Crosslinked Epoxy Polymers Research Articles

Aging of Adhesives of External Reinforcement Systems of Building Structures. Part 2. Structural Studies

The structural changes of cross-linked epoxy polymer adhesive for external reinforcement systems of building structures after exposure to artificial climatic factors have been studied by the method of IR Fourier spectroscopy. The aim of the study was to determine the significance of influencing factors (UV radiation, temperature, water and alkaline environment) for the development of a method for accelerated assessment of the durability of selected systems. The objectives of the study were to identify the mechanism of aging and obtain differential spectra of aged and initial samples. The processes of destruction of samples of cross-linked epoxy polymer of amine curing under the influence of artificial climatic factors of aging have been studied by IR spectroscopy. It has been established that the most significant factors influencing structural changes in the polymer are UV radiation and an alkaline solution of NaOH and KOH.

A silica/epoxy resin nanocomposite exhibiting high thermal stability and low thermal expansion based on the uniform dispersion of hydrophilic colloidal silica nanospheres

The present study fabricated high-performance silica/epoxy resin nanocomposites having a low coefficient of linear thermal expansion (CTE) and a high glass transition temperature ( Tg). This was accomplished by dispersing colloidal silica nanospheres having hydrophilic surfaces in epoxy resins, which limited the motion of the polymer chains. Nanocomposites were produced wherein isolated primary particles of colloidal silica without silane surface modification were dispersed uniformly. These particles were generated via the breakdown of loosely bound agglomerates of spherical silica particles during the agitation of a dispersion in an epoxy resin solution. Hydrogen bonding between hydroxyl groups on the hydrophilic surfaces of the dispersed silica nanoparticles and the cross-linked epoxy polymer network evidently limited thermally-induced motion of the polymer chains, resulting in a considerable reduction in the CTE and an increase in the Tg for the nanocomposite.

A Reactive Molecular Dynamics Study on Crosslinked Epoxy Resin Decomposition under High Electric Field and Thermal Aging Conditions.

To reveal the microscopic mechanism of synergetic thermal-electrical degradation during a partial discharge process in epoxy insulation materials, the decomposition of crosslinked epoxy resin is investigated using reactive molecular dynamics simulations under high electric field and thermal degradation conditions. Bond-boost acceleration method is employed in reactive molecular dynamics simulations to successfully establish epoxy polymer models with a crosslink degree of 93%. Active molecular species derived from electrical partial discharges are considered in the current work. Small molecule products and decomposition temperature in the degradation process under an electric field are calculated to elucidate the effect of nitric acid and ozone molecules, being the active products generated by electrical partial discharges, on the synergetic thermal-electrical degradation of epoxy resin. Both nitric acid and ozone exacerbate thermal impact decomposition of crosslinked epoxy polymer by decreasing initial decomposition temperature from 1050 K to 940 K and 820 K, respectively. It is found that these active products can oxidize hydroxyl groups and carbon-nitrogen bridge bonds in epoxy molecular chains, leading to the aggravation of epoxy resin decomposition, as manifested by the significant increase in the decomposed molecular products. In contrast, thermal degradation of the epoxy resin without the active species is not expedited by increasing electric field. These strongly oxidative molecules are easily reduced to negative ions and able to obtain kinetic energies from electric field, which result in chemical corrosion and local temperature increase to accelerate decomposition of epoxy insulation materials.

Microscopic deformation and failure modes of high‐functionality epoxy resins from bond breaking molecular dynamics simulations and experimental investigation

AbstractThe material behavior of the API‐60 epoxy resin was investigated using atomistic molecular dynamics simulations. In this study, a hybrid, reactive force field (M‐GAFF2), which combined Morse and harmonic bond potentials from GAFF2, was developed to understand the elastic/plastic response of the cured epoxy polymer. We found that elastic deformation was caused by epoxy network rearrangement at low strain while plastic deformation was observed by stretching and breaking covalent bonds after the yield point. Ab initio calculations at the CASPT2(2,2)/6‐311+G** level of theory were performed to compute parameters for M‐GAFF2. These are necessary for breaking covalent bonds in the epoxy network backbone when they are deformed. The M‐GAFF2 was effective in revealing the ductile‐like deformation behavior of the crosslinked epoxy polymer at the microscopic level, including elastic/plastic deformations and progressive failure. The effect of testing temperature at 300 and 400 K and degree of cure (DoC) on tensile properties such as Young's modulus, ultimate strength, and failure strain was evaluated to verify the feasibility of the M‐GAFF2 in different thermodynamic constraints. This new approach is easy to use for nonequilibrium MD simulations and computationally efficient for studying the microscopic deformation and failure behavior of thermosetting polymers at the atomistic scale.

