We present a new coarse-grained (CG) model that captures directional interactions between graft and matrix polymer chains in polymer nanocomposites (PNCs) comprising polymer grafted spherical nanoparticles in a matrix polymer. In this CG model we incorporate acceptor and donor CG beads along with graft and matrix monomer CG beads and optimize the bonded and nonbonded interactions to mimic directional and specific H-bonding between the acceptor and donor sites on graft and matrix chains, respectively. Using this CG model and molecular dynamics simulations we show that H-bonding interactions between graft and matrix polymer chains increase the grafted layer wetting by matrix chains compared to that at the purely entropic limit. One can achieve equivalent grafted layer wetting in PNCs with directional acceptor–donor interactions and PNCs with isotropic graft–matrix interactions, but the directional acceptor–donor interaction strength needs to be much stronger than the isotropic graft–matrix monomer attraction strength. Strikingly, despite equivalent grafted layer wetting and graft chain conformations, on average, each graft chain interacts with fewer matrix chains and has a lower free volume in PNCs with H-bonding interaction as compared to PNCs with isotropic graft-matrix attraction. These trends are seen both at high (brush-like) and low grafting densities, and in PNCs with equal graft and matrix chain lengths as well as PNCs with matrix chain length three times the graft chain length.

In this paper, we study symmetric polymer blends comprised of two polymer chemistries, one containing hydrogenbonding (H-bonding) acceptor groups and another containing Hbonding donor groups to predict the blend morphology (i.e., twophase, ordered/lamellar, disordered, disordered microphase-separated, and bicontinuous microemulsion or BμE) for varying compositions (i.e., fraction of monomers containing hydrogenbonding groups along the polymer chain) and placements of hydrogen-bonding groups along the polymer chains. We use molecular dynamics (MD) simulations with a previously developed coarse-grained (CG) model that captures relevant macromolecular length and time scales and both the attractive directional interactions between H-bonding acceptor and donor groups and isotropic polymer−polymer interactions. We first validate our CG MD simulation approach by reproducing the published theoretical phase diagram for end-associating polymer chains at varying Hbonding strengths vs polymer segregation strengths. We also show that with increasing H-bonding strength, end-associating blends with short-chain lengths transition from two-phase to BμE or from disordered blends to BμE depending on the polymer segregation strength and finally to disordered microphase morphologies. End-associating blends with longer-chain lengths transition from twophase to ordered lamellar phase at high polymer segregation strengths and from two-phase to disordered microphase-separated state at low polymer segregation strengths. Next, we study blends with the center placement of a single H-bonding group in each polymer chain as well as random and regular placements of multiple H-bonding groups per polymer chain. Regardless of the number and placement of H-bonding groups, with increasing H-bonding strength, the fraction of associated H-bonding groups increases with the system transitioning from blends of unassociated polymers to a mixture of associated copolymers and unassociated polymers and finally to a melt of fully associated supramolecular copolymers. At intermediate strengths of H-bonding, we observe BμE morphologies in all systems with end, center, random, and regular placements of H-bonding group(s). At high strengths of Hbonding, the blend morphology is disordered microphase-separated with domain sizes being smallest for the center placement, followed by the end, regular, and then random placements. We find that this variation in the placement of H-bonding groups leads to a greater change in domain sizes than with variation in the strength of the isotropic polymer−polymer interaction at constant Hbonding attraction. These trends in disordered microphase domain sizes with varying compositions and placements of H-bonding groups are linked to the supramolecular copolymer architecture formed upon the association of the two homopolymer chemistries. The polymers with the center placement of H-bonding groups form miktoarm star copolymers upon association, which show smaller domain sizes compared to diblock copolymers formed by polymers with end p...

