Molecular Simulation Of Growth And Aggregation Structure Evolution Of In-situ Polyethylene

Polyethylene is a widely used general plastic,and its mechanical and serviceability can largely depend on the crystallinity,crystal orientation,entanglement and other aggregation structural properties of the product.Although the aggregation structure of polyethylene is related to growth,heat treatment and molding process,the aggregation structure of nascent polyethylene is the key to the final properties of the product and some of the structural features cannot be completely eliminated or changed by post-processing processes.Therefore,it is important to investigate the formation,evolution and regulation of the aggregation structure of nascent polyethylene.For nascent polyethylene chains grown on supported catalysts,the active sites on the catalyst surface constrain the movement and orientation of the nascent chains.At the same time,the growth of nascent polyethylene occurs in parallel with crystallization,and the competing coordination between the two significantly influences the evolution of the aggregation structure.Therefore,compared with the post-processing process,the aggregation structure evolution of nascent polyethylene is influenced by more factors and controlled by more complex mechanisms.At present,micron-scale nascent membrane morphology and atomiclevel single chain growth and adsorption can be observed in the experiment,but the chain conformation and aggregation structure evolution that accompany the growth process are still difficult to obtain by experimental means.Molecular dynamics simulation is one of the most important mathematical methods to study the microstructure of polymer systems.However,a large amount of computational simulation work has focused on the transition of the aggregation structure in postprocessing,but modelled descriptions of the in-situ polymer growth are still lacking.This is because the in-situ growth not only needs to consider the characteristics of the catalyst active site,but also needs to reasonably describe the coupling relationship between chain growth and conformational transformation.Therefore,the construction of polyethylene in-situ growth model,the analysis of the nascent polyethylene conformational changes,and then the study of catalyst support structure and reaction conditions on the evolution of the aggregation structure,are important directions for the study of the aggregation structure of polymer.In this work,for the growth and crystallization process of nascent polyethylene on the supported catalyst,the molecular dynamics simulation method is used to construct a polyethylene chain model and polyethylene in-situ growth model under different spatial constraints.The evolution of the aggregation structure of in-situ polyethylene has been systematically studied by extracting parameters such as chain length distribution,chain conformation,and crystal orientation.The main research works and results of this thesis are as follows:(1)Established a model of polyethylene chains with single end fixed to simulate the constraints of active sites on the catalyst surface of the nascent polyethylene chains,and to study the effects of single end confinement on the conformation and chain motion of polymer chains at high temperatures,and on the crystallization temperature and crystal orientation of the system at low temperatures.The results show that the single end confinement in the XY plane causes the chains to be oriented in the Z direction,while the chain center-of-mass motion no longer takes the form of random motion due to the confinement in the X and Y directions.The consistency of the chain conformation and the constraint of the chain motion make the system more ordered and easier to crystallize compared to the normal melt and form regular lamellae structure with larger average stem lengths during the growth of the crystal zone.Further,a temperature gradient is set in the system along the Z-direction from the fixed surface of chain end to the top free end,and the region near the fixed surface above the crystallization temperature is the high-temperature region,and the region away from fixed surface is the low-temperature region,simulating the temperature difference caused by the exothermic polymerization reaction and the formation of the liquid state near the fixed surface.The results show that nucleation occurs in the lowtemperature region,and the range of the liquid state gradually decreases with the growth of the crystal zone;in addition,although the temperature of the liquid state is higher,chains here are restrained by the bottom site and the crystal zone respectively,and thus the movement of the chain segments gradually decreases with the growth of the crystal.