Atomistic to continuum scale investigations on mechanical properties of epoxy bonded fiber reinforced polymer composite systems under hygro-thermal exposures

Epoxy polymers are widely used as adhesives in bonded composite systems. In this study, multi-scale computational investigations from molecular dynamics (MD) to finite element (FE) simulations are carried out to understand the material behaviour at atomistic level and to evaluate the performance at macro level. MD investigations are carried out to understand the extent of degradation of mechanical (such as strength, modulus) and transport properties (such as moisture diffusion coefficient) due to different environmental conditions like moisture ingress and high temperature. The influence of degree of curing of epoxy (diglycidyl ether of bisphenol-A) and type of hardener (diethyltoluene diamine, diethylene triamine, trimethylene hexadiamine) on the mechanical performance of the cross-linked epoxy polymer systems is also investigated. In-depth investigations are also carried out to identify the factors contributing to the total potential energy of cross-linked epoxy polymers and their role during the process of curing. Diffusion coefficient (key transport property) of epoxy polymer under moisture exposure is also determined using MD simulations. It has been demonstrated that high temperature causes the increase in diffusion in epoxy polymer systems. The observations on the performance of the epoxy system under moisture exposure obtained from MD simulations are translated to the higher length scale. The moisture ingress (in turn, significant reduction in mechanical properties) in epoxy polymer used for bonded composite system is also investigated using FE simulations. It is found that the extent of moisture ingress in the epoxy adhesive increases rapidly when epoxy polymer is environmentally exposed (under high temperature), and thus, the whole system gradually loses its effectiveness due to mechanical degradation. The findings of this study will help in understanding crucial parameters and aid to better use/engineering of epoxy polymers.

Enhanced electrical property of graphite/Al2O3 composite fabricated by reductive sintering of gel-casted body using cross-linked epoxy polymer

In the present study, graphite/alumina composites are fabricated via reductive sintering of gel-casted green bodies with structurally controlled cross-linked epoxy polymers for the first time. The cross-linking degrees of polymers are tuned by the amount ratio of epoxy monomer/polyvinyl alcohol cross-linker utilized in gel-casting process. Superior electrical properties with respect to 5-fold enhanced electrical conductivity and 2-fold higher carrier mobility are successfully achieved in graphite/alumina composite fabricated from cross-linked epoxy polymer, whose phenomenon is attributed to the excellent conductive path in ceramic matrix established by highly uniform network with improved graphitization degree.

A multiscale framework for the elasto-plastic constitutive equations of crosslinked epoxy polymers considering the effects of temperature, strain rate, hydrostatic pressure, and crosslinking density

In this study, a multiscale approach was constructed for the multi-axial plastic deformations of epoxy polymers to develop a macroscopic constitutive model at a quasi-static strain rate (consistent with laboratory tests) without any experiments. The 3-dimensional constitutive model was fully characterized, considering the effects of strain rate, temperature, hydrostatic pressure, and crosslinking density. The quasi-static yield stress was predicted by conducting molecular dynamics simulations based on the Argon theory and internal stress law established by thermomechanical properties. For the proper description of nonlinear dependences of the obtained yield stresses on the strain rate and temperature, which have not been considered in previous studies, a cooperative model was adopted as a link between the simulations and experiments. Taking into account characteristic quasi-static yield behavior, one-dimensional quasi-static constitutive equations were derived by developing hardening laws considering strain rate dependences of the yield strain and yield stress established at the quasi-static level. Finite element analysis was then performed on these equations considering the influence of temperature, hydrostatic pressure, and crosslinking density by implementing a continuum model based on the paraboloidal yield function. Finite element simulations included a 1-element patch test conducted for one-dimensional loading paths and open-hole test performed for both the validation and practical use of the model describing the structural behavior of the studied polymer under combined loading. The results of open-hole simulations revealed that the proposed multiscale framework successfully considered the microscopic structural characteristics of amorphous polymers in establishing elasto-plastic finite element models.

Light-Healable Epoxy Polymer Networks via Anthracene Dimer Scission of Diamine Crosslinker

Two anthracene-based diamine crosslinkers were used to cure a range of commercially available monomers to produce four highly photoreversible crosslinked epoxy polymers. Through careful selection of the epoxy monomers used, the properties of the resultant polymer networks were varied to create a coating material that possessed room-temperature light-stimulated healing. Of the four coatings created, the best healing performance was exhibited by the two most flexible systems, both of these also exhibited the thermal and mechanical performance necessary for coatings. By using anthracene, the utilization of a wide range of wavelengths in the healing process is possible, which in applications such as industrial coatings would be of significant benefit.

Coarse-Grained Molecular Dynamics Study of the Curing and Properties of Highly Cross-Linked Epoxy Polymers.

