Nanovoid formation and mechanics: a comparison of poly(dicyclopentadiene) and epoxy networks from molecular dynamics simulations

Noncovalent Interactions in Mechanical Response of Thermoset Epoxy Resin

Free volume in polymers is known to influence the mechanical response of the polymers. Noncovalent interactions such as hydrogen bonds, Coulombic electrostatic interactions, and van der Waals interactions are present within these free volume regions. The manuscript presents a comprehensive identification, characterization, and evolution of noncovalent interactions as a thermoset epoxy resin (typically used as an interfacial adhesive material) is subjected to uniaxial tension, shear, and shock loading. Even though noncovalent interactions dominate uniaxial tension and shear response (up to strain levels of 50% wherein covalent bond dissociation is not observed), both covalent and noncovalent interactions define response for shock loading. Van der Waals interactions dominate the response as the samples are subjected to strain levels of 50% in tension and shear. In contrast, hydrogen bonds influence shock response.

Thermal Conductivity of Bottle-Brush Polymers

Using molecular dynamics (MD) simulations of a generic model, we investigated heat propagation in bottle-brush polymers (BBP). An architecture is referred to as a BBP when a linear (backbone) polymer is grafted with the side chains of different length Ns and grafting density ρg, which control the bending stiffness of a backbone. Investigating κ-behavior in BBP is of particular interest due to two competing mechanics: increased backbone stiffness, via Ns and ρg, increases the thermal transport coefficient κ, while the presence of side chains provides additional pathways for heat leakage. We show how a delicate competition between these two effects controls κ. These results reveal that going from a weakly grafting (ρg < 1) to a highly grafting (ρg ≥ 1) regime, κ changes non-monotonically that is independent of Ns. The effect of side chain mass on κ and heat flow in the BBP melts is also discussed.

A Self-Healing System for Polydicyclopentadiene Thermosets

Self-healing offers promise for addressing structural failures, increasing lifespan, and improving durability in polymeric materials. Implementing self-healing in thermoset polymers faces significant manufacturing challenges, especially due to the elevated temperature requirements of thermoset processing. To introduce self-healing into structural thermosets, the self-healing system must be thermally stable and compatible with the thermoset chemistry. This article demonstrates a self-healing microcapsule-based system stable to frontal polymerization (FP), a rapid and energy-efficient manufacturing process with a self-propagating exothermic reaction (≈200 °C). A thermally latent Grubbs-type complex bearing two N-heterocyclic carbene ligands addresses limitations in conventional G2-based self-healing approaches. Under FP's elevated temperatures, the catalyst remains dormant until activated by a Cu(I) co-reagent, ensuring efficient polymerization of the dicyclopentadiene (DCPD) upon damage to the polyDCPD matrix. The two-part microcapsule system consists of one capsule containing the thermally latent Grubbs-type catalyst dissolved in the solvent, and another capsule containing a Cu(I) coagent blended with liquid DCPD monomer. Using the same chemistry for both matrix fabrication and healing results in strong interfaces as demonstrated by lap-shear tests. In an optimized system, the self-healing system restores the mechanical properties of the tough polyDCPD thermoset. Self-healing efficiencies greater than 90% via tapered double cantilever beam tests are observed.

Understanding the Effect of Grain Boundaries on the Mechanical Properties of Epoxy/Graphene Composites

