Polymer semicrystalline texture made by interplay of crystal growth

Effects of Mechanical Stress on Insulation Structure and Performance of HV Cable

Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, new traps are created by broken molecular chains to limit the movement of charges, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective.

Investigation on the influence of fold conformation on PLLA lamellar splaying by film crystallization in supercritical CO2

Lamellar splaying is an important non-crystallographic phenomenon in polymer spherulite. We prepared three chain conformations (extended, tight-folded and loose-folded) by supercritical CO₂ to discuss the origin of lamellar splaying.

Intramolecular Crystal Nucleation Favored by Polymer Crystallization: Monte Carlo Simulation Evidence

We performed dynamic Monte Carlo simulations of half-half binary blends of symmetric (double and mutual) crystallizable polymers. We separately enhanced the driving forces for polymer-uniform and polymer-staggered crystals. Under parallel enhancements, polymer-uniform crystals exhibit faster nucleation and growth, with more chain folding and less lamellar thickening, than those in polymer-staggered crystals. We attributed the results to intramolecular crystal nucleation, ruined by enhanced polymer-staggered crystallization. Our observations provide direct molecular-level evidence to support the fact that intramolecular crystal nucleation is favored by polymer crystallization in quiescent solutions and melt, which yields chain folding for the characteristic β-sheet or lamellar morphology of macromolecular crystals.