In this work, a coarse-grained model is developed for highly cross-linked bisphenol A diglycidyl ether epoxy resin with diaminobutane hardener. In this model, all conformationally relevant coarse-grained degrees of freedom are accounted for by sampling over the free-energy surfaces of the atomic structures using quantum mechanical simulations. The interaction potentials between nonbonded coarse-grained particles are optimized to accurately predict the experimentally measured density and glass-transition temperature of the system. In addition, a new curing algorithm is also developed to model the creation of highly cross-linked epoxy networks. In this algorithm, to create a highly cross-linked network, the reactants are redistributed from regions with an excessive number of reactive molecules to regions with a lower number of reactants to increase the chances of cross-linking. This new algorithm also dynamically controls the rate of cross-linking at each local region to ensure uniformity of the resulting network. The curing simulation conducted using this algorithm is able to develop polymeric networks having a higher average degree of cross-linking, which is more uniform throughout the simulation cell as compared to that in the networks cured using other curing algorithms. The predicted gel point from the current curing algorithm is in the acceptable theoretical and experimental range of measured values. Also, the resulting cross-linked microstructure shows a volume shrinkage of 5%, which is close to the experimentally measured volume shrinkage of the cured epoxy. Finally, the thermal expansion coefficients of materials in the glassy and rubbery states show good agreement with the experimental values.

Comparative investigation of thermal and mechanical properties of cross-linked epoxy polymers with different curing agents by molecular dynamics simulation

Molecular dynamics (MD) simulations were carried out to predict the thermal and mechanical properties of the cross-linked epoxy system composed of DGEBA resin and the curing agent TETA. To investigate the effects of curing agents, a comprehensive and comparative study was also performed on the thermal and mechanical properties of DGEBA/TETA and DGEBA/DETDA epoxy systems such as density, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and elastic properties of different cross-linking densities and different temperatures. The results indicated that the glass transition temperature of DGEBA/TETA system calculated through density–temperature data, ∼385–395°K, for the epoxy system with the cross-linking density of 62.5% has a better agreement with the experimental value (Tg, ∼400°K) in comparison to the value calculated through the variation of cell volume in terms of temperature, 430–440°K. They also indicated that CTE related parameters and elastic properties including Young, Bulk, and shear’s moduli, and Poisson’s ratio have a relative agreement with the experimental results. Comparison between the thermal and mechanical properties of epoxy systems of DGEBA/TETA and DGEBA/DETDA showed that the DGEBA/DETDA has a higher Tg in all cross linking densities than that of DGEBA/TETA, while higher mechanical properties was observed in the case of DGEBA/TETA in almost all cross linking densities.

A Multiscale Model for the Quasi-Static Thermo-Plastic Behavior of Highly Cross-Linked Glassy Polymers

We present experimentally validated molecular dynamics predictions of the quasi-static yield and postyield behavior for a highly cross-linked epoxy polymer under general stress states and for different temperatures. In addition, a hierarchical multiscale model is presented where the nanoscale simulations obtained from molecular dynamics were homogenized to a continuum thermoplastic constitutive model for the epoxy that can be used to describe the macroscopic behavior of the material. Three major conclusions were achieved: (1) the yield surfaces generated from the nanoscale model for different temperatures agree well with the paraboloid yield criterion, supporting previous macroscopic experimental observations; (2) rescaling of the entire yield surfaces to the quasi-static case is possible by considering Argon’s theoretical predictions for pure compression of the polymer at absolute zero temperature; (3) nanoscale simulations can be used for an experimentally free calibration of macroscopic continuum models, opening new avenues for the design of materials and structures through multiscale simulations that provide structure–property–performance relationships.

Repeatable self-healing of an epoxy matrix using imidazole initiated polymerization

For the first time, repeatable self-healing was achieved in a cross-linked epoxy polymer by incorporating 2-ethyl-4-methylimidazole (24-EMI) into the matrix as a latent polymerization initiator. Upon material damage and infiltration of liquid EPON 8132 epoxy monomer healing agent into the crack plane, polymerization occurs in the damaged region with a moderate application of heat in the presence of the latent imidazole initiator. Using tapered double cantilever beam (TDCB) fracture testing, greater than 90% recovery of fracture toughness was observed over multiple healing cycles in samples containing 10 wt% 24-EMI, with up to 11 repeat healing cycles possible. The effect of incorporating the imidazole on the host epoxy fracture toughness, complex moduli and glass transition temperature was also investigated. As imidazole concentration increases, a reduction in glass transition temperature and an increase in fracture toughness of the host epoxy is observed.

Effect of C12-14 Alcohols on the Thermomechanical Properties of Epoxy Shape-Memory Polymer

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its Tg, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by introducing flexible aliphatic alcohol. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with Tg’s above room temperature were found to possess shape memory properties.