In this work, we examine binary and ternary nanocomposites of poly(methyl methacrylate) grafted silica nanoparticles (PMMA-NP), in poly(styrene-ran-acrylonitrile) (SAN), and poly(methyl methacrylate) matrices as a platform to directly probe governing parameters guiding phase behavior and nanoparticle assembly in composite materials. Through the addition of PMMA matrix chains similar in molecular weight to the grafted PMMA chains and significantly smaller than the SAN matrix chains, we observe increased nanoparticle miscibility in off-critical compositions due to interfacial segregation of PMMA matrix chains. A simple interfacial model provides a general guideline for predicting the extent of compatibilization. Further insights on compatibilization behavior are provided by polymer particle pair correlation functions and structure factors obtained using Polymer Reference Interaction Site Model theory calculations as well as polymer concentration profiles from molecular dynamics simulations. This study serves as a guideline to facilitate PNC processing and design of materials for a broad range of technological applications.

We use molecular dynamics (MD) simulations and Polymer Reference Interaction Site Model (PRISM) theory with a coarse-grained (CG) model to study polymer nanocomposites (PNCs) comprised of polymer grafted nanoparticles in a polymer matrix. Specifically, we describe the impact of increasing graft–matrix attraction on the PNC structure quantified in terms of the extent of interpenetration of matrix and graft chains (i.e., grafted layer wetting) and dispersion/aggregation of the grafted particles in the matrix via intermolecular pair correlation functions and structure factors. Past work on PNCs with attractive graft–matrix interactions had already established that grafted layer wetting–dewetting and dispersion-aggregation are two distinct phase transitions with the former being a continuous transition and the latter being a first-order transition with increasing graft–matrix attraction. In this paper, we go beyond that previous work and show that the dispersion and aggregation of polymer grafted particles in PNCs is driven by the hardness and size of the grafted layer which is tuned by the strength of the attractive graft–matrix interactions. As the strength of the graft–matrix attraction increases, graft polymer chains adopt extended conformations to form energetically favorable graft–matrix contacts leading to enhanced grafted layer wetting by matrix chains. This extended grafted layer and increased grafted layer wetting with increasing graft–matrix attraction leads to larger and harder grafted particles compared to analogous PNCs with purely entropic (athermal) graft–matrix interactions. The increased size and hardness of the grafted particles causes the PNC morphology to change from an entropically driven aggregated/dispersed morphology at athermal graft–matrix interaction to a dispersed morphology due to energetically favorable weak graft–matrix attraction, and ultimately, to a correlated fluid of hard grafted particles at strong graft–matrix attraction. We see the above trends in PNCs with isotropic as well as directional attraction between graft and matrix chains, grafted particles at high (densely grafted) and low grafting densities, and with equal graft and matrix chain lengths as well as matrix chain lengths greater than the graft chain lengths.

In this paper, we identify the modifications needed in a recently developed generic coarse-grained (CG) model that captured directional interactions in polymers to specifically represent two exemplary hydrogen bonding polymer chemistries—poly(4-vinylphenol) and poly(2-vinylpyridine). We use atomistically observed monomer-level structures (e.g., bond, angle and torsion distribution) and chain structures (e.g., end-to-end distance distribution and persistence length) of poly(4-vinylphenol) and poly(2-vinylpyridine) in an explicitly represented good solvent (tetrahydrofuran) to identify the appropriate modifications in the generic CG model in implicit solvent. For both chemistries, the modified CG model is developed based on atomistic simulations of a single 24-mer chain. This modified CG model is then used to simulate longer (36-mer) and shorter (18-mer and 12-mer) chain lengths and compared against the corresponding atomistic simulation results. We find that with one to two simple modifications (e.g., incorporating intra-chain attraction, torsional constraint) to the generic CG model, we are able to reproduce atomistically observed bond, angle and torsion distributions, persistence length, and end-to-end distance distribution for chain lengths ranging from 12 to 36 monomers. We also show that this modified CG model, meant to reproduce atomistic structure, does not reproduce atomistically observed chain relaxation and hydrogen bond dynamics, as expected. Simulations with the modified CG model have significantly faster chain relaxation than atomistic simulations and slower decorrelation of formed hydrogen bonds than in atomistic simulations, with no apparent dependence on chain length.