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The rheology of molecular brushes remains challenging to control due to the multiple length scales and relaxation processes involved and the lack of direct observation of molecular conformation during flow. We use molecular dynamics simulations to determine the shear thinning of unentangled bottlebrush polymers with varying architecture, from linear chains to combs, to densely grafted bottlebrushes, to star-like and star polymers. We find shear thinning exponents in line with theoretical and experimental results and characterize the shape and orientation of bottlebrushes in steady-state flow. Many shape parameters derived from the gyration tensor show molecular alignment with the flow for all systems. Yet, the orientation of individual bonds is what most strongly correlates with the architecture-dependent shear-thinning exponents. In densely grafted bottlebrushes, the packing of side chains prevents alignment with the flow, causing a reduction in shear thinning. The molecular insight from our simulations is useful to tune the architecture of bottlebrushes to control their rheology.

The dispersion of graphene sheets within a polymer matrix is critical for the performance of graphene-reinforced polymer nanocomposites, particularly in achieving optimal percolation and electrical conductivity. However, accurately characterizing and controlling the dispersion of 2D graphene in polymer melts remains a significant challenge due to the complex and varied configurations that graphene sheets can adopt. Herein, we employ coarse-grained molecular dynamics simulations to investigate how the grafting density (g) and grafted chain length (n) of poly(methyl methacrylate), p(MMA), affect graphene dispersion, where graphene is classified into three distinct morphologies, i.e., “aggregation”, “intercalated”, and “unbound”. We find that increasing gand nenhances graphene dispersion, evidenced by a higher dispersity parameter (fd), stronger interfacial interactions, greater Gaussian surface area of graphene clusters, and lower aggregation energy (EAggregation). Our results also indicate that a higher fdis linked to a higher Young’s modulus in the nanocomposite, reaching a maximum of 4.18 GPa. However, the electrical conductivity of nanocomposites initially rises with increasing gand nbut declines beyond g> 5% and n> 10 due to reduced conductive pathways caused by graphene overdispersion, as revealed by the conductive edge analysis. Additionally, the free polymer fraction and chain length significantly influence toughness, and grafting p(MMA) chains on graphene slows down the dynamics of the surrounding polymer due to the intrinsic stiffness of graphene, an effect more pronounced at higher fd(well dispersed). These findings present an effective approach for tuning and precisely characterizing graphene dispersity, clarifying its influence on material properties and forming the interfacial design of advanced nanocomposites reinforced with functional two-dimensional nanofillers.

This work presents a detailed kinetic study on the Diels-alder reaction between furan and maleimide moieties based on variable temperature infrared spectroscopy. BMI–689 was mixed with a multifunctional furan monomer. The furan functionality of the furan-bearing monomers was varied to observe any differences in reaction conversion and the resulting kinetic description of these networks. The endo and exo stereo isomers being formed during the Diels-Alder reaction are considered in this description and have been observed through ex-situ 1H NMR spectroscopy. The kinetic parameters were further used to predict the required percolation time for each system at any temperature. The activation energy of the forward Diels-Alder reactions were between 46 and 60 kJ mol−1and backward activation energies between 87 and 119 kJ mol−1. Only minor differences were found in the final Diels-Alder conversion developments by comparing systems with a varying monomer functionality, implying that the percolation process does not influence the kinetics for these systems. Finally, we shed light on the percolation process and occurrence of looping throughout the reaction of the used bismaleimide and various furan-bearing constituents via coarse-grained molecular dynamics simulations. The results of this study form a basis for understanding the temperature dependent self-healing behavior of such materials, and for defining temperature programs in 3D printing and extrusion applications, where rapid solidification while ensuring reprocessability are the main challenges.

While most calibration methods focus on inferring a set of model parameters that are unknown but assumed to be constant, many models have parameters that have a functional relation with the controllable input variables. Formulating a low-dimensional approximation of these calibration functions allows modelers to use low-fidelity models to explore phenomena at lengths and time scales unattainable with their high-fidelity sources. While functional calibration methods are available for low-dimensional problems (e.g., one to three unknown calibration functions), exploring high-dimensional spaces of unknown calibration functions (e.g., more than ten) is still a challenging task due to its computational cost and the risk for identifiability issues. To address this challenge, we introduce a semi-parametric calibration method that uses an approximate Bayesian computation scheme to quantify the uncertainty in the unknown calibration functions and uses this insight to identify what functions can be replaced with low-dimensional approximations. Through a test problem and a coarse-grained model of an epoxy resin, we demonstrate that the introduced method enables the identification of a low-dimensional set of calibration functions with a limited compromise in calibration accuracy. The novelty of the presented method is the ability to synthesize domain knowledge from various sources (i.e., physical experiments, simulation models, and expert insight) to enable high-dimensional functional calibration without the need for prior knowledge on the class of unknown calibration functions.

Understanding and predicting the mechanical and conformational properties of conjugated polymer (CP) thin films are a central focus in flexible electronic device research. Employing molecular dynamics simulations with an architecture-transferable chemistry-specific coarse-grained (CG) model of poly(3-alkylthiophene)s (P3ATs), developed by using an energy renormalization approach, we investigate the mechanical and conformational behavior of P3AT thin films during deformation. The density profiles and measures of local mobility identify a softer interfacial layer for all films, the thickness of which does not depend on Mwor side-chain length. Remarkably, Young’s modulus measured via nanoindentation is more sensitive to Mwthan for tensile tests, which we attribute to distinct deformation mechanisms. High-Mwthin films show increased toughness, whereas longer side-chain lengths of P3AT resulted in lower Young’s modulus. Fractures in low-Mwthin films occur through chain pullout due to insufficient chain entanglement and crazing in the plastic region. Importantly, stretching promoted both chain alignment and longer conjugation lengths of P3AT, potentially enhancing its electronic properties. For instance, at room temperature, stretching P3HT thin films to 150% increases the conjugated length of P3HT thin films from 2.7 nm to 4.7 nm, aligning with previous experimental findings and all-atom simulation results. Furthermore, high-Mwthin films display elevated friction forces due to the chain accumulation on the indenter, with negligible variations in the friction coefficient across all thin film systems. These findings offer valuable insights that enhance our understanding and guide the rational design of CP thin films in flexible electronics.

