Molecular Dynamics Simulation Of Crystallization Of Copolyethylenes Containing Cyclic Units

Molecular dynamics (MD) solves the classical equations of motion(Newton equation) for a system of N atoms interacting according to a force field.In this way an MD simulation generates a trajectory that describes how thedynamic variables change with time, from which time averages of macroscopicproperties (energy, pressure, etc.) can be calculated. Molecular dynamicssimulation is a deterministic method, by which the state of the system at anyfuture time can be predicted from its current state. The simulation results showthe 'real'dynamics behavior of the system. Nowadays, molecular dynamicssimulation has been extensively used to investigate various subjects of polymersystems, such as glass transition temperature, conformation of polymer chain,distribution of free volume, diffusion of small molecules in polymer, process ofpolymer crystallization, and polymer blend, etc.The physical properties of polymer, such as mechanics, electrology, andoptical properties, are influenced directly by the crystal structure of polymer.Therefore, for molecular design and physical property improvement of polymermaterials, it is extraordinarily meaningful to obtain the relationships between thecrystal structure and the physical property of polymers. Due to complex chainstructures and different crystallization conditions, the polymer crystallizationwith various morphologies, for example, the single crystal, the chain-foldedlamellar crystal, the spherulites, and the super-molecule structure, etc, may beformed. Moreover, there is still much limitation in experimental technology andtheoretical model. All this made it difficult to study the crystal property ofpolymer materials systematically. In order to understand and reveal the dynamicsbehavior and the microscopic mechanism of polymer crystallization, it isnecessary to study the relation between the crystal structure and the physicalproperty of polymer at the molecular level. Previous work has proved that themolecular dynamics simulation is expected to be a powerful tool for the study.Being one of the most valuable polymer materials in industrial applicationand academic study, polyethylene ( PE ) and its copolymers have beenintensively investigated by experiments and theories. The copolymers containingcyclopentane units in the main chain are highly valued for high glass-transitiontemperature and excellent transparency. The properties of these copolymers canbe widely controlled by changing content and structure of the cyclic unit.In this thesis, molecular dynamics simulations on several copolyethylene(containing different disubstitued cyclic units) models are performed to studysystematically the crystallization mechanism of polymer lamella and the effectsof different cyclic structures on polymer crystallization at the molecular level.The main results of the thesis are as follows:1) By means of MD simulations, the isothermal crystallization process of aseries of full extended linear copolyethylene chain models is performed, and theeffects of different types of disubstitued cyclic units are investigated. It is foundthat the copolymer chains containing cyclic units generally collapse into aglobule via a local collapse process, while pure PE via a global one. Moreover,the copolymer chains containing 1,2-disubstitued cyclic units, especially the onecontaining 1,2-disubstitued cyclopentane units, have faster collapse rate thanpure PE. And near the substituted atoms the chain segments kinks can easilyoccur. However, for copolymers containing other types of cyclic structures, theircollapse rates are similar to pure PE's, and in some cases are even lower thanpure PE's.2) By means of anneal MD simulations, the lamellar crystal conformations ofthose copolyethylene chain models are obtained. From their snapshots it is seenthat the 1,3-and 1,4-disubstituted cycloparaffin structures are partially excludedfrom the crystalline phase (chain segments arranging parallel with each other),while the 1,2-disubstituted cycloparaffin structures are rejected to amorphousregion (the fold surface) completely. From the trans dihedral angel population(Ptrans ), it can be seen that the more easily the cyclic units are excluded toamorphous region, the larger Ptrans is and the more perfect the crystal is. Then,through the comparison of the degrees of adjacent reentry( Par ), it is found that...