This work presents a molecular dynamics (MD) simulation study on the effect of grain boundaries (GBs) on the mechanical properties of epoxy/graphene composites. Ten types of GB models were constructed and comparisons were made for epoxy/graphene composites containing graphene with GBs. The results showed that the tensile and compressive behaviors, the glass transition temperature (Tg), and the configurations of epoxy/graphene composites were significantly affected by GBs. The tensile yield strength of epoxy/graphene composites could be either enhanced or weakened by GBs under a tensile load parallel to the graphene sheet. The underlying mechanisms may be attributed to multi-factor coupling, including the tensile strength of the reinforcements, the interfacial interaction energy, and the inflection degree of reinforcements. A balance exists among these effect factors, resulting in the diversity in the tensile yield strength of epoxy/graphene composites. The compressive yield strength for epoxy/graphene composites is higher than their counterpart in tension. The tensile/compressive yield strength for the same configuration presents diversity in different directions. Both an excellent interfacial interaction and the appropriate inflection degree of wrinkles for GB configurations restrict the translational and rotational movements of epoxy chains during volume expansion, which eventually improves the overall Tg. Understanding the reinforcing mechanism for graphene with GBs from the atomistic level provides new physical insights to material design for epoxy-based composites containing defective reinforcements.

Relative effects of polymer composition and sample preparation on glass dynamics

Modern design of common adhesives, composites and polymeric parts makes use of polymer glasses that are stiff enough to maintain their shape under a high stress while still having a ductile behavior after the yield point. Typically, material compositions are tuned with co-monomers, polymer blends, plasticizers, or other additives to arrive at a tradeoff between the elastic modulus and toughness. In contrast, strong changes to the mechanics of a glass are possible by changing only the molecular packing during vitrification or even deep in the glassy state. For example, physical aging or processing techniques such as physical vapor deposition increase the density, embrittle the material, and increase elastic modulus. Here, we use molecular simulations, validated by positron annihilation lifetime spectroscopy (PALS) and quasi-elastic neutron scattering, to understand the free volume distribution and the resulting dynamics of glassy co-polymers where the composition is systemically varied between polar 5-norbornene-2-methanol (NBOH) and non-polar ethylidene norbornene (ENB) monomers. In these polymer glasses, we analyze the structural features of the unoccupied volume using clustering analysis, where the clustering is parameterized to reproduce experimental measurements of the same features from PALS. Further, we analyze the dynamics, quantified by the Debye-Waller factor, and compare the results with softer, lower density states. Our findings indicate that faster structural relaxations and potentially improved ductility are possible through changes to the geometric structure and fraction of the free volume, and that the resulting changes to the glass dynamics are comparable to large changes in the monomer composition.

Preparation and properties of an interpenetrating network polymer based on polydicyclopentadiene and phenolic resin

In this paper, phenolic resin (PF) and dicyclopentadiene (DCPD) monomers were mixed in different proportions. Under the action of a new generation of ruthenium carbene catalysts, DCPD was polymerised in situ. Polydicyclopentadiene (PDCPD)/PF interpenetrating polymer networks (IPNs) were prepared using the casting curing moulding process. The structure and properties of the prepared IPNs were characterised using Fourier infrared spectroscopy (FT-IR), apparent crosslinking degree, thermal weight loss, mechanical properties, impact resistance and scanning electron microscopy (SEM). The study results showed that the conversion of DCPD did not change with the addition of PF. But when its content exceeds 10%, the crosslinking degree of PDCPD decreases. When the PF content is 10%, the maximum bending strength of PDCPD/PF IPNs is (104.5 ± 1.3) MPa, maximum tensile strength is (74.5 ± 1.4) MPa, and maximum-notched impact strength is (4.2 ± 0.2) kJ/m2. Compared with PDCPD, the bending strength is increased by 22.7%, tensile strength is increased by 32.6%, and notched impact strength is increased by 31.3%, but the thermal stability has no major impact at this time. PF has good dispersibility and compatibility in DCPD. Due to the interpenetrating network structure of PF and PDCPD, the interpenetrating interlocking of the PF molecular chain and PDCPD crosslinked network restricts its movement. Its performance reached the optimum, and the PDCPD/PF IPNs with excellent performance was successfully prepared.