(2)Established a lateral confined polyethylene chain model,and side walls of different sizes were set as spatial barriers along the Z-direction to simulate the surface structure of the supported catalyst with barriers,and a temperature gradient was set to construct the proposed liquid state,to investigated the effect of barriers on the nucleation and crystallization process of nascent polyethylene.The local order parameter and density distribution of chain particles during the nucleation induction period indicate that there is a competition between high subcooling and heterogeneous nucleation medium in reducing the nucleation free energy during the nucleation process,which leads to two different nucleation mechanisms.When the wall height reaches the height of the low-temperature region,the local orientation consistency of chain segments near the wall makes the conformational entropy lower;along with the high subcooling in the low-temperature zone,the system has a shorter nucleation induction period.The initial nucleation occurs near the wall surface and is oriented along the wall surface.When the wall is only in the high-temperature region,the local orientation-ordered structure near the wall in the high-temperature region cannot reduce the conformational entropy to crystallization,so the initial nucleation still occurs in the low-temperature region,independent of the wall,and the nucleation location and orientation are random.In response to the contact between the lamellae thickening direction and the wall during the growth of the crystalline zone,the thickening of the sheet crystal will be hindered,while the contact between the lamellae growth front and the wall can change the tilt angle of the lamellae to make it oriented along the Z-axis.Based on these results,the introduction of a suitable height of Z-directional spatial barrier in the nascent polyethylene model results in regular crystalline zones,and the consistency of the crystal orientation helps to merge the crystal clusters within the system,thus reducing chain entanglement within the amorphous phase.(3)Established a model that can describe the growth and motion of polymer chains simultaneously and keep the active site at the initial position in the XY plane while the chains are growing.Realized molecular dynamics simulation of ethylene polymerization on the surface of supported catalyst,and investigated the coupling effects of growth rate,chain conformation,and monomer diffusion resistance.The results show that the polymer chains cannot fully relax at high growth rates,which makes the longitudinal component of the mean square radius of gyration Rgz2 increase,and the chains tend to be oriented along the Z axis.For the same chain length,this conformation provides a "diffusion pathway" through the top and bottom,reducing the diffusion resistance of the monomer from the bulk phase to the active site.Further,a small number of chains were induced to change their chain conformation by lateral gravitational force,the chains tend to be oriented along the Z axis,resulting in an increased growth rate,again confirming the hypothesis that chain conformation affects the "diffusion pathway" of monomers.For the system with non-uniform distribution of active sites,the aggregation of local active sites causes the spatial aggregation pattern of molecular chains to change abruptly with chain growth.At the beginning of the simulation,the gravitational potential between the molecular chains causes the chains to aggregate with each other and form a cluster structure,and as the cluster volume increases,the surface tension of the clusters is insufficient to maintain the shape,causing the clusters to "break" and extend twice along the plane.At the same time,the cluster-like accumulation of chain segments makes the active sites at different locations in completely different microscopic environments,and the diffusion paths and diffusion resistance between the bulk phase monomer and the active sites are different,resulting in a distribution of chain lengths that are long at the edge sites and short at the central sites.(4)Established a lateral confined polyethylene in-situ growth model,sidewall was set along the Z-direction to simulate the spatial barrier,and the aggregation and conformation of chains under different gravitational potentials of the wall were studied.A growth model with a constant chain growth rate was established to simulate polyethylene’s in-situ growth and crystallization process,and to study the correlation between reaction conditions and crystal orientation.In the growth model with lateral confinement,the competing between intermolecular chain cohesion energy and different wall gravitational potentials cause the molecular chains to exhibit differential aggregation states in space.The molten polymer chain segments are classified as flow-constrained"mimetic liquids",and as the wall gravitational potential energy rises,the upper part goes from a "convex liquid surface" to a "flat liquid surface" until it is transformed into a "concave liquid surface".When the gravitational potential energy ε increases to 0.3,the negative tension between the contact surface of the molecular chain and the side wall makes the chain segment climb along the wall.In the constant growth rate model,the chain conformation during the nucleation induction period determines the nucleation orientation and thus the final crystal zone orientation.As the chain length increases during the growth process,the chain conformation changes from orientation along the XY plane to orientation along the Z axis,so that the tilt angle of the final crystal zone is larger if nucleation occurs earlier.At the same time,the induction of high subcooling or wide walls will result in multiple nuclei with different orientations,although the crystallization rate can be accelerated by these means,the compression of the liquid state can not result in a uniformly oriented crystal zone.The induction of narrow walls can reduce the nucleation barrier but cannot change the overall chain conformation.Considering the crystal orientation and crystallinity,obtaining a uniformly oriented single crystal zone is easier when the average chain length reaches 300～500,relying on the induction of nucleation by narrow walls at low subcooling.