Simulation Of Rheological Properties Of Polymers Via Dissipative Particle Dynamics

In recent years, nano particles are becoming a hot area of studies. In the area of polymer materials, more than 1/3 of current researches are focusing on nanocomposites. In order to fabricate polymer composites of high quality, it is required that nano particles are efficiently incorporated into polymer matrix under shear flows. However, since polymer moleculars are of large size, there is still lack of fully understandings in experiments of polymer composites. And such large systems are hard to be modelled in molecule simulations.Dispersion and alignment of carbon nanotubes in shear polymer flow are of key importance in fabricating carbon nanotube polymer composites. There are unique advantages in dissipative particle dynamics(DPD) of modeling the properties of dispersion and alignment over macro and micro simulations: a. simple modeling process – most models and potential functions in molecule dynamics can be directly used in DPD; b. the parameterization of solubility of different components of composites; c. comparatively larger size; d. mature tools, such as large-scale atomic/molecule massively parallel simulator LAMMPS and Materials Studio. This thesis successfully built the meso model of nanotube polymer composites, quantitatively computed the degree of dispersion and alignment, and simulated the process of heat conduction of the composites via smoothed particle hydrodynamics.The main results are as follows:1. Morphologies of CNT/polymer composites with various degrees of dispersion and alignment were studied and the degrees of dispersion and alignment were quantatitively measured by dispersion possibility and average angle. Polymers are coarse-grained as chain connected by finitely extensible non-linear elastic force and nanotubes are coarse-grained as tubes composed of triple rings. Rheology was built by applying reverse Poiseuille flow, which was validated by computing viscosity of polymer. Concepts of dispersion possibility and average angle were proposed to quantitatively measure degrees of dispersion and alignment.2. Nanotubes were distributed in the area of higher velocity in shear flows. In order to achieve higher quality of dispersion, it is recommended the functionalizing the surface of carbon nanotubes and properly applying shear stress. The quality of alignment was improved by enlarging the length and volume fraction of carbon nanotubes. For well dispersed long nanotubes, the quality of alignment was continuously improved by enlarging the volume fraction of carbon nanotubes. For short nanotubes, the quality of alignment improved little with the increase of volume fraction of nanotubes.3. The process of heat conduction of carbon nanotube composites was studied by applying the energy equation of Navier-Stocks to smoothed particle hydrodynamics. We investigated the effect of dispersion quality, volume fraction, length of nanotubes and length of polymer on thermal conductivity of composites without and with shear flows.4. Thermal conductivity of aligned composites increased linearly versus the square of CNT volume fraction and the logarithm of CNT length. The increase of volume fraction, dispersion degree and length of CNT efficiently enhances the thermal conductivity of aligned composites. The effect of increasing polymer size was comparative to the effect of decreasing the quality of dispersion.