Simulation Study of the Effects of Interfacial Bonds on Adhesion and Fracture Behavior of Epoxy Resin Layers

The adhesion and fracture behavior of tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM)/4,4'-diaminodiphenyl sulfone (44DDS)-bisphenol A diglycidyl ether (DGEBA)/44DDS layer interfaces were investigated by molecular dynamics (MD) simulation, mainly focusing on the role of covalent and noncovalent interactions. To accurately investigate the bond dissociation processes, the force field parameters of several bond potentials of the epoxy resin polymers were optimized by density functional theory calculations. In the MD simulations under a tensile load, small voids gradually developed without covalent bond dissociation in the plateau region. In the final large strain region, the stress rapidly increased with bond breaking, leading to failure. When the chemical bonds across the interface between the two layers were removed, the stress-strain curve in the initial elastic region was almost the same as that with interfacial bonds. This showed that the nonbonded interactions governed adhesion strength in the initial elastic region. In contrast, the bonded interactions at interfaces played important roles in the hardening regions because the bonded interactions made the major contribution to the fracture energies. We also investigated the effect of the etherification reaction in cross-linking. It was found that the etherification reaction mainly contributed to the behavior in the late region with large strain. These simulation results revealed that the nonbonded interactions, especially, van der Waals interactions, are important factors for adhesion of the different polymer layers in the small strain region up to the yield point.

Effect of curing reaction types on the structures and properties of acetylene-containing thermosets: towards optimization of curing procedure

High-temperature thermosets are usually prepared from resins containing alkynyl groups, and their properties depend much upon the curing process containing various types of curing reactions. However, how the curing process affects the properties remains unclear due to the complicated curing reactions. We used molecular dynamics simulations to investigate the effect of curing reaction types, including cyclotrimerization, Diels-Alder reaction, and radical reaction, on the structures and properties of imide oligomers terminated with alkynyl groups. The results show that the cycloadditions such as cyclotrimerization and Diels-Alder reaction endow the thermosets with rigid structures and high moduli. Compared with the cycloadditions, the radical reaction enables the formation of flexible cured structures, which can enhance the toughness of thermosets. The differences in thermal and mechanical properties caused by different curing types were elucidated by the relaxation processes of fragments in these cured systems and were explained by the variation of torsion energy in different curing forms. As this work aims to optimize the curing procedure to obtain high-performance resins with desired properties, different curing procedures were finally designed according to the theoretical studies, and the obtained cured polymers show significant differences in the properties from different curing ways. The results can guide the preparation of desired thermosetting resins by tuning the curing procedure.

Universal G' ∼ L-3 Law for the Low-Frequency Shear Modulus of Confined Liquids

Liquids confined to sub-millimeter scales have remained poorly understood. One of the most striking effects is the large elasticity revealed using good wetting conditions, which grows upon further decreasing the confinement length, L. These systems display a low-frequency shear modulus in the order of 1-10³ Pa, contrary to our everyday experience of liquids as bodies with a zero low-frequency shear modulus. While early experimental evidence of this effect was met with skepticism and abandoned, further experimental results and, most recently, a new atomistic theoretical framework have confirmed that liquids indeed possess a finite low-frequency shear modulus G', which scales with the inverse cubic power of confinement length L. We show that this law is universal and valid for a wide range of materials (liquid water, glycerol, ionic liquids, non-entangled polymer liquids, isotropic liquids crystals). Open questions and potential applications in microfluidics mechanochemistry, energy, and other fields are highlighted.

A highly efficient metal-free protocol for the synthesis of linear polydicyclopentadiene

We have developed a highly efficient synthesis of linear polydicyclopentadiene (pDCPD) via photoredox mediated metal-free ring-opening metathesis polymerization (MF-ROMP) and investigated the T_g–M_n dependence of linear pDCPD.