Synthesis of a diamine cross-linker containing Diels–Alder adducts to produce self-healing thermosetting epoxy polymer from a widely used epoxy monomer

A diamine cross-linker with thermal reversible Diels–Alder (DA) adducts of furan and maleimide groups has been designed and synthesised, which can be used to crosslink commercial epoxy monomers to achieve self-healing, cross-linked epoxy polymers. This new strategy to make self-healing epoxy polymer was successfully proved by incorporating the cross-linker with a widely used epoxy monomer, the diglycidyl ether of bisphenol A. The self-healing property of the new epoxy polymer was shown by complete elimination of cracks after the process of retro-DA and DA reactions which occurred in the cross-linker component of the network.

Method of scale bridging for thermoelasticity of cross-linked epoxy/SiC nanocomposites at a wide range of temperatures

In this study, we propose a sequential multiscale modeling approach to describe the thermoelastic behavior of cross-linked epoxy/silicon carbide (SiC) nanocomposites embedding different radii of nanofiller through molecular dynamics (MD) simulations and a continuum micromechanics constitutive model. In MD simulations, the coefficients of thermal expansion (CTEs), the elastic moduli, and the glass transition temperatures of different nanocomposites having different particle sizes are obtained at temperatures from 250 K to 550 K, the range in which cross-linked epoxy polymers generally experience the glassy-to-rubbery phase transition and consequently, their CTEs and elastic moduli change dramatically. In the equivalent continuum model, an interphase is defined between the particle and the matrix to account for the contribution of the polymer densification in the vicinity of the nanoparticle to the size-dependent CTE and elastic modulus at each temperature. Based on the thermal strain field defined in the micromechanics constitutive model, a physically meaningful description of the thermal expansion behavior of the interphase is obtained to reproduce the MD simulation results from fully continuum-based approaches for nanocomposites in rubbery state as well as in glassy state. Finally, the accuracy of the proposed multiscale approach is confirmed from finite element analysis.

Nanostructures in Cross-linked Epoxy Polymers and their Influence on Mechanical Properties

The methods of nanostructure s adjustment in cross-linked epoxy polymers were considered. The large po- tential of changing mechanical properties of these materials by nanoclusters relative fraction variation was shown.

Molecular modeling of crosslinked epoxy polymers: The effect of crosslink density on thermomechanical properties

Molecular dynamics and molecular mechanics simulations are used to establish well-equilibrated, validated molecular models of the EPON 862-DETDA epoxy system with a range of crosslink densities using a united atom force field. Molecular dynamics simulations are subsequently used to predict the glass transition temperature, thermal expansion coefficients, and elastic properties of each of the crosslinked systems. The results indicate that glass transition temperature and elastic properties increase with increasing levels of crosslink density and the thermal expansion coefficient decreases with crosslink density, both above and below the glass transition temperature. The results demonstrate reasonable agreement with thermomechanical properties in the literature. The results also indicate that there may be a range of crosslink densities in epoxy systems beyond which there are limited changes in thermomechanical properties.

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its Tg, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with Tg's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the Tg of epoxy SMP polymers ranging from room temperature to 89°C.

Curing of epoxy oligomers by the eutectic mixture of aromatic amines

Kinetics of curing of structurally different epoxy oligomers (ED-20 and PDI-3AK resins) in a mixture with other low-molecular-weight epoxy oligomers and plasticizers by the eutectic mixture of aromatic amines UP-0638/1 is studied by the DSC method. The activation energy and the heats of curing reactions are determined. It is established that crosslinked epoxy polymers cured at moderate temperatures (40–80°C) are strong moisture-resistant compositions with different mechanical characteristics. Plasicized elastomers based on PDI-3AK resin with glass transition temperatures of −78 and −95°C are freeze-and heat-resistant materials.

Novel pyridinium salts as cationic thermal and photoinitiators and their photosensitization properties

AbstractNovel pyridinium salts [N‐(α‐phenylbenzyl)‐, N‐(1‐naphthylmethyl)‐, or N‐cinnamyl p‐ or o‐cyanopyridinium hexafluoroantimonates] were synthesized by the reaction of p‐ or o‐cyanopyridine and the corresponding bromides followed by anion exchange with KSbF6. These pyridinium salts polymerized epoxy monomers at lower temperatures than previously reported for N‐benzyl‐2‐cyanopyridinium hexafluoroantimonate. The o‐substituted pyridinium salts showed higher activity than the p‐substituted ones, and the crosslinked epoxy polymers cured with these initiators showed higher glass‐transition temperatures. These pyridinium salts photoinitiated radical polymerization as well as cationic polymerization. The photopolymerization was accelerated by the addition of aromatic ketones as photosensitizers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1037–1046, 2002