Differences in the Solid-State Structures of Single-Site and Ziegler−Natta Linear Low-Density Polyethylenes As Revealed by Molecular Dynamics Simulation

Abstract

Molecular dynamics (MD) simulations were carried out to study the solid-state structures of singlesite (ss) and Ziegler-Natta (ZN) linear low-density polyethylenes (LLDPE) at a temperature slightly below their melting temperatures. The two bulk state models, used to represent the polymers, possessed the same average branch content (10 hexyl branches per 1000 backbone carbons) but with different degrees of interchain branch distribution homogeneity. Both models were first equilibrated at 463 K (i.e., 190 °C) for several nanoseconds, and the resultant structures, which were found to be representative of the corresponding liquid-state structures, were then used as the initial structures for the subsequent quenching process. The quenching temperature was 373 K (i.e., 100 °C), and the structures were then equilibrated at the same temperature for a period of about 10 ns. The structures of the two polymers formed after the low-temperature equilibrations were considerably different. In particular, the ZN-LLDPE model exhibited a higher amount of order, as quantified by a higher trans/gauche ratio, and a longer "stem length" than those of the ss-LLDPE model. The hexyl branches in the ss-LLDPE model distributed more or less evenly in its interphase and amorphous phase while the branches in the ZN-LLDPE model concentrated in the amorphous phase. The concentration of tie molecules in the ss-LLDPE model was significantly higher than that of ZN-LLDPE. We believe that the structures revealed by the MD simulations correspond to those formed in the early stages of the crystallization process since the models and simulation times used precluded us from modeling the complete crystallization process. However, it is also believed that these structures should resemble the chain conformations of the polymers in their solid state because the available thermal energy at 373 K was not sufficient for further significant conformational rearrangements. The results found are consistent with the experimental findings that ZN-LLDPE solids tend to have a higher degree of crystallinity than ss-LLDPE with similar or even lower average branch content and that ss-LLDPE solids possess a higher concentration of tie molecules. Our simulation results also indicated that with the presence of highly branched chains linear chains tended to crystallize faster than the chains with branches, and this is consistent with the experimental observation of Mirabella that thicker lamellae form before the thinner ones [J.

Effect of Short Chain Branching on the Interlamellar Structure of Semicrystalline Polyethylene

“…This observation is similar to that observed by Wang et al in their experiments. This trend also appears to be consistent with the observations of Zhang et al from MD simulations, but probably for very different reasons, since the distributions of branch points in the current work were determined by thermodynamic considerations and not fixed by a certain topological distribution and short time kinetics. The observed trends are at odds with the lattice simulation results of Mattice et al − and with many proposed interpretations of experimental data.…”

“…However, lattice models are limited in their ability to represent accurately the conformers and packing interactions of real systems over distances of only a few bonds, and the model of Mattice and co-workers did not capture the difference in density between the crystalline and noncrystalline regions. Zhang et al 34 subsequently carried out molecular dynamics (MD) simulations of branched polyethylenes using a united atom (UA) model, with chains constructed to mimic material obtained using either ss or ZN catalysts. Upon quenching to 375 K, they observed the formation of small crystallites in both cases.…”

Predicting experimental results for polyethylene by computer simulation

“…In all these works, the characteristic general trends observed experimentally in SCB polyolefins are noted, such as rejection to the amorphous regions of branches longer than methyl, and a decrease in crystal size and melting temperatures with SCB content [121-123, 243, 244, 335-337].Very few simulations can be found to study the effects of SCB on polymer crystallization from the melt. Choi et al examined solid-state structures formed in the early stage of crystallization of different homogeneous and heterogeneous LLDPE models by means of MD simulations using the DREIDING FF [338,339]. These simulation results show that for homogenously branched systems, SCBs are distributed in both the interphase and the amorphous regions.…”

Dominant Effects of Short-Chain Branching on the Initial Stage of Nucleation and Formation of Tie Chains for Bimodal Polyethylene as Revealed by Molecular Dynamics Simulation

“…Ramos et al [43] reported that the lamellar thickening processes of the branched polyethylene samples were impeded when compared with that of the linear polyethylene system. Choi et al [44,45,46] investigated the branch effects on the structure of linear low-density polyethylene, and identified a critical branch content (38.5 SCB/1000 backbone carbons) where the branch length began to influence the order parameter. Rutledge et al [47] observed that the interfacial region’s thicknesses of linear polyethylene systems were bigger than those found for branched systems.…”