Gunja Pandav

Fluctuation effects on the order-disorder transition in polydisperse copolymer melts

Using single chain in mean field simulation approach, we examine the influence of fluctuation effects on the order-disorder transition (ODT) of polydisperse copolymer systems. We consider two model systems: (i) molecular weight polydisperse systems represented by AB diblock copolymer melts having monodisperse A blocks and polydisperse B blocks; and (ii) compositionally polydisperse symmetric diblock copolymer systems. In each case, we present results for the fluctuation-induced shift in the ODT from the corresponding mean-field predictions. In both models, an increase in polydispersity enhances the influence of fluctuations. Moreover, for compositionally polydisperse systems, we observe that the effects of fluctuations show similar trends in systems containing quenched and annealed representation of sequences.

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.

Complexation between weakly basic dendrimers and linear polyelectrolytes: effects of grafts, chain stiffness, and pOH

We develop and implement a new hybrid methodology combining self-consistent field theory (SCFT) and Monte Carlo simulations to study the complexation between negatively charged semiflexible linear polyelectrolyte (LPE) molecules and a positively charged dendrimer containing grafts of neutral polymers. We examine the influence of LPE stiffness, length of the dendrimer grafts, and solution pOH upon the characteristics of the resulting complexes. Our results indicate that increasing LPE stiffness reduces the dendrimer–LPE binding affinity and results in an overall higher net charge carried within the dendrimer molecule. When we varied the size of the grafts, the dendrimer–LPE binding strength was seen to decrease with increasing grafting chain length for the flexible LPE chains. In contrast, for stiff LPE chains, the binding strength was not seen to vary significantly with the grafting lengths. Overall, longer grafting lengths were seen to reduce the fraction of exposed LPE molecules, suggesting that grafted dendrimers may better shield nucleic acid material from serum nucleases. Lastly, we found that increasing the solution pOH was seen to enhance both the binding between the dendrimer and LPE molecules and the total positive charge carried by the complex.

Phase behavior of gradient copolymer solutions: a Monte Carlo simulation study

We use computer simulations to study the phase separation behavior of amphiphilic linear gradient copolymer solution under poor solvent conditions. Using the bond fluctuation model and parallel tempering algorithm, we explore the influence of the gradient strength (the largest difference in the instantaneous composition along the copolymer) upon the phase separation characteristics. Under poor solvent conditions, the chains collapse to form micelle-like aggregates. We find that the critical temperature for this transition exhibits a linear dependence on the gradient strength of the copolymers. A systematic quantification of the cluster characteristics formed during the phase separation also reveals a strong dependence of aggregation numbers and the bridging statistics upon the gradient strength of the copolymers. Analysis of our results reveals that the critical parameter determining the thermodynamic behavior of gradient copolymers is in fact the average length of the hydrophobic sequences in the gradient copolymers. We demonstrate that the latter provides a useful measure to quantitatively predict the critical transition temperature of the gradient copolymer solution. We also present a few results from the framework of an annealed representation of the sequences of the gradient copolymer to demonstrate the limitations arising from such a model representation.

Directed self assembly of block copolymers using chemical patterns with sidewall guiding lines, backfilled with random copolymer brushes

Recently, alignment of block copolymer domains has been achieved using a topographically patterned substrate with a sidewall preferential to one of the blocks. This strategy has been suggested as an option to overcome the patterning resolution challenges facing chemoepitaxy strategies, which utilize chemical stripes with a width of about half the period of block copolymer to orient the equilibrium morphologies. In this work, single chain in mean field simulation methodology was used to study the self assembly of symmetric block copolymers on topographically patterned substrates with sidewall interactions. Random copolymer brushes grafted to the background region (space between patterns) were modeled explicitly. The effects of changes in pattern width, film thicknesses and strength of sidewall interaction on the resulting morphologies were examined and the conditions which led to perpendicular morphologies required for lithographic applications were identified. A number of density multiplication schemes were studied in order to gauge the efficiency with which the sidewall pattern can guide the self assembly of block copolymers. The results indicate that such a patterning technique can potentially utilize pattern widths of the order of one-two times the period of block copolymer and still be able to guide ordering of the block copolymer domains up to 8X density multiplication.

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.