Tyler B. Martin

Wetting-Dewetting and Dispersion-Aggregation Transitions Are Distinct for Polymer Grafted Nanoparticles in Chemically Dissimilar Polymer Matrix.

Simulations and experiments are conducted on mixtures containing polymer grafted nanoparticles in a chemically distinct polymer matrix, where the graft and matrix polymers exhibit attractive enthalpic interactions at low temperatures that become progressively repulsive as temperature is increased. Both coarse-grained molecular dynamics simulations, and X-ray scattering and neutron scattering experiments with deuterated polystyrene (dPS) grafted silica and poly(vinyl methyl ether) PVME matrix show that the sharp phase transition from (mixed) dispersed to (demixed) aggregated morphologies due to the increasingly repulsive effective interactions between the blend components is distinct from the continuous wetting-dewetting transition. Strikingly, this is unlike the extensively studied chemically identical graft-matrix composites, where the two transitions have been considered to be synonymous, and is also unlike the free (ungrafted) blends of the same graft and matrix homopolymers, where the wetting-dewetting is a sharp transition coinciding with the macrophase separation.

Polydisperse homopolymer grafts stabilize dispersions of nanoparticles in a chemically identical homopolymer matrix: an integrated theory and simulation study

This paper presents a computational study of the effect of polydispersity in grafted polymers on the effective interactions between polymer grafted nanoparticles in a polymer matrix, when graft and matrix polymers are chemically identical. The potential of mean force (PMF) between grafted particles, calculated using a self-consistent PRISM theory-Monte Carlo simulation approach, shows that graft polydispersity weakens the attractive well at intermediate inter-particle distances, eliminating the well completely at high polydispersity index (PDI). The elimination of the mid-range attractive well is due to the longer grafts in the polydisperse distribution that introduce steric repulsion at large distances, and the increased wetting of the grafted layer by matrix chains arising from reduced monomer crowding within the polydisperse grafted layer. Trends in how the PMF changes as a function of grafting density, ratio of matrix to graft length, and packing fraction of polymer matrix seen for monodisperse grafts are preserved for polydisperse grafts. Comparison of a log-normal distribution to a bidisperse distribution of chain lengths (with equal number of short and long chains) with the same PDI and average length, shows that the polydisperse distribution can better stabilize dispersions than the bidisperse distributions because of the longer chains in the polydisperse distribution. Additionally, in a bidisperse distribution, with all chains shorter than the matrix chain length, there is a reduction in the mid-range attraction, thus confirming the role of reduced monomer crowding in the bidisperse grafted layer in increasing the grafted layer wetting by the matrix chains, and, as a result, improving miscibility of grafted particles and matrix.

Assembly of copolymer functionalized nanoparticles: a Monte Carlo simulation study

Functionalizing nanoparticles with copolymer ligands is an attractive method to tailor the assembly of the nanoparticles. We use Monte Carlo simulation to demonstrate how the monomer sequence in the grafted copolymers is a tuning parameter to control assembly of nanoparticles, and the shapes, sizes and structures of the assembled nanoclusters. We have studied spherical nanoparticles grafted with AB copolymers with alternating or diblock sequences, and a range of monomer chemistries by varying strengths of like-monomer (A–A and/or B–B) attractive interactions in the presence of either relatively strong or negligible unlike-monomer (A–B) repulsive interaction. In the presence of negligible A–B repulsions the alternating sequence produces nanoclusters that are relatively isotropic regardless of whether A–A or B–B monomers are attractive, while the diblock sequence produces nanoclusters that are smaller and more compact when the block closer to the surface (A–A) is attractive and larger loosely held together clusters when the outer block (B–B) is attractive. In the presence of strong A–B repulsions the alternating sequence leads to either particle dispersion or smaller clusters than those at negligible A–B repulsions; for the diblock sequence strong and negligible A–B repulsions exhibit similar cluster characteristics. Additionally, diblock copolymer grafted particles tend to assemble into anisotropic shapes despite the isotropic grafting of the copolymer chains on the particle surface. Particle size and graft length balance enthalpic gain and entropic losses coming from inter-grafted particle contacts and/or inter- and intra-grafted chain contacts within the same grafted particle, and in turn dictates the shape and size of the cluster. For constant graft length and when A–A attractions are stronger than B–B attractions, diblock copolymer grafted particles form long “caterpillar-like” structures with large particle diameters, and short nanowires with small particle diameters. In the dilute concentration regime a small increase in the particle concentration does not change the cluster characteristics confirming that the structure within a cluster is primarily governed by the copolymer functionalization imparting a “valency” to the nanoparticle “atom”. This work illustrates how copolymer functionalization and tuning the grafted copolymer sequence could be an exciting new route experimentalists can take to tailor self-assembly of nanoparticles into target nanostructures.

