Numerical Study Of The Nucleation And Crystallization Of Polymers

The annual output of polymer products exceeds 400 million metric tons,and the trade volume of related industries exceeds 500 billion US dollars.Due to the characteristics of light weight,high strength,and easy processing of polymer materials,it has been widely utilized in production,life,research and other aspects.In all polymer products,70%are semi-crystalline polymers,that is,the polymer material is partially crystalline.The micro structure of semi-crystalline polymers such as crystal size,crystal orientation,crystallinity and other parameters fundamentally determine the performance of the product and affect the performance of the product in mechanical service,electrical devices,and optical components.Therefore,it is particularly important to understand the crystallization behavior of polymers,which helps us to control the micro structure of products and further design materials to improve product quality.In the past half century,the research on polymer crystallization has experienced vigorous development and achieved many results.Based on the classic nucleation theory,the Huffman-Lauritzen theory describing secondary nucleation was experimentally confirmed under certain conditions.The multi-step crystallization model and spinodal decomposition asisted crystallization developed at the end of the 20th century propose more possible explanation.However,there is no consensus on the crystallization behavior of polymers.The molecular mechanism of the transition from flexible coil to conformational ordered segments during crystallization has not yet been determined.What is the effect of long-chain connectivity on the crystallization?Moreover,previous discussions on the polymer crystallization have focused on the crystalline domains,and the amorphous part accompanying the crystal is equally important but rarely mentioned.What role does the amorphous play in the polymer crystallization process?These are all urgent questions.The crystallization behavior of spherical particles(such as colloidal particles,metal particles)and small molecule systems provides more ideas for the crystallization research of polymers.Taking colloidal particles as an example,the temporal and spatial resolution of experimental equipment enable to direct observation of nucleation process,and the state of art computer simulation technologies,molecular dynamics simulation and Monte Carlo simulation are able to study the nucleation process.Experimental and simulation studies have revealed the two-step or even multi-step nucleation process which are different with the classical nucleation theory.Among them,bond-orientational order or density fluctuation occur before nucleation.Polymer chain are more complex than spherical particles,and their crystallization is more difficult,which may be an orderly process of two or even more steps.Based on the above background,this thesis mainly uses molecular dynamics,Monte Carlo and other molecular simulation techniques to study the nucleation and crystallization behavior of long-chain polymers.The Monte Carlo method can directly sample in the phase space,so that the thermodynamic properties of the system can be directly studied,and the molecular dynamics can track the spatial trajectory of each particle,providing molecular-scale data for the study of crystallization dynamics.The specific research content and research results are as follows:(1)The homogeneous nucleation process of polyethylene under quiescent conditions is studied using all-atom molecular dynamics simulation.Considering the polymer's unique intra-chain conformation order and inter-chain orientation order structure,a local order parameter OCB was proposed to identify solid-like monomers,hexagonal-like structure and orthogonal nucleus,in the crystallization.We found that during the crystallization of the supercooled melt,the hexagonal-like local ordered structures first appeared.After that,the orthorhombic nuclei emergied inside the hexagonal-like structures,and the appearance of the orthorhombic nuclei was caused by the coalescing of neighboring hexagonal-like structures rather than the standard nucleation process.Therefore,we propose a two-step nucleation model for quiescent crystallization of polymer melts,that is,a nucleation process assisted by local ordered structure fluctuations.(2)The non-equilibrium molecular dynamics simulation is performed to study the nucleation process of polyethylene under shear flow.Similar to the quiescent case,the polyethylene crystallization also exhibits a two-step nucleation process under external flow.The density fluctuations occur before polyethylene nucleation,and nucleation takes happen within high-density regions.We also found that the density fluctuations originated from the coupling effect of intar-chain conformational ordering and inter-chain orientational ordering.The flow field forces the molecular chain to change from flexible coil to conformational ordered segments,which overcomes the free energy barrier of conformational transformation.At the same time,the flow field causes the conformationally ordered segments to aggregate into high-density regions,which promotes nucleation occur.The order of magnitude increase in nucleation rate under the flow field may be mainly attributed to the unique dynamic path of conformation/orientation order-density fluctuation-nucleation.(3)The crystallisation of long-chain polymers from their melt is investigated using computer simulation with a coarse-grained model.Using hybrid Monte Carlo simulations enables us to probe the behaviour of long polymer chains.We identify solidlike beads with a centrosymmetry local order parameter and compute the nucleation free-energy barrier at relatively high supercooling with adaptive-bias windowed umbrella sampling.Our results demonstrate that the critical nucleus sizes and the heights of free-energy barriers do not significantly depend on the molecular weight of the polymer;however,the nucleation rate decreases with increasing molecular weight.Moreover,an analysis of the composition of the critical nucleus suggests that intra-molecular growth of the nucleated cluster does not contribute significantly to crystallisation for this system.(4)We investigate the nucleation and crystallization behavior of a coarse-grained polymer melt within a two-component system,rigid rods and polymers.The structure factor of the initial stage of polymer crystallization suggests that so-called spinodal decomposition assisted nucleation did not occur in our simulation.Meanwhile,we study the influence of length and volume fraction of rigid rod on polymer crystallization,the results indicate that the crystallization kinetics is promoted when the length of rigid rod(lr)is longer than the persistence length of polymers(lp),and is depressed otherwise.Besides,it is observed that the thickening of lamellae would be suppressed by the increase in both the volume fraction and length of rigid rods.The main innovations are summarized as follows:(1)Combining intra-chain conformational order and inter-chain orientation order,a local order parameter for polyethylene systems was proposed,and the two-step nucleation process of polyethylene under quiescent condition was observed accordingly.(2)The non-equilibrium molecular dynamics simulations are used to study the nucleation pathway under the shear flow,and the molecular images of density fluctuations induced by the flow field are intuitively provided,which provides simulation support for the interpretation of experimental phenomena.(3)For the first time,the nucleation free energy profiles of long-chain polymers was calculated using the Hybrid Monte Carlo method.The molecular weight of the polymer does not affect the height of the free energy barrier of the primary nucleation,but the molecular weight has a significant effect on the nucleation rate due to changes in the mobility of molecular chains.(4)The initial stage of polymer crystallization was studied within a two-component system without phase separation,and the nucleation growth mode rather than spinodal decomposition upon polymer crystallization was confirmed.Meanwhile,the effects of rigid rods on polymer crystallization were systematically studied.