A model for the prediction of structure–property relations in cross-linked polymers (original) (raw)
Related papers
Macromolecules, 2012
Molecular dynamics and molecular mechanics simulations have been used to study thermo-mechanical response of highly cross-linked polymers composed of epoxy resin DGEBA and hardener DETDA. The effective cross-linking approach used in this work allowed construction of a set of stress-free molecular models with high conversion degree containing up to 35000 atoms. The generated structures were used to investigate the influence of model size, length of epoxy strands, and degree of cure on thermo-mechanical properties. The calculated densities, coefficients of thermal expansion, and glass transition temperatures of the systems are found to be in good agreement with experimental data. The computationally efficient static deformation approach we used to calculate elastic constants of the systems successfully compensated for the large scattering of the mechanical properties data due to nanoscopically small volume of simulation cells and allowed comparison of properties of similar polymeric networks having minor differences in structure or chemistry. However, some of the elastic constants obtained using this approach were found to be higher than in real macroscopic samples. This can be attributed to both finite-size effect and to the limitations of the static deformation approach to account for dynamic effects. The observed dependence of properties on system size, in this work, can be used to estimate the contribution of large-scale defects and relaxation events into macroscopic properties of the thermosetting materials.
2012
: We use molecular dynamics (MD) both at the atomistic and coarse-grained level to predict the mechanical and thermal properties of thermosetting polymers. The coarse-grained simulations, where the polymer network is treated as a bead-spring system can capture several important general behaviors of thermosets such as the role of chain length of the resin strands, degree of curing, strain rate and temperature on the thermo-mechanical response of a cured polymer system. Atomistic simulations, on the other hand, can provide detailed microscopic information on the physical properties of thermosetting polymers and can lead to predictions in quantitative agreement with experiments. Recently a number of MD models of thermosets were developed and investigated by some researchers [1-7]. While generating useful insight into the dependence of the physical properties of thermosetting polymers on their cross-link networks,these studies have either not been able to provide specific correlation wi...
Molecular dynamics predictions of thermomechanical properties of an epoxy thermosetting polymer
Polymer, 2020
This paper reports the thermomechanical properties of a thermosetting polymer formed by curing a DGEBA resin with a Jeffamine D230 agent predicted by molecular dynamics (MD) simulations. A multistep crosslinking approach is used to form the crosslinked network of the thermosetting polymer. The radial distribution function and X-ray diffraction pattern of the MD predicted crosslinked structure are calculated and compared with experimental results to validate the epoxy network system. Thermomechanical properties such as mass density, gel point, glass transition temperature (g), elastic moduli (Young's modulus and shear modulus), and yield strength in shear and tension are calculated at different temperatures and crosslinking conversions by employing the DREIDING and AMBER force fields. The MD predicted results are in good agreement with theoretical studies and existing experimental data. We find a significant increase in g and yield strength with crosslinking conversion. The elastic modulus is less sensitive to the strain rate, but the yield strength is significantly strain-rate dependent. The high-quality digital epoxy configurations developed in this work are available in LAMMPS data format from the journal website.
Polymers
This work is focused on the comparison of macro-, micro- and nanomechanical properties of a series of eleven highly homogeneous and chemically very similar polymer networks, consisting of diglycidyl ether of bisphenol A cured with diamine terminated polypropylene oxide. The main objective was to correlate the mechanical properties at multiple length scales, while using very well-defined polymeric materials. By means of synthesis parameters, the glass transition temperature (Tg) of the polymer networks was deliberately varied in a broad range and, as a result, the samples changed their mechanical behavior from very hard and stiff (elastic moduli 4 GPa), through semi-hard and ductile, to very soft and elastic (elastic moduli 0.006 GPa). The mechanical properties were characterized in macroscale (dynamic mechanical analysis; DMA), microscale (quasi-static microindentation hardness testing; MHI) and nanoscale (quasi-static and dynamic nanoindentation hardness testing; NHI). The stiffnes...
Macromolecules
We present experimentally validated molecular dynamics predictions of the quasistatic yield and post-yield 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 nano-scale 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.
