| Polymer crystallization is one of the important issues in polymer science,which can control the structural formation process of polymer materials,thereby further controlling the performance of final polymer products.With the development of nanotechnology,confined polymers have gradually attracted the attention of scientific researchers and have been widely used in nano-materials and nano-devices.When polymers are confined,their crystallization behavior may differ greatly from those of unconfined polymers,which can significantly impact the processing and application performance of materials.In addition,in polymer processing,the application of external field is often inevitable.Therefore,studying the evolution mechanism of molecular chain conformation,nucleation mechanism,and changes in crystal structure and morphology in systems under restricted conditions and external field conditions is of great significance to further improve and deepen polymer crystallization theory and guide the production of high-performance polymer materials.So far,some experimental work has investigated the crystallization behavior of confined polymers,but due to the limitations of experimental observation scale,the micro-mechanisms of their nucleation and crystal growth processes have not been systematically and effectively revealed.Computer simulation methods can effectively make up for the shortcomings of the experiment and reproduce the nucleation and crystal growth processes of polymer systems.Based on the above background,this paper first studied the crystallization behavior of structure confined polymer systems and space confined polymer systems using Monte Carlo simulation method.On this basis,the crystallization behavior of polymer systems with both structural and spatial constraints was studied.Subsequently,the stretch-induced crystallization behavior of confined polymer systems was studied by molecular dynamics simulation method.The main research contents and conclusions are as follows:1.The crystallization behavior of structurally confined polymer systems was studied using Monte Carlo simulation method,including the crystallization behavior of structurally confined polymer systems grafted on the surface of two-dimensional filler and the crystallization behavior of structurally confined polymer systems grafted on the surface of one-dimensional filler.For the structurally confined polymer system grafted on the surface of two-dimensional filler,the simulation results show that the grafted chains can be used as nucleating agents to improve crystallization ability.The increase of the grafted chain contents leads to the increase of the loss of conformation entropy,melting temperature,and the number of crystal nuclei,resulting in the improvement of the crystallization rate and crystallinity.However,when the interface interaction between chains and filler is changed,with the enhancement of the interface interaction,the number of crystal nuclei and the mobility of free chains decrease,resulting in a slower crystallization rate.The simulation results also indicate that almost all the crystal regions formed by free chains in the system are distributed at the edges of the crystal regions formed by grafted chains.For the structurally confined polymer system grafted on the surface of one-dimensional filler,the existence of more grafted chains increases the crystal nuclei number in the systems,which shortens the nucleation induction time.At the same time,with the increase of the content of grafted chains,the nucleation mode tends to change from intramolecular nucleation to intermolecular nucleation.When the interface interaction is changed,the chain elongation increases with the enhancement of interface interaction,which leads to the increase of the loss of conformational entropy and is conducive to the formation of crystal nucleus.However,the chain mobility slows down with the enhancement of interface interaction,which will hinder the formation of crystal nucleus.By tailoring the interface interaction,the crystal morphologies of polymer nanocomposites can be adjusted to form the unique nanohybrid shish-kebab structures,that is,the filler is used as shish,the crystal regions generated by free chains are attached to the outside of crystal regions generated by restricted chains to form kebabs.The results also showed that the length of grafted chains has different effects on the crystallization depending on the contents of grafted chains.2.The crystallization behavior of spatially confined polymer systems was studied using Monte Carlo simulation method,that is,the crystallization behavior of polymers confined within the columnar region.The simulation results show that for confined polymer systems with different molecular weights,the evolution of crystallinity with Monte Carlo time shows first-order kinetics,which means that homogeneous nucleation controls the whole crystallization process.At the same time,the nucleation mechanism changes from intermolecular nucleation to intramolecular nucleation with the increase of molecular weights.For confined polymer systems with different interface interactions,the enhancement of interface interactions changes the nucleation mode of the system from homogeneous to heterogeneous nucleation.When the lateral size of the nanocylinder is changed,the nucleation mode of the system changes from homogeneous nucleation to heterogeneous nucleation with the decrease of lateral sizes.During the crystallization,crystal orientation and morphology are also closely related to molecular weights,interface