Influence of Interfacial Bonding on the Mechanical and Impact Properties Ring-Opening Metathesis Polymer (ROMP) Silica Composites

Polymers formed by ring-opening metathesis polymerization (ROMP) such as poly(dicyclopentadiene) (pDCPD) exhibit a technologically desirable combination of high toughness, high glass transition temperature, and outstanding low-temperature performance. However, because of their nonpolar molecular structure, they tend to suffer from relatively low elastic moduli and poor adhesion to common fillers, fibers, and substrates, limiting their utility as adhesives and composite binders without specialized bonding agents. Here, we investigate the mechanical properties of a pDCPD-based copolymer filled with well-defined spherical microparticles having four distinct surface chemistries capable of strong, moderate, or weak bonding to the matrix with surfaces ranging from polar to nonpolar. Measurements in uniaxial tension, quasi-static fracture, and high-velocity impact are complemented by digital image correlation (DIC), scanning electron microscopy (SEM) fractography, and X-ray computed tomography (X-μCT) of subcritically loaded crack tips, yielding insight into the complex roles played by interfacial bonding in strength, stiffness, and toughening mechanisms of an already tough polymer. Analysis using quantitative fracture and impact mechanism models provided valuable guidelines for designing heterogeneous systems that combine structural and tough performance.

Seeded laser-induced cavitation for studying high-strain-rate irreversible deformation of soft materials

Characterizing the high-strain-rate and high-strain mechanics of soft materials is critical to understanding the complex behavior of polymers and various dynamic injury mechanisms, including traumatic brain injury. However, their dynamic mechanical deformation under extreme conditions is technically difficult to quantify and often includes irreversible damage. To address such challenges, we investigate an experimental method, which allows quantification of the extreme mechanical properties of soft materials using ultrafast stroboscopic imaging of highly reproducible laser-induced cavitation events. As a reference material, we characterize variably cross-linked polydimethylsiloxane specimens using this method. The consistency of the laser-induced cavitation is achieved through the introduction of laser absorbing seed microspheres. Based on a simplified viscoelastic model, representative high-strain-rate shear moduli and viscosities of the soft specimens are quantified across different degrees of crosslinking. The quantified rheological parameters align well with the time-temperature superposition prediction of dynamic mechanical analysis. The presented method offers significant advantages with regard to quantifying high-strain rate, irreversible mechanical properties of soft materials and tissues, compared to other methods that rely upon the cyclic dynamics of cavitation. These advances are anticipated to aid in the understanding of how damage and injury develop in soft materials and tissues.

On the Nature of Epoxy Resin Post-Curing

Post-curing is intended to improve strength, elevate glass transition, and reduce residual stress and outgassing in thermosets. Also, experiments indicate post-curing temperatures lead to ether crosslinks and backbone dehydration. These results informed molecular dynamics methods to represent them and compare the resulting thermomechanical effects. Diglycidyl ether of bisphenol A (DGEBA)-diamino diphenyl sulfone (DDS) systems were examined. Independent variables were resin length, stoichiometry, and reaction type (i.e., amine addition, etherification, and dehydration). Etherification affected excess epoxide systems most. These were strengthened and became strain hardening. Systems which were both etherified and dehydrated were most consistent with results of post-curing experiments. Dehydration stiffened and strengthened systems with the longer resin molecules due to their intermediate hydroxyl groups for crosslinking. Changes in the concavity of functions fit to the specific volume versus temperature were used to detect thermal transitions. Etherification generally increased transition temperatures. Dehydration resulted in more transitions.

Identifying Nonaffine Softening Modes in Glassy Polymer Networks: A Pathway to Chemical Design

Using molecular simulations and theory, we develop an explicit mapping of the contribution of molecular relaxation modes in glassy thermosets to the shear modulus, where the relaxations were tuned by altering the polarity of side groups. Specifically, motions at the domain, segmental, monomer, and atomic levels are taken from molecular dynamics snapshots and directly linked with the viscoelasticity through a framework based in the lattice dynamics of amorphous solids. This unique approach provides direct insight into the roles of chemical groups in the stress response, including the time scale and spatial extent of relaxations during mechanics. Two thermoset networks with differing concentrations of polar side groups were examined, dicyclopentadiene (DCPD) and 5-norbornene-2-methanol (NBOH). A machine learning method is found to be effective for quantifying large-scale correlated motions, while more local chemical relaxations are readily identified by direct inspection. The approach is broadly applicable and enables rapid predictions of the frequency-dependent modulus for any glass.

