Computational modeling of fluctuations and phase behavior in polymeric systems

textThis research focuses on the computational modeling of fluctuations, interactions, phase behavior and structural characteristics of multicomponent polymeric systems. The role of fluctuations is studied in the context of block copolymer melts and polymer blends stabilized using copolymers exhibiting different sequence architectures. The relationship between interparticle interactions and structural characteristics of the aggregates formed in particle-polymer solutions is examined for charged nanoparticle-polymer and charged dendrimer-polyelectrolyte system. A hybrid Monte Carlo and self consistent field theory approach employed in single chain in mean field simulations (SCMF) is utilized in order to achieve the equilibrium morphologies/aggregates in such polymeric systems. We examine the effect of composition fluctuations on the phase behavior of polydisperse block copolymer melts quantified in terms of fluctuation-induced shift in the order-disorder transition temperature from the corresponding mean-field predictions. Fluctuation effects can also play an important role in stabilizing bicontinuous microemulsions phases. To study this effect, we examine polymer blend systems compatibilized by a copolymer having different sequence architectures such as monodisperse and polydisperse block copolymer, and gradient copolymer. We systematically assess the efficiency of such system in forming bicontinuous microemulsions phases. We also study the effect of sequence architecture on the phase behavior of gradient copolymer solutions. We extend above framework to account for electrostatic effects arising from charged polymers and dendrimers. Using such a framework, we characterize the clusters formed due to electrostatic binding between oppositely charged dendrimers and polyelectrolytes. Our results indicate that, the binding is maximum when the charge on dendrimers is balanced by the charge on the polyelectrolytes. We extend the above study to probe the phase behavior of charged nanoparticles suspended in polymer solutions. We examine the influence of polymer concentration, particle volume fraction, and particle charge on the structure and size of clusters. We also examine the influence of multibody effects on the resulting structure of nanoparticle clusters. The charged nanoparticle-polymer solution is seen to exhibit significant multibody effects and the effective two-body interparticle potentials are seen to be a function of nanoparticle density.Chemical Engineerin

Computer Simulations of Dendrimer–Polyelectrolyte Complexes

We carry out a systematic analysis of static properties of the clusters formed by complexation between charged dendrimers and linear polyelectrolyte (LPE) chains in a dilute solution under good solvent conditions. We use single chain in mean-field simulations and analyze the structure of the clusters through radial distribution functions of the dendrimer, cluster size, and charge distributions. The effects of LPE length, charge ratio between LPE and dendrimer, the influence of salt concentration, and the dendrimer generation number are examined. Systems with short LPEs showed a reduced propensity for aggregation with dendrimers, leading to formation of smaller clusters. In contrast, larger dendrimers and longer LPEs lead to larger clusters with significant bridging. Increasing salt concentration was seen to reduce aggregation between dendrimers as a result of screening of electrostatic interactions. Generally, maximum complexation was observed in systems with an equal amount of net dendrimer and LPE charges, whereas either excess LPE or dendrimer concentrations resulted in reduced clustering between dendrimers

Efficacy of Different Block Copolymers in Facilitating Microemulsion Phases in Polymer Blend Systems

Polymeric microemulsions are formed in a narrow range of phase diagram when a blend of immiscible homopolymers is compatibilized by copolymers. In this study, we consider the ternary blend system of A and B homopolymers mixed with block copolymers containing A and B segments and probe the efficacy of different copolymer configurations in promoting the formation of microemulsion phases. Specifically, we consider (a) monodisperse diblock copolymers (D), (b) diblock copolymers with bidisperse molecular weights (MW) (BDL), (c) block copolymers having MW polydispersity in one of the blocks (PD), (d) diblock copolymers having monodisperse MW but bidispersity in average composition (BDC), and (e) gradient copolymers exhibiting a linear variation in the average composition (G). Using single chain in mean field simulations effected in two dimensions, we probe the onset of formation and the width of the bicontinuous microemulsion channel in the ternary phase diagram of homopolymer blended with compatibilizer. We observed that diblock copolymers having bidisperse composition are most efficient (i.e., microemulsion phases occupy the largest area of phase diagram) in forming microemulsions. On the other hand, monodisperse diblock copolymers and diblock copolymers having bidisperse MW distribution form microemulsions with the least amount of compatibilizers. We rationalize our results by explicitly quantifying the interfacial activity and the influence of fluctuation effects in the respective copolymer systems

Interactions and Aggregation of Charged Nanoparticles in Uncharged Polymer Solutions

We employ an extension of the single chain in mean field simulation method to study mixtures of charged particles and uncharged polymers. We examine the effect of particle charge, polymer concentration, and particle volume fraction on the resulting particle aggregates. The structures of aggregates were characterized using particle–particle radial distribution functions and cluster size distributions. We observe that the level of aggregation between particles increases with increasing particle volume fraction and polymer concentration and decreasing particle charge. At intermediate regimes of particle volume fraction and polymer concentrations, we observe the formation of equilibrium clusters with a preferred size. We also examined the influence of manybody effects on the structure of a charged particle–polymer system. Our results indicate that the effective two-body approximation overpredicts the aggregation between particles even at dilute particle concentrations. Such effects are thought to be a consequence of the interplay between the respective manybody effects on the depletion and electrostatic interactions

Multibody Interactions, Phase Behavior, and Clustering in Nanoparticle–Polyelectrolyte Mixtures

We present the results of a computational study of the interactions, phase-behavior and aggregation characteristics of charged nanoparticles (CNPs) suspended in solution of oppositely charged polyelectrolytes (PEs). We used an extension of the mean-field polymer self-consistent field theory (SCFT) model presented in our earlier work (Macromolecules, 2014, 47, 6095−6112) to explicitly characterize the multibody interactions in such systems. For dilute–moderate particle volume fractions, the magnitudes of three and higher multibody interactions were seen to be weak relative to the contributions from pair interactions. On the basis of such results, we embeded the pair-interaction potentials within a thermodynamic perturbation theory approach to identify the phase behavior of such systems. The results of such a framework suggested that the gas and FCC crystal phases were thermodynamically stable, whereas the fluidlike phase was metastable in such systems. To complement the parameters studied using SCFT, we used a recently developed multibody simulation approach to study the aggregation and cluster morphologies in CNP–PE mixtures. For low particle charges, such systems mainly exhibited clusters arising from direct contact aggregation between CNPs. However, for higher particle and PE charges and low PE concentrations, large regions of PE-bridged clusters were seen to form. We present a morphological phase diagram summarizing such results