Hybrid hydrogels are hydrogels that exhibit heterogeneity in the network architecture by means of chemical composition and/or microstructure. The different types of interactions, together with structural heterogeneity, which can be created on different length scales, determine the mechanical properties of the final material to a large extent. In this work, the microstructure–mechanical property relationships for a hybrid hydrogel that contains both electrostatic and covalent interactions are investigated. The hybrid hydrogel is composed of a microphase-separated polyelectrolyte complex network (PEC) made of poly(4-styrenesulfonate) and poly(diallyldimethylammonium chloride) within a soft and elastic polyacrylamide hydrogel network. The system exhibits a granular structure, which is attributed to the liquid–liquid phase separation into complex coacervate droplets induced by the polymerization and the subsequent crowding effect of the polyacrylamide chains. The coacervate droplets are further hardened into PEC granules upon desalting the hydrogel. The structure formation is confirmed by a combination of electron microscopic imaging and molecular dynamics simulations. The interpenetration of both networks is shown to enhance the toughness of the resulting hydrogels due to the dissipative behavior of the PEC through the rupture of electrostatic interactions. Upon cyclic loading–unloading, the hydrogels show recovery of up to 80% of their original dissipative behavior in less than 300 s of rest with limited plasticity. The granular architecture and the tough and self-recoverable properties of the designed hybrid networks make them good candidates for applications, such as shape-memory materials, actuators, biological tissue mimics, and elastic substrates for soft sensors.

Improving the toughness of brittle epoxy while keeping its high strength-to-weight ratio is challenging, as these two properties work against each other. Fracture processes are difficult to ascertain with experiments, as they occur at nanoscopic lengths and time scales and require higher efficiency than what can be attained with atomistic simulations. To overcome this challenge, we utilize a recently developed chemistry specific coarse-grained model to examine two different hardeners, diamine 4,4′-methylenebis(cyclohexylamine) (PACM) and propylene oxide diamine (Jeffamine), to cure bisphenol A diglycidyl ether (DGEBA) at varying stoichiometries and understand how hardener composition influences the elasticity, yield strength, and fracture toughness of epoxy resins. The results indicate that PACM mainly contributes to the modulus and that long Jeffamine chains increase fracture toughness by making the epoxy ductile, whereas short Jeffamine chains significantly improve the yield strength. Longer Jeffamines also lead to larger voids and the formation of fibrils that carry a significant amount of stress and contribute to toughness. Interestingly, the Ashby plots reveal that epoxies with intermediate-length Jeffamine chains (D800 and D2000) outperform other systems, as the toughness enhancement from flexible Jeffamine chains and the stiffness due to PACM help to overcome the strength–toughness trade-off. Our modeling framework and findings establish a path toward resin design from predictive multiscale models with no empirical input and reveal new insights into the molecular failure mechanisms of epoxy resins.

Understanding the interactions between polymer-grafted nanoparticles is imperative to predict the macroscale mechanical properties of the nanocomposites they form. Molecular dynamics simulations capture the interfacial effects of grafting on structure and mobility, but directly linking these features to macroscale constitutive relations for nanocomposites remains challenging. As a step toward addressing this challenge, we develop a computational framework to predict the effective pairwise interparticle interactions between polymer-grafted nanoparticles with different design parameters, that is, polymer chain length, grafting density, and polymer chemistry. Using coarse-grained molecular dynamics simulations, we evaluate the potential of mean force between two nanoparticles by varying their radial distance under uniaxial deformations, from which an effective interaction can be derived. We find that the repulsive part of the interaction can be expressed as an exponential repulsion term, whereas the attractive part is best captured using a sigmoidal form. The empirical constants of these equations depend linearly on the chain length and quadratically on the grafting density. To ensure that the finite rate of deformation does not affect our conclusions, we also take into account the strain rate dependence of the effective interaction, using a Cowper–Symonds model to extrapolate the zero-rate limit of our results. With the development of this interatomic potential between the nanoparticles, we propose a mesoscopic model for nanoparticle assemblies that circumvents the need to explicitly simulate polymer chains, significantly improving the computational efficiency by extending the spatiotemporal scales by 6–7 orders of magnitude.

Polymeric films with greater impact and ballistic resistance are highly desired for numerous applications, but molecular configurations that best address this need remain subject to debate. We study the resistance to ballistic impact of thin polymer films using coarse-grained molecular dynamics simulations, investigating melts of linear polymer chains and star polymers with varying number (2≤f≤16) and degree of polymerization (10≤M≤50) of the arms. We show that increasing the number of arms for the length of the arms Mboth result in greater specific penetration energy within the parameter ranges studied. Greater interpenetration of chains in stars with larger fallows energy to be dissipated predominantly through rearrangement of the stars internally, rather than chain sliding. During film deformation, stars with large fshow higher energy absorption rates soon after contact with the projectile, whereas stars with larger Mhave a delayed response where dissipation arises primarily from chain sliding, which results in significant back face deformation. Our results suggest that stars may be advantageous for tuning energy dissipation mechanisms of ultra-thin films. These findings set the stage for a topology-based strategy for the design of impact-resistant polymer films.