Dissipative Particle Dynamics Simulation Study On Controlling Molecular Weight Distribution In Emulsion Polymerization

Emulsion polymerization is a very important industrial method for the synthesis of polymers for a wide variety of applications ranging from coatings and adhesives to biomedical applications. Water-based products via emulsion polymerization are the current direction of development. Countries in the world compete in dedicating to the research, development and application of the polymer latex because of its wide potential market and the applications. As the most important parameters characterizing the polymer products, molecular weight and molecular weight distribution (MWD) are the nevertheless target of emulsion polymerization science and industry. They can be influenced by many factors, such as various reactions in the aqueous and polymer phases, monomer concentrations, surfactants, initiator concentrations and polymerization rate. Exploring the molecular weight and molecular weight distribution dependent on the emulsion polymerization mechanism and the contents is very important for the design of polymer materials to meet the target of improving the materials properties.Computation simulations may be helpful to tackle the problem by exactly controlling a single factor to probe the detailed information on microscopic level and dynamic process, and it can visualize these physical processes directly to helps us understand and explore the laws in these natural phenomena. In this dissertation, we carry out comprehensive dissipative particle dynamics simulations (DPD) to study the emulsion polymerization. Within the DPD method, all the particles interact with each other through three pairwise forces: a conservative force, a dissipative force, and a random force, and the pair-wise interactions result the momentum of the system being conserved. These forces are very soft, so the integration time steps can be very large, the time scale in DPD simulation can be at milliseconds. It's also due to the soft repulsions, we can unite some molecules or polymer segments into one DPD bead, thus the DPD model can be used to study the systems at mesoscopic length scale. DPD method has been applied on the study of polymer blends, microphase separation of the block copolymers, self-organizing of amphiphilic molecules into membrane, and the budding and fission of bionic micelles. These studies become the research foundation of the model of micelles formation. In addition, DPD model by incorporating Monte Carlo reaction model has been occurred. A combination of these methods makes it possible to study the emulsion polymerization process.In this study, the MWD on emulsion polymerization with four different factors has been investigated by DPD simulations, including monomer concentration, initiator concentration, polymerization rate and surfactant chain length. Chain propagation and bimolecular termination were considered. Here we have omitted initiation process. The main results are as follows:1. In the beginning of the simulations, we need to know the interaction parameters between different species control the structure and stability of self-assembled micelles. Including the interaction parameterαWA between water (W) and the hydrophilicity of the solvable surfactant A block, the interaction parameterαWB between water (W) and the hydrophibicity of the solvable surfactant B block and the interaction parameterαWM between water (W) and the monomer (M). We can conclude that, the hydrophilicity of the solvable surfactant A block (αWA) plays a predominate role in controlling the micelle size, increasing the hydrophibicity of surfactant B and monomer improves the wrapping of monomer beads inside the micelles. Following these criteria, in our simulations of emulsion polymerization, we chooseαWA= 24,αWB= 30,αWM= 100 to obtain the micelles with both good dispersion and stable structure while the monomers are wrapped well by the surfactants.2. The influences we focused on are monomer concentration, initiator concentration, polymerization, surfactant chain length on the MWD.(1) The effect of monomer concentration: The polydispersity index (PDI) increases with increasing monomer conversion at the same monomer concentration. But at the same conversion, PDI varies differently with different monomer concentration. In the early stage of polymerization, the monomer conversion is low, and the difference among the values of PDI for the five different concentrations is not very obvious. The difference between PDI enlarges when the monomer conversion is high. The ratio of long chain increases with increasing monomer concentration and the MWD become wide. We can conclude that monomer concentration plays an important role on obtaining high molecular weight.(2) The effect of initiation concentration: Initiator concentration is also an efficient way to control the molecular weight distribution. Higher initiator concentration decreases the molecular weight by creating more the polymer chain number with a more widen MWD. That because more initiation concentration can create more radical number, and then create more polymer chain number. With increasing initiator concentration, the ratio of short chain increases and the ratio of long chain decreases at the same conversion. Then we obtain the decrease of average molecular weight and number average molecular weight at the same time.(3) The effect of initiator activity: The ratio of medium chain increases with increasing initiator activity with a narrowed MWD. The narrowed MWD is contributed to the increase of the ratio of medium chain.(4) The effect of surfactant chain length: There is not obvious law at the average molecular weight, number average molecular weight and MWD with increasing the surfactant chain length, except for micelle size. That indicate the surfactant chain length determined the micelle size completely. In this part, micelle size completely depends on the surfactant chain length and concentration, so it is necessary to select appropriate surfactant for appropriate micelle size.