| Polymer materials are gradually replacing other materials,such as metal materials,in many fields.However,its main weakness is its relatively low mechanical strength,so it is necessary to improve the mechanical properties of polymer materials.For semi-crystalline polymer materials,their physical properties are mainly determined by their crystal structure and morphology.Under the circumstance,it becomes very important to realize the control of the polymer crystal structure and morphology for the control of its physical properties.The special cyclic topology of the molecular chain,the different molecular chain lengths of the polymer melt,and the addition of carbon nanotube(CNT)fillers will all affect the final morphology of the crystal,especially under the action of the flow field,the combined action of many factors makes the crystallization performance in the system more complicated.In order to reveal the microscopic mechanism of the above crystallization behavior,the evolution of the aggregate structure of different polymer systems during the stretching process was investigated from a microscopic perspective by Molecular Dynamics simulations.details as follows:1: Molecular Dynamics Simulation of Tensile-Induced Crystallization Behavior of Cyclic Polyethylene(PE),five groups of cyclic PE systems with different chain lengths were established and uniaxially stretched at 350 K.In addition,a system with a chain length of C200 combined atomic length was selected,and the effect of the stretching rate on the stretchinginduced crystallization behavior of the cyclic PE system was investigated.Studies have shown that the coupling of segment conformational transitions to local segment orientations plays an important role in the flow-induced polymer nucleation process.The nucleation induction phase disappeared with increasing molecular weight.And there is a critical value below which an increase in molecular weight leads to an increase in the crystallization rate;above this critical value,an increase in molecular weight leads to a decrease in the crystallization rate.At the same time,the PE is easier to crystallize and orient with a lower stretching rate.2: Molecular dynamics simulation study of the relationship between entanglement,orientation and crystallization during the crystallization of long-chain and short-chain polymer blends induced by stretching.Long chains with molecular chain lengths of C400 and C1000 content of 10%,30% and 60% were added to the short chain system with a length of C50.The different systems were uniaxially stretched to strain 4 while slowly cooling from 600 K to 300 K after a period of relaxation at 600 K.The experimental results show that both long-chain and short-chain are involved in the early crystallization,and the initial crystallization strain of the system with more long-chain content is smaller.Stretching promotes the orientation of molecular chains,and long chains drive the orientation of short chains by forming an entangled network,thereby affecting the crystallization process.3: Molecular dynamics simulation study of the crystal structure evolution process of tensile-induced CNT/PE composite system.The effects of the length and content of CNTs on the static crystallization of the composite system and the evolution of the crystal structure of the composite system during stretching were investigated.The selected CNTs have a diameter of 9.49 ? and lengths of 36.89 ? and 73.78 ?,respectively.The study shows that there is no obvious nucleation induction period in the crystallization of the system during the static crystallization process.In early crystallization,nuclei first appeared in the surface area of CNTs.During the recrystallization process,the crystal at the CNT interface will be partially ablated due to the movement of the CNT,and the degree of ablation is related to the orientation of the initial state of CNT stretching.During the stretching process,there are phenomena such as the overall movement,ablation and crystal splitting of the crystal.The simulation results in this paper reveal the evolution law of the aggregate structure in different polymer systems induced by stretching,and provide theoretical support for the control of crystal morphology in experiments. |
