Atomistic Molecular Simulations Of Crosslinked Epoxy Resins

In this dissertation, molecular simulation methods were employed to investigate structure and properties of a typical crosslinked epoxy resin. This polymer was studied mainly because of the excellent intergrated performances, such as high modulus and fracture strength, low creep and high-temperature performance, which led to the versatile applications as coatings, adhesives, composites, etc. in electronics and aerospace industries. Those molecular simulations can provide very detailed information about structure, and interactions, and dynamics of this kind of polymers.Crosslinked epoxy resins exhibit much more complexity in molecular structure than the linear polymers (homopolymers and copolymers), which poses significant difficulty to the experimental characterization. In this regard, molecular simulations can be great helpful to the resolution procedure due to the complete control of model detail. A new algorithm was thus developed for constructing the molecular model of polymeric network. This algorithm uses the combined procedure of energy minimization (MM) and molecular dynamics (MD) based on a generic DREIDING2.21 forcefield, and dynamically forms network in close proximation. And high efficiency and great flexibility in building models make this algorithm superior to other similar methods. This algorithm was applied to a typical epoxy polymeric network based on diglycidyl ether bisphenol A (DGEBA) and isophorone diamine (IPD), obtaining a highly crosslinked infinite network model with the conversion up to 93.7%. Several circles of MM and MD were performed on this molecular model, obtaining a stable configuration. Molecular mechanics calculations based on this configuration and COMPASS forcefield reproduce the reasonable density and elastic constants compared to the experimental data, which in some degree validate the model and algorithm.In order to simulate the system in more realistic way, a series of of MD simulations were carried out on the built model at eight temperatures including both the glassy and rubbery regions. It can be seen that the stepwise cooling constant-NPT MD procedure widely used for the linear polymers can reproduce the densities and volumetric expansive coefficient in glassy region but not in rubbery region for this polymeric network. So this procedure was not practical to predict the glass transition temperature (T_g) of this polymer, which may be mainly caused by the highly crosslinked infinte network feature. However, some interesting features were reflected by following constant-NVT MD of nanosecond scales when the configurations with the experimental densities appropriate for the temperatures were used as the initial inputs: some typical radial distribution functions and dihedral distribution functions had more structural features in the glassy region than in the rubbery region. One of important findings is that this polymeric network demonstrates a different mechanism in glass transition from the linear polymers: the non-bonding interactions and the mobility of the crosslinks play the key roles in glass transition of the polymeric network. The dynamics of the segments between the crosslinks can be also associated with the glass transition.Despite many excellent performances, crosslinked epoxy resins would be significantly degraded when they were exposed to the humid environment and susceptible to the moisture absorption, so was always the practical case. In order to study the effects of water on the properties of the epoxy polymer at the molecular level, four moist polymeric network models with different water concentrantions were developed from the dry polymeric network built previously by randomly inserting some water molecules into the free spaces in the dry one. A series of of molecular dynamics simulations were carried out on all the mimimizated molecular models. Main finding is that the effect of water on structure and properties of the epoxy polymer is non-monotonic: on one hand, more water can lead to the plasticization of the polymeric network, that is, decreased density, increased fractional free volume, accelerated local chains; on the other hand, less water can lead to the contrast trends of the said above properties, that is, antiplasticization. These results can be well explained by the hydrogen bonds present in the polymeric system. The simulated results also indicate that water molecules prefer to locate in the vinicity of the polar groups on the polymeric network, and the more the water molecules are the easier the water clusters form.In conclusions, the simulated results can generally be in reasonable agreements with the experimental observations and related theories, at least qualitatively, which confirms the success of all these simulations. Molecular simulation methods surely played the important roles in understanding the structure-property relationships of this kind of complex polymers. With the advent of computer hardware and software, and with the developments of modified model and algorithm, these molecular simulation methods can be used to accurately predict those desirable properties, and turn the molecular design of epoxy polymer products into reality.