Effect of blockiness in grafted monomer sequences on assembly of copolymer grafted nanoparticles: a Monte Carlo simulation study

We use Monte Carlo simulations to study AB copolymer-grafted nanoparticles to elucidate the effect of blockiness (length of contiguous blocks of like-monomers) in the grafted monomer sequence on the shape, size, and structure of assembled nanoparticle clusters for a range of monomer–monomer and monomer–particle interactions. The graft sequence dictates the ease or difficulty of the grafted chains to form attractive like-monomer (A–A or B–B) contacts while minimizing repulsive unlike-monomer (A–B) contacts within an assembled cluster or a dispersed state. When A–B repulsion is negligible, with increasing blockiness at constant graft length the cluster size and average coordination number decrease in the presence of A–A or B–B attractions, and are approximately constant in the presence of A–A and B–B attractions. When A–B repulsion is strong, the cluster size and average coordination number increase with increasing blockiness for small and large particles. For small particles, with strong B–B attraction and A–B repulsion, increasing blockiness leads to increasing anisotropy in cluster shape, while strong A–A attraction produces isotropic clusters regardless of the graft sequence. The effect of graft sequence on cluster shape is reduced for large particles as compared to small particles, at constant graft length. Lastly, the extent to which monomer–particle attractive interactions change the above trends is highly dependent on the relative strength of monomer–particle to monomer–monomer interactions, in addition to the ratio of particle size to graft length, and the grafting density.

Tuning the wetting–dewetting and dispersion–aggregation transitions in polymer nanocomposites using composition of graft and matrix polymers

Recent simulation and experimental work on polymer nanocomposites composed of polymer grafted particles and free matrix polymers, where the graft and matrix homopolymers are chemically dissimilar and exhibit lower critical solution temperature behavior with temperature, has shown that wetting to dewetting is a gradual and distinct transition from the sharp particle dispersion–aggregation transition. In this study, using coarse-grained molecular simulations, we demonstrate that the extent of wetting of the grafted polymer layer and the particle dispersion–aggregation transition are tuned using the composition of graft and matrix polymers. Specifically, we study composites where the graft and matrix chains are random copolymers composed of attractive and athermal monomers. We maintain a dense grafting density on the spherical particles of diameter five times the monomer diameter and study matrix lengths five times that of the graft chain length or equal graft and matrix chain lengths. We vary the fraction of attractive monomers in the graft and matrix chains, graft–matrix chain composition ratio and the graft–matrix interaction strength, as characterized by the Flory–Huggins interaction parameter between graft and matrix attractive monomers: When is negative, decreasing and/or decreases the extent of grafted layer wetting by matrix chains because the enthalpic driving force for wetting is reduced. As the increases and becomes positive, the extent of wetting decreases gradually till it reaches the wetting of analogous athermal composites. That value of where the extent of wetting is the same as that of an analogous athermal polymer nanocomposite marks the onset of dispersion–aggregation transition. For symmetric graft and matrix chain compositions , the magnitude of and tunes the overall extent of wetting of the grafted particles in the dispersed state but not the dispersion–aggregation transition. Varying the asymmetry of the graft and matrix chain composition (i.e. f G/f M) tunes both the extent of wetting of the grafted layer and the dispersion–aggregation transition.

Design and Implementation of pyPRISM: A Polymer Liquid-State Theory Framework

In this work, we describe the code structure, implementation, and usage of a Python-based, open-source framework, pyPRISM, for conducting polymer liquid-state theory calculations. Polymer Reference Interaction Site Model (PRISM) theory describes the equilibrium spatial-correlations, thermodynamics, and structure of liquid-like polymer systems and macromolecular materials. pyPRISM provides data structures, functions, and classes that streamline predictive PRISM calculations and can be extended for other tasks such as the coarse-graining of atomistic simulation force-fields or the modeling of experimental scattering data. The goal of providing this framework is to reduce the barrier to correctly and appropriately using PRISM theory and to provide a platform for rapid calculations of the structure and thermodynamics of polymeric fluids and polymer nanocomposites.