PLoS ONE, 2012
The construction of molecular models of crosslinked polymers is an area of some difficulty and considerable interest. We report here a new method of constructing these models and validate the method by modelling three epoxy systems based on the epoxy monomers bisphenol F diglycidyl ether (BFDGE) and triglycidyl-p-amino phenol (TGAP) with the curing agent diamino diphenyl sulphone (DDS). The main emphasis of the work concerns the improvement of the techniques for the molecular simulation of these epoxies and specific attention is paid towards model construction techniques, including automated model building and prediction of glass transition temperatures (T g ). Typical models comprise some 4200-4600 atoms (ca. 120-130 monomers). In a parallel empirical study, these systems have been cast, cured and analysed by dynamic mechanical thermal analysis (DMTA) to measure T g . Results for the three epoxy systems yield good agreement with experimental T g ranges of 200-220uC, 270-285uC and 285-290uC with corresponding simulated ranges of 210-230uC, 250-300uC, and 250-300uC respectively.
2017
Thermoset polymers have gained interest in recent years due to their low cost, ease of processing and unique physical properties. Molecular simulation represents an avenue to explore the chemical structure-function relationship of these polymers by leveraging advances in the speed and accuracy of molecular dynamics (MD) simulations, due to high performance computing (CPU/GPU), efficient algorithms and modern force fields. We have developed a cross linking algorithm that allows for any chemistry to be defined to break two bonds and form new ones. This feature greatly increases the applicability in forming polymers with different crosslinking chemistries. System properties can be monitored during a cross linking simulation within a single interface, allowing the user to estimate properties like theoretical gel points and reactive group concentrations as curing occurs. After curing, glass transition temperatures (Tg) can be predicted using long MD cooling simulations in excess of 1 mic...
2016
Polymers are characterized by a rich variety of mechanical properties originating from their complex chains network. To capture such intricate structure properties, a number of polymer constitutive models have been proposed and implemented into finite element codes in an effort to solve complex engineering problems. Recent effort by Billon [1,2] focused on proposing alternative route for constitutive equations to model time-dependent mechanical behavior of polymers. This approach is based on a statistical equivalent network concept modified to account for the effect of microstructure evolution associated to inelastic processes taking place during polymer deformation. The model considers microstructure at a mesoscopic level through internal state variables (ISV) evolving in an equivalent statistical network according to classical physic-chemical approaches. Inelastic phenomena are assumed to result from state variables evolution associated to evolution of the network.
Use of Mechanical Modeling To Study Multiphase Polymeric Materials
Macromolecules, 2001
Viscoelastic properties of several multiphase polymeric materials were investigated in connection with their morphologies. Both mechanical modeling and mechanical spectrometry were used as tools for extracting information about morphology, molecular mobility, and interfacial interactions in such heterogeneous systems. The use of self-consistent mechanical models in direct and reverse modes was shown to be of interest for in depth discussion of the possibilities and limitations of the theoretical mechanical approach in the understanding of experimental dynamic mechanical data of complex polymeric materials. Christensen and Lo's model was used in direct mode to highlight mechanical coupling effects in binary thermoset/thermoplastic polymer blends. It was shown that the magnitude of these effects between phases in such blends, as in composite materials, depends not only on mechanical properties and relative content of each phase but also on the geometric arrangement of the polymeric phases. Furthermore, a new way of presenting experimental dynamic mechanical data and simulations resulting from direct mechanical approach, was also proposed as a qualitative, well-suited probe of morphology of multicomponent polymeric materials. The models of Christensen and Lo and of Herve and Zaoui were both used in reverse mode. It was demonstrated that such a new approach for mechanical modeling allows the extraction of the actual viscoelastic properties of one phase among others in multiphase polymeric materials. That is of particular interest for investigating the actual properties of the interfacial area in such complex systems, whose experimental in situ detection and characterization remain problem.
International Journal of Plasticity, 2015
A visco-hyperelastic constitutive model, based on an original approach initially developed by (Billon, 2012) and applied to amorphous rubbery polymers for a one-dimensional formalism, was extended in this study to three-dimensional constitutive equations based on a thermodynamic framework. The model was applied to a semi-crystalline polyamide polymer, PA66. The experiments included tension and shear testing coupled with synchronized digital image correlation and infrared measurements device for capturing the time, temperature, and stress state dependence, as well as the complex thermomechanical coupling exhibited by the material under large deformation. A notion of equivalent strain rate (based on the timetemperature principle superposition) was also introduced to show its capability to build master curves and therefore decrease the number of testing needed to build a material database. The model is based on the Edward Vilgis theory (1986) and accounts for chains network reorganization under external loading through the introduction of an evolution equation for the internal state variable, η, representing the degree of mobility of entanglement points. The model accounting for the equivalent strain rate notion was calibrated using master curves. The thermomechanical model agreed well with the experimental mechanical and temperature measurements under tension and shear conditions. The approach developed in this study may open a different way to model the polymer behavior.