Mechanics and nanovoid nucleation dynamics: effects of polar functionality in glassy polymer networks

We use molecular simulations and experiments to rationalize the properties of a class of networks based on dicyclopentadiene (DCPD), a polymer with excellent fracture toughness and a high glass transition temperature (Tg), copolymerized with 5-norbornene-2-methanol (NBOH). DCPD is a highly non-polar hydrocarbon, while NBOH contains a hydroxy group, introducing polar functionality and hydrogen bonds (H-bonds). NBOH thus represents a possible route to improve the chemical compatibility of DCPD-based networks with less-hydrophobic materials. We systematically vary the NBOH content (polar chemistry) in DCPD networks, while keeping other network parameters nearly constant, including the molecular weight between cross-links, chain rigidity, and Tg. Using molecular dynamics (MD) simulations, we quantify the thermovolumetric and mechanical properties, including Tg, cohesive energy density, stiffness, and yield strength. We compare these results with experiments on networks of similar composition, finding good agreement. The relation between these properties and polar chemistry are studied by examining a secondary network of physical cross-links, formed by hydrogen bonds between NBOH units. Further, we examine nanovoid formation, an energy dissipation mechanism hypothesized to contribute to the toughness of pDCPD. Using metadynamics to accelerate sampling, we quantify the nanovoid nucleation rate under hydrostatic tension, similar to the stress state in the plastic zone preceding a crack tip. Small additions of NBOH have minimal effect, but the rate drops steeply with larger amounts. Several properties are mapped at nanometer scales, including stiffness and mobility, and associated with void nucleation. Estimates of the length- and time-scale of the plastic zone near a crack tip are used in discussing nanovoid formation as a plausible toughening mechanism in these materials. Overall, the results suggest that pDCPD tolerates the addition of some polar chemistry without degrading its excellent mechanical properties.

Influence of molecular weight between crosslinks on the mechanical properties of polymers formed via ring-opening metathesis

The apparent molecular weight between crosslinks (Mc,a) in a polymer network plays a fundamental role in the network mechanical response. We systematically varied Mc,a independent of strong noncovalent bonding by using ring-opening metathesis polymerization (ROMP) to co-polymerize dicyclopentadiene (DCPD) with a chain extender that increases Mc,a or a di-functional crosslinker that decreases Mc,a. We compared the ROMP series quasi-static modulus (E), tensile yield stress (σy), and fracture toughness (KIC and GIC) in the glassy regime with literature data for more polar thermosets. ROMP resins showed high KIC (>1.5 MPa m0.5), high GIC (>1000 J m-2), and 4-5 times higher high rate impact resistance than typical polar thermosets with similar Tg values (100 °C to 178 °C). The overall E values were lower for ROMP systems. The σy dependence on Mc,a and T-Tg for ROMP resins was qualitatively similar to more polar thermosets, but the overall σy values were lower. In contrast to more polar thermosets, the KIC and GIC values of the ROMP resins showed strong Mc,a and T-Tg dependence. High rate impact (∼104-105 s-1) trends were similar to the KIC and GIC behavior, but were also correlated to σy. Overall, a ductile failure mode was observed for quasi-static and high rate results for a linear ROMP polymer (Mc,a = 1506 g mol-1 due to chain entanglement), and this gradually transitioned to a fully brittle failure mode for highly crosslinked ROMP polymers (Mc,a ≤ 270 g mol-1). Molecular dynamics (MD) simulations showed that low Mc,a ROMP resins were more likely to form molecular scale nanovoids. The higher chain stiffness in low Mc,a ROMP resins inhibited stress relaxation in the vicinity of these nanovoids, which correlated with brittle mechanical responses. Overall, these differences in mechanical properties were attributed to the weak non-covalent interactions in ROMP resins.