Anisotropic Three-Particle Interactions between Spherical Polymer-Grafted Nanoparticles in a Polymer Matrix

Spherical nanoparticles (NPs) uniformly grafted with polymer chains have recently been shown to assemble into anisotropic phases like strings and sheets. Here we investigated the underlying basis for anisotropic interactions between polymer-grafted NPs in a polymer matrix by computing via molecular dynamics simulations the potential of mean force (PMF), and its three-body contribution, for a test NP interacting with a NP-dimer along a set of reaction coordinates differing in their orientation with respect to the dimer axis. The polymer-mediated portions of the PMF and of the three-body contribution were both found to be highly repulsive and anisotropic with the degree of repulsion rising with increasing angular deviation from the dimer axis. The anisotropy was shown to arise from the expulsion of polymer grafts from in between the dimer NPs which leads to a gradient in the graft segmental density around the dimer from its contact point to its poles. This effect produces a concomitant gradient in steric repulsion between test and dimer NP grafts, a significant portion of which is however negated by an opposing gradient in depletion attraction between NPs due to the matrix. The anisotropy in the polymer-mediated PMF was observed to be particularly strong for NP–polymer systems with long grafts, high grafting densities, and short matrix chains. The overall PMFs allowed us to compute the free energies of formation of two- and three-particle clusters, yielding a phase diagram in graft length–grafting density parameter space analogous to that observed experimentally for the dispersed, stringlike, and sheetlike phases of NPs. The PMFs also revealed possible existence of a stable dimer phase that remains to be tested experimentally. Taken together, this study illustrates how the deformability of NP grafts can introduce novel anisotropic interactions between otherwise isotropic NPs with far-reaching consequences in NP assembly

Viscoelastic Properties of Polymer-Grafted Nanoparticle Composites from Molecular Dynamics Simulations

To provide insights into how polymer-grafted nanoparticles (NPs) enhance the viscoelastic properties of polymers, we have computed the frequency-dependent storage and loss modulus of coarse-grained models of polymer nanocomposites by means of molecular dynamics simulations. Nanocomposites containing NPs grafted with chains similar to those comprising the host polymer matrix exhibit considerably higher moduli than nanocomposites containing bare NPs across the entire frequency range investigated. This effect is shown to arise from the additional distortion of the shear field in the polymer matrix resulting from the grafted chains and from the slower relaxation time of the grafted chains compared to the matrix chains when the former are at least half as long as the latter. Increasing the attraction between the grafted and matrix chains results in further enhancement in the two moduli, but only at frequencies slower than those corresponding to the longest relaxation time of the chains. This effect is shown to arise from a dramatic slowdown in the relaxation dynamics of both the matrix and grafted chains. In addition, the nanocomposite moduli are found to increase with decreasing NP size and increasing NP loading, grafted chain length, and grafting density with varying frequency dependence. These parametric effects are also explained in terms of shear distortion effects and chain relaxation times. Based on these results, a phenomenological model is proposed to estimate the storage and loss modulus of such nanocomposites as a function of the Rouse relaxation times of the grafted and matrix chains and the volume fractions of the NPs, grafted chains, and matrix chains. The model captures the observed dependence of the moduli with the examined parameters of the grafted NPs and yields moduli predictions that agree quantitatively with those computed from the simulations at low frequencies

Viscoelastic Properties and Shock Response of Coarse-Grained Models of Multiblock versus Diblock Copolymers: Insights into Dissipative Properties of Polyurea

We compare and contrast the microstructure, viscoelastic properties, and shock response of coarse-grained models of multiblock copolymer and diblock copolymers using molecular dynamics simulations. This study is motivated by the excellent dissipative and shock-mitigating properties of polyurea, speculated to arise from its multiblock chain architecture. Our microstructural analyses reveal that the multiblock copolymer microphase-separates into small, interconnected, rod-shaped, hard domains surrounded by a soft matrix, whereas the diblock copolymer forms larger, unconnected, hard domains. Our viscoelastic analyses indicate that compared with the diblock copolymer, the multiblock copolymer is not only more elastic but also more dissipative, as signified by its larger storage and loss modulus at low to intermediate frequencies. Our shock simulations and slip analyses reveal that shock waves propagate slower in the multiblock copolymer in comparison with the diblock copolymer, most likely due to the more deformable hard domains in the former system. These results suggest that the multiblock architecture of polyurea might impart polyurea with smaller, more deformable, and interconnected hard domains that lead to improved energy dissipation and lower shock speeds

Predicting the Mechanical Properties of Organic Semiconductors Using Coarse-Grained Molecular Dynamics Simulations

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. Here, we describe the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly­(3-hexyl­thiophene) (P3HT) and its blend with C₆₀. In particular, we show that the resolution of the coarse-grained model has a strong effect on the predicted properties. We find that a one-site model, in which each 3-hexyl­thiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm^–3, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chains as well as the vitrification of P3HT by C₆₀ and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K). We also observe a decrease in the radius of gyration and the density of entanglements of the P3HT chains with the addition C₆₀ which may contribute to the experimentally noted brittleness of the composite material. Although extension of the model to poly­(3-alkyl­thiophenes) (P3ATs) containing side chains longer than hexyl groupsnonyl (N) and dodecyl (DD) groupscorrectly predicts the trend of decreasing modulus with increasing length of the side chain measured experimentally, obtaining absolute agreement for P3NT and P3DDT could not be accomplished by a straightforward extension of the three-site coarse-grained model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C₆₀ blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures

Biomimetic Material-Assisted Delivery of Human Embryonic Stem Cell Derivatives for Enhanced In Vivo Survival and Engraftment

The ability of human embryonic stem cells (hESCs) and their derivatives to differentiate and contribute to tissue repair has enormous potential to treat various debilitating diseases. However, improving the in vivo viability and function of the transplanted cells, a key determinant of translating cell-based therapies to the clinic, remains a daunting task. Here, we develop a hybrid biomaterial consisting of hyaluronic acid (HA) grafted with 6-aminocaproic acid moieties (HA-6ACA) to improve cell delivery and their subsequent in vivo function using skeletal muscle as a model system. Our findings show that the biomimetic material-assisted delivery of hESC-derived myogenic progenitor cells into cardiotoxin-injured skeletal muscles of NOD/SCID mice significantly promotes survival and engraftment of transplanted cells in a dose-dependent manner. The donor cells were found to contribute to the regeneration of damaged muscle fibers and to the satellite cell (muscle specific stem cells) compartment. Such biomimetic cell delivery vehicles that are cost-effective and easy-to-synthesize could play a key role in improving the outcomes of other stem cell-based therapies

Metallic Nanoislands on Graphene as Highly Sensitive Transducers of Mechanical, Biological, and Optical Signals

This article describes an effect based on the wetting transparency of graphene; the morphology of a metallic film (≤20 nm) when deposited on graphene by evaporation depends strongly on the identity of the substrate supporting the graphene. This control permits the formation of a range of geometries, such as tightly packed nanospheres, nanocrystals, and island-like formations with controllable gaps down to 3 nm. These graphene-supported structures can be transferred to any surface and function as ultrasensitive mechanical signal transducers with high sensitivity and range (at least 4 orders of magnitude of strain) for applications in structural health monitoring, electronic skin, measurement of the contractions of cardiomyocytes, and substrates for surface-enhanced Raman scattering (SERS, including on the tips of optical fibers). These composite films can thus be treated as a platform technology for multimodal sensing. Moreover, they are low profile, mechanically robust, semitransparent and have the potential for reproducible manufacturing over large areas