Molecular Dynamics Simulations of Stretch-Induced Crystal Changes in Crystallized Polyethylene/Carbon Nanotubes Nanocomposites

Understanding deformation mechanisms in semi-crystalline polymers during stretching is useful for guiding the processing of highperformance polymer products. In the current work, molecular dynamics simulations were performed to investigate the crystal changes in crystallized polyethylene/carbon nanotube nanocomposites during uniaxial stretching. Both crystal fragmentation and melting occur at low strains. Crystals with small sizes are easier to melt, while those with large sizes would break into smaller crystals. In addition, crystals in interfacial regions are more likely to melt or break due to the orientation motion of carbon nanotubes. It was also found that the recrystallization process is closely related to the stretch-induced orientation of chain segments. After orientation of chain segments along stretching direction is saturated,the recrystallization of highly oriented segments dominates. The current simulation findings are effective complements to the theories of the mechanism of plastic deformation in semicrystalline polymers.

Significantly Improved Stereocomplexation Ability in Cyclic Block Copolymers

Stereocomplex crystallization in cyclic polymer blend and cyclic block copolymers was investigated by means of dynamic Monte Carlo simulations.Five polymer systems(linear polymer blend,linear diblock copolymer,cyclic polymer blend,cyclic diblock copolymer and tetrablock copolymer)were established.It was interestingly found that the cyclic polymer blend exhibited the weakest stereocomplexation ability,while the two cyclic block copolymers showed stronger stereocomplexation ability than the linear diblock copolymer.This abnormal improved stereocomplexation ability of the cyclic block copolymers can be attributed to the synergy between the ring chain topology and the block copolymer structure.Compared with the linear block copolymers,the ring chain topology confined segmental motions of cyclic polymer chains to smaller regions,and then the segments belonging to the different blocks in the cyclic block copolymers have more chance to contact with each other.In this way,the cyclic block copolymers had better miscibility between segments belonging to different types of blocks,leading to the stronger stereocomplexation ability.

Stereocomplex Crystallization in Asymmetric Diblock Copolymers Studied by Dynamic Monte Carlo Simulations

Stereocomplex crystallization in asymmetric diblock copolymers was studied using dynamic Monte Carlo simulations,and the key factor dominating the formation of stereocomplex crystallites(SCs)was uncovered.The asymmetric diblock copolymers with higher degree of asymmetry exhibit larger difference between volume fractions of beads of different blocks,and local miscibility between different kinds of beads is lower,leading to lower SC content.To minimize the interference from volume fraction of beads,the SC formation in blends of asymmetric diblock copolymers was also studied.For the cases where the volume fractions of beads of different blocks are the same,similar local miscibility between beads of different blocks and similar SC content was observed.These findings indicate that the volume fraction of beads of different blocks is a key factor controlling the SC formation in the asymmetric diblock copolymers.The SC content can be regulated by adjusting the difference between the contents of beads of different blocks in asymmetric diblock copolymers.

Differences in Crystallization Behaviors between Cyclic and Linear Polymer Nanocomposites

Cyclic polymers exhibit fascinating crystallization behaviors owing to the absence of chain ends and more compact conformations.In the current simulation,dynamic Monte Carlo simulations were performed to reveal the underlying mechanism of the effect of chain topology and chain length on crystallization of polymer in solutions containing one-dimensional nanofiller.Simulation results suggested that the filled cyclic polymers exhibit higher melting temperature,higher crystallization temperature,and faster crystallization rate than the analogous linear polymers of identical chain length,especially in the systems with relatively short chains.Based on the Thomson-Gibbs equation,we theoretically analyzed the difference in the melting point between the cyclic and linear polymers under different chain lengths,and derived the dependence of the ratio of the melting point of the linear polymers to that of its cyclic analogs on chain length.In addition,it was also observed that the nanofiller can induce the formation of nanohybrid shish-kebab structure during isothermal crystallization of all systems.