Molecular Dynamics Simulation Of Polyethylene Chain Crystallization

Polyethylene (PE) is one of the most important and most common commercial polymer products due to its good performance-price ratio, light weight and great end-use properties. Its annual production capacity is the largest and over 90 million tons in the world polymer market. Polyethylene properties are dominantly affected by chain structures, including chain length, branch content (BC), branch length (BL) and branch distribution. Mechanistic studies of chain structures effects are very important to elucidate structure-property relationship and further develop high-performance polyethylene. Molecular dynamics (MD) simulation is a powerful and advantageous method to reveal the polyethylene crystallization process at molecular level.The MD simulations in this work have provided a molecular level insight into the isothermal crystallization processes of a linear PE chain with chain length from C1000 to C10000 and short chains systems at a wide range of crystallization temperatures. Three crystallization states were observed in terms of a dominant and combined effects from the chain length with decreasing temperature:(i) nucleation controlled state, (ii) Competitive state of crystal growth process and new nuclei formation and (iii) crystal growth controlled state, and the crystallization state could be easily clarified by evolution of nuclei number. The crystallization states were determined by size effect:for a single chain crystallization in vacuum with different size effect, the crystallization states changed from state (i) to (iii) as chain length increased; while for short chains system with same size effect, chain length did not influence the crystallization states. The conformational changes were also in consistent with the changing of crystallization states. Long chain generated more nucleation site than short one and short chain favored the crystal growth process in both single chain and short chains system simulations.The non-isothermal crystallization processes with different cooling rate of linear PE chain with chain length from C100 to C14000 was also studied by MD simulation. The crystallization behavior was influenced by chain length and cooling rate:(a) single chain system:C100 and C150 was unable to fold to form the crystal. For C1000, C2000 and C3000, crystallization abilities strengthened as chain length increased. From C5000 to C14000, chain length has almost no influences on crystallization abilities. Final morphologies changed from rotator phase to single crystal domain, and to multi crystal domains as chain length increased, (b) Multi chains system:At slow cooling rate, longer PE chain gained better crystallization ability. At medium cooling rate, longer PE chain performed better crystallization ability in initial stage and shorter PE chain performed better crystallization ability in the final stage. In both isothermal and non-isothermal crystallization, the longer chain was more easily to form multi crystal domains structure than the shorter one at the same crystallization condition.The MD simulations in this work have also provided a molecular level insight into branch effects on precisely branched polyethylene crystallization. Crystallization kinetics and final morphologies were both dominantly influenced by branch content. Branch acts as a defect both in nucleation and crystal growth processes. Crystallization rate and crystallinity decreased as branch content increased (methylene sequence length decreased), and the morphologies became disordered. Final morphologies changed from lamellae crystal to bundle crystal. Branch length was important for the final morphologies when more inclusion of branches happened as branch length increased, but branch length had no obvious effect on crystallization kinetics.Trans-rich phenomenon in pre-crystalline state was observed for all chains in pre-crystalline state. The increase of trans state population was prior to the increase of crystallinity. Crystallization process begin when trans state population reaches a critical value at the end of induction stage, and this value is independent of branch content and branch length, both in isothermal and non-isothermal crystallization.