interactions and lateral sizes.3.The crystallization behavior of polymer systems with both structural and spatial constraints was studied using Monte Carlo simulation method,that is the crystallization behavior of confined polymers grafted into the columnar region.The simulation results show that the crystallinity and melting temperature of confined systems increase with the increase of free chain content.And,the crystallinity and melting temperature of confined systems with lager lateral size are higher than those with smaller lateral size.For the confined polymer system with the lateral size of 8lattice points,with the increase of free chain content,the surface free energy of the nuclei and the diffusion activation energy of the chains decrease due to the combined effects of chain conformation size and chain movement ability,which leads to the enhancement of nucleation ability of polymers.However,for the confined polymer system with lateral size of 12 lattice points,with the increase of free chain content,the nucleation sites decrease and the critical free energy barrier increases,which are not conducive to nucleation.In addition,the simulation results also confirmed that the grafting point can be used as the nucleation site for polymer crystallization.4.The stretch-induced crystallization behavior of structurally confined polymer systems was studied using molecular dynamics simulation method,including the crystallization behavior of graphene grafted polyethylene nanocomposite systems stretched to different strains at a strain rate of 1010s-1,as well as the effect of graphene content and size on the crystallization behavior of graphene grafted polyethylene nanocomposite systems stretched at a strain rate of 1010s-1 to a strain of 4.For graphene grafted polyethylene nanocomposite system under stretch induction,the results showed that stretching caused changes in the conformation and orientation of molecular chains in the system.During the stretching process,the degree of ordered orientation of chain segments in the structurally confined polymer system is higher than that in the unconfined polymer system,and the number of chain segments with trans conformation is also higher than that in the unconfined polymer system.At the same time,compared with the unconfined polymer system,the structurally confined polymer system formed more segments with longer all-trans conformational segments and higher orientation during the stretching process.Therefore,the content of precursors is also high,which leads to more crystal nuclei in the structurally confined polymer system than in the unconfined polymer system,and faster crystallization rate.In addition,stretching also affects the nucleation process and microstructure.At low strain,the molecular chains in the system tend to form crystal nuclei by intramolecular nucleation,while at high strain,the molecular chains in the system tend to form oriented microcrystals by intermolecular nucleation.For graphene grafted polyethylene nanocomposite systems with different graphene content and size,the results show that the extension degree and orientational order degree of chains first increased and then decreased with the increase of graphene content and size.This is because when the content of graphene in the system is too high,the distance between the fillers will be reduced,and the extension of the molecular chain will be limited,which is not conducive to the orderly arrangement of the molecular chains.And when the size of graphene in the system is too large,its movement rate will be reduced,which will affect the movement ability of the chains grafted on its surfaces,and is not conducive to the extension and orderly arrangement of the molecular chains.There is a critical value of filler content and a critical value of filler size in the system,at which the molecular chains in the system tend to form nuclei by intermolecular nucleation.Below this critical value,with the increase of graphene content or size,the local segments of the molecular chains tend to change from the gauche conformation to the trans conformation.At the same time,the segments with longer all-trans conformational segments and higher orientation in the system also increased,which increased the content of precursors in the system,and further led to the increase of the number of crystal nuclei and the acceleration of crystallization rate.In this paper,the crystallization behavior of confined polymer systems was systematically studied using computer simulation methods.The effects of factors such as the content and length of grafted chains,nanocylinder size and interfacial interaction on the crystallization rate,nucleation mode,chain conformation and motion,melting temperature,crystal orientation and morphology,and overall crystallinity of structural and spatial confined polymer systems were clarified.The nucleation site effect of the grafting point in the system where both structural and spatial constraints exist was confirmed,and the microscopic mechanism of the influence of the restricted chain content on the crystallization ability of the spatially confined polymer was revealed.The changes in molecular chain conformation and orientation,the formation process of precursors,and the influence of the presence of precursors on the nucleation ability of polymer systems under the combined effects of stretching,fillers and grafted chains were clarified.The simulation results obtained are conducive to the design and preparation of high-performance polymer nanocomposites in experiments and industrial production. |
