Research On Molecular Simulation Of Polymer Crystallization Behavior Under Effect Of Complex External Fields

Due to excellent mechanical and processing properties,polymer materials,which have been widely used in modern industry and life,have become one kind of the indispensable materials in the development of the national economy.At present,polymer materials with crystalline characteristics account for more than two-thirds of the total volume.In the process of molding,under influence of temperature,stress,pressure and actuation duration,the microstructure of polymer materials will be different.The performance of the polymer is determined by its microstructure,which formed during crystallization.Therefore,the prediction concerning variation tendency of the microstructure and properties of polymers,during the process of molding,will provide a basis for controlling processing conditionsIn terms of the research on the microstructure transformation in the molding process of polymers,experimental methods are usually adopted,which not only expensive,time-consuming and laborious,but also has some limitations in the control of experimental parameters.Since many advanced testing methods are difficult to be directly introduced in the process of experiment,we can only speculate on the process according to experimental results,so that qualitative descriptions are always conducted without quantitative judgments.The introduction of computer simulation technology has erected the bridge between experiment and theory.eEspecially the studies of molecular-scale which will help to understand the relationship between structure and properties more comprehensively.In this thesis,the crystallization behavior of the polymer under effect of complex external field during the processing was studied.In this paper,the simulation algorithm for the crystallization behavior of polymers was developed independently based on the lattice Monte Carlo method The Monte Carlo method was used to simulate the microscopic motion of polymer chain segments.And the effects on the crystallization behavior of the polymer under different cooling conditions and complex stress field were investigatedThe key technologies of lattice Monte Carlo method were studied,including model types,interaction parameters and the generate method of random number;a simulation algorithm of polymer crystallization has been developed independently.the direct and indirect methods were proposed to build a coarse grained polymer model.The relevant parameters that reflect the static and dynamic properties of the polyethylene system,such as mean square end-to-end distance,mean square radius of gyration,mean square displacement,were discussed,and the modal has been verified to coincide with physical scale of De Gennes and Rouse theory.Based on lattice Monte Carlo method,the static crystallization behavior of polymer under different cooling conditions,such as isothermal and non-isothermal,was studied.The effects of temperature and cooling rate on the crystallization behavior of polymers with different chain lengths were analyzed,and their changing rules were analyzed.We predicted Tthe equilibrium melting point of the polymer was determined based on the fluctuation of energy during crystallization,which consistent with the Flory-Vrij theory.In addition to the effect of the cooling conditions,the polymer is inevitably affected by the external stress during the polymer processing,which can change the accumulative structure and chemical structure.A shear lattice MC model was established based on the Kramers potential field,in order to deeply study the crystallization behavior and mechanism under the influence of complex stress fields According to the characteristics of processing,the continuous shear-induced crystallization and pre-shear-induced crystallization behaviors were studied respectively.The effects of shear conditions,cooling conditions,etc.on the crystallinity and the orientation degree of the polymer were studied.Besides,the mechanism of the influence of the shear temperature on the procursor was analyzed The crystallization of polymer under tensile stress was studied.And with analysis concerning the influence rules of stretch temperature and strain on the crystallization behavior of polymer.And the aggregation structure of polymer under the stretch conditions was studied.The crystallization behaviors under static conditions and effects of complex external fields were compared,and the influential mechanism of the stress conditions on crystallization behavior of polymers was ascertainedIn this thesis,a molecular model and algorithm for the crystallization behavior of polymers were established based on the lattice Monte Carlo method.The crystallization behavior of polymer under static conditions and effect of complex stress fields have been studied systematically.The structure properties,crystallization and orientation of polymer were analyzed and discussed which has a certain significance for further research and exploration of the crystallization behavior of polymers.