-angle neutron scattering enables us to focus on one compartment of the worm-like micelles. The results validate the existence of the patchy structure also in solution.

Electrophoresis and dielectric dispersion of spherical polyelectrolyte brushes

Spherical polyelectrolyte brushes (SPBs) consist of a rigid core on which polyelectrolyte chains are grafted in such a way that in certain conditions (low ionic strength and high charge of the chains) the polymer chains extend radially toward the liquid medium. Because of the hairy-like structure of the polymer brushes, the typical soft-particle approach used for explaining the behavior of polyelectrolyte-coated particles must be modified, using the assumptions that the density of charged segments in the polymer chains decreases with the squared distance to the rigid core surface and that the same happens to the friction between the brushes and the surrounding fluid. Interest in clarifying the electrokinetics of these systems is not just academic. It has recently been found experimentally (Jiménez et al., Soft Matter 2011, 7, 3758-3762) that the response of concentrated suspensions of spherical polyelectrolyte brushes in the presence of alternating electric fields shows a number of unexpected features. Both dielectric and dynamic electrophoretic mobility spectra (respectively, dependences of the electric permittivity and the AC electrophoretic mobility on the frequency of the applied field) showed very special aspects, with giant values of the mobility and an unusually strong dielectric relaxation in the kHz region. In the present paper we give a full account of the electrodynamics of such systems, based on a cell model for describing the hydrodynamic and electrical interactions between the particles. It is found that the low-frequency dynamic mobility of SPBs is much higher than that of rigid particles of comparable size and charge, making any interpretation based on zeta potential estimations of very limited applicability. The very characteristic feature of SPBs in concentrated suspensions, namely, the enhanced alpha relaxation, can be explained by considering an adequate description of the field-induced perturbations in the counterion and co-ion concentrations, well developed both outside and inside the soft layer in the case of brush-coated particles. It can be also pointed out that the dynamic electrophoretic mobility of SPBs increases with the volume fraction of particles, as a consequence of the large thickness of the brush. Predictions are also shown for the effects of friction coefficient and charge of the polyelectrolyte layer. The results compare well with experimental spectra of the dynamic mobility and electric permittivity of moderately concentrated suspensions of SPBs consisting of a 50 nm polystyrene core with grafted poly(styrene sulfonate) chains some 140 nm in length.

The Structure of Fluids with Impurities

The influence of dilute impurities on the structure of a fluid solvent is investigated theoretically. General arguments, which do not rely on particular models, are used to derive an extension of the Ornstein–Zernike form for the solvent structure factor at small scattering vectors. It is shown that dilute impurities can influence the solvent structure only if they are composed of ions with significantly different sizes. Non-ionic impurities or ions of similar size are shown to not alter the solvent structure qualitatively. This picture is compatible with available experimental data. The derived form of the solvent structure factor is expected to be useful to infer information on the impurity-solvent interactions form measured scattering intensities.

Structural properties of dendrimer-colloid mixtures

We consider binary mixtures of colloidal particles and amphiphilic dendrimers of the second generation by means of Monte Carlo simulations. By using the effective interactions between monomer-resolved dendrimers and colloids, we compare the results of simulations of mixtures stemming from a full monomer-resolved description with the effective two-component description at different densities, composition ratios, colloid diameters and interaction strengths. Additionally, we map the two-component system onto an effective one-component model for the colloids in the presence of the dendrimers. Simulations based on the resulting depletion potentials allow us to extend the comparison to yet another level of coarse graining and to examine under which conditions this two-step approach is valid. In addition, a preliminary outlook into the phase behavior of this system is given.

Computational insights into the interactions between DNA and siRNA with "rigid" and "flexible" triazine dendrimers

In this study, simulation challenges intuitive models of "flexible" and "rigid" generation two triazine dendrimers as it pertains to solution conformation and conformation on binding DNA or siRNA sequences. These results derive from structural and energetic analyses of the binding events. Simulations of the rigid structure reinforce the role of the constrained piperazine linker in positioning the peripheral groups at significant distance from each other and the core of the dendrimer. In contrast, the flexible dendrimer, characterized by triethyleneglycol-like linkers, collapses in solution. On binding DNA and siRNA, these conformations are largely retained. The rigid dendrimer undergoes reorganization of peripheral groups to generate a large number of contacts to the nucleic acid. In contrast, the flexible dendrimer, originally conceived to create multivalent interactions with nucleic acids, generates only a few contacts and collapses further. This paper provides unique insight in the role played by molecular flexibility in the binding phenomenon.