Simulation of a Symmetric Binary Mixture of Charged Dendrimers Under Varying Electrostatic Interactions: Static and Dynamic Aspects

Abstract

A symmetric binary mixture of terminally charged trifunctional-core/difunctional-branched dendrimers of the third and the fourth generation is examined in explicit solvent solution by means of molecular dynamics simulations. The static and dynamic response of the components is monitored under a varying strength of electrostatic interactions. The response of the dendrimer constituents shares common features as the intensity of Coulombic interactions increases, but characteristic differences are noted in the individual structural arrangement and their dynamic response. An effective-charge modulation develops across the dendritic structure and at the immediate vicinity of each molecule. The asymmetry in size and in the overall charge between the two components affects the packing properties of the molecules resulting in different morphologies compared to the respective single component systems. In addition, individual translational and rotational motion of the dendrimers exhibits distinct behavior in different Coulombic regimes. The motional contrast between the two components differentiates their relative dynamic response as the electrostatic interactions grow stronger. The increase of Coulombic interactions drives the components of the mixture to a dynamically arrested state, but the eventual vitrification takes place at different levels of electrostatic interactions depending on dendrimer generation. The findings of the present study elucidate several aspects of the behavior of these charged soft-colloidal materials and offer new insight toward the control of their long-range order and of their dynamic properties.

Modeling the formation of ordered nano-assemblies comprised by dendrimers and linear polyelectrolytes: The role of Coulombic interactions

“…14,84,85 Under the examined conditions of dendrimer size and charging pattern, our systems appear to form films of thickness comparable to two dendrimer diameters (i.e., in actual units of approximately 2 nm). This finding, combined with those of previous studies 10, 26,58 implies that under appropriate changes in factors such as the dendrimer size, their charging density, and the length of the linear polyelectrolytes, the characteristics of the resulted morphology in the polymer-rich region might be controlled down to the nanoscale.…”

“…In particular, the obstruction of the overall dendrimer reorientation in the present systems is manifested already within the weak electrostatic regime (i.e., at l B /σ well below 10) as a result of the thermodynamically preferable complexation of the dendrimer molecules with LPEs, even at relatively low electrostatic levels (see Figure 4 and Figures S5 and S6 in the supplementary material 55 ). This observation is to be contrasted to the behavior found in systems comprised solely by charged dendrimers, 56,58 where the charge reversal responsible for the effective like-charge attraction between dendrimer molecules takes place only in the regime of strong electrostatic interactions.…”

“…For the monomers of the polymeric species as well as for the counterions and the solvent beads, we have adopted a bead-spring representation (with flexible bonds), as in our previous works. [56][57][58] The dendrimer molecules were modeled with a trifunctional core and a bifunctional branching pattern (see Figure S2 in the supplementary material 55 ), which was grown up to the 3rd generation. According to this branching pattern each dendrimer was comprised by 91 monomers, 24 of which (those located at the outer generational shell) were assigned a charge of +1, while each monomer (of unit length) of a linear chain was oppositely charged.…”

“…Previous studies in colloidal systems 68,90,91 and in dendrimer polyelectrolyte solutions 58,92 revealed that the dynamic response of such systems to changes in thermodynamic parameters of their microenvironment, affect decisively the characteristics of the final (equilibrium or nonequilibrium) morphology. The degrees of freedom available to the colloidal particles, the nature of the effective interactions between them and the existence of spatial or ther-FIG.…”

“…[96][97][98][99] Systems comprised solely by dendritic components similar to those of the present study were found to exhibit a characteristic slow-down of their molecular-scale dynamics upon increase of the strength of electrostatic interactions, resembling that observed when approaching a colloidal glass transition. 58,75 The gradual freezing-in of translational and rotational degrees of freedom, however, was realized only at the strong electrostatic regime, i.e., at l B /σ ≥ 10. To compare this behavior to the analogous dynamic response of the dendritic components of our systems at the molecular-level, we have monitored the